ALBERT

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Description: Lakes may function as either sinks or sources of CO 2 . Their response to climate change is uncertain, as we lack continuous data of lake CO 2 efflux and its drivers. This is especially true in the littoral zone of lakes, which can be very dynamic from the continuous injection and remobilization of terrestrial nutrients. This study used high-frequency measurements of CO 2 exchange during the ice-free season by prototype low-power floating Forced Diffusion (FD) autochambers. We quantified the net surface flux of CO 2 across a transect of the littoral zone of a small deep oligotrophic lake in eastern Nova Scotia, Canada, and examined potential drivers. The littoral zone was a net source for CO 2 , on average emitting 0.171 ± 0.023 µmol CO 2 m -2 s -1 , but we did observe significant temporal variation across diel and seasonal periods, as well as with distance from shore. While no pelagic environmental driver appeared to explain this variability in CO 2 exchange, our study suggests that factors which vary on a fine spatial scale within the littoral zone may effectively regulate CO 2 exchange. If environmental drivers of pelagic CO 2 exchange are unrelated to CO 2 exchange in the littoral zone, this may have large implications for current mechanistic understandings of lake carbon dynamics, and for upscalings of fluxes. This work shows the spatial and temporal variability of littoral CO 2 efflux, as well as the utility of low-power FD automated chambers for observing lake-atmosphere net CO 2 exchange.

Description: Water relations in plant communities are influenced both by contrasting functional groups (grasses, shrubs) and by climate change via complex effects on interception, uptake and transpiration. We modelled the effects of functional group replacement and biomass increase, both of which can be outcomes of invasion and vegetation management, and climate change on ecological drought (soil water potential below which photosynthesis stops) in 340 semiarid grassland sites over 30-year periods. Relative to control vegetation (climate and site-determined mixes of functional groups), the frequency and duration of drought were increased by shrubs and decreased by annual grasses. The rankings of shrubs, control vegetation, and annual grasses in terms of drought effects were generally consistent in current and future climates, suggesting that current differences among functional groups on drought effects predict future differences. Climate change accompanied by experimentally-increased biomass (i.e. the effects of invasions that increase community biomass, or management that increases productivity through fertilization or respite from grazing) increased drought frequency and duration, and advanced drought onset. Our results suggest that the replacement of perennial temperate semiarid grasslands by shrubs, or increased biomass, can increase ecological drought both in current and future climates.

Description: The ability to forecast ecological carbon cycling is imperative to land management in a world where past carbon fluxes are no longer a clear guide in the Anthropocene. However, carbon-flux forecasting has not been practiced routinely like numerical weather prediction. This study explored: (1) the relative contributions of model forcing data and parameters to uncertainty in forecasting flux- vs. pool-based carbon cycle variables; and (2) time points when temperature and CO 2 treatments may cause statistically detectable differences in those variables. We developed an online forecasting workflow (EcoPAD), which facilitates iterative data-model integration. EcoPAD automates data transfer from sensor networks, data assimilation and ecological forecasting. We used SPRUCE data collected from 2011-2014 to constrain the parameters in the Terrestrial ECOsystem Model (TECO), forecast carbon cycle responses to elevated CO 2 and a gradient of warming from 2015 to 2024, and specify uncertainties in the model output. Our results showed that data assimilation substantially reduces forecasting uncertainties. Interestingly, we found that the stochasticity of future external forcing contributed more to the uncertainty of forecasting future dynamics of C flux-related variables than model parameters. However, the parameter uncertainty primarily contributes to the uncertainty in forecasting C pool-related response variables. Given the uncertainties in forecasting carbon fluxes and pools, our analysis showed that statistically different responses of fast-turnover pools to various CO 2 and warming treatments were observed sooner than slow-turnover pools. Our study has identified the sources of uncertainties in model prediction and thus leads to improve ecological carbon cycling forecasts in the future.

Description: Extreme events such as storm surges, intense precipitation, and supermoons cause anomalous and large fluctuations in water level in tidal salt marshes, which impacts the sediment biogeochemistry that dictates arsenic (As) cycling. In addition to changes in water level, which impacts soil redox potential, these extreme events may also change salinity due to freshwater inputs from precipitation or saltwater inputs due to surge. It is currently unknown how As mobility in tidal salt marshes will be impacted by extreme events, as fluctuations in salinity and redox potential may act synergistically to mobilize As. To investigate impacts of extreme events on As cycling in tidal salt marshes, we conducted a combined laboratory and field investigation. We monitored pore water and soil samples before, during, and after two extreme events: a supermoon lunar eclipse followed by a storm surge and precipitation induced by Hurricane Joaquin in Fall 2015 at the St. Jones Reserve in Dover, Delaware, a representative tidal salt marsh in the Mid-Atlantic United States. We also conducted soil incubations of marsh sediments in batch and in flow-through experiments in which redox potential and/or salinity were manipulated. Field investigations showed that pore water As was inversely proportional to redox potential. During the extreme events, a distinct pulse of As was observed in the pore water with maximum salinity. Combined field and laboratory investigations revealed that this As pulse is likely due to rapid changes in salinity. These results have implications for As mobility in the face of extreme weather variability.

Description: Ongoing global temperature rise has caused significant thaw and degradation of permafrost soils on the Qinghai-Tibetan Plateau (QTP). Leaching of organic matter from permafrost soils to aquatic systems is highly complex and difficult to reproduce in a laboratory setting. We collected samples from natural seeps of active and permafrost layers in an alpine swamp meadow on the QTP to shed light on the composition of mobilized dissolved organic matter (DOM) by combining optical measurements, ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry, radiocarbon ( 14 C) and solid-state 13 C nuclear magnetic resonance spectroscopy. Our results show that even though the active layer soils contain large amounts of proteins and carbohydrates, there is a selective release of aromatic components, whereas, in the deep permafrost layer, carbohydrate and protein components are preferentially leached during the thawing process. Given these different chemical characteristics of mobilized DOM, we hypothesize that photomineralization contributes significantly to the loss of DOM that is leached from the seasonally thawed surface layer. However, with continued warming, biodegradation will become more important since biolabile materials such as protein and carbohydrate are preferentially released from deep-layer permafrost soils. This transition in DOM leachate source and associated chemical composition has ramifications for downstream fluvial networks on the QTP particularly in terms of processing of carbon and associated fluxes.

Description: Terrestrial carbon (C) cycling in high Arctic tundra depends on ecosystem responses to climatic warming and concurrent changes in environmental conditions. There are very few studies to quantify long-term C budget in high Arctic tundra due to lack of sufficient measurements. Here, based on well-established multi-year measurements, we calibrated a process-oriented model (CoupModel) to quantify various components of the C budget at a Cassiope tetragona heath ecosystem in northeast Greenland. Net ecosystem exchange of CO 2 (NEE) for 2000-2014 was estimated at -15 ± 10 g C m -2 yr -1 . Ecosystem respiration (ER) for non-growing seasons was estimated at 10.3 ± 5.3 g C m -2 yr -1 , representing around 13 % of the annual ER. Significant trends for interannual variations of above- and below-ground C fluxes and stocks were found for the sub-periods (i.e. 2000-2008 and 2008-2014) but not for the entire period. Interannual variations of NEE largely relied on the response of gross primary production (GPP) and ER to seasonal changes in climate. Moreover, the model showed that interannual variations of GPP, ER, and NEE had a much higher linear correlation with July temperature and annual maximum thawing depth (ALD max ) than other climatic and site characteristics. ALD max had the highest correlation with the decomposition rate of humus C. Overall, this modeling study suggests that a sink-source transition of the studied ecosystem depends on ecosystem responses to interannual variations of climate and that the net C balance may be sensitive to summer warmth and active layer thickness.

Description: Biogeochemical experiments and modelling were coupled to investigate how nutrient leaching to aquatic ecosystems changes the dynamics of microbial community on suspended sediment and how these changes modulate the nitrogen cycle. Mineral suspensions amended with inorganic nitrogen ( and ) and inoculated with native sedimentary microbial strains were tested in a settling column under continuous water quality measurements. Experiments were then used to calibrate and validate a kinetic model that described abiotic and biotic processes, including chemical adsorption on minerals, aqueous complexation, gas dissolution, microbial metabolism, necromass dynamics, and microbial competition for limiting substrates. Modelling revealed that the interactions between microbial functional groups were highly non-linear and highly sensitive to changes in nutrient and dissolved oxygen concentrations, showing shifts in regimes where a functional group became prevalent over the others. Our results suggested that necromass dynamics played a major role in sustaining microbial growth in low nutrient conditions and had an important control over the N cycle dynamics. The reaction network and model structure presented in this study provide a tool to analyse and predict the long-term dynamics of both natural and engineered aquatic ecosystems.

Description: This study aims to explain effects of soil textural class, topography, land-use and land-use history on soil GHG fluxes in the Lake Victoria region. We measured GHG fluxes from intact soil cores collected in Rakai, Uganda, an area characterized by low-input smallholder (〈2 ha) farming systems, typical for the East African highlands. The soil cores were air dried and re-wetted to water holding capacities (WHC) of 30, 55 and 80%. Soil CO 2 , CH 4 and N 2 O fluxes were measured for 48 hours following re-wetting. Cumulative N 2 O fluxes were highest from soils under perennial crops and the lowest from soils under annual crops ( P 〈 0.001 for all WHC). At WHC of 55% or 80%, the sandy clay loam soils had lower N 2 O fluxes than the clay soils ( P 〈 0.001 and P = 0.041 respectively). Cumulative soil CO 2 fluxes were highest from eucalyptus plantations and lowest from annual crops across multiple WHC ( P = 0.014 at 30% WHC and P 〈 0.001 at both 55 and 80% WHC). Methane fluxes were below detectable limits, a shortcoming for using soil cores from the top soil. This study reveals that land-use and soil type have strong effects on GHG fluxes from agricultural land in the study area. Field monitoring of fluxes is needed to confirm whether these findings are consistent with what happens in situ .

Description: To evaluate the risk of nitrate (NO 3 - -N) in groundwater, it is necessary to know the denitrification capacity. In this study, observations were carried out for two years to investigate the groundwater denitrification capacity below three arable land systems in Eastern China. Denitrification capacity was assessed by measuring the concentration and distribution patterns of nitrous oxide (N 2 O) and dinitrogen (N 2 ) in groundwater. The results revealed that groundwater denitrification activity was high and consumed 76%, 83%, and 65% of the NO 3 - -N in the vineyard (VY), vegetable field (VF), and paddy field (PF), respectively. The high denitrification activity might be attributed to the strong reducing conditions, with high dissolved oxygen concentrations in groundwater, which promotes denitrification. During the sampling period, we observed high dinitrogen (excess N 2 ) accumulation in groundwater, which exceeded the total reactive nitrogen (N) in the deep layer. The large amount of excess N 2 observed in VY and VF indicated that considerable N was stored as gaseous N 2 in groundwater and should not be ignored when balancing N budgets in aquifers where denitrification is high.