ALBERT

Description: Hyrrokkin sarcophaga is a parasitic foraminifera that is commonly found in cold-water coral reefs where it infests the file clam Acesta excavata and the scleractinian coral Desmophyllum pertusum (formerly known as Lophelia pertusa). Here, we present measurements of the trace element and isotopic composition of these parasitic foraminifera, analyzed by inductively coupled optical emission spectrometry (ICP-OES), electron probe microanalysis (EPMA) and mass spectrometry (gas-source MS and inductively-coupled-plasma MS). Our results reveal that the geochemical signature of H. sarcophaga depends on the host organism it infests. Sr / Ca ratios are 1.1 mmol mol−1 higher in H. sarcophaga that infest D. pertusum, which could be an indication that dissolved host carbonate material is utilized in shell calcification, given that the aragonite of D. pertusum has a naturally higher Sr concentration compared to the calcite of A. excavata. Similarly, we measure 3.1 ‰ lower δ13C and 0.25 ‰ lower δ18O values in H. sarcophaga that lived on D. pertusum, which might be caused by the direct uptake of the host's carbonate material with a more negative isotopic composition or different pH regimes in these foraminifera (pH can exert a control on the extent of CO2 hydration/hydroxylation) due to the uptake of body fluids of the host. We also observe higher Mn / Ca ratios in foraminifera that lived on A. excavata but did not penetrate the host shell compared to specimen that penetrated the shell, which could be interpreted as a change in food source, changes in the calcification rate, Rayleigh fractionation or changing oxygen conditions. While our measurements provide an interesting insight into the calcification process of this unusual foraminifera, these data also indicate that the geochemistry of this parasitic foraminifera is unlikely to be a reliable indicator of paleoenvironmental conditions using Sr / Ca, Mn / Ca, δ18O or δ13C unless the host organism is known and its geochemical composition can be accounted for.

Description: Determining controls on the temperature sensitivity of heterotrophic soil respiration remains critical to incorporating soil–climate feedbacks into climate models. Most information on soil respiratory responses to temperature comes from laboratory incubations of isolated soils and typically subsamples of individual horizons. Inconsistencies between field and laboratory results may be explained by microbial priming supported by cross-horizon exchange of labile C or N. Such exchange is feasible in intact soil profiles but is absent when soils are isolated from surrounding depths. Here we assess the role of soil horizon connectivity, by which we mean the degree to which horizons remain layered and associated with each other as they are in situ, on microbial C and N substrate use and its relationship to the temperature sensitivity of respiration. We accomplished this by exploring changes in C : N, soil organic matter composition (via C : N, amino acid composition and concentration, and nuclear magnetic resonance spectroscopy), and the δ13C of respiratory CO2 during incubations of organic horizons collected across boreal forests in different climate regions where soil C and N compositions differ. The experiments consisted of two treatments: soil incubated (1) with each organic horizon separately and (2) as a whole organic profile, permitting cross-horizon exchange of substrates during the incubation. The soils were incubated at 5 and 15 ∘C for over 430 d. Enhanced microbial use of labile C-rich, but not N-rich, substrates were responsible for enhanced, whole-horizon respiratory responses to temperature relative to individual soil horizons. This impact of a labile C priming mechanism was most emergent in soils from the warmer region, consistent with these soils' lower C bioreactivity relative to soils from the colder region. Specifically, cross-horizon exchange within whole soil profiles prompted increases in mineralization of carbohydrates and more 13C-enriched substrates and increased soil respiratory responses to warming relative to soil horizons incubated in isolation. These findings highlight that soil horizon connectivity can impact microbial substrate use in ways that affect how soil effluxes of CO2 are controlled by temperature. The degree to which this mechanism exerts itself in other soils remains unknown, but these results highlight the importance of understanding mechanisms that operate in intact soil profiles – only rarely studied – in regulating a key soil–climate feedback.

Description: The deep chlorophyll maximum (DCM) is a well-known feature of the global ocean. However, its description and the study of its formation are a challenge, especially in the peculiar environment that is the Black Sea. The retrieval of chlorophyll a (chl a) from fluorescence (Fluo) profiles recorded by Biogeochemical Argo (BGC-Argo) floats is not trivial in the Black Sea, due to the very high content of coloured dissolved organic matter (CDOM) which contributes to the fluorescence signal and produces an apparent increase in the chl a concentration with depth. Here, we revised Fluo correction protocols for the Black Sea context using co-located in situ high-performance liquid chromatography (HPLC) and BGC-Argo measurements. The processed set of chl a data (2014–2019) is then used to provide a systematic description of the seasonal DCM dynamics in the Black Sea and to explore different hypotheses concerning the mechanisms underlying its development. Our results show that the corrections applied to the chl a profiles are consistent with HPLC data. In the Black Sea, the DCM begins to form in March, throughout the basin, at a density level set by the previous winter mixed layer. During a first phase (April–May), the DCM remains attached to this particular layer. The spatial homogeneity of this feature suggests a hysteresis mechanism, i.e. that the DCM structure locally influences environmental conditions rather than adapting instantaneously to external factors. In a second phase (July–September), the DCM migrates upward, where there is higher irradiance, which suggests the interplay of biotic factors. Overall, the DCM concentrates around 45 % to 65 % of the total chlorophyll content within a 10 m layer centred around a depth of 30 to 40 m, which stresses the importance of considering DCM dynamics when evaluating phytoplankton productivity at basin scale.

Description: Vegetation optical depth (VOD) retrieved from microwave radiometry correlates with the total amount of water in vegetation, based on theoretical and empirical evidence. Because the total amount of water in vegetation varies with relative water content (as well as with biomass), this correlation further suggests a possible relationship between VOD and plant water potential, a quantity that drives plant hydraulic behavior. Previous studies have found evidence for that relationship on the scale of satellite pixels tens of kilometers across, but these comparisons suffer from significant scaling error. Here we used small-scale remote sensing to test the link between remotely sensed VOD and plant water potential. We placed an L-band radiometer on a tower above the canopy looking down at red oak forest stand during the 2019 growing season in central Massachusetts, United States. We measured stem xylem and leaf water potentials of trees within the stand and retrieved VOD with a single-channel algorithm based on continuous radiometer measurements and measured soil moisture. VOD exhibited a diurnal cycle similar to that of leaf and stem water potential, with a peak at approximately 05:00 eastern daylight time (UTC−4). VOD was also positively correlated with both the measured dielectric constant and water potentials of stem xylem over the growing season. The presence of moisture on the leaves did not affect the observed relationship between VOD and stem water potential. We used our observed VOD–water-potential relationship to estimate stand-level values for a radiative transfer parameter and a plant hydraulic parameter, which compared well with the published literature. Our findings support the use of VOD for plant hydraulic studies in temperate forests.

Description: Dimethyl sulfide (DMS), dimethylsulfoniopropionate (DMSP) and dimethyl sulfoxide (DMSO) were measured at the Boknis Eck Time Series Station (BE, Eckernförde Bay, SW Baltic Sea) during the period February 2009–December 2018. Our results show considerable interannual and seasonal variabilities in the mixed-layer concentrations of DMS, total DMSP (DMSPt) and total DMSO (DMSOt). Positive correlations were found between particulate DMSP (DMSPp) and particulate DMSO (DMSOp) as well as DMSPt and DMSOt in the mixed layer, suggesting a similar source for both compounds. The decreasing long-term trends, observed for DMSPt and DMS in the mixed layer, were linked to the concurrent trend of the sum of 19′-hexanoyloxyfucoxanthin and 19′-butanoyloxy-fucoxanthin, which are the marker pigments of prymnesiophytes and chrysophytes, respectively. Major Baltic inflow (MBI) events influenced the distribution of sulfur compounds due to phytoplankton community changes, and sediment might be a potential source for DMS in the bottom layer during seasonal hypoxia/anoxia at BE. A modified algorithm based on the phytoplankton pigments reproduces the DMSPp : Chl a ratios well during this study and could be used to estimate future surface (5 m) DMSPp concentrations at BE.

Description: Multiple global change drivers affect plant productivity of grasslands and thus ecosystem services like forage production and the soil carbon sink. Subalpine grasslands seem particularly affected and may serve as a proxy for the cold, continental grasslands of the Northern Hemisphere. Here, we conducted a 4-year field experiment (AlpGrass) with 216 turf monoliths, subjected to three global change drivers: warming, moisture, and N deposition. Monoliths from six different subalpine pastures were transplanted to a common location with six climate scenario sites (CSs). CSs were located along an altitudinal gradient from 2360 to 1680 m a.s.l., representing an April–October mean temperature change of −1.4 to +3.0 ∘C, compared to CSreference with no temperature change and with climate conditions comparable to the sites of origin. To uncouple temperature effects along the altitudinal gradient from soil moisture and soil fertility effects, an irrigation treatment (+12 %–21 % of ambient precipitation) and an N-deposition treatment (+3 kg and +15 kg N ha−1 a−1) were applied in a factorial design, the latter simulating a fertilizing air pollution effect. Moderate warming led to increased productivity. Across the 4-year experimental period, the mean annual yield peaked at intermediate CSs (+43 % at +0.7 ∘C and +44 % at +1.8 ∘C), coinciding with ca. 50 % of days with less than 40 % soil moisture during the growing season. The yield increase was smaller at the lowest, warmest CS (+3.0 ∘C) but was still 12 % larger than at CSreference. These yield differences among CSs were well explained by differences in soil moisture and received thermal energy. Irrigation had a significant effect on yield (+16 %–19 %) in dry years, whereas atmospheric N deposition did not result in a significant yield response. We conclude that productivity of semi-natural, highly diverse subalpine grassland will increase in the near future. Despite increasingly limiting soil water content, plant growth will respond positively to up to +1.8 ∘C warming during the growing period, corresponding to +1.3 ∘C annual mean warming.

Description: The sea surface microlayer (SML) represents the boundary layer at the air–sea interface. Microbial eukaryotes in the SML potentially influence air–sea gas exchange directly by taking up and producing gases and indirectly by excreting and degrading organic matter, which may modify the viscoelastic properties of the SML. However, little is known about the distribution of microbial eukaryotes in the SML. We studied the composition of the microbial community, transparent exopolymer particles and polysaccharides in the SML during the PEACETIME cruise along a west–east transect in the Mediterranean Sea, covering the western basin, Tyrrhenian Sea and Ionian Sea. At the stations located in the Ionian Sea, fungi – likely of continental origin and delivered by atmospheric deposition – were found in high relative abundances, making up a significant proportion of the sequences recovered. Concomitantly, bacterial and picophytoplankton counts decreased from west to east, while transparent exopolymer particle (TEP) abundance and total carbohydrate (TCHO) concentrations remained constant in all basins. Our results suggest that the presence of substrates for fungi, such as Cladosporium, known to take up phytoplankton-derived polysaccharides, in combination with decreased substrate competition by bacteria, might favor fungal dominance in the neuston of the Ionian Sea and other low-nutrient, low-chlorophyll (LNLC) regions.

Description: Extreme events in the ocean severely impact marine organisms and ecosystems. Of particular concern are compound events, i.e., when conditions are extreme for multiple potential ocean ecosystem stressors such as temperature and chlorophyll. Yet, little is known about the occurrence, intensity, and duration of such compound high-temperature (a.k.a. marine heatwaves – MHWs) and low-chlorophyll (LChl) extreme events, whether their distributions have changed in the past decades, and what the potential drivers are. Here we use satellite-based sea surface temperature and chlorophyll concentration estimates to provide a first assessment of such compound extreme events. We reveal hotspots of compound MHW and LChl events in the equatorial Pacific, along the boundaries of the subtropical gyres, in the northern Indian Ocean, and around Antarctica. In these regions, compound events that typically last 1 week occur 3 to 7 times more often than expected under the assumption of independence between MHWs and LChl events. The occurrence of compound MHW and LChl events varies on seasonal to interannual timescales. At the seasonal timescale, most compound events occur in summer in both hemispheres. At the interannual timescale, the frequency of compound MHW and LChl events is strongly modulated by large-scale modes of natural climate variability such as the El Niño–Southern Oscillation, whose positive phase is associated with increased compound event occurrence in the eastern equatorial Pacific and in the Indian Ocean by a factor of up to 4. Our results provide a first understanding of where, when, and why compound MHW and LChl events occur. Further studies are needed to identify the exact physical and biological drivers of these potentially harmful events in the ocean and their evolution under global warming.

Description: Continental shelf sediments are places of both rapid organic carbon turnover and accumulation, while at the same time increasingly subjected to human-induced disturbances. Recent research suggests that shelf sediments might have a role to play as a natural climate solution, e.g. by storing organic carbon if left undisturbed from anthropogenic activity. However, we have an incomplete understanding about the centres of organic carbon accumulation and storage on continental shelves. To better constrain the rate of accumulation and the mass of organic carbon that is stored in sediments, we developed and applied a spatial modelling framework that allows us to estimate those quantities from sparse observations and predictor variables known or suspected to influence the spatial patterns of these parameters. This paper presents spatial distribution patterns of organic carbon densities and accumulation rates in the North Sea and Skagerrak. We found that organic carbon stocks and accumulation rates are highest in the Norwegian Trough, while large parts of the North Sea are characterised by low stocks and zero net accumulation. The total stock of organic carbon that is stored in the upper 0.1 m of sediments amounted to 230.5 ± 134.5 Tg C, of which approximately 26 % is stored in the Norwegian Trough. Rates of organic carbon accumulation in the Norwegian Trough are comparable with those reported from nearby fjords. We provide baseline datasets that could be used in marine management, e.g. for the establishment of “carbon protection zones”. Additionally, we highlight the complex nature of continental shelves with zones of rapid carbon cycling and accumulation juxtaposed, which will require further detailed and spatially explicit analyses to constrain sedimentary organic carbon stocks and accumulation rates globally.

Description: Complex microbial communities facilitate iron and methane transformations in anoxic methanic sediments of freshwater lakes, such as Lake Kinneret (the Sea of Galilee, Israel). The phylogenetic and functional diversity of these consortia are not fully understood, and it is not clear which lineages perform iron reduction and anaerobic oxidation of methane (AOM). Here, we investigated microbial communities from both natural Lake Kinneret iron-rich methanic sediments (〉20 cm depth) and iron-amended slurry incubations from this zone using metagenomics, focusing on functions associated with iron reduction and methane cycling. Analyses of the phylogenetic and functional diversity indicate that consortia of archaea (mainly Bathyarchaeia, Methanomicrobia, Thermoplasmata, and Thermococci) and bacteria (mainly Chloroflexi (Chloroflexota), Nitrospirae (Nitrospirota), and Proteobacteria) perform key metabolic reactions such as amino acid uptake and dissimilation, organic matter fermentation, and methanogenesis. The Deltaproteobacteria, especially Desulfuromondales (Desulfuromonadota), have the potential to transfer electrons extracellularly either to iron mineral particles or to microbial syntrophs, including methanogens. This is likely via transmembrane cytochromes, outer-membrane hexaheme c-type cytochrome (OmcS) in particular, or pilin monomers (PilA), all of which were attributed to this lineage. Bona fide anaerobic oxidizers of methane (ANME) and denitrifying methanotrophs Methylomirabilia (NC10) may mediate AOM in these methanogenic sediments; however we also consider the role of methanogens in active AOM or back flux of methanogenesis. Putative aerobes, such as methane-oxidizing bacteria Methylomonas and their methylotrophic syntrophs Methylotenera, are found among the anaerobic lineages in Lake Kinneret iron-amended slurries and are also involved in the oxidation of methane or its intermediates, as suggested previously. We propose a reaction model for the metabolic interactions in these sediments, linking the potential players that interact via intricate metabolic tradeoffs and direct electron transfer between species. Our results highlight the metabolic complexity of microbial communities in an energy-limited environment, where aerobe and anaerobe communities may co-exist and facilitate AOM as one strategy for survival.

Description: Boreal forest soils are globally an important sink for methane (CH4), while these soils are also capable of emitting CH4 under favourable conditions. Soil wetness is a well-known driver of CH4 flux, and the wetness can be estimated with several terrain indices developed for the purpose. The aim of this study was to quantify the spatial variability of the forest floor CH4 flux with a topography-based upscaling method connecting the flux with its driving factors. We conducted spatially extensive forest floor CH4 flux and soil moisture measurements, complemented by ground vegetation classification, in a boreal pine forest. We then modelled the soil moisture with a random forest model using digital-elevation-model-derived topographic indices, based on which we upscaled the forest floor CH4 flux. The modelling was performed for two seasons: May–July and August–October. Additionally, we evaluated the number of flux measurement points needed to get an accurate estimate of the flux at the whole study site merely by averaging. Our results demonstrate high spatial heterogeneity in the forest floor CH4 flux resulting from the soil moisture variability as well as from the related ground vegetation. The mean measured CH4 flux at the sample points was −5.07 µmol m−2 h−1 in May–July and −8.67 µmol m−2 h−1 in August–October, while the modelled flux for the whole area was −7.42 and −9.91 µmol m−2 h−1 for the two seasons, respectively. The spatial variability in the soil moisture and consequently in the CH4 flux was higher in the early summer (modelled range from −12.3 to 6.19 µmol m−2 h−1) compared to the autumn period (range from −14.6 to −2.12 µmol m−2 h−1), and overall the CH4 uptake rate was higher in autumn compared to early summer. In the early summer there were patches emitting high amounts of CH4; however, these wet patches got drier and smaller in size towards the autumn, changing their dynamics to CH4 uptake. The mean values of the measured and modelled CH4 fluxes for the sample point locations were similar, indicating that the model was able to reproduce the results. For the whole site, upscaling predicted stronger CH4 uptake compared to simply averaging over the sample points. The results highlight the small-scale spatial variability of the boreal forest floor CH4 flux and the importance of soil chamber placement in order to obtain spatially representative CH4 flux results. To predict the CH4 fluxes over large areas more reliably, the locations of the sample points should be selected based on the spatial variability of the driving parameters, in addition to linking the measured fluxes with the parameters.

Description: Changes in the vegetation of Brazilian Cerrado may occur over time. However, long-term dynamics are not fully understood yet, especially woody plant encroachment (WPE). The objective of this study was to examine changes in vegetation structure in a preserved area in Triângulo Mineiro region, within the southern Brazilian Cerrado domain, over 32 years (1987, 2005, and 2019). We based the study on field and literature surveys, as well as satellite imagery, and hypothesized that, due to the absence of periodic fires or grazing, Cerrado open formations (i.e., grassland or savanna) tend to become denser due to WPE. Shrubby grassland cover assessed in 1987 disappeared in the following periods (from 30.0 % to 0.0 % in 2019) while forest formations increased (from 7.0 % in 1987 to 11.0 % in 2019). Changes between 2005 and 2019 occurred within the stricto sensu cerrado subdivisions, with reduction of sparse cerrado (from 34.2 % to 7.7 %) and an increase in dense cerrado (from 6.9 % to 39.8 %). Normalized difference vegetation index (NDVI) applied for similar periods indicates a progressive increase of values over time (from 1986 (0.61±0.10) to 2004 (0.65±0.06) and 2018 (0.78±0.05)) and corroborates the WPE process. These patterns imply the loss of biodiversity in open plant formation. Another major consequence was the reduction of wetlands and possible impact on water supply. Such patterns are important to support plant management plans for the threatened Cerrado open plant formations.

Description: The El Niño‐-Southern Oscillation (ENSO) influences the global climate and the variability in the terrestrial carbon cycle on interannual timescales. Two different expressions of El Niño have recently been identified: (i) central Pacific (CP) and (ii) eastern Pacific (EP). Both types of El Niño are characterised by above-average sea surface temperature anomalies at the respective locations. Studies exploring the impact of these expressions of El Niño on the carbon cycle have identified changes in the amplitude of the concentration of interannual atmospheric carbon dioxide (CO2) variability following increased tropical near-surface air temperature and decreased precipitation. We employ the dynamic global vegetation model LPJ-GUESS (Lund–Potsdam–Jena General Ecosystem Simulator) within a synthetic experimental framework to examine the sensitivity and potential long-term impacts of these two expressions of El Niño on the terrestrial carbon cycle. We manipulated the occurrence of CP and EP events in two climate reanalysis datasets during the latter half of the 20th and early 21st century by replacing all EP with CP and separately all CP with EP El Niño events. We found that the different expressions of El Niño affect interannual variability in the terrestrial carbon cycle. However, the effect on longer timescales was small for both climate reanalysis datasets. We conclude that capturing any future trends in the relative frequency of CP and EP El Niño events may not be critical for robust simulations of the terrestrial carbon cycle.

Description: The Mg/Ca and Sr/Ca ratios of marine shells have been widely used in environmental paleoreconstructions to understand past marine conditions. Temperature calibrations to ostracod Mg/Ca ratios are known to be species-specific but only available for a few species, despite the large number of known ostracod species. Here, we develop temperature calibrations for two shallow marine ostracods, Sinocytheridea impressa and Neomonoceratina delicata, using modern sediment samples. Our results show that adult specimens of these two species might be useful as a paleothermometer. We observed significant correlations using the Mg/Ca ratios of both species to the annual (Mg/CaS. impressa=3.7 ⋅ T−62.7; Mg/CaN. delicata=1.6 ⋅ T−16.8) and April (Mg/CaS. impressa=2.8 ⋅ T−39.2; Mg/CaN. delicata=1.6 ⋅ T−15.7) temperatures. The correlation of temperature to the Mg/Ca ratio of S. impressa is more significant and therefore should be preferred for paleoreconstructions. Re-analysis from satellite data allows us to validate our temperature calibration to an extended area around the Pearl River estuary. Our results show that Mg/Ca of S. impressa and N. delicata ostracods can be used to reconstruct water temperature at a regional scale, which provides information on the oceanic circulation in coastal areas of the South China Sea. Sr/Ca ratios of both species do not correlate with any of the 24 water parameters recorded by the Environmental Protection Department of Hong Kong, including temperature (21.7–24.1 ∘C), salinity (23.8–33.7 PSU), dissolved oxygen (4.3–7.1 mg L−1), suspended solids (1.9–35.4 mg L−1) and pH (7.7–8.2).

Description: Evolution of organic carbon content in soils has the potential to be a major driver of atmospheric greenhouse gas concentrations over the next century. Understanding soil carbon dynamics is a challenge due to a wide range of residence times of soil organic matter and limited constraints on the mechanisms influencing its persistence. In particular, large uncertainties exist regarding the persistence of pyrogenic organic carbon in soils. In order to characterize organic matter with varying degrees of persistence and to distinguish pyrogenic organic carbon, we combined Rock-Eval analysis, a thermo-chemical method, with the benzene polycarboxylic acid molecular marker method and Raman spectroscopy to characterize samples from long-term bare-fallow experiments, progressively depleted in the most labile organic carbon over time. Considering the heterogeneity of soil samples, size fractions have been separated to distinguish pools of organic carbon with distinct properties. We observe that organic carbon dynamics is dependent on granulometry. A pool of organic carbon with intermediate residence times, from years to a few decades, representing ca. 65 % of the bulk soil organic carbon stock, is mainly associated with fine fractions ( 20 µm) are rich in centennially persistent organic carbon, representing ca. 20 % of the initial organic carbon stock, due to the chemical recalcitrance of organic matter in these fractions, dominated by pyrogenic organic carbon. A second pool of persistent organic carbon, representing ca. 15 % of the initial organic carbon stock, is associated with the clay fraction, indicating mechanisms of protection occurring at the submicron scale (

Description: Relative to their surface area, estuaries make a disproportionately large contribution of dissolved organic carbon (DOC) to the global carbon cycle, but it is unknown how this will change under a future climate. As such, the response of DOC fluxes from microbially dominated unvegetated sediments to individual and combined future climate stressors of temperature change (from Δ−3 to Δ+5 ∘C compared to ambient mean temperatures) and ocean acidification (OA, ∼ 2× current CO2 partial pressure, pCO2) was investigated ex situ. Warming alone increased sediment heterotrophy, resulting in a proportional increase in sediment DOC uptake; sediments became net sinks of DOC (3.5 to 8.8 mmol C m−2 d−1) at warmer temperatures (Δ+3 and Δ+5 ∘C, respectively). This temperature response changed under OA conditions, with sediments becoming more autotrophic and a greater sink of DOC (up to 4× greater than under current pCO2 conditions). This response was attributed to the stimulation of heterotrophic bacteria with the autochthonous production of labile organic matter by microphytobenthos. Extrapolating these results to the global area of unvegetated subtidal estuarine sediments, we find that the future climate of warming (Δ+3 ∘C) and OA may decrease estuarine export of DOC by ∼ 80 % (∼ 150 Tg C yr−1) and have a disproportionately large impact on the global DOC budget.

Description: Tropical ecosystems contribute significantly to global emissions of methane (CH4), and landscape topography influences the rate of CH4 emissions from wet tropical forest soils. However, extreme events such as drought can alter normal topographic patterns of emissions. Here we explain the dynamics of CH4 emissions during normal and drought conditions across a catena in the Luquillo Experimental Forest, Puerto Rico. Valley soils served as the major source of CH4 emissions in a normal precipitation year (2016), but drought recovery in 2015 resulted in dramatic pulses in CH4 emissions from all topographic positions. Geochemical parameters including (i) dissolved organic carbon (C), acetate, and soil pH and (ii) hydrological parameters like soil moisture and oxygen (O2) concentrations varied across the catena. During the drought, soil moisture decreased in the slope and ridge, and O2 concentrations increased in the valley. We simulated the dynamics of CH4 emissions with the Microbial Model for Methane Dynamics-Dual Arrhenius and Michaelis–Menten (M3D-DAMM), which couples a microbial functional group CH4 model with a diffusivity module for solute and gas transport within soil microsites. Contrasting patterns of soil moisture, O2, acetate, and associated changes in soil pH with topography regulated simulated CH4 emissions, but emissions were also altered by rate-limited diffusion in soil microsites. Changes in simulated available substrate for CH4 production (acetate, CO2, and H2) and oxidation (O2 and CH4) increased the predicted biomass of methanotrophs during the drought event and methanogens during drought recovery, which in turn affected net emissions of CH4. A variance-based sensitivity analysis suggested that parameters related to aceticlastic methanogenesis and methanotrophy were most critical to simulate net CH4 emissions. This study enhanced the predictive capability for CH4 emissions associated with complex topography and drought in wet tropical forest soils.

Description: Can experimental studies on the behavioural impacts of ocean acidification be trusted? That question was raised in early 2020 when a high-profile paper failed to corroborate previously observed responses of coral reef fish to high CO2. New information on the methodologies used in the “replicated” studies now provides a plausible explanation: the experimental conditions were substantially different. High sensitivity to test conditions is characteristic of ocean acidification research; such response variability shows that effects are complex, interacting with many other factors. Open-minded assessment of all research results, both negative and positive, remains the best way to develop process-based understanding. As in other fields, replication studies in ocean acidification are most likely to contribute to scientific advancement when carried out in a spirit of collaboration rather than confrontation.

Description: Researchers have known for decades that silicon plays a major role in biogeochemical and plant–soil processes in terrestrial systems. Meanwhile, plant biologists continue to uncover a growing list of benefits derived from silicon to combat abiotic and biotic stresses, such as defense against herbivory. Yet despite growing recognition of herbivores as important ecosystem engineers, many major gaps remain in our understanding of how silicon and herbivory interact to shape biogeochemical processes, particularly in natural systems. We review and synthesize 119 available studies directly investigating silicon and herbivory to summarize key trends and highlight research gaps and opportunities. Categorizing studies by multiple ecosystem, plant, and herbivore characteristics, we find substantial evidence for a wide variety of important interactions between plant silicon and herbivory but highlight the need for more research particularly in non-graminoid-dominated vegetation outside of the temperate biome as well as on the potential effects of herbivory on silicon cycling. Continuing to overlook silicon–herbivory dynamics in natural ecosystems limits our understanding of potentially critical animal–plant–soil feedbacks necessary to inform land management decisions and to refine global models of environmental change.

Description: Arctic coastal ecosystems are rapidly changing due to climate warming. This makes modeling their productivity crucially important to better understand future changes. System primary production in these systems is highest during the pronounced spring bloom, typically dominated by diatoms. Eventually the spring blooms terminate due to silicon or nitrogen limitation. Bacteria can play an important role for extending bloom duration and total CO2 fixation through ammonium regeneration. Current ecosystem models often simplify the effects of nutrient co-limitations on algal physiology and cellular ratios and simplify nutrient regeneration. These simplifications may lead to underestimations of primary production. Detailed biochemistry- and cell-based models can represent these dynamics but are difficult to tune in the environment. We performed a cultivation experiment that showed typical spring bloom dynamics, such as extended algal growth via bacterial ammonium remineralization, reduced algal growth and inhibited chlorophyll synthesis under silicate limitation, and gradually reduced nitrogen assimilation and chlorophyll synthesis under nitrogen limitation. We developed a simplified dynamic model to represent these processes. Overall, model complexity in terms of the number of parameters is comparable to the phytoplankton growth and nutrient biogeochemistry formulations in common ecosystem models used in the Arctic while improving the representation of nutrient-co-limitation-related processes. Such model enhancements that now incorporate increased nutrient inputs and higher mineralization rates in a warmer climate will improve future predictions in this vulnerable system.

Description: Continental shelf regions in the ocean play an important role in the global cycling of carbon and nutrients, but their responses to global change are understudied. Global Earth system models (ESMs), as essential tools for building understanding of ocean biogeochemistry, are used extensively and routinely for projections of future climate states; however, their relatively coarse spatial resolution is likely not appropriate for accurately representing the complex patterns of circulation and elemental fluxes on the shelves along ocean margins. Here, we compared 29 ESMs used in the Intergovernmental Panel on Climate Change (IPCC)'s Assessment Reports (ARs) 5 and 6 and a regional biogeochemical model for the northwest North Atlantic (NWA) shelf to assess their ability to reproduce surface observations of temperature, salinity, nitrate and chlorophyll. The NWA region is biologically productive, influenced by the large-scale Gulf Stream and Labrador Current systems and particularly sensitive to climatically induced changes in large-scale circulation. Most ESMs compare relatively poorly to observed surface nitrate and chlorophyll and show differences with observed surface temperature and salinity that suggest spatial mismatches in their large-scale current systems. Model-simulated nitrate and chlorophyll compare better with available observations in AR6 than in AR5, but none of the models perform equally well for all four parameters. The ensemble means of all ESMs, and of the five best-performing ESMs, strongly underestimate observed chlorophyll and nitrate. The regional model has a much higher spatial resolution and reproduces the observations significantly better than any of the ESMs. It also simulates reasonably well vertically resolved observations from gliders and bi-monthly ship-based monitoring observations. A ranking of the ESMs indicates that only one ESM has good and consistent performance for all variables. An additional evaluation of the ESMs along the regional model boundaries shows larger variability but is generally consistent with the ranking on the shelf. Overall, 11 ESMs were deemed satisfactory for use in the NWA, either directly or for regional downscaling.

Description: Coccolithophores and other haptophyte algae acquire the carbon required for metabolic processes from the water in which they live. Whether carbon is actively moved across the cell membrane via a carbon concentrating mechanism, or passively through diffusion, is important for haptophyte biochemistry. The possible utilization of carbon concentrating mechanisms also has the potential to over-print one proxy method by which ancient atmospheric CO2 concentration is reconstructed using alkenone isotopes. Here I show that carbon concentrating mechanisms are likely used when aqueous carbon dioxide concentrations are below 7 µmol L−1. I compile published alkenone-based CO2 reconstructions from multiple sites over the Pleistocene and recalculate them using a common methodology, which allows comparison to be made with ice core CO2 records. Interrogating these records reveals that the relationship between proxy CO2 and ice core CO2 breaks down when local aqueous CO2 concentration falls below 7 µmol L−1. The recognition of this threshold explains why many alkenone-based CO2 records fail to accurately replicate ice core CO2 records, and it suggests the alkenone proxy is likely robust for much of the Cenozoic when this threshold was unlikely to be reached in much of the global ocean.

Description: Forest steppes are dynamic ecosystems, highly susceptible to changes in climate, disturbances and land use. Here we examine the Holocene history of the European forest steppe ecotone in the lower Danube Plain to better understand its sensitivity to climate fluctuations, fire and human impact, and the timing of its transition into a cultural forest steppe. We used multi-proxy analyses (pollen, n-alkanes, coprophilous fungi, charcoal and geochemistry) of a 6000-year sequence from Lake Oltina (southeastern Romania) combined with a REVEALS (Regional Estimates of Vegetation Abundance from Large Sites) model of quantitative vegetation cover. We found a greater tree cover, composed of xerothermic (Carpinus orientalis and Quercus) and temperate (Carpinus betulus, Tilia, Ulmus and Fraxinus) tree taxa, between 6000 and 2500 cal yr BP. Maximum tree cover (∼ 50 %), dominated by C. orientalis occurred between 4200 and 2500 cal yr BP at a time of wetter climatic conditions and moderate fire activity. Compared to other European forest steppe areas, the dominance of C. orientalis represents the most distinct feature of the woodland's composition at this time. Tree loss was underway by 2500 yr BP (Iron Age), with the REVEALS model indicating a fall to ∼ 20 % tree cover from the Late Holocene forest maximum, linked to clearance for agriculture, while climate conditions remained wet. Biomass burning increased markedly at 2500 cal yr BP, suggesting that fire was regularly used as a management tool until 1000 cal yr BP when woody vegetation became scarce. A sparse tree cover, with only weak signs of forest recovery, then became a permanent characteristic of the lower Danube Plain, highlighting more or less continuous anthropogenic pressure. The timing of anthropogenic ecosystem transformation here (2500 cal yr BP) falls between that in central-eastern (between 3700 and 3000 cal yr BP) and eastern (after 2000 cal yr BP) Europe. Our study is the first quantitative land cover estimate at the forest steppe ecotone in southeastern Europe spanning 6000 years. It provides critical empirical evidence that, at a broad spatial scale, the present-day forest steppe and woodlands reflect the potential natural vegetation in this region under current climate conditions. However, the extent of tree cover and its composition have been neither stable in time nor shaped solely by the climate. Consequently, vegetation change must be seen as dynamic and reflecting wider changes in environmental conditions including natural disturbances and human impact.

Description: Close-to-nature management (CTNM) has been proposed as a promising forestry management approach to improve the structure and quality of forests, which integrates wood production and ecological service functions. Research on the effect of CTNM on the univariate and bivariate distribution of the spatial structure of forest stands provides a scientific basis for the evaluation of CTNM implemented in forestry. Here, we analyzed and compared the spatial-structure characteristics of Masson pine (Pinus massoniana) plantations (young, middle-age, and near-mature stages) under CTNM 8 years after selective cutting and unmanaged control. We used univariate and bivariate distribution of three spatial-structure parameters: mingling (M), dominance (U), and uniform-angle index (W). Results showed that the effect of CTNM on spatial structure was more remarkable in middle-aged and near-mature Masson pine forests compared with the young forest. CTNM significantly improved mingling degree and promoted the horizontal distribution, thereby changing from a cluster to a random distribution. Moreover, CTNM improved the proportion of trees with a high mixing degree and random distribution and the proportion of trees having a micro-structure of random distribution with a high degree of mixture and dominance with a high degree of mixture in middle-aged and near-mature Masson pine forest. Overall, the implementation of CTNM 8 years ago showed a positive effect on the improvement of the spatial structure of Masson pine forest, but the present spatial structure is suboptimal. Further implementation of CTNM to adjust the mingling and uniform-angle index is necessary, and CTNM according to this method of frequency distribution of stand structure parameters can improve the success of forest management.

Description: The prediction of nitrous oxide (N2O) and of dinitrogen (N2) emissions formed by biotic denitrification in soil is notoriously difficult due to challenges in capturing co-occurring processes at microscopic scales. N2O production and reduction depend on the spatial extent of anoxic conditions in soil, which in turn are a function of oxygen (O2) supply through diffusion and O2 demand by respiration in the presence of an alternative electron acceptor (e.g. nitrate). This study aimed to explore controlling factors of complete denitrification in terms of N2O and (N2O + N2) fluxes in repacked soils by taking micro-environmental conditions directly into account. This was achieved by measuring microscale oxygen saturation and estimating the anaerobic soil volume fraction (ansvf) based on internal air distribution measured with X-ray computed tomography (X-ray CT). O2 supply and demand were explored systemically in a full factorial design with soil organic matter (SOM; 1.2 % and 4.5 %), aggregate size (2–4 and 4–8 mm), and water saturation (70 %, 83 %, and 95 % water-holding capacity, WHC) as factors. CO2 and N2O emissions were monitored with gas chromatography. The 15N gas flux method was used to estimate the N2O reduction to N2. N gas emissions could only be predicted well when explanatory variables for O2 demand and O2 supply were considered jointly. Combining CO2 emission and ansvf as proxies for O2 demand and supply resulted in 83 % explained variability in (N2O + N2) emissions and together with the denitrification product ratio [N2O / (N2O + N2)] (pr) 81 % in N2O emissions. O2 concentration measured by microsensors was a poor predictor due to the variability in O2 over small distances combined with the small measurement volume of the microsensors. The substitution of predictors by independent, readily available proxies for O2 demand (SOM) and O2 supply (diffusivity) reduced the predictive power considerably (60 % and 66 % for N2O and (N2O+N2) fluxes, respectively). The new approach of using X-ray CT imaging analysis to directly quantify soil structure in terms of ansvf in combination with N2O and (N2O + N2) flux measurements opens up new perspectives to estimate complete denitrification in soil. This will also contribute to improving N2O flux models and can help to develop mitigation strategies for N2O fluxes and improve N use efficiency.

Description: Carbon dioxide (CO2) emissions from running waters represent a key component of the global carbon cycle. However, quantifying CO2 fluxes across air–water boundaries remains challenging due to practical difficulties in the estimation of reach-scale standardized gas exchange velocities (k600) and water equilibrium concentrations. Whereas craft-made floating chambers supplied by internal CO2 sensors represent a promising technique to estimate CO2 fluxes from rivers, the existing literature lacks rigorous comparisons among differently designed chambers and deployment techniques. Moreover, as of now the uncertainty of k600 estimates from chamber data has not been evaluated. Here, these issues were addressed by analysing the results of a flume experiment carried out in the Summer of 2019 in the Lunzer:::Rinnen – Experimental Facility (Austria). During the experiment, 100 runs were performed using two different chamber designs (namely, a standard chamber and a flexible foil chamber with an external floating system and a flexible sealing) and two different deployment modes (drifting and anchored). The runs were performed using various combinations of discharge and channel slope, leading to variable turbulent kinetic energy dissipation rates (1.5×10-3

Description: In the ocean, remineralization rate associated with sinking particles is a crucial variable. Since the 1990s, particulate biogenic barium (Baxs) has been used as an indicator of carbon remineralization by applying a transfer function relating Baxs to O2 consumption (Dehairs's transfer function, Southern Ocean-based). Here, we tested its validity in the Mediterranean Sea (ANTARES/EMSO-LO) for the first time by investigating connections between Baxs, prokaryotic heterotrophic production (PHP) and oxygen consumption (JO2-Opt; optodes measurement). We show that (1) higher Baxs (409 pM; 100–500 m) occurs in situations where integrated PHP (PHP100/500=0.90) is located deeper, (2) higher Baxs occurs with increasing JO2-Opt, and (3) there is similar magnitude between JO2-Opt (3.14 mmol m−2 d−1; 175–450 m) and JO2-Ba (4.59 mmol m−2 d−1; transfer function). Overall, Baxs, PHP and JO2 relationships follow trends observed earlier in the Southern Ocean. We conclude that such a transfer function could apply in the Mediterranean Sea.

Description: Eastern boundary upwelling ecosystems (EBUEs) are among the most productive marine regions in the world's oceans. Understanding the degree of interannual to decadal variability in the Mauritania upwelling system is crucial for the prediction of future changes of primary productivity and carbon sequestration in the Canary Current EBUE as well as in similar environments. A multiyear sediment trap experiment was conducted at the mooring site CBmeso (“Cape Blanc mesotrophic”, ca. 20∘ N, ca. 20∘40′ W) in the highly productive coastal waters off Mauritania. Here, we present results on fluxes of diatoms and the species-specific composition of the assemblage for the time interval between March 1988 and June 2009. The temporal dynamics of diatom populations allows the proposal of three main intervals: (i) early 1988–late 1996, (ii) 1997–1999, and (iii) early 2002–mid 2009. The Atlantic Multidecadal Oscillation (AMO) appears to be an important driver of the long-term dynamics of diatom population. The long-term AMO-driven trend is interrupted by the occurrence of the strong 1997 El Niño–Southern Oscillation (ENSO). The extraordinary shift in the relative abundance of benthic diatoms in May 2002 suggests the strengthening of offshore advective transport within the uppermost layer of filament waters and in the subsurface and in deeper and bottom-near layers. It is hypothesized that the dominance of benthic diatoms was the response of the diatom community to the intensification of the slope and shelf poleward undercurrents. This dominance followed the intensification of the warm phase of AMO and the associated changes of the Atlantic Meridional Overturning Circulation. Transported valves (siliceous remains) from shallow Mauritanian coastal waters into the bathypelagic should be considered for the calculation and model experiments of bathy- and pelagic nutrients budgets (especially Si), the burial of diatoms, and the paleoenvironmental signal preserved in downcore sediments. Additionally, our 1988–2009 data set contributes to the characterization of the impact of low-frequency climate forcings in the northeastern Atlantic and will be especially helpful for establishing the scientific basis for forecasting and modeling future states of the Canary Current EBUE and its decadal changes.

Description: The deep chlorophyll maximum (DCM) is a ubiquitous feature of phytoplankton vertical distribution in stratified waters that is relevant to our understanding of the mechanisms that underpin the variability in photoautotroph ecophysiology across environmental gradients and has implications for remote sensing of aquatic productivity. During the PEACETIME (Process studies at the air-sea interface after dust deposition in the Mediterranean Sea) cruise, carried out from 10 May to 11 June 2017, we obtained 23 concurrent vertical profiles of phytoplankton chlorophyll a, carbon biomass and primary production, as well as heterotrophic prokaryotic production, in the western and central Mediterranean basins. Our main aims were to quantify the relative role of photoacclimation and enhanced growth as underlying mechanisms of the DCM and to assess the trophic coupling between phytoplankton and heterotrophic prokaryotic production. We found that the DCM coincided with a maximum in both the biomass and primary production but not in the growth rate of phytoplankton, which averaged 0.3 d−1 and was relatively constant across the euphotic layer. Photoacclimation explained most of the increased chlorophyll a at the DCM, as the ratio of carbon to chlorophyll a (C:Chl a) decreased from ca. 90–100 (g:g) at the surface to 20–30 at the base of the euphotic layer, while phytoplankton carbon biomass increased from ca. 6 mg C m−3 at the surface to 10–15 mg C m−3 at the DCM. As a result of photoacclimation, there was an uncoupling between chlorophyll a-specific and carbon-specific productivity across the euphotic layer. The ratio of fucoxanthin to total chlorophyll a increased markedly with depth, suggesting an increased contribution of diatoms at the DCM. The increased biomass and carbon fixation at the base of the euphotic zone was associated with enhanced rates of heterotrophic prokaryotic activity, which also showed a surface peak linked with warmer temperatures. Considering the phytoplankton biomass and turnover rates measured at the DCM, nutrient diffusive fluxes across the nutricline were able to supply only a minor fraction of the photoautotroph nitrogen and phosphorus requirements. Thus the deep maxima in biomass and primary production were not fuelled by new nutrients but likely resulted from cell sinking from the upper layers in combination with the high photosynthetic efficiency of a diatom-rich, low-light acclimated community largely sustained by regenerated nutrients. Further studies with increased temporal and spatial resolution will be required to ascertain if the peaks of deep primary production associated with the DCM persist across the western and central Mediterranean Sea throughout the stratification season.

Description: Trace elements (TEs) play important roles as micronutrients in modulating marine productivity in the global ocean. The South Atlantic around 40∘ S is a prominent region of high productivity and a transition zone between the nitrate-depleted subtropical gyre and the iron-limited Southern Ocean. However, the sources and fluxes of trace elements to this region remain unclear. In this study, the distribution of the naturally occurring radioisotope 228Ra in the water column of the South Atlantic (Cape Basin and Argentine Basin) has been investigated along a 40∘ S zonal transect to estimate ocean mixing and trace element supply to the surface ocean. Ra-228 profiles have been used to determine the horizontal and vertical mixing rates in the near-surface open ocean. In the Argentine Basin, horizontal mixing from the continental shelf to the open ocean shows an eddy diffusion of Kx=1.8±1.4 (106 cm2 s−1) and an integrated advection velocity w=0.6±0.3 cm s−1. In the Cape Basin, horizontal mixing is Kx=2.7±0.8 (107 cm2 s−1) and vertical mixing Kz = 1.0–1.7 cm2 s−1 in the upper 600 m layer. Three different approaches (228Ra diffusion, 228Ra advection, and 228Ra/TE ratio) have been applied to estimate the dissolved trace element fluxes from the shelf to the open ocean. These approaches bracket the possible range of off-shelf fluxes from the Argentine Basin margin to be 4–21 (×103) nmol Co m−2 d−1, 8–19 (×104) nmol Fe m−2 d−1 and 2.7–6.3 (×104) nmol Zn m−2 d−1. Off-shelf fluxes from the Cape Basin margin are 4.3–6.2 (×103) nmol Co m−2 d−1, 1.2–3.1 (×104) nmol Fe m−2 d−1, and 0.9–1.2 (×104) nmol Zn m−2 d−1. On average, at 40∘ S in the Atlantic, vertical mixing supplies 0.1–1.2 nmol Co m−2 d−1, 6–9 nmol Fe m−2 d−1, and 5–7 nmol Zn m−2 d−1 to the euphotic zone. Compared with atmospheric dust and continental shelf inputs, vertical mixing is a more important source for supplying dissolved trace elements to the surface 40∘ S Atlantic transect. It is insufficient, however, to provide the trace elements removed by biological uptake, particularly for Fe. Other inputs (e.g. particulate or from winter deep mixing) are required to balance the trace element budgets in this region.

Description: We present a new natural carbon dioxide (CO2) system located off the southern coast of the island of La Palma (Canary Islands, Spain). Like CO2 seeps, these CO2 submarine groundwater discharges (SGDs) can be used as an analogue to study the effects of ocean acidification (OA) on the marine realm. With this aim, we present the chemical characterization of the area, describing the carbon system dynamics, by measuring pH, AT and CT and calculating Ω aragonite and calcite. Our explorations of the area have found several emission points with similar chemical features. Here, the CT varies from 2120.10 to 10 784.84 µmol kg−1, AT from 2415.20 to 10 817.12 µmol kg−1, pH from 7.12 to 8.07, Ω aragonite from 0.71 to 4.15 and Ω calcite from 1.09 to 6.49 units. Also, the CO2 emission flux varies between 2.8 and 28 kg CO2 d−1, becoming a significant source of carbon. These CO2 emissions, which are of volcanic origin, acidify the brackish groundwater that is discharged to the coast and alter the local seawater chemistry. Although this kind of acidified system is not a perfect image of future oceans, this area of La Palma is an exceptional spot to perform studies aimed at understanding the effect of different levels of OA on the functioning of marine ecosystems. These studies can then be used to comprehend how life has persisted through past eras, with higher atmospheric CO2, or to predict the consequences of present fossil fuel usage on the marine ecosystem of the future oceans.

Description: Previous studies have established links between biodiversity and soil geochemistry in the McMurdo Dry Valleys, Antarctica, where environmental gradients are important determinants of soil biodiversity. However, these gradients are not well established in the central Transantarctic Mountains, which are thought to represent some of the least hospitable Antarctic soils. We analyzed 220 samples from 11 ice-free areas along the Shackleton Glacier (∼ 85∘ S), a major outlet glacier of the East Antarctic Ice Sheet. We established three zones of distinct geochemical gradients near the head of the glacier (upper), its central part (middle), and at the mouth (lower). The upper zone had the highest water-soluble salt concentrations with total salt concentrations exceeding 80 000 µg g−1, while the lower zone had the lowest water-soluble N:P ratios, suggesting that, in addition to other parameters (such as proximity to water and/or ice), the lower zone likely represents the most favorable ecological habitats. Given the strong dependence of geochemistry on geographic parameters, we developed multiple linear regression and random forest models to predict soil geochemical trends given latitude, longitude, elevation, distance from the coast, distance from the glacier, and soil moisture (variables which can be inferred from remote measurements). Confidence in our random forest model predictions was moderately high with R2 values for total water-soluble salts, water-soluble N:P, ClO4-, and ClO3- of 0.81, 0.88, 0.78, and 0.74, respectively. These modeling results can be used to predict geochemical gradients and estimate salt concentrations for other Transantarctic Mountain soils, information that can ultimately be used to better predict distributions of soil biota in this remote region.

Description: Headspace analysis of CO2 frequently has been used to quantify the concentration of CO2 in fresh water. According to basic chemical theory, not considering chemical equilibration of the carbonate system in the sample vials will result in a systematic error. By analysing the potential error for different types of water and experimental conditions, we show that the error incurred by headspace analysis of CO2 is less than 5 % for typical samples from boreal systems which have low alkalinity (

Description: In Southeast Asia, oil palm (OP) plantations have largely replaced tropical forests. The impact of this shift in land use on greenhouse gas (GHG) fluxes remains highly uncertain, mainly due to a relatively small pool of available data. The aim of this study is to quantify differences of nitrous oxide (N2O) and methane (CH4) fluxes as well as soil carbon dioxide (CO2) respiration rates from logged forests, oil palm plantations of different ages, and an adjacent small riparian area. Nitrous oxide fluxes are the focus of this study, as these emissions are expected to increase significantly due to the nitrogen (N) fertilizer application in the plantations. This study was conducted in the SAFE (Stability of Altered Forest Ecosystems) landscape in Malaysian Borneo (Sabah) with measurements every 2 months over a 2-year period. GHG fluxes were measured by static chambers together with key soil physicochemical parameters and microbial biodiversity. At all sites, N2O fluxes were spatially and temporally highly variable. On average the largest fluxes (incl. 95 % CI) were measured from OP plantations (45.1 (24.0–78.5) µg m−2 h−1 N2O-N), slightly smaller fluxes from the riparian area (29.4 (2.8–84.7) µg m−2 h−1 N2O-N), and the smallest fluxes from logged forests (16.0 (4.0–36.3) µg m−2 h−1 N2O-N). Methane fluxes were generally small (mean ± SD): −2.6 ± 17.2 µg CH4-C m−2 h−1 for OP and 1.3 ± 12.6 µg CH4-C m−2 h−1 for riparian, with the range of measured CH4 fluxes being largest in logged forests (2.2 ± 48.3 µg CH4-C m−2 h−1). Soil respiration rates were larger from riparian areas (157.7 ± 106 mg m−2 h−1 CO2-C) and logged forests (137.4 ± 95 mg m−2 h−1 CO2-C) than OP plantations (93.3 ± 70 mg m−2 h−1 CO2-C) as a result of larger amounts of decomposing leaf litter. Microbial communities were distinctly different between the different land-use types and sites. Bacterial communities were linked to soil pH, and fungal and eukaryotic communities were linked to land use. Despite measuring a large number of environmental parameters, mixed models could only explain up to 17 % of the variance of measured fluxes for N2O, 3 % of CH4, and 25 % of soil respiration. Scaling up measured N2O fluxes to Sabah using land areas for forest and OP resulted in emissions increasing from 7.6 Mt (95 % confidence interval, −3.0–22.3 Mt) yr−1 in 1973 to 11.4 Mt (0.2–28.6 Mt) yr−1 in 2015 due to the increasing area of forest converted to OP plantations over the last ∼ 40 years.

Description: A key challenge for biological oceanography is relating the physiological mechanisms controlling phytoplankton growth to the spatial distribution of those phytoplankton. Physiological mechanisms are often isolated by varying one driver of growth, such as nutrient or light, in a controlled laboratory setting producing what we call “intrinsic relationships”. We contrast these with the “apparent relationships” which emerge in the environment in climatological data. Although previous studies have found machine learning (ML) can find apparent relationships, there has yet to be a systematic study examining when and why these apparent relationships diverge from the underlying intrinsic relationships found in the lab and how and why this may depend on the method applied. Here we conduct a proof-of-concept study with three scenarios in which biomass is by construction a function of time-averaged phytoplankton growth rate. In the first scenario, the inputs and outputs of the intrinsic and apparent relationships vary over the same monthly timescales. In the second, the intrinsic relationships relate averages of drivers that vary on hourly timescales to biomass, but the apparent relationships are sought between monthly averages of these inputs and monthly-averaged output. In the third scenario we apply ML to the output of an actual Earth system model (ESM). Our results demonstrated that when intrinsic and apparent relationships operate on the same spatial and temporal timescale, neural network ensembles (NNEs) were able to extract the intrinsic relationships when only provided information about the apparent relationships, while colimitation and its inability to extrapolate resulted in random forests (RFs) diverging from the true response. When intrinsic and apparent relationships operated on different timescales (as little separation as hourly versus daily), NNEs fed with apparent relationships in time-averaged data produced responses with the right shape but underestimated the biomass. This was because when the intrinsic relationship was nonlinear, the response to a time-averaged input differed systematically from the time-averaged response. Although the limitations found by NNEs were overestimated, they were able to produce more realistic shapes of the actual relationships compared to multiple linear regression. Additionally, NNEs were able to model the interactions between predictors and their effects on biomass, allowing for a qualitative assessment of the colimitation patterns and the nutrient causing the most limitation. Future research may be able to use this type of analysis for observational datasets and other ESMs to identify apparent relationships between biogeochemical variables (rather than spatiotemporal distributions only) and identify interactions and colimitations without having to perform (or at least performing fewer) growth experiments in a lab. From our study, it appears that ML can extract useful information from ESM output and could likely do so for observational datasets as well.

Description: Coccolithophores play a key role in the marine carbon cycle and ecosystem. The carbonate shells produced by coccolithophore, named as coccolith, could be well preserved in the marine sediment for millions of years and become an excellent archive for paleoclimate studies. The micro-filtering and sinking–decanting methods have been successfully designed for coccolith separation and promoted the development of geochemistry studies on coccolith, such as the stable isotopes and Sr / Ca ratio. However, these two methods are still not efficient enough for the sample-consuming methods. In this study, the trajectory of coccolith movement during a centrifugation process was calculated in theory and carefully tested by separations in practice. We offer a MATLAB code to estimate the appropriate parameter, angular velocity at a fixed centrifugation duration, for separating certain coccolith size fractions from bulk sediment. This work could improve the efficiency of coccolith separation, especially for the finest size fraction, and make it possible to carry out the clumped isotope and radio carbon analyses on coccoliths in sediment.

Description: Alpine grasslands sustain local economy by providing fodder for livestock. Intensive fertilization is common to enhance their yields, thus creating negative externalities on water quality that are difficult to evaluate without reliable estimates of nutrient fluxes. We apply a mechanistic ecosystem model, seamlessly integrating land-surface energy balance, soil hydrology, vegetation dynamics, and soil biogeochemistry, aiming at assessing the grassland response to fertilization. We simulate the major water, carbon, nutrient, and energy fluxes of nine grassland plots across the broad European Alpine region. We provide an interdisciplinary model evaluation by confirming its performance against observed variables from different datasets. Subsequently, we apply the model to test the influence of fertilization practices on grassland yields and nitrate (NO3-) losses through leaching under both current and modified climate scenarios. Despite the generally low NO3- concentration in groundwater recharge, the variability across sites is remarkable, which is mostly (but not exclusively) dictated by elevation. In high-Alpine sites, short growing seasons lead to less efficient nitrogen (N) uptake for biomass production. This combined with lower evapotranspiration rates results in higher amounts of drainage and NO3- leaching to groundwater. Scenarios with increased temperature lead to a longer growing season characterized by higher biomass production and, consequently, to a reduction of water leakage and N leaching. While the intersite variability is maintained, climate change impacts are stronger on sites at higher elevations. The local soil hydrology has a crucial role in driving the NO3- use efficiency. The commonly applied fixed threshold limit on fertilizer N input is suboptimal. We suggest that major hydrological and soil property differences across sites should be considered in the delineation of best practices or regulations for management. Using distributed maps informed with key soil and climatic attributes or systematically implementing integrated ecosystem models as shown here can contribute to achieving more sustainable practices.

Description: Monitoring leaf phenology tracks the progression of climate change and seasonal variations in a variety of organismal and ecosystem processes. Networks of finite-scale remote sensing, such as the PhenoCam network, provide valuable information on phenological state at high temporal resolution, but they have limited coverage. Satellite-based data with lower temporal resolution have primarily been used to more broadly measure phenology (e.g., 16 d MODIS normalized difference vegetation index (NDVI) product). Recent versions of the Geostationary Operational Environmental Satellites (GOES-16 and GOES-17) can monitor NDVI at temporal scales comparable to that of PhenoCam throughout most of the western hemisphere. Here we begin to examine the current capacity of these new data to measure the phenology of deciduous broadleaf forests for the first 2 full calendar years of data (2018 and 2019) by fitting double-logistic Bayesian models and comparing the transition dates of the start, middle, and end of the season to those obtained from PhenoCam and MODIS 16 d NDVI and enhanced vegetation index (EVI) products. Compared to these MODIS products, GOES was more correlated with PhenoCam at the start and middle of spring but had a larger bias (3.35 ± 0.03 d later than PhenoCam) at the end of spring. Satellite-based autumn transition dates were mostly uncorrelated with those of PhenoCam. PhenoCam data produced significantly more certain (all p values ≤0.013) estimates of all transition dates than any of the satellite sources did. GOES transition date uncertainties were significantly smaller than those of MODIS EVI for all transition dates (all p values ≤0.026), but they were only smaller (based on p value

Description: Global spread of hypoxia and less frequent mixing in lakes is a major growing environmental concern. Climate change and human impact are expected to increasingly deteriorate aquatic ecosystems. The study of processes and drivers of such changes in the past provides a great asset for prevention and remediation in the future. We used a multiproxy approach combining high-resolution bulk pigment data measured by hyperspectral imaging (HSI) with lower-resolution specific chlorophyll types and carotenoids measured by HPLC to examine Holocene trophic state changes and anoxia evolution in the meromictic Lake Jaczno, NE Poland. A redundancy analysis (RDA) including pollen-inferred vegetation cover, temperature and human impacts provides insight into specific conditions and drivers of changing trophic and redox states in the lake. Anoxic and sulfidic conditions were established in Lake Jaczno after initial basin infilling 9500 years ago. Until 6700 cal BP, lake trophy was relatively low, water turbidity was high and green sulfur bacteria (GSB) were abundant within the phototrophic community, suggesting a deep oxic–anoxic boundary and weak stratification. The period between 6700–500 cal BP is characterized by constantly increasing lake production and a gradual shift from GSB to purple sulfur bacteria (PSB), suggesting a shallower oxic–anoxic boundary and pronounced stratification. Yet, the presence of spheroidene and speroidenone in the sediments indicates intermittent anoxia. After 500 cal BP, increasing human impact, deforestation and intensive agriculture promoted lake eutrophication, with a shift to PSB dominance and establishment of permanent anoxia and meromixis. Our study unambiguously documents the legacy of human impact on processes determining eutrophication and anoxia.

Description: Coastal lagoons are important sites for nitrogen (N) removal via sediment burial and denitrification. Blooms of heterocystous cyanobacteria may diminish N retention as dinitrogen (N2) fixation offsets atmospheric losses via denitrification. We measured N2 fixation in the Curonian Lagoon, Europe's largest coastal lagoon, to better understand the factors controlling N2 fixation in the context of seasonal changes in phytoplankton community composition and external N inputs. Temporal patterns in N2 fixation were primarily determined by the abundance of heterocystous cyanobacteria, mainly Aphanizomenon flos-aquae, which became abundant after the decline in riverine nitrate inputs associated with snowmelt. Heterocystous cyanobacteria dominated the summer phytoplankton community resulting in strong correlations between chlorophyll a (Chl a) and N2 fixation. We used regression models relating N2 fixation to Chl a, along with remote-sensing-based estimates of Chl a to derive lagoon-scale estimates of N2 fixation. N2 fixation by pelagic cyanobacteria was found to be a significant component of the lagoon's N budget based on comparisons to previously derived fluxes associated with riverine inputs, sediment–water exchange, and losses via denitrification. To our knowledge, this is the first study to derive ecosystem-scale estimates of N2 fixation by combining remote sensing of Chl a with empirical models relating N2 fixation rates to Chl a.

Description: Turbidity flows – underwater avalanches – are large-scale physical disturbances that are believed to have profound and lasting impacts on benthic communities in the deep sea, with hypothesized effects on both productivity and diversity. In this review we summarize the physical characteristics of turbidity flows and the mechanisms by which they influence deep-sea benthic communities, both as an immediate pulse-type disturbance and through longer-term press-type impacts. Further, we use data from turbidity flows that occurred hundreds to thousands of years ago as well as three more recent events to assess published hypotheses that turbidity flows affect productivity and diversity. We find, unlike previous reviews, that evidence for changes in productivity in the studies was ambiguous at best, whereas the influence on regional and local diversity was more clear-cut: as had previously been hypothesized, turbidity flows decrease local diversity but create mosaics of habitat patches that contribute to increased regional diversity. Studies of more recent turbidity flows provide greater insights into their impacts in the deep sea, but without pre-disturbance data, the factors that drive patterns in benthic community productivity and diversity, be they physical, chemical, or a combination thereof, still cannot be identified. We propose criteria for data that would be necessary for testing these hypotheses and suggest that studies of Kaikōura Canyon, New Zealand, where an earthquake-triggered turbidity flow occurred in 2016, will provide insights into the impacts of turbidity flows on deep-sea benthic communities as well as the impacts of other large-scale disturbances such as deep-sea mining.

Description: Wildfires in sagebrush (Artemisia spp.)-dominated semi-arid ecosystems in the western United States have increased dramatically in frequency and severity in the last few decades. Severe wildfires often lead to the loss of native sagebrush communities and change the biogeochemical conditions which make it difficult for sagebrush to regenerate. Invasion of cheatgrass (Bromus tectorum) accentuates the problem by making the ecosystem more susceptible to frequent burns. Managers have implemented several techniques to cope with the cheatgrass–fire cycle, ranging from controlling undesirable fire effects by removing fuel loads either mechanically or via prescribed burns to seeding the fire-affected areas with shrubs and native perennial forbs. There have been a number of studies at local scales to understand the direct impacts of wildfire on vegetation; however there is a larger gap in understanding these impacts at broad spatial and temporal scales. This need highlights the importance of dynamic global vegetation models (DGVMs) and remote sensing. In this study, we explored the influence of fire on vegetation composition and gross primary production (GPP) in the sagebrush ecosystem using the Ecosystem Demography (EDv2.2) model, a dynamic global vegetation model. We selected the Reynolds Creek Experimental Watershed (RCEW) to run our simulation study, an intensively monitored sagebrush-dominated ecosystem in the northern Great Basin. We ran point-based simulations at four existing flux tower sites in the study area for a total of 150 years after turning on the fire module in the 25th year. Results suggest dominance of shrubs in a non-fire scenario; however under the fire scenario we observed contrasting phases of high and low shrub density and C3 grass growth. Regional model simulations showed a gradual decline in GPP for fire-introduced areas through the initial couple of years instead of killing all the vegetation in the affected area in the first year itself. We also compared the results from EDv2.2 with satellite-derived GPP estimates for the areas in the RCEW burned by a wildfire in 2015 (Soda Fire). We observed moderate pixel-level correlations between maps of post-fire recovery EDv2.2 GPP and MODIS-derived GPP. This study contributes to understanding the application of ecosystem models to investigate temporal dynamics of vegetation under alternative fire regimes and post-fire ecosystem restoration.

Description: Soil organic carbon (SOC) accounts for two-thirds of terrestrial carbon. Yet, the role of soil physicochemical properties in regulating SOC stocks is unclear, inhibiting reliable SOC predictions under land use and climatic changes. Using legacy observations from 141 584 soil profiles worldwide, we disentangle the effects of biotic, climatic and edaphic factors (a total of 31 variables) on the global spatial distribution of SOC stocks in four sequential soil layers down to 2 m. The results indicate that the 31 variables can explain 60 %–70 % of the global variance of SOC in the four layers, to which climatic variables and edaphic properties each contribute ∼35 % except in the top 20 cm soil. In the top 0–20 cm soil, climate contributes much more than soil properties (43 % vs. 31 %), while climate and soil properties show the similar importance in the 20–50, 50–100 and 100–200 cm soil layers. However, the most important individual controls are consistently soil-related and include soil texture, hydraulic properties (e.g. field capacity) and pH. Overall, soil properties and climate are the two dominant controls. Apparent carbon inputs represented by net primary production, biome type and agricultural cultivation are secondary, and their relative contributions were ∼10 % in all soil depths. This dominant effect of individual soil properties challenges the current climate-driven framework of SOC dynamics and needs to be considered to reliably project SOC changes for effective carbon management and climate change mitigation.

Description: Jellyfish are increasingly recognised as important components of the marine ecosystem, yet their specific role is poorly defined compared to that of other zooplankton groups. This paper presents the first global ocean biogeochemical model that includes an explicit representation of jellyfish and uses the model to gain insight into the influence of jellyfish on the plankton community. The Plankton Type Ocean Model (PlankTOM11) model groups organisms into plankton functional types (PFTs). The jellyfish PFT is parameterised here based on our synthesis of observations on jellyfish growth, grazing, respiration and mortality rates as functions of temperature and jellyfish biomass. The distribution of jellyfish is unique compared to that of other PFTs in the model. The jellyfish global biomass of 0.13 PgC is within the observational range and comparable to the biomass of other zooplankton and phytoplankton PFTs. The introduction of jellyfish in the model has a large direct influence on the crustacean macrozooplankton PFT and influences indirectly the rest of the plankton ecosystem through trophic cascades. The zooplankton community in PlankTOM11 is highly sensitive to the jellyfish mortality rate, with jellyfish increasingly dominating the zooplankton community as its mortality diminishes. Overall, the results suggest that jellyfish play an important role in regulating global marine plankton ecosystems across plankton community structure, spatio-temporal dynamics and biomass, which is a role that has been generally neglected so far.

Description: Forest edges change micro-environmental conditions, thereby affecting the ecology of many forest-dwelling species. Understanding such edge effects is particularly important for Malagasy primates because many of them live in highly fragmented forests today. The aim of our study was to assess the influence of forest edge effects on activity budgets, feeding ecology, and stress hormone output (measured as faecal glucocorticoid metabolite – fGCM – levels) in wild Verreaux's sifakas (Propithecus verreauxi), a group living, arboreal lemur. We observed five habituated groups: three living in the forest interior and two at an established forest edge. There was no difference in average daily temperatures between edge and interior habitats; however, within the edge site, the average daily temperature incrementally increased over 450 m from the forest edge towards the interior forest of the edge habitat, and the population density was lower at the edge site. Activity budgets differed between groups living in the two microhabitats, with individuals living near the edge spending more time travelling and less time feeding. Groups living near the edge also tended to have smaller home ranges and core areas than groups in the forest interior. In addition, edge groups had elevated average fGCM concentrations, and birth rates were lower for females living in the edge habitat. Combined with lower levels of fruit consumption at the edge, these results suggest that nutritional stress might be a limiting factor for Verreaux's sifakas when living near a forest edge. Hence, Verreaux's sifakas appear to be sensitive to microhabitat characteristics linked to forest edges; a result with implications for the conservation of this critically endangered lemurid species.

Description: The rewetting of peatlands is regarded as an important nature-based climate solution and intended to reconcile climate protection with the restoration of self-regulating ecosystems that are resistant to climate impacts. Although the severity and frequency of droughts are predicted to increase as a consequence of climate change, it is not well understood whether such extreme events can jeopardize rewetting measures. The goal of this study was to better understand drought effects on vegetation development and the exchange of the two important greenhouse gases CO2 and CH4, especially in rewetted fens. Based on long-term reference records, we investigated anomalies in vegetation dynamics, CH4 emissions, and net CO2 exchange, including the component fluxes of ecosystem respiration (Reco) and gross ecosystem productivity (GEP), in a rewetted fen during the extreme European summer drought in 2018. Drought-induced vegetation dynamics were derived from remotely sensed data. Since flooding in 2010, the fen was characterized by a patchy mosaic of open-water surfaces and vegetated areas. After years of stagnant vegetation development, drought acted as a trigger event for pioneer species such as Tephroseris palustris and Ranunculus sceleratus to rapidly close persistent vegetation gaps. The massive spread of vegetation assimilated substantial amounts of CO2. In 2018, the annual GEP budget increased by 20 % in comparison to average years (2010–2017). Reco increased even by 40 %, but enhanced photosynthetic CO2 sequestration could compensate for half of the drought-induced increase in respiratory CO2 release. Altogether, the restored fen remained a net CO2 sink in the year of drought, though net CO2 sequestration was lower than in other years. CH4 emissions were 20 % below average on an annual basis, though stronger reduction effects occurred from August onwards, when daily fluxes were 60 % lower than in reference years. Our study reveals an important regulatory mechanism of restored fens to maintain their net CO2 sink function even in extremely dry years. It appears that, in times of more frequent climate extremes, fen restoration can create ecosystems resilient to drought. However, in order to comprehensively assess the mitigation prospects of peatland rewetting as a nature-based climate solution, further research needs to focus on the long-term effects of such extreme events beyond the actual drought period.

Description: We developed a simple method to refine existing open-ocean maps and extend them towards different coastal seas. Using a multi-linear regression we produced monthly maps of surface ocean fCO2 in the northern European coastal seas (the North Sea, the Baltic Sea, the Norwegian Coast and the Barents Sea) covering a time period from 1998 to 2016. A comparison with gridded Surface Ocean CO2 Atlas (SOCAT) v5 data revealed mean biases and standard deviations of 0 ± 26 µatm in the North Sea, 0 ± 16 µatm along the Norwegian Coast, 0 ± 19 µatm in the Barents Sea and 2 ± 42 µatm in the Baltic Sea. We used these maps to investigate trends in fCO2, pH and air–sea CO2 flux. The surface ocean fCO2 trends are smaller than the atmospheric trend in most of the studied regions. The only exception to this is the western part of the North Sea, where sea surface fCO2 increases by 2 µatm yr−1, which is similar to the atmospheric trend. The Baltic Sea does not show a significant trend. Here, the variability was much larger than the expected trends. Consistently, the pH trends were smaller than expected for an increase in fCO2 in pace with the rise of atmospheric CO2 levels. The calculated air–sea CO2 fluxes revealed that most regions were net sinks for CO2. Only the southern North Sea and the Baltic Sea emitted CO2 to the atmosphere. Especially in the northern regions the sink strength increased during the studied period.

Description: The interest in organic nitrogen and particularly in quantifying and studying the fate of amino acids (AAs) has been growing in the atmospheric-science community. However very little is known about biotic and abiotic transformation mechanisms of amino acids in clouds. In this work, we measured the biotransformation rates of 18 amino acids with four bacterial strains (Pseudomonas graminis PDD-13b-3, Rhodococcus enclensis PDD-23b-28, Sphingomonas sp. PDD-32b-11, and Pseudomonas syringae PDD-32b-74) isolated from cloud water and representative of this environment. At the same time, we also determined the abiotic (chemical, OH radical) transformation rates within the same solutions mimicking the composition of cloud water. We used a new approach by UPLC–HRMS (ultra-performance liquid chromatography–high-resolution mass spectrometry) to quantify free AAs directly in the artificial-cloud-water medium without concentration and derivatization. The experimentally derived transformation rates were used to compare their relative importance under atmospheric conditions with loss rates based on kinetic data of amino acid oxidation in the aqueous phase. This analysis shows that previous estimates overestimated the abiotic degradation rates and thus underestimated the lifetime of amino acids in the atmosphere, as they only considered loss processes but did not take into account the potential transformation of amino acids into each other.

Description: The western Arctic Ocean, including its shelves and coastal habitats, has become a focus in ocean acidification research over the past decade as the colder waters of the region and the reduction of sea ice appear to promote the uptake of excess atmospheric CO2. Due to seasonal sea ice coverage, high-frequency monitoring of pH or other carbonate chemistry parameters is typically limited to infrequent ship-based transects during ice-free summers. This approach has failed to capture year-round nearshore carbonate chemistry dynamics which is modulated by biological metabolism in response to abundant allochthonous organic matter to the narrow shelf of the Beaufort Sea and adjacent regions. The coastline of the Beaufort Sea comprises a series of lagoons that account for 〉 50 % of the land–sea interface. The lagoon ecosystems are novel features that cycle between “open†and “closed†phases (i.e., ice-free and ice-covered, respectively). In this study, we collected high-frequency pH, salinity, temperature, and photosynthetically active radiation (PAR) measurements in association with the Beaufort Lagoon Ecosystems – Long Term Ecological Research program – for an entire calendar year in Kaktovik Lagoon, Alaska, USA, capturing two open-water phases and one closed phase. Hourly pH variability during the open-water phases are some of the fastest rates reported, exceeding 0.4 units. Baseline pH varied substantially between the open phase in 2018 and open phase in 2019 from ∼ 7.85 to 8.05, respectively, despite similar hourly rates of change. Salinity–pH relationships were mixed during all three phases, displaying no correlation in the 2018 open phase, a negative correlation in the 2018/19 closed phase, and a positive correlation during the 2019 open phase. The high frequency of pH variability could partially be explained by photosynthesis–respiration cycles as correlation coefficients between daily average pH and PAR were 0.46 and 0.64 for 2018 and 2019 open phases, respectively. The estimated annual daily average CO2 efflux (from sea to atmosphere) was 5.9 ± 19.3 mmolm-2d-1, which is converse to the negative influx of CO2 estimated for the coastal Beaufort Sea despite exhibiting extreme variability. Considering the geomorphic differences such as depth and enclosure in Beaufort Sea lagoons, further investigation is needed to assess whether there are periods of the open phase in which lagoons are sources of carbon to the atmosphere, potentially offsetting the predicted sink capacity of the greater Beaufort Sea.

Description: Pyrogenic carbon (PyC) is produced by the incomplete combustion of vegetation during wildfires and is a major and persistent pool of the global carbon (C) cycle. However, its redistribution in the landscape after fires remains largely unknown. Therefore, we conducted rainfall simulation experiments on 0.25 m2 plots with two distinct Swiss forest soils (Cambisol (clay loam) and Luvisol (sandy silt)). We applied PyC produced from wood (Picea abies) labeled under FACE conditions and C4 grass (Miscanthus sinensis) to the soil surface to study PyC redistribution by runoff and splash and the vertical mobility of PyC in a 10 cm unsaturated soil column based on the differences in δ13C of soils and PyC. We assessed the effect of soil texture, slope angle and PyC characteristics (feedstock and particle size) on the mobility of PyC during 30 min of intense rainfall (102 mm h−1). Our results highlight that PyC is highly mobile. Surface runoff transported between 0.2 % and 36.0 % of the total added PyC. Erosion by splash further redistributed 10.3 % to 25.3 % of the added PyC. Soil type had a substantial impact on the redistribution of PyC by both runoff and splash: on average, we recovered 10.5 % of the added PyC in runoff and splashed material for the clay-rich Cambisol and 61.3 % of the added PyC for the sandy silt Luvisol combined. PyC feedstock had a clear but contrasting effect on PyC redistribution: relocation in the runoff and splashed material was greater for wood PyC (43.4 % of total added PyC) than grass PyC (28.4 %). However, more wood PyC (11.5 %; fraction of organic C derived from the PyC) remained where it was initially applied compared to grass PyC (7.4 %). The results further suggest that the effect of PyC characteristics on its mobility can be highly variable and depend not only on the material from which it was derived, but also on other factors (e.g., particle size, porosity, density). In particular, the mobility of PyC was almost twice as large for fine-grained PyC (

Description: Coccolithophores are globally important marine calcifying phytoplankton that utilize a haplo-diplontic life cycle. The haplo-diplontic life cycle allows coccolithophores to divide in both life cycle phases and potentially expands coccolithophore niche volume. Research has, however, to date largely overlooked the life cycle of coccolithophores and has instead focused on the diploid life cycle phase of coccolithophores. Through the synthesis and analysis of global scanning electron microscopy (SEM) coccolithophore abundance data (n=2534), we find that calcified haploid coccolithophores generally constitute a minor component of the total coccolithophore abundance (≈ 2 %–15 % depending on season). However, using case studies in the Atlantic Ocean and Mediterranean Sea, we show that, depending on environmental conditions, calcifying haploid coccolithophores can be significant contributors to the coccolithophore standing stock (up to ≈30 %). Furthermore, using hypervolumes to quantify the niche of coccolithophores, we illustrate that the haploid and diploid life cycle phases inhabit contrasting niches and that on average this allows coccolithophores to expand their niche by ≈18.8 %, with a range of 3 %–76 % for individual species. Our results highlight that future coccolithophore research should consider both life cycle stages, as omission of the haploid life cycle phase in current research limits our understanding of coccolithophore ecology. Our results furthermore suggest a different response to nutrient limitation and stratification, which may be of relevance for further climate scenarios. Our compilation highlights the spatial and temporal sparsity of SEM measurements and the need for new molecular techniques to identify uncalcified haploid coccolithophores. Our work also emphasizes the need for further work on the carbonate chemistry niche of the coccolithophore life cycle.

Description: For the assessment of evapotranspiration, near-surface airborne thermography offers new opportunities for studies with high numbers of spatial replicates and in a fine spatial resolution. We tested drone-based thermography and the subsequent application of the DATTUTDUT energy balance model using the widely accepted eddy covariance technique as a reference method. The study site was a mature oil palm plantation in lowland Sumatra, Indonesia. For the 61 flight missions, latent heat flux estimates of the DATTUTDUT (Deriving Atmosphere Turbulent Transport Useful To Dummies Using Temperature) model with measured net radiation agreed well with eddy covariance measurements (r2 = 0.85; MAE = 47; RMSE = 60) across variable weather conditions and times of day. Confidence intervals for slope and intercept of a model II Deming regression suggest no difference between drone-based and eddy covariance methods, thus indicating interchangeability. The DATTUTDUT model is sensitive to the configuration of the net radiation assessment. Overall, we conclude that drone-based thermography with energy balance modeling is a reliable method complementing available methods for evapotranspiration studies. It offers promising, additional opportunities for fine grain and spatially explicit studies.

Description: Eutrophication-driven coastal hypoxia has been of great interest for decades, though its mechanisms remain not fully understood. Here, we showed elevated concentrations of particulate and dissolved polyunsaturated aldehydes (PUAs) associated with the hypoxic waters in the bottom layer of a salt-wedge estuary. Bacterial respiration within the hypoxic waters was mainly contributed by particle-attached bacteria (PAB) (〉 0.8 µm), with free-living bacteria (0.2–0.8 µm) only accounting for 25 %–30 % of the total rate. The concentrations of particle-adsorbed PUAs (∼ 10 µmol L−1) in the hypoxic waters were directly quantified for the first time based on large-volume filtration and subsequent on-site PUA derivation and extraction. PUA-amended incubation experiments for PAB (〉 25 µm) associated with sinking or suspended particles retrieved from the low-oxygen waters were also performed to explore the impacts of PUAs on the growth and metabolism of PAB and associated oxygen utilization. We found an increase in cell growth of PAB in response to low-dose PUAs (1 µmol L−1) but an enhanced cell-specific bacterial respiration and production in response to high-dose PUAs (100 µmol L−1). Improved cell-specific metabolism of PAB in response to high-dose PUAs was also accompanied by a shift of PAB community structure with increased dominance of the genus Alteromonas within the Gammaproteobacteria. We thus conclude that a high PUA concentration associated with aggregate particles within the bottom layer may be crucial for some species within Alteromonas to regulate PAB community structure. The change in bacteria community could lead to an enhancement of oxygen utilization during the degradation of particulate organic matter and thus likely contribute to the formation of coastal hypoxia. These findings are potentially important for coastal systems with large river inputs, intense phytoplankton blooms driven by eutrophication, and strong hypoxia developed below the salt-wedge front.

Description: Ecosystems play a fundamental role in climate change mitigation by photosynthetically fixing carbon from the atmosphere and storing it for a period of time in organic matter. Although climate impacts of carbon emissions by sources can be quantified by global warming potentials, the appropriate formal metrics to assess climate benefits of carbon removals by sinks are unclear. We introduce here the climate benefit of sequestration (CBS), a metric that quantifies the radiative effect of fixing carbon dioxide from the atmosphere and retaining it for a period of time in an ecosystem before releasing it back as the result of respiratory processes and disturbances. In order to quantify CBS, we present a formal definition of carbon sequestration (CS) as the integral of an amount of carbon removed from the atmosphere stored over the time horizon it remains within an ecosystem. Both metrics incorporate the separate effects of (i) inputs (amount of atmospheric carbon removal) and (ii) transit time (time of carbon retention) on carbon sinks, which can vary largely for different ecosystems or forms of management. These metrics can be useful for comparing the climate impacts of carbon removals by different sinks over specific time horizons, to assess the climate impacts of ecosystem management, and to obtain direct quantifications of climate impacts as the net effect of carbon emissions by sources versus removals by sinks.

Description: The element silicon (Si) is required for the growth of silicified organisms in marine environments, such as diatoms. These organisms consume vast amounts of Si together with N, P, and C, connecting the biogeochemical cycles of these elements. Thus, understanding the Si cycle in the ocean is critical for understanding wider issues such as carbon sequestration by the ocean's biological pump. In this review, we show that recent advances in process studies indicate that total Si inputs and outputs, to and from the world ocean, are 57 % and 37 % higher, respectively, than previous estimates. We also update the total ocean silicic acid inventory value, which is about 24 % higher than previously estimated. These changes are significant, modifying factors such as the geochemical residence time of Si, which is now about 8000 years, 2 times faster than previously assumed. In addition, we present an updated value of the global annual pelagic biogenic silica production (255 Tmol Si yr−1) based on new data from 49 field studies and 18 model outputs, and we provide a first estimate of the global annual benthic biogenic silica production due to sponges (6 Tmol Si yr−1). Given these important modifications, we hypothesize that the modern ocean Si cycle is at approximately steady state with inputs =14.8(±2.6) Tmol Si yr−1 and outputs =15.6(±2.4) Tmol Si yr−1. Potential impacts of global change on the marine Si cycle are discussed.

Description: The largest share of total soil organic carbon (OC) is associated with minerals. However, the factors that determine the amount and turnover of slower- versus faster-cycling components of mineral-associated carbon (MOC) are still poorly understood. Bioavailability of MOC is thought to be regulated by desorption, which can be facilitated by displacement and mobilization by competing ions. However, MOC stability is usually determined by exposure to chemical oxidation, which addresses the chemical stability of the organic compounds rather than the bonding strength of the OC–mineral bond. We used a solution of NaOH, a strong agent for desorption due to high pH, and NaF, adding F−, a strongly sorbing anion that can replace anionic organic molecules on mineral surfaces, to measure the maximum potentially desorbable MOC. For comparison, we measured maximal potential oxidation of MOC using heated H2O2. We selected MOC samples (〉 1.6 g cm3) obtained from density fractionation of samples from three soil depth increments (0–5, 10–20, and 30–40 cm) of five typical soils of central Europe, with a range of clay and pedogenic oxide contents, and under different ecosystem types (one coniferous forest, two deciduous forests, one grassland, and one cropland). Extracts and residues were analysed for OC and 14C contents, and further chemically characterized by cross-polarization magic angle spinning 13C-nuclear magnetic resonance (CPMAS-13C-NMR). We expected that NaF–NaOH extraction would remove less and younger MOC than H2O2 oxidation and that the NaF–NaOH extractability of MOC is reduced in subsoils and soils with high pedogenic oxide contents. The results showed that a surprisingly consistent proportion of 58 ± 11 % (standard deviation) of MOC was extracted with NaF–NaOH across soils, independent of depth, mineral assemblage, or land use conditions. NMR spectra revealed strong similarities in the extracted organic matter, with more than 80 % of OC in the O/N (oxygen and/or nitrogen) alkyl and alkyl C region. Total MOC amounts were correlated with the content of pedogenic oxides across sites, independent of variations in total clay, and the same was true for OC in extraction residues. Thus, the uniform extractability of MOC may be explained by dominant interactions between OC and pedogenic oxides across all study sites. While Δ14C values of bulk MOC suggested differences in OC turnover between sites, these were not linked to differences in MOC extractability. As expected, OC contents of residues had more negative Δ14C values than extracts (an average difference between extracts and residues of 78 ± 36 ‰), suggesting that non-extractable OC is older. Δ14C values of extracts and residues were strongly correlated and proportional to Δ14C values of bulk MOC but were not dependent on mineralogy. Neither MOC extractability nor differences in Δ14C values between extracts and residues changed with depth along soil profiles, where declining Δ14C values might indicate slower OC turnover in deeper soils. Thus, the 14C depth gradients in the studied soils were not explained by increasing stability of organic–mineral associations with soil depth. Although H2O2 removed 90 ± 8 % of the MOC, the Δ14C values of oxidized OC (on average −50 ± 110 ‰) were similar to those of OC extracted with NaF–NaOH (−51 ± 122 ‰), but oxidation residues (−345 ± 227 ‰) were much more depleted in 14C than residues of the NaF–NaOH extraction (−130 ± 121 ‰). Accordingly, both chemical treatments removed OC from the same continuum, and oxidation residues were older than extraction residues because more OC was removed. In contrast to the NaF–NaOH extractions, higher contents of pedogenic oxides slightly increased the oxidation resistance of MOC, but this higher H2O2 resistance did not coincide with more negative Δ14C values of MOC nor its oxidation residues. Therefore, none of the applied chemical fractionation schemes were able to explain site-specific differences in Δ14C values. Our results indicate that total MOC was dominated by OC interactions with pedogenic oxides rather than clay minerals, as we detected no difference in bond strength between clay-rich and clay-poor sites. This suggests that site-specific differences in Δ14C values of bulk MOC and depth profiles are driven by the accumulation and exchange rates of OC at mineral surfaces.

Description: Endolithic microhabitats have been described as the last refuge for life in arid and hyper-arid deserts where life has to deal with harsh environmental conditions. A number of rock substrates from the hyper-arid Atacama Desert, colonized by endolithic microbial communities such as halite, gypsum crusts, gypcrete, calcite, granite and ignimbrite, have been characterized and compared using different approaches. In this work, three different endolithic microhabitats are described, each one with a particular origin and architecture, found within a lithic substrate known as gypcrete. Gypcrete, an evaporitic rock mainly composed of gypsum (CaSO4 ⋅ 2H2O) and collected in the Cordón de Lila area of the desert (Preandean Atacama Desert), was found to harbour cryptoendolithic (within pore spaces in the rock), chasmoendolithic (within cracks and fissures) and hypoendolithic (within microcave-like pores in the bottom layer of rock) microhabitats. A combination of microscopy investigation and high-throughput sequencing approaches were used to characterize the endolithic communities and their habitats at the microscale within the same piece of gypcrete. Microscopy techniques revealed differences in the architecture of the endolithic microhabitats and the distribution of the microorganisms within those microhabitats. Cyanobacteria and Proteobacteria were dominant in the endolithic communities, of which the hypoendolithic community was the least diverse and hosted unique taxa, as a result of less access to sun radiation. These results show, for the first time, that the differences in the architecture of a microhabitat, even within the same piece of a lithic substrate, play an essential role in shaping the diversity and composition of endolithic microbial communities.

Description: American bison (Bison bison L.) have recovered from the brink of extinction over the past century. Bison reintroduction creates multiple environmental benefits, but impacts on greenhouse gas emissions are poorly understood. Bison are thought to have produced some 2 Tg yr−1 of the estimated 9–15 Tg yr−1 of pre-industrial enteric methane emissions, but few measurements have been made due to their mobile grazing habits and safety issues associated with measuring non-domesticated animals. Here, we measure methane and carbon dioxide fluxes from a bison herd on an enclosed pasture during daytime periods in winter using eddy covariance. Methane emissions from the study area were negligible in the absence of bison (mean ± standard deviation = −0.0009 ± 0.008 µmol m−2 s−1) and were significantly greater than zero, 0.048 ± 0.082 µmol m−2 s−1, with a positively skewed distribution, when bison were present. We coupled bison location estimates from automated camera images with two independent flux footprint models to calculate a mean per-animal methane efflux of 58.5 µmol s−1 per bison, similar to eddy covariance measurements of methane efflux from a cattle feedlot during winter. When we sum the observations over time with conservative uncertainty estimates we arrive at 81 g CH4 per bison d−1 with 95 % confidence intervals between 54 and 109 g CH4 per bison d−1. Uncertainty was dominated by bison location estimates (46 % of the total uncertainty), then the flux footprint model (33 %) and the eddy covariance measurements (21 %), suggesting that making higher-resolution animal location estimates is a logical starting point for decreasing total uncertainty. Annual measurements are ultimately necessary to determine the full greenhouse gas burden of bison grazing systems. Our observations highlight the need to compare greenhouse gas emissions from different ruminant grazing systems and demonstrate the potential for using eddy covariance to measure methane efflux from non-domesticated animals.

Description: Oxygen-depleted regions of the global ocean are rapidly expanding, with important implications for global biogeochemical cycles. However, our ability to make projections about the future of oxygen in the ocean is limited by a lack of empirical data with which to test and constrain the behavior of global climatic and oceanographic models. We use depth-stratified plankton tows to demonstrate that some species of planktic foraminifera are adapted to life in the heart of the pelagic oxygen minimum zone (OMZ). In particular, we identify two species, Globorotaloides hexagonus and Hastigerina parapelagica, living within the eastern tropical North Pacific OMZ. The tests of the former are preserved in marine sediments and could be used to trace the extent and intensity of low-oxygen pelagic habitats in the fossil record. Additional morphometric analyses of G. hexagonus show that tests found in the lowest oxygen environments are larger, more porous, less dense, and have more chambers in the final whorl. The association of this species with the OMZ and the apparent plasticity of its test in response to ambient oxygenation invites the use of G. hexagonus tests in sediment cores as potential proxies for both the presence and intensity of overlying OMZs.

Description: Climate warming perturbs ecosystem carbon (C) cycling, causing both positive and negative feedbacks on greenhouse gas emissions. In 2016, we began a tidal marsh field experiment in two vegetation communities to investigate the mechanisms by which whole-ecosystem warming alters C gain, via plant-driven sequestration in soils, and C loss, primarily via methane (CH4) emissions. Here, we report the results from the first 4 years. As expected, warming of 5.1 ∘C more than doubled CH4 emissions in both plant communities. We propose this was caused by a combination of four mechanisms: (i) a decrease in the proportion of CH4 consumed by CH4 oxidation, (ii) more C substrates available for methanogenesis, (iii) reduced competition between methanogens and sulfate-reducing bacteria, and (iv) indirect effects of plant traits. Plots dominated by Spartina patens consistently emitted more CH4 than plots dominated by Schoenoplectus americanus, indicating key differences in the roles these common wetland plants play in affecting anaerobic soil biogeochemistry and suggesting that plant composition can modulate coastal wetland responses to climate change.

Description: The Canary upwelling system (CanUS) is a productive coastal region characterized by strong seasonality and an intense offshore transport of organic carbon (Corg) to the adjacent oligotrophic offshore waters. There, the respiration of this Corg substantially modifies net community production (NCP). While this transport and the resulting coupling of the biogeochemistry between the coastal and open ocean has been well studied in the annual mean, the temporal variability, and especially its seasonality, has not yet been investigated. Here, we determine the seasonal variability of the offshore transport of Corg, its mesoscale component, latitudinal differences, and the underlying physical and biological drivers. To this end, we employ the Regional Ocean Modeling System (ROMS) coupled to a nutrient–phytoplankton–zooplankton–detritus (NPZD) ecosystem model. Our results reveal the importance of the mesoscale fluxes and of the upwelling processes (coastal upwelling and Ekman pumping) in modulating the seasonal variation of the offshore Corg transport. We find that the region surrounding Cape Blanc (21∘ N) hosts the most intense Corg offshore flux in every season, linked to the persistent, and far reaching Cape Blanc filament and its interaction with the Cape Verde Front. Coastal upwelling filaments dominate the seasonality of the total offshore flux up to 100 km from the coast, contributing in every season at least 80 % to the total flux. The seasonality of the upwelling modulates the offshore Corg seasonality hundreds of kilometers from the CanUS coast via lateral redistribution of nearshore production. North of 24.5∘ N, the sharp summer–fall peak of coastal upwelling results in an export of more than 30 % of the coastal Corg at 100 km offshore due to a combination of intensified nearshore production and offshore fluxes. To the south, the less pronounced upwelling seasonality regulates an overall larger but farther-reaching and less seasonally varying lateral flux, which exports between 60 % and 90 % of the coastal production more than 100 km offshore. Overall, we show that the temporal variability of nearshore processes modulates the variability of Corg and NCP hundreds of kilometers offshore from the CanUS coast via the offshore transport of the nearshore production.

Description: Plant phenology plays a fundamental role in land–atmosphere interactions, and its variability and variations are an indicator of climate and environmental changes. For this reason, current land surface models include phenology parameterizations and related biophysical and biogeochemical processes. In this work, the climatology of the beginning and end of the growing season, simulated by the land component of seven state-of-the-art European Earth system models participating in the CMIP6, is evaluated globally against satellite observations. The assessment is performed using the vegetation metric leaf area index and a recently developed approach, named four growing season types. On average, the land surface models show a 0.6-month delay in the growing season start, while they are about 0.5 months earlier in the growing season end. The difference with observation tends to be higher in the Southern Hemisphere compared to the Northern Hemisphere. High agreement between land surface models and observations is exhibited in areas dominated by broadleaf deciduous trees, while high variability is noted in regions dominated by broadleaf deciduous shrubs. Generally, the timing of the growing season end is accurately simulated in about 25 % of global land grid points versus 16 % in the timing of growing season start. The refinement of phenology parameterization can lead to better representation of vegetation-related energy, water, and carbon cycles in land surface models, but plant phenology is also affected by plant physiology and soil hydrology processes. Consequently, phenology representation and, in general, vegetation modelling is a complex task, which still needs further improvement, evaluation, and multi-model comparison.

Description: The Arctic is rapidly changing, disrupting biogeochemical cycles and the processing, delivery and sedimentation of carbon (C), in linked terrestrial–aquatic systems. In this investigation, we coupled a hydrogeomorphic assessment of catchment soils, sediments and plants with a recent lake sediment sequence to understand the source and quality of organic carbon present in three Arctic upland lake catchments on Disko Island, located just south of the low–high Arctic transition zone. This varied permafrost landscape has exposed soils with less vegetation cover at higher altitudes, and lakes received varying amounts of glacial meltwater inputs. We provide improved isotope and biomarker source identifications for palaeolimnological studies in high-latitude regions, where terrestrial vegetation is at or close to its northerly and altitudinal range limit. The poorly developed catchment soils lead to lake waters with low dissolved organic carbon (DOC) concentrations (≤1.5 mg L−1). Sedimentary carbon/nitrogen (C/N) ratios, the C isotope composition of organic matter (δ13Corg) and biomarker ratios (n-alkanes, n-alkanols, n-alkanoic acids and sterols) showed that sedimentary organic matter (OM) in these lakes is mostly derived from aquatic sources (algae and macrophytes). We used a 210Pb-dated sediment core to determine how carbon cycling in a lake–catchment system (Disko 2) had changed over recent centuries. Recent warming since the end of the Little Ice Age (LIA∼1860 CE), which accelerated after ca. 1950, led to melt of glacier ice and permafrost, releasing nutrients and DOC to the lake and stimulating pronounced aquatic algal production, as shown by a 〉10-fold increase in β-carotene, indicative of a major regime shift. We also demonstrate that recent increases in catchment terrestrial vegetation cover contributed to the autochthonous response. Our findings highlight that in Arctic lakes with sparsely developed catchment vegetation and soils, recent Anthropocene warming results in pronounced changes to in-lake C processing and the deposition of more reactive, predominately autochthonous C, when compared with extensively vegetated low-Arctic systems.

Description: The ecosystem of the Baltic Sea is endangered by eutrophication. This has triggered expensive international management efforts. Some of these efforts are impeded by natural processes such as nitrogen-fixing cyanobacteria blooms that add bioavailable nitrogen to the already over-fertilized system and thereby enhance primary production, export of organic matter to depth, and associated oxygen consumption. Controls of cyanobacteria blooms are not comprehensively understood, and this adds to the uncertainty of model-based projections into the warming future of the Baltic Sea. Here we review our current understanding of cyanobacteria bloom dynamics. We summarize published field studies and laboratory experiments and dissect the basic principles ingrained in state-of-the-art coupled ocean–circulation biogeochemical models.

Description: High biogenic silica (BSi) concentrations occur sporadically in lake sediments throughout the world; however, the processes leading to high BSi concentrations vary. We explored the factors responsible for the high BSi concentration in sediments of a small, high-latitude subarctic lake (Lake 850). The Si budget of this lake had not been fully characterized before to establish the drivers of BSi accumulation in this environment. To do this, we combined measurements of variations in stream discharge, dissolved silica (DSi) concentrations, and stable Si isotopes in both lake and stream water with measurements of BSi content in lake sediments. Water, radon, and Si mass balances revealed the importance of groundwater discharge as a main source of DSi to the lake, with groundwater-derived DSi inputs 3 times higher than those from ephemeral stream inlets. After including all external DSi sources (i.e., inlets and groundwater discharge) and estimating the total BSi accumulation in the sediment, we show that diatom production consumes up to 79 % of total DSi input. Additionally, low sediment accumulation rates were observed based on the dated gravity core. Our findings thus demonstrate that groundwater discharge and low mass accumulation rate can account for the high BSi accumulation during the last 150 cal yr BP. Globally, lakes have been estimated to retain one-fifth of the annual DSi terrestrial weathering flux that would otherwise be delivered to the ocean. Well-constrained lake mass balances, such as presented here, bring clarity to those estimates of the terrestrial Si cycle sinks.

Description: Understanding the dependencies of the terrestrial carbon and water cycle with meteorological conditions is a prerequisite to anticipate their behaviour under climate change conditions. However, terrestrial ecosystems and the atmosphere interact via a multitude of variables across temporal and spatial scales. Additionally these interactions might differ among vegetation types or climatic regions. Today, novel algorithms aim to disentangle the causal structure behind such interactions from empirical data. The estimated causal structures can be interpreted as networks, where nodes represent relevant meteorological variables or land-surface fluxes and the links represent the dependencies among them (possibly including time lags and link strength). Here we derived causal networks for different seasons at 119 eddy covariance flux tower observations in the FLUXNET network. We show that the networks of biosphere–atmosphere interactions are strongly shaped by meteorological conditions. For example, we find that temperate and high-latitude ecosystems during peak productivity exhibit biosphere–atmosphere interaction networks very similar to tropical forests. In times of anomalous conditions like droughts though, both ecosystems behave more like typical Mediterranean ecosystems during their dry season. Our results demonstrate that ecosystems from different climate zones or vegetation types have similar biosphere–atmosphere interactions if their meteorological conditions are similar. We anticipate our analysis to foster the use of network approaches, as they allow for a more comprehensive understanding of the state of ecosystem functioning. Long-term or even irreversible changes in network structure are rare and thus can be indicators of fundamental functional ecosystem shifts.

Description: To achieve the Paris Agreement requires aggressive mitigation strategies alongside negative emission technologies. Recent studies suggest that increasing tree cover can make a substantial contribution to negative emissions, with the tropics being the most suitable region from a biogeophysical perspective. Yet these studies typically do not account for subsequent carbon cycle and climate responses to large-scale land-use change. Here we quantify the maximum potential temperature and CO2 benefits from pantropical forest restoration, including the Earth system response, using a fully coupled, emission-driven Earth system model (HadGEM2-ES). We perform an idealised experiment where all land use in the tropics is stopped and vegetation is allowed to recover, on top of an aggressive mitigation scenario (RCP2.6). We find that tropical restoration of 1529 Mha increases carbon stored in live biomass by 130 Pg C by 2100 CE. Whilst avoiding deforestation and tropical restoration in the tropics removes 42 Pg C compared to RCP2.6, the subsequent reduction in extratropical and ocean carbon uptake means that carbon in the atmosphere only reduces by 18 Pg C by 2100. The resulting small CO2 (9 ppm) benefit does not translate to a detectable reduction in global surface air temperature compared to the control experiment. The greatest carbon benefit is achieved 30–50 years after restoration before the Earth system response adjusts to the new land-use regime and declining fossil fuel use. Comparing our results with previous modelling studies, we identify two model-independent key points: (i) in a world where emission reductions follow the Paris Agreement, restoration is best deployed immediately, and (ii) the global carbon cycle response to reduced emissions limits the efficacy of negative emissions technologies by more than half. We conclude that forest restoration can reduce peak CO2 mid-century, but it can only modestly contribute to negative emissions.

Description: A model of the radionuclide accumulation in fish taking into account the contribution of different tissues and allometry is presented. The basic model assumptions are as follows. (i) A fish organism is represented by several compartments in which radionuclides are homogeneously distributed. (ii) The compartments correspond to three groups of organs or tissues: muscle, bones and organs (kidney, liver, gonads, etc.) differing in metabolic function. (iii) Two input compartments include gills absorbing contamination from water and digestive tract through which contaminated food is absorbed. (iv) The absorbed radionuclide is redistributed between organs or tissues according to their metabolic functions. (v) The elimination of assimilated elements from each group of organs or tissues differs, reflecting differences in specific tissues or organs in which elements were accumulated. (vi) The food and water uptake rates, elimination rate, and growth rate depend on the metabolic rate, which is scaled by fish mass to the 3/4 power. The analytical solutions of the system of model equations describing dynamics of the assimilation and elimination of 134Cs, 57Co, 60Co, 54Mn and 65Zn, which are preferably accumulated in different tissues, exhibited good agreement with the laboratory experiments. The developed multi-compartment kinetic–allometric model was embedded into the box model POSEIDON-R (Maderich et al., 2018b), which describes transport of radionuclides in water, accumulation in the sediment and transfer of radionuclides through the pelagic and benthic food webs. The POSEIDON-R model was applied for the simulation of the transport and fate of 60Co and 54Mn routinely released from Forsmark Nuclear Power Plant (NPP) located on the Baltic Sea coast of Sweden and for calculation of 90Sr concentration in fish after the accident at Fukushima Dai-ichi NPP. Computed concentrations of radionuclides in fish agree with the measurements much better than calculated using standard whole-body model and target tissue model. The model with the defined generic parameters could be used in different marine environments without calibration based on a posteriori information, which is important for emergency decision support systems.

Description: Lithogenic elements such as aluminum (Al), iron (Fe), rare earth elements (REEs), thorium (232Th and 230Th, given as Th) and protactinium (Pa) are often assumed to be insoluble. In this study, their dissolution from Saharan dust reaching Mediterranean seawater was studied through tank experiments over 3 to 4 d under controlled conditions including controls without dust addition as well as dust seeding under present and future climate conditions (+3 ∘C and −0.3 pH). Unfiltered surface seawater from three oligotrophic regions (Tyrrhenian Sea, Ionian Sea and Algerian Basin) were used. The maximum dissolution was low for all seeding experiments: less than 0.3 % for Fe, 1 % for 232Th and Al, about 2 %–5 % for REEs and less than 6 % for Pa. Different behaviors were observed: dissolved Al increased until the end of the experiments, Fe did not dissolve significantly, and Th and light REEs were scavenged back on particles after a fast initial release. The constant 230Th/232Th ratio during the scavenging phase suggests that there is little or no further dissolution after the initial Th release. Quite unexpectedly, comparison of present and future conditions indicates that changes in temperature and/or pH influence the release of Th and REEs in seawater, leading to lower Th release and a higher light REE release under increased greenhouse conditions.

Description: Unraveling the environmental controls influencing Arctic tundra productivity is paramount for advancing our predictive understanding of the causes and consequences of warming in tundra ecosystems and associated land–atmosphere feedbacks. This study focuses on aquatic emergent tundra plants, which dominate productivity and methane fluxes in the Arctic coastal plain of Alaska. In particular, we assessed how environmental nutrient availability influences production of biomass and greenness in the dominant aquatic tundra species: Arctophila fulva and Carex aquatilis. We sampled a total of 17 sites distributed across the Barrow Peninsula and Atqasuk, Alaska, following a nutrient gradient that ranged from sites with thermokarst slumping or urban runoff to sites with relatively low nutrient inputs. Employing a multivariate analysis, we explained the relationship of soil and water nutrients to plant leaf macro- and micro-nutrients. Specifically, we identified soil phosphorus as the main limiting nutrient factor given that it was the principal driver of aboveground biomass (R2=0.34, p=0.002) and normalized difference vegetation index (NDVI) (R2=0.47, p=0.002) in both species. Plot-level spectral NDVI was a good predictor of leaf P content for both species. We found long-term increases in N, P and Ca in C. aquatilis based on historical leaf nutrient data from the 1970s of our study area. This study highlights the importance of nutrient pools and mobilization between terrestrial–aquatic systems and their potential influence on productivity and land–atmosphere carbon balance. In addition, aquatic plant NDVI spectral responses to nutrients can serve as landscape hot-spot and hot-moment indicators of landscape biogeochemical heterogeneity associated with permafrost degradation, nutrient leaching and availability.

Description: Soil bacteria rank among the most diverse groups of organisms on Earth and actively impact global processes of carbon cycling, especially in the emission of greenhouse gases like methane, CO2 and higher gaseous hydrocarbons. An abundant group of soil bacteria are the mycobacteria, which colonize various terrestrial, marine and anthropogenic environments due to their impermeable cell envelope that contains remarkable lipids. These bacteria have been found to be highly abundant at petroleum and gas seep areas, where they might utilize the released hydrocarbons. However, the function and the lipid biomarker inventory of these soil mycobacteria are poorly studied. Here, soils from the Fuoco di Censo seep, an everlasting fire (gas seep) in Sicily, Italy, were investigated for the presence of mycobacteria via 16S rRNA gene sequencing and fatty acid profiling. The soils contained high relative abundances (up to 34 % of reads assigned) of mycobacteria, phylogenetically close to the Mycobacterium simiae complex and more distant from the well-studied M. tuberculosis and hydrocarbon-utilizing M. paraffinicum. The soils showed decreasing abundances of mycocerosic acids (MAs), fatty acids unique for mycobacteria, with increasing distance from the seep. The major MAs at this seep were tentatively identified as 2,4,6,8-tetramethyl tetracosanoic acid and 2,4,6,8,10-pentamethyl hexacosanoic acid. Unusual MAs with mid-chain methyl branches at positions C-12 and C-16 (i.e., 2,12-dimethyl eicosanoic acid and 2,4,6,8,16-pentamethyl tetracosanoic acid) were also present. The molecular structures of the Fuoco di Censo MAs are different from those of the well-studied mycobacteria like M. tuberculosis or M. bovis and have relatively δ13C-depleted values (−38 ‰ to −48 ‰), suggesting a direct or indirect utilization of the released seep gases like methane or ethane. The structurally unique MAs in combination with their depleted δ13C values identified at the Fuoco di Censo seep offer a new tool to study the role of soil mycobacteria as hydrocarbon gas consumers in the carbon cycle.

Description: The world map of anthropogenic atmospheric nitrogen deposition and its effects on natural ecosystems is not described with equal precision everywhere. In this paper, we report atmospheric nutrient, sulfate and spheroidal carbonaceous particle (SCP) deposition rates, based on snowpack analyses of a formerly unexplored Siberian mountain region. Then, we discuss their potential effects on lake phytoplankton biomass limitation. We estimate that the nutrient depositions observed in the late-season snowpack (40 ± 16 mg NO3-N m−2 and 0.58 ± 0.13 mg TP-P m−2; TP for total phosphorous) would correspond to yearly depositions lower than 119 ± 71 mg NO3-N m−2 yr−1 and higher than 1.71 ± 0.91 mg TP-P m−2 yr−1. These yearly deposition estimates would approximately fit the predictions of global deposition models and correspond to the very low nutrient deposition range, although they are still higher than world background values. In spite of the fact that such a low atmospheric nitrogen deposition rate would be enough to induce nitrogen limitation in unproductive mountain lakes, phosphorus deposition was also extremely low, and the resulting lake water N : P ratio was unaffected by atmospheric nutrient deposition. In the end, the studied lakes' phytoplankton appeared to be split between phosphorus and nitrogen limitation. We conclude that these pristine lakes are fragile sensitive systems exposed to the predicted climate warming, increased winter precipitation, enhanced forest fires and shifts in anthropogenic nitrogen emissions that could finally couple their water chemistry to that of atmospheric nutrient deposition and unlock temperature-inhibited responses of phytoplankton to nutrient shifts.

Description: A 5-year greenhouse gas (GHG) exchange study of the three major gas species (CO2, CH4 and N2O) from an intensively managed permanent grassland in Switzerland is presented. Measurements comprise 2 years (2010 and 2011) of manual static chamber measurements of CH4 and N2O, 5 years of continuous eddy covariance (EC) measurements (CO2–H2O – 2010–2014), and 3 years (2012–2014) of EC measurement of CH4 and N2O. Intensive grassland management included both regular and sporadic management activities. Regular management practices encompassed mowing (three to five cuts per year) with subsequent organic fertilizer amendments and occasional grazing, whereas sporadic management activities comprised grazing or similar activities. The primary objective of our measurements was to compare pre-plowing to post-plowing GHG exchange and to identify potential memory effects of such a substantial disturbance on GHG exchange and carbon (C) and nitrogen (N) gains and losses. In order to include measurements carried out with different observation techniques, we tested two different measurement techniques jointly in 2013, namely the manual static chamber approach and the eddy covariance technique for N2O, to quantify the GHG exchange from the observed grassland site. Our results showed that there were no memory effects on N2O and CH4 emissions after plowing, whereas the CO2 uptake of the site considerably increased when compared to pre-restoration years. In detail, we observed large losses of CO2 and N2O during the year of restoration. In contrast, the grassland acted as a carbon sink under usual management, i.e., the time periods 2010–2011 and 2013–2014. Enhanced emissions and emission peaks of N2O (defined as exceeding background emissions 0.21 ± 0.55 nmol m−2 s−1 (SE = 0.02) for at least 2 sequential days and the 7 d moving average exceeding background emissions) were observed for almost 7 continuous months after restoration as well as following organic fertilizer applications during all years. Net ecosystem exchange of CO2 (NEECO2) showed a common pattern of increased uptake of CO2 in spring and reduced uptake in late fall. NEECO2 dropped to zero and became positive after each harvest event. Methane (CH4) exchange fluctuated around zero during all years. Overall, CH4 exchange was of negligible importance for both the GHG budget and the carbon budget of the site. Our results stress the inclusion of grassland restoration events when providing cumulative sums of C sequestration potential and/or global warming potential (GWP). Consequently, this study further highlights the need for continuous long-term GHG exchange observations as well as for the implementation of our findings into biogeochemical process models to track potential GHG mitigation objectives as well as to predict future GHG emission scenarios reliably.

Description: Arctic regions and their water bodies are affected by a rapidly warming climate. Arctic lakes and small ponds are known to act as an important source of atmospheric methane. However, not much is known about other types of water bodies in permafrost regions, which include major rivers and coastal bays as a transition type between freshwater and marine environments. We monitored dissolved methane concentrations in three different water bodies (Lena River, Tiksi Bay, and Lake Golzovoye, Siberia, Russia) over a period of 2 years. Sampling was carried out under ice cover (April) and in open water (July–August). The methane oxidation (MOX) rate and the fractional turnover rate (k′) in water and melted ice samples from the late winter of 2017 was determined with the radiotracer method. In the Lena River winter methane concentrations were a quarter of the summer concentrations (8 nmol L−1 vs. 31 nmol L−1), and mean winter MOX rate was low (0.023 nmol L−1 d−1). In contrast, Tiksi Bay winter methane concentrations were 10 times higher than in summer (103 nmol L−1 vs. 13 nmol L−1). Winter MOX rates showed a median of 0.305 nmol L−1 d−1. In Lake Golzovoye, median methane concentrations in winter were 40 times higher than in summer (1957 nmol L−1 vs. 49 nmol L−1). However, MOX was much higher in the lake (2.95 nmol L−1 d−1) than in either the river or bay. The temperature had a strong influence on the MOX (Q10=2.72±0.69). In summer water temperatures ranged from 7–14 ∘C and in winter from −0.7 to 1.3 ∘C. In the ice cores a median methane concentration of 9 nM was observed, with no gradient between the ice surface and the bottom layer at the ice–water interface. MOX in the (melted) ice cores was mostly below the detection limit. Comparing methane concentrations in the ice with the underlaying water column revealed methane concentration in the water column 100–1000 times higher. The winter situation seemed to favor a methane accumulation under ice, especially in the lake with a stagnant water body. While on the other hand, in the Lena River with its flowing water, no methane accumulation under ice was observed. In a changing, warming Arctic, a shorter ice cover period is predicted. With respect to our study this would imply a shortened time for methane to accumulate below the ice and a shorter time for the less efficient winter MOX. Especially for lakes, an extended time of ice-free conditions could reduce the methane flux from the Arctic water bodies.

Description: In order to determine the origins of dissolved organic matter (DOM) occurring in the seawater of Sihwa Lake, we measured the stable carbon isotope ratios of dissolved organic carbon (DOC-δ13C) and the optical properties (absorbance and fluorescence) of DOM in two different seasons (March 2017 and September 2018). Sihwa Lake is enclosed by a dike along the western coast of South Korea, and the water is exchanged with the Yellow Sea twice a day through the sluice gates. The DOC concentrations were generally higher in lower-salinity waters in both periods, and excess of DOC was also observed in 2017 in high-salinity waters. Here, the excess DOC represents any DOC concentrations higher than those in the incoming open-ocean seawater. The excess DOC occurring in the lower-salinity waters originated mainly from marine sediments of tidal flats, based on the DOC-δ13C values (-20.7±1.2 ‰) and good correlations among the DOC, humic-like fluorescent DOM (FDOMH), and NH4+ concentrations. However, the origins of the excess DOC observed in 2017 appear to be from two different sources: one mainly from marine sources such as biological production based on the DOC-δ13C values (−19.1 ‰ to −20.5 ‰) and the other mainly from terrestrial sources by land–seawater interactions based on its depleted DOC-δ13C values (−21.5 ‰ to −27.8 ‰). This terrestrial DOM source observed in 2017 was likely associated with DOM on the reclaimed land, which experienced extended exposure to light and bacterial degradation as indicated by the higher spectral slope ratio (SR) of light absorbance and no concurrent increases in the FDOMH and NH4+ concentrations. Our study demonstrates that the combination of these biogeochemical tools can be a powerful tracer of DOM sources and characteristics in coastal environments.

Description: The North Atlantic north of 50∘ N is one of the most intense ocean sink areas for atmospheric CO2 considering the flux per unit area, 0.27 Pg-C yr−1, equivalent to −2.5 mol C m−2 yr−1. The northwest Atlantic Ocean is a region with high anthropogenic carbon inventories. This is on account of processes which sustain CO2 air–sea fluxes, in particular strong seasonal winds, ocean heat loss, deep convective mixing, and CO2 drawdown by primary production. The region is in the northern limb of the global thermohaline circulation, a path for the long-term deep-sea sequestration of carbon dioxide. The surface water masses in the North Atlantic are of contrasting origins and character, with the northward-flowing North Atlantic Drift, a Gulf Stream offspring, on the one hand and on the other hand the cold southward-moving low-salinity Polar and Arctic waters with signatures from Arctic freshwater sources. We have studied by observation the CO2 air–sea flux of the relevant water masses in the vicinity of Iceland in all seasons and in different years. Here we show that the highest ocean CO2 influx is to the Arctic and Polar waters, respectively, -3.8±0.4 and -4.4±0.3 mol C m−2 yr−1. These waters are CO2 undersaturated in all seasons. The Atlantic Water is a weak or neutral sink, near CO2 saturation, after poleward drift from subtropical latitudes. These characteristics of the three water masses are confirmed by data from observations covering 30 years. We relate the Polar Water and Arctic Water persistent undersaturation and CO2 influx to the excess alkalinity derived from Arctic sources. Carbonate chemistry equilibrium calculations clearly indicate that the excess alkalinity may support at least 0.058 Pg-C yr−1, a significant portion of the North Atlantic CO2 sink. The Arctic contribution to the North Atlantic CO2 sink which we reveal was previously unrecognized. However, we point out that there are gaps and conflicts in the knowledge about the Arctic alkalinity and carbonate budgets and that future trends in the North Atlantic CO2 sink are connected to developments in the rapidly warming and changing Arctic. The results we present need to be taken into consideration for the following question: will the North Atlantic continue to absorb CO2 in the future as it has in the past?

Description: The north-western Mediterranean deep convection plays a crucial role in the general circulation and biogeochemical cycles of the Mediterranean Sea. The DEWEX (DEnse Water EXperiment) project aimed to better understand this role through an intensive observation platform combined with a modelling framework. We developed a three-dimensional coupled physical and biogeochemical model to estimate the cycling and budget of dissolved oxygen in the entire north-western Mediterranean deep-convection area over the period September 2012 to September 2013. After showing that the simulated dissolved oxygen concentrations are in a good agreement with the in situ data collected from research cruises and Argo floats, we analyse the seasonal cycle of the air–sea oxygen exchanges, as well as physical and biogeochemical oxygen fluxes, and we estimate an annual oxygen budget. Our study indicates that the annual air-to-sea fluxes in the deep-convection area amounted to 20 molm-2yr-1. A total of 88 % of the annual uptake of atmospheric oxygen, i.e. 18 mol m−2, occurred during the intense vertical mixing period. The model shows that an amount of 27 mol m−2 of oxygen, injected at the sea surface and produced through photosynthesis, was transferred under the euphotic layer, mainly during deep convection. An amount of 20 mol m−2 of oxygen was then gradually exported in the aphotic layers to the south and west of the western basin, notably, through the spreading of dense waters recently formed. The decline in the deep-convection intensity in this region predicted by the end of the century in recent projections may have important consequences on the overall uptake of atmospheric oxygen in the Mediterranean Sea and on the oxygen exchanges with the Atlantic Ocean, which appear necessary to better quantify in the context of the expansion of low-oxygen zones.

Description: The phosphorus (P) concentration of soil solution is of key importance for plant nutrition. During large rainfall events, the P concentration is altered by lateral and vertical subsurface storm flow (SSF) that facilitates P mobilization, redistribution within the soil profile and potential P export from the ecosystem. These processes are not well studied under field conditions. Important factors of the replenishment of P concentrations in soil solutions are the rate of P replenishment (by biotic and abiotic processes) and the P buffering capacity of soils. Lab experiments have shown that replenishment times can vary between minutes and months. The question remains of how P concentrations in lateral and vertical SSF vary under natural field conditions. We present results of large-scale sprinkling experiments simulating 150 mm throughfall at 200 m2 plots on hillslopes at three beech forests in Germany. We aimed at quantifying lateral and vertical SSF and associated P concentrations on the forest floor, in the mineral soil and in the saprolite during sprinkling experiments in spring and summer. The sites differed mainly in terms of soil depth, skeleton content and soil P stock (between 189 and 624 g/m2 in the top 1 m soil depth). Vertical SSF in the mineral soil and in the saprolite was at least 2 orders of magnitude larger than lateral SSF at the same depth. Vertical and lateral SSF consisted mainly of pre-event water that was replaced by sprinkling water. Higher P concentrations in SSF in the first 1 to 2 h after the onset of SSF indicated nutrient flushing, but P concentrations in the mineral soil and saprolite were nearly constant thereafter for most of the experiment despite a strong increase in SSF. This suggests that P in the soil solution at all three sites was replenished fast by mineral or organic sources. If chemostatic transport conditions would dominate in SSF, annual P losses at the lateral and vertical boundary of a forest plot could be approximated by knowing the average P concentration and the water fluxes in forest soils. A rough estimation of the annual P loss based on this simplified assumption for one of our sites with longer SSF data resulted in an annual P loss of 3.16 mg/m2/a. This P loss is similar to estimates from a previous study at the same site using bi-weekly groundwater samples. Our approximated annual P loss in SSF was in a similar order of magnitude as P input by dry and wet deposition and by mineral weathering. Despite the fact that P losses from the ecosystem seem to be small, the translocation of P from the forest floor to the mineral soil might be of high relevance at sites with low P stocks where the forest floor is the dominant source for the P nutrition of trees.

Description: To better understand the nutrient assimilation characteristics of subtropical phytoplankton, deep-water addition incubation experiments were carried out on surface waters collected at seven stations across the subtropical North and South Pacific Ocean. These deep-water additions induced phytoplankton blooms with nutrient drawdown at all stations. The drawdown ratios of dissolved inorganic nitrogen (DIN) to phosphate (PO4) varied from 14.1 to 30.7 at the PO4-replete stations in the central North Pacific (CNP) and eastern South Pacific (ESP). These ratios were similar to the range represented by the canonical Redfield ratio (16) through to typical particulate N:P ratios in the surface subtropical ocean (28). In contrast, lower DIN:PO4 drawdown ratios (7.7–13.3) were observed in induced blooms at the PO4-depleted stations in the western North Pacific (WNP). The DIN:PO4 drawdown ratios in the PO4-replete ESP were associated with eukaryote-dominated blooms, while those in the PO4-depleted WNP were associated with eukaryotic and cyanobacterial blooms. The surplus PO4 assimilation, relative to DIN, by phytoplankton in the WNP was not expected based on their typical cellular N:P ratio and was likely due to the high PO4 uptake capability as induced by low-PO4-adapted phytoplankton. The low- and high-P* (=PO4- DIN/16) regimes geographically corresponded to the low and high DIN:PO4 drawdown ratios in the WNP and the CNP or ESP, respectively. The basin-wide P* distribution in the oligotrophic Pacific surface waters showed a clear regional trend from low in the WNP (100 nM). These results suggest that the subtropical phytoplankton blooms as observed in our experiments could be an important factor controlling P* as well as the commonly recognized dinitrogen fixation and denitrification characteristics.

Description: An expansion of bioenergy has been proposed to help reduce fossil-fuel greenhouse gas emissions, and short-rotation forestry (SRF) can contribute to this expansion. However, SRF plantations could also be sources of biogenic volatile organic compound (BVOC) emissions, which can impact atmospheric air quality. In this study, emissions of isoprene and 11 monoterpenes from the branches and forest floor of hybrid aspen, Italian alder and Sitka spruce stands in an SRF field trial in central Scotland were measured during two years (2018–2019) and used to derive emission potentials for different seasons. Sitka spruce was included as a comparison as it is the most extensive plantation species in the UK. Winter and spring emissions of isoprene and monoterpenes were small compared to those in summer. Sitka spruce had a standardised mean emission rate of 15 µgCg-1h-1 for isoprene in the dry and warm summer of 2018 – more than double the emissions in 2019. However, standardised mean isoprene emissions from hybrid aspen were similar across both years, approximately 23 µgCg-1h-1, and standardised mean isoprene emissions from Italian alder were very low. Mean standardised total monoterpene emissions for these species followed a similar pattern of higher standardised emissions in the warmer year: Sitka spruce emitting 4.5 and 2.3 µgCg-1h-1 for 2018 and 2019, aspen emitting 0.3 and 0.09 µgCg-1h-1, and Italian alder emitting 1.5 and 0.2 µgCg-1h-1, respectively. In contrast to these foliage emissions, the forest floor was only a small source of monoterpenes, typically 1 or 2 orders of magnitude lower than foliage emissions on a unit of ground area basis. Estimates of total annual emissions from each plantation type per hectare were derived using the MEGAN 2.1 model. The modelled total BVOC (isoprene and monoterpenes) emissions of SRF hybrid aspen plantations were approximately half those of Sitka spruce for plantations of the same age. Italian alder SRF emissions were 20 times smaller than from Sitka spruce. The expansion of bioenergy plantations to 0.7 Mha has been suggested for the UK to help achieve net-zero greenhouse gas emissions by 2050. The model estimates show that, with such an expansion, total UK BVOC emissions would increase between

Description: Grasslands provide many important ecosystem services globally, and projecting grassland productivity in the coming decades will provide valuable information to land managers. Productivity models can be well calibrated at local scales but generally have some maximum spatial scale in which they perform well. Here we evaluate a grassland productivity model to find the optimal spatial scale for parameterization and thus for subsequently applying it in future productivity projections for North America. We also evaluated the model on new vegetation types to ascertain its potential generality. We find the model most suitable when incorporating only grasslands, as opposed to also including agriculture and shrublands, and only in the Great Plains and eastern temperate forest ecoregions of North America. The model was not well suited to grasslands in North American deserts or northwest forest ecoregions. It also performed poorly in agriculture vegetation, likely due to management activities, and shrubland vegetation, likely because the model lacks representation of deep water pools. This work allows us to perform long-term projections in areas where model performance has been verified, with gaps filled in by future modeling efforts.

Description: The induction of biocrusts through inoculation-based techniques has gained increasing scientific attention in the last 2 decades due to its potential to address issues related to soil degradation and desertification. The technology has shown the most rapid advances in the use of biocrust organisms, particularly cyanobacteria and mosses, as inoculants and biocrust initiators. Cyanobacteria and mosses are poikilohydric organisms – i.e., desiccation-tolerant organisms capable of reactivating their metabolism upon rehydration – that can settle on bare soils in abiotically stressing habitats, provided that selected species are used and an appropriate and customized protocol is applied. The success of inoculation of cyanobacteria and mosses depends on the inoculant's physiology, but also on the ability of the practitioner to identify and control, with appropriate technical approaches in each case study, those environmental factors that most influence the inoculant settlement and its ability to develop biocrusts. This review illustrates the current knowledge and results of biocrust induction biotechnologies that use cyanobacteria or mosses as inoculants. At the same time, this review's purpose is to highlight the current technological gaps that hinder an efficient application of the technology in the field.

Description: River deltas, with their mosaic of ponds, channels and seasonally inundated areas, act as the last continental hot spots of carbon turnover along the land–ocean aquatic continuum. There is increasing evidence for the important role of riparian wetlands in the transformation and emission of terrestrial carbon to the atmosphere. The considerable spatial heterogeneity of river deltas, however, forms a major obstacle for quantifying carbon emissions and their seasonality. The water chemistry in the river reaches is defined by the upstream catchment, whereas delta lakes and channels are dominated by local processes such as aquatic primary production, respiration or lateral exchange with the wetlands. In order to quantify carbon turnover and emissions in the complex mosaic of the Danube Delta, we conducted monthly field campaigns over 2 years at 19 sites spanning river reaches, channels and lakes. Here we report on the greenhouse gas fluxes (CO2 and CH4) from the freshwater systems of the Danube Delta and present the first seasonally resolved estimates of its freshwater carbon emissions to the atmosphere. Furthermore, we quantify the lateral carbon transport of the Danube River to the Black Sea. We estimate the delta's CO2 and CH4 emissions to be 65 GgC yr−1 (30–120 GgC yr−1, a range calculated using 25 to 75 percentiles of observed fluxes), of which about 8 % are released as CH4. The median CO2 fluxes from river branches, channels and lakes are 25, 93 and 5.8 mmol m−2 d−1, respectively. Median total CH4 fluxes amount to 0.42, 2.0 and 1.5 mmol m−2 d−1. While lakes do have the potential to act as CO2 sinks in summer, they are generally the largest emitters of CH4. Small channels showed the largest range in emissions, including a CO2 and CH4 hot spot sustained by adjacent wetlands. Thereby, the channels contribute disproportionately to the delta's emissions, considering their limited surface area. In terms of lateral export, we estimate the net total export (the sum of dissolved inorganic carbon, DIC, dissolved organic carbon, DOC, and particulate organic carbon, POC) from the Danube Delta to the Black Sea to be about 160 ± 280 GgC yr−1, which only marginally increases the carbon load from the upstream river catchment (8490 ± 240 GgC yr−1) by about 2 %. While this contribution from the delta seems small, deltaic carbon yield (45.6 gC m−2 yr−1; net export load/surface area) is about 4 times higher than the riverine carbon yield from the catchment (10.6 gC m−2 yr−1).

Description: The number and quality of ocean pH measurements have increased substantially over the past few decades such that trends, variability, and spatial patterns of change are now being evaluated. However, comparing pH changes across domains with different initial pH values can be misleading because a pH change reflects a relative change in the hydrogen ion concentration ([H+], expressed in mol kg−1) rather than an absolute change in [H+]. We recommend that [H+] be used in addition to pH when describing such changes and provide three examples illustrating why.

Description: Landscape fires, often referred to as biomass burning (BB), emit substantial amounts of (greenhouse) gases and aerosols into the atmosphere each year. Frequently burning savannas, mostly in Africa, Australia, and South America are responsible for over 60 % of total BB carbon emissions. Compared to many other sources of emissions, fires have a strong seasonality. Previous research has identified the mitigation potential of prescribed fires in savanna ecosystems; by burning cured fuels early in the dry season when landscape conditions still provide moist buffers against fire spread, fires are in general smaller, patchier, and less intense. While it is widely accepted that burned area (BA) and the total carbon consumed are lower when fires are ignited early in the dry season, little is known about the intraseasonal variability of emission factors (EFs). This is important because potentially, higher EFs in the early dry season (EDS) could offset some of the carbon benefits of EDS burning. Also, a better understanding of EF intraseasonal variability may improve large-scale BB assessments, which to date rely on temporally static EFs. We used a sampling system mounted on an unmanned aerial vehicle (UAV) to sample BB smoke in the Estação Ecológica Serra Geral do Tocantins in the Brazilian states of Tocantins and Bahia. The protected area contains all major Cerrado vegetation types found in Brazil, and EDS burning has been implemented since 2014. Over 800 smoke samples were collected and analysed during the EDS of 2018 and late dry season (LDS) of 2017 and 2018. The samples were analysed using cavity ring-down spectroscopy, and the carbon balance method was used to estimate CO2, CO, CH4, and N2O EFs. Observed EF averages and standard deviations were 1651 (±50) g kg−1 for CO2, 57.9 (±28.2) g kg−1 for CO, 0.97 (±0.82) g kg−1 for CH4, and 0.096 (±0.174) g kg−1 for N2O. Averaged over all measured fire prone Cerrado types, the modified combustion efficiency (MCE) was slightly higher in the LDS (0.961 versus 0.956), and the CO and CH4 were 10 % and 2.3 % lower in the LDS compared to the EDS. However, these differences were not statistically significant using a two-tailed t test with unequal variance at a 90 % significance level. The seasonal effect was larger in more wood-dominated vegetation types. N2O EFs showed a more complex seasonal dependency, with opposite intraseasonal trends for savannas that were dominated by grasses versus those with abundant shrubs. We found that the N2O EF for the open Cerrado was less than half the EF suggested by literature compilations for savannas. This may indicate a substantial overestimation of the contribution of fires in the N2O budget. Overall, our data imply that in this region, seasonal variability in greenhouse gas emission factors may offset only a small fraction of the carbon mitigation gains in fire abatement programmes.

Description: Foraminifera are unicellular organisms that play an important role in marine organic matter cycles. Some species are able to isolate chloroplasts from their algal food source and incorporate them as kleptoplasts into their own metabolic pathways, a phenomenon known as kleptoplastidy. One species showing this ability is Elphidium excavatum, a common foraminifer in the Kiel Fjord, Germany. The Kiel Fjord is fed by several rivers and thus forms a habitat with strongly fluctuating salinity. Here, we tested the effects of the food source, salinity and light regime on the food uptake (via 15N and 13C algal uptake) in this kleptoplast-bearing foraminifer. In our study E. excavatum was cultured in the lab at three salinity levels (15, 20 and 25) and uptake of C and N from the food source Dunaliella tertiolecta (Chlorophyceae) and Leyanella arenaria (Bacillariophyceae) were measured over time (after 3, 5 and 7 d). The species was very well adapted to the current salinity of the sampling region, as both algal N and C uptake was highest at a salinity of 20. It seems that E. excavatum coped better with lower than with higher salinities. The amount of absorbed C from the green algae D. tertiolecta showed a tendency effect of salinity, peaking at a salinity of 20. Nitrogen uptake was also highest at a salinity of 20 and steadily increased with time. In contrast, C uptake from the diatom L. arenaria was highest at a salinity of 15 and decreased at higher salinities. We found no overall significant differences in C and N uptake from green algae vs. diatoms. Furthermore, the food uptake at a light–dark rhythm of 16:8 h was compared to continuous darkness. Darkness had a negative influence on algal C and N uptake, and this effect increased with incubation time. Starving experiments showed a stimulation of food uptake after 7 d. In summary, it can be concluded that E. excavatum copes well with changes of salinity to a lower level. For changes in light regime, we showed that light reduction caused a decrease of C and N uptake by E. excavatum.

Description: The ocean and inland waters are two separate regimes, with concentrations in greenhouse gases differing on orders of magnitude between them. Together, they create the land–ocean aquatic continuum (LOAC), which comprises itself largely of areas with little to no data with regards to understanding the global carbon system. Reasons for this include remote and inaccessible sample locations, often tedious methods that require collection of water samples and subsequent analysis in the lab, and the complex interplay of biological, physical and chemical processes. This has led to large inconsistencies, increasing errors and has inevitably lead to potentially false upscaling. A set-up of multiple pre-existing oceanographic sensors allowing for highly detailed and accurate measurements was successfully deployed in oceanic to remote inland regions over extreme concentration ranges. The set-up consists of four sensors simultaneously measuring pCO2, pCH4 (both flow-through, membrane-based non-dispersive infrared (NDIR) or tunable diode laser absorption spectroscopy (TDLAS) sensors), O2 and a thermosalinograph at high resolution from the same water source. The flexibility of the system allowed for deployment from freshwater to open ocean conditions on varying vessel sizes, where we managed to capture day–night cycles, repeat transects and also delineate small-scale variability. Our work demonstrates the need for increased spatiotemporal monitoring and shows a way of homogenizing methods and data streams in the ocean and limnic realms.

Description: During the past 20 years, hypoxic areas have expanded rapidly in the Baltic Sea, which has become one of the largest marine “dead zones” in the world. At the same time, the most important commercial fish population of the region, the eastern Baltic cod, has experienced a drastic reduction in mean body condition, but the processes behind the relation between deoxygenation and condition remain elusive. Here we use extensive long-term monitoring data on cod biology and distribution as well as on hydrological variations to investigate the processes that relate deoxygenation and cod condition during the autumn season. Our results show that the depth distribution of cod has increased during the past 4 decades at the same time of the expansion, and shallowing, of waters with oxygen concentrations detrimental to cod performance. This has resulted in a progressively increasing spatial overlap between the cod population and low-oxygenated waters after the mid-1990s. This spatial overlap and the actual oxygen concentration experienced by cod therein statistically explained a large proportion of the changes in cod condition over the years. These results complement previous analyses on fish otolith microchemistry that also revealed that since the mid-1990s, cod individuals with low condition were exposed to low-oxygen waters during their life. This study helps to shed light on the processes that have led to a decline of the eastern Baltic cod body condition, which can aid the management of this population currently in distress. Further studies should focus on understanding why the cod population has moved to deeper waters in autumn and on analyzing the overlap with low-oxygen waters in other seasons to quantify the potential effects of the variations in physical properties on cod biology throughout the year.

Description: Freshwaters are important emitters of carbon dioxide (CO2) and methane (CH4), two potent greenhouse gases (GHGs). While aquatic surface GHG fluxes have been extensively measured, there is much less information about their underlying sources. In lakes and reservoirs, surface GHG can originate from horizontal riverine flow, the hypolimnion, littoral sediments, and water column metabolism. These sources are generally studied separately, leading to a fragmented assessment of their relative role in sustaining CO2 and CH4 surface fluxes. In this study, we quantified sources and sinks of CO2 and CH4 in the epilimnion along a hydrological continuum in a permanently stratified tropical reservoir (Borneo). Results showed that horizontal inputs are an important source of both CO2 and CH4 (〉90 % of surface emissions) in the upstream reservoir branches. However, this contribution fades along the hydrological continuum, becoming negligible in the main basin of the reservoir, where CO2 and CH4 are uncoupled and driven by different processes. In the main basin, vertical CO2 inputs and sediment CH4 inputs contributed to on average 60 % and 23 % respectively to the surface fluxes of the corresponding gas. Water column metabolism exhibited wide amplitude and range for both gases, making it a highly variable component, but with a large potential to influence surface GHG budgets in either direction. Overall our results show that sources sustaining surface CO2 and CH4 fluxes vary spatially and between the two gases, with internal metabolism acting as a fluctuating but key modulator. However, this study also highlights challenges and knowledge gaps related to estimating ecosystem-scale CO2 and CH4 metabolism, which hinder aquatic GHG flux predictions.

Description: Ocean acidification (OA), which is a major environmental change caused by increasing atmospheric CO2, has considerable influences on marine phytoplankton. But few studies have investigated interactions of OA and seasonal changes in temperature and photoperiod on marine diatoms. In the present study, a marine diatom Skeletonema costatum was cultured under two different CO2 levels (LC, 400 µatm; HC, 1000 µatm) and three different combinations of temperature and photoperiod length (8:16 L:D with 5 ∘C, 12:12 L:D with 15 ∘C, 16:8 L:D with 25 ∘C), simulating different seasons in typical temperate oceans, to investigate the combined effects of these factors. The results showed that specific growth rate of S. costatum increased with increasing temperature and day length. However, OA showed contrasting effects on growth and photosynthesis under different combinations of temperature and day length: while positive effects of OA were observed under spring and autumn conditions, it significantly decreased growth (11 %) and photosynthesis (21 %) in winter. In addition, OA alleviated the negative effect of low temperature and short day length on the abundance of RbcL and key photosystem II (PSII) proteins (D1 and D2). These data indicated that future ocean acidification may show differential effects on diatoms in different clusters of other factors.

Description: Bottom trawling in shelf seas can occur more than 10 times per year for a given location. This affects the benthic metabolism, through a mortality of the macrofauna, resuspension of organic matter from the sediment, and alterations of the physical sediment structure. However, the trawling impacts on organic carbon mineralization and associated processes are not well known. Using a modelling approach, the effects of increasing trawling frequencies on early diagenesis were studied in five different sedimentary environments, simulating the effects of a deeper-penetrating gear (e.g. a tickler chain beam trawl) versus a shallower, more variable penetrating gear (e.g. an electric pulse trawl). Trawling events strongly increased oxygen and nitrate concentrations in surface sediment layers and led to significantly lower amounts of ammonium (43 %–99 % reduction) and organic carbon in the top 10 cm of the sediment (62 %–96 % reduction). As a result, total mineralization rates in the sediment were decreased by up to 28 %. The effect on different mineralization processes differed both between sediment types and between trawling frequencies. The shallow-penetrating gear had a slightly smaller effect on benthic denitrification than the deeper-penetrating gear, but there were no statistically different results between gear types for all other parameters. Denitrification was reduced by 69 % in a fine sandy sediment, whereas nitrogen removal nearly doubled in a highly eutrophic mud. This suggests that even relatively low penetration depths from bottom fishing gears generate significant biogeochemical alterations. Physical organic carbon removal through trawl-induced resuspension of sediments, exacerbated by a removal of bioturbating macrofauna, was identified as the main cause of the changes in the mineralization process.

Description: Hypersaline tidal flats (HTFs) are coastal ecosystems with freshwater deficits often occurring in arid or semi-arid regions near mangrove supratidal zones with no major fluvial contributions. Here, we estimate that organic carbon (OC), total nitrogen (TN) and total phosphorus (TP) were buried at rates averaging 21 (±6), 1.7 (±0.3) and 1.4 (±0.3) gm-2yr-1, respectively, during the previous century in three contrasting HTF systems, one in Brazil (eutrophic) and two in Australia (oligotrophic). Although these rates are lower than those from nearby mangrove, saltmarsh and seagrass systems, the importance of HTFs as sinks for OC, TN and TP may be significant given their extensive coverage. Despite the measured short-term variability between net air–saltpan CO2 influx and emission estimates found during the dry and wet season in the Brazilian HTF, the only site with seasonal CO2 flux measurements, the OC sedimentary profiles over several decades suggest efficient OC burial at all sites. Indeed, the stable isotopes of OC and TN (δ13C and δ15N) along with C:N ratios show that microphytobenthos are the major source of the buried OC in these HTFs. Our findings highlight a previously unquantified carbon as well as a nutrient sink and suggest that coastal HTF ecosystems could be included in the emerging blue carbon framework.