ALBERT

Beschreibung: [1]  A small hot spring that is informally called “Fe-waterfall spring” and is located in the Rehai geothermal area discharges hot (42 to 73 °C), near-neutral (pH =7.65) Fe-rich water. Submerged reddish precipitates are composed largely of ferrihydrite, goethite, lepidocrocite, opal-A, quartz, and anorthite, as revealed by X-ray diffraction (XRD) and Mössbauer spectroscopy. Molecular phylogenetic analysis demonstrates that the bacterial community in these precipitates is mainly composed of Cyanobacteria, Planctomycetes, β-proteobacteria, Deinococci-Thermus and Chlorobi. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) examinations show that abundant sheath-like Fe oxyhydroxides, which exhibit different morphologies and sizes, are present in Fe-rich precipitates. These sheath-like structures are composed of ferrihydrite rather than more crystalline lepidocrocite or goethite. Energy dispersive X-ray spectrometer (EDS), scanning transmission electron microscopy (STEM) and nano secondary ion mass spectrometry (Nano-SIMS) reveal that they are mainly composed of Fe, Si and O, together with some trace elements. Most of the sheath-like structures are not morphologically comparable to biogenic Fe oxyhydroxides produced by known chemolithotrophic Fe oxidizers, which is consistent with the fact that no chemolithotrophic Fe oxidizers were identified by molecular analysis in the precipitates. We suggest that the sheath-like Fe oxyhydroxides are formed through passive Fe sorption and nucleation onto the cell walls of various thermophiles rather than by the direct metabolic activities of chemolithotrophic Fe oxidizers. Biogenic sheath-like Fe oxyhydroxides in Fe-waterfall spring have important implications for geochemical cycles driven by microorganisms, the origin of microfossils and the formation of banded iron formations (BIFs) in the Archean ocean.

Beschreibung: [1]  We present the concept of the Carbon Cycle Data Assimilation System and describe its evolution over the last two decades from an assimilation system around a simple diagnostic model of the terrestrial biosphere to a system for the calibration and initialization of the land component of a comprehensive earth system model. We critically review the capability of this modeling framework to integrate multiple data streams, to assess their mutual consistency and with the model, to reduce uncertainties in the simulation of the terrestrial carbon cycle, to provide, in a traceable manner, reanalysis products with documented uncertainty, and to assist the design of the observational network. We highlight some of the challenges we met and experience we gained, give recommendations for operating the system and suggest directions for future development.

Beschreibung: [1]  Hypolithic microbial communities are productive niches in deserts worldwide, but many facets of their basic ecology remain unknown. The Namib Desert is an important site for hypolith study because it has abundant quartz rocks suitable for colonization and extends west to east across a transition from fog to rain dominated moisture sources. We show fog sustains and impacts hypolithic ecology in several ways: (1) fog effectively replaces rainfall in the western zone of the central Namib to enable high (≥95%) hypolithic abundance at landscape (1- 10 km) and larger scales; and (2) high water availability, through fog (western zone) and/or rainfall (eastern zone), results in smaller size-class rocks being colonized (mean 6.3 ± 1.2 cm) at higher proportions (e.g. 98% vs. ca . 3%) than in previously studied hyperarid deserts. We measured 0.1% of incident sunlight as the lower limit for hypolithic growth on quartz rocks in the Namib, and found uncolonized ventral rock surfaces were limited by light rather than moisture. In-situ monitoring showed that, although rainfall supplied more liquid water (36 hrs) per event than fog (mean, 4 hrs), on an equivalent annual basis, fog provided nearly twice as much liquid water as rainfall to the hypolithic zone. Hypolithic abundance reaches 100% at a mean annual precipitation (MAP) ca . 40-60 mm, but at a much lower MAP ( ca . 25 mm) when moisture from fog is available.

Beschreibung: [1]  Mountain pine beetle ( Dendroctonus ponderosae ) outbreaks in North America are widespread and have potentially-large scale impacts on albedo and associated radiative forcing. Mountain pine beetle outbreaks in Colorado and southern Wyoming have resulted in persistent and significant increases in both winter albedo (change peaked 10 years post-outbreak at 0.06 ± 0.01 and 0.05 ± 0.01, in lodgepole pine ( Pinus contorta ) and ponderosa pine ( Pinus ponderosa ) stands, respectively) and spring albedo (change peaked 10 years post-outbreak at 0.06 ± 0.01 and 0.04 ± 0.01, in lodgepole pine and ponderosa pine stands, respectively). Instantaneous top-of-atmosphere radiative forcing peaked for both lodgepole pine and ponderosa pine stands in winter at 10 years post-outbreak at −1.7 ± 0.2 W m -2 and −1.4 ± 0.2 W m -2 , respectively. The persistent increase in albedo with time since mountain pine beetle disturbance combined with the continued progression of the attack across the landscape from 1994–2011, resulted in an exponential increase in winter and annual radiative cooling (MW) over time. In 2011 the rate of radiative forcing within the study area reached −982.7 ± 139.0 MW, -269.8 ± 38.2 MW, -31.1 ± 4.4 MW and −147.8 ± 20.9 MW in winter, spring, summer and fall, respectively. An increase in radiative cooling has the potential to decrease sensible and/or latent heat flux by reducing available energy. Such changes could affect current mountain pine beetle outbreaks which are influenced by climatic conditions.

Beschreibung: [1]  Characteristics of the natural fire regime are poorly resolved in the Arctic, even though fire may play an important role cycling carbon stored in tundra vegetation and soils to the atmosphere. In the course of studying vegetation and permafrost-terrain characteristics along a chronosequence of tundra burn sites from AD 1977, 1993, and 2007 on the North Slope of Alaska, we discovered two large, previously unrecognized tundra fires. The Meade River fire burned an estimated 500 km 2 and the Ketik River fire burned an estimated 1,200 km 2 . Based on radiocarbon dating of charred twigs, analysis of historic aerial photography, and regional climate proxy data, these fires likely occurred between AD 1880 and 1920. Together, these events double the estimated burn area on the North Slope of Alaska over the last ~100 to 150 years. Assessment of vegetation succession along the century-scale chronosequence of tundra fire disturbances demonstrates for the first time on the North Slope of Alaska that tundra fires can facilitate the invasion of tundra by shrubs. Degradation of ice-rich permafrost was also evident at the fire sites and likely aided in the presumed changes of the tundra vegetation post-fire. Other previously unrecognized tundra fire events likely exist in Alaska and other Arctic regions and identification of these sites is important for better understanding disturbance regimes and carbon cycling in Arctic tundra.

Beschreibung: [1]  Carbon sequestration occurs only when terrestrial ecosystems are at nonsteady states. Despite of their ubiquity in the real world, the nonsteady states of ecosystems have not been well quantified, especially at regional and global scales. In this study, we developed a two-step data assimilation scheme to estimate carbon sink strength in China's forest ecosystems. Specifically, the two-step scheme consists of a steady-state step and a nonsteady-state step. In the steady-state step, we constrained a process-based model (TECO-R model) against biometric data (NPP, biomass, litter, and soil organic carbon) in mature forests. With a subset of the parameter values estimated from the steady-state data assimilation being fixed, the nonsteady-state data assimilation was performed to estimate carbon sequestration in China's forest ecosystems. Our results indicated that 17 out of the 22 total parameters in the TECO-R model were well constrained by the biometric data with the steady-state data assimilation. When observations from both mature and developing forests were used, all the 10 parameters related to carbon sequestration in vegetation and soil carbon pools were well constrained at the nonsteady-state step. The estimated mean vegetation carbon sink in China's forests is 89.7 ± 16.8 gC m -2  yr -1 , comparable with the values estimated from the forest inventory and other process-based regional models. The estimated mean soil and litter carbon sinks in China's forests are 14.1 ± 20.7 and 4.7 ± 6.5 gCm -2  yr -1 . This study demonstrated that a two-step data assimilation scheme can be a potent tool to estimate regional carbon sequestration in nonsteady-state ecosystems.

Beschreibung: ABSTRACT [1]  It has often been hypothesized that the dissolved organic carbon (DOC) pool of algal origin in lakes is more bioavailable than its terrestrial counterpart, but this hypothesis has seldom been directly tested. Here we test this hypothesis by tracking the production and isotopic signature of bacterial respiratory CO 2 in 2-week lake water incubations, and use the resulting data to reconstruct and model the bacterial consumption dynamics of algal and terrestrial DOC. The proportion of algal DOC respired decreased systematically over time in all experiments, suggesting a rapid consumption and depletion of this substrate. Our results further show that the algal DOC pool was used in proportions and at rates twice and ten-time as high as the terrestrial DOC pool, respectively. On the other hand, the absolute amount of labile terrestrial DOC was on average four-time higher than labile algal DOC, accounting for almost the entire long-term residual C metabolism, but also contributing to short-term bacterial C consumption. The absolute amount of labile algal DOC increased with chlorophyll a concentrations, whereas total phosphorus appeared to enhance the amount of terrestrial DOC that bacteria could consume, suggesting that the degradation of these pools is not solely governed by their respective chemical properties, but also by interactions with nutrients. Our study shows that there is a highly reactive pool of terrestrial DOC that is processed in parallel to algal DOC, and because of interactions with nutrients, terrestrial DOC likely supports high levels of bacterial metabolism and CO 2 production even in more productive lakes.

Beschreibung: [1]  N-fertilization significantly increases N 2 O and NO soil fluxes to the atmosphere. In spite of the expansion of agricultural activities in tropical managed soils from the developing-world, there is little information about the loss of applied nitrogen (LAN) as NO and N 2 O from these areas. In this work, we determined LAN-N 2 O and LAN-NO from different crops during the growing season at a sandy soil experimental field and two active farms with loamy and clayed soils, respectively. Tillage (T) and no-tillage (NT) farming were separately evaluated. All of the evaluated areas were located in the Venezuela savanna region. [2]  A large range of LAN-N 2 O (0.30-6.1%) and LAN-NO (0.26-2.1%) were recorded, with overall average values of 1.9% and 0.9%, respectively. LAN values were mainly affected by soil texture and rainfall pattern, which affected soil moisture and WFPS. Also, soil management (T and NT) and the chemical composition of the N-fertilizer played important roles. The overall average of LAN-N 2 O is about two times higher than the IPCC default value of 1%, therefore, our results suggest that a higher factor should be considered for cropping systems in tropical savanna regions.

Beschreibung: [1]  Model-data fusion is a process in which field observations are used to constrain model parameters. How observations are used to constrain parameters has a direct impact on the carbon cycle dynamics simulated by ecosystem models. In this study, we present an evaluation of several options for the use of observations in modeling regional carbon dynamics and explore the implications of those options. We calibrated the Terrestrial Ecosystem Model (TEM) on a hierarchy of three vegetation classification levels for the Alaskan boreal forest: species-level, plant-functional-type-level (PFT-level) and biome-level, and we examined the differences in simulated carbon dynamics. Species specific field-based estimates were directly used to parameterize the model for species-level simulations, while weighted-averages based on species percent cover were used to generate estimates for PFT- and biome-level model parameterization. We found that calibrated key ecosystem process parameters differed substantially among species and overlapped for species that are categorized into different PFTs. Our analysis of parameter sets suggests that the PFT-level parameterizations primarily reflected the dominant species and that functional information of some species were lost from the PFT-level parameterizations. The biome-level parameterization was primarily representative of the needleleaf PFT and lost information on broadleaf species or PFT function. Our results indicate that PFT-level simulations may be potentially representative of the performance of species-level simulations while biome-level simulations may result in biased estimates. Improved theoretical and empirical justifications for grouping species into PFTs or biomes are needed to adequately represent the dynamics of ecosystem functioning and structure.

Beschreibung: [1]  Spatial and temporal heterogeneity of methane flux from boreal wetlands make prediction and up-scaling challenging, both within and among wetland systems. Drivers of methane production and emissions are also highly variable, making empirical model development difficult and leading to uncertainty in methane emissions estimates from wetlands. Previous studies have examined this problem using point-scale (static chamber method) and ecosystem-scale (flux tower methods) measurements, but few studies have investigated whether different processes are observed at these scales. We analyzed methane emissions from a boreal fen, measured by both techniques, using data from the Boreal Ecosystem-Atmosphere Study (BOREAS). We sought to identify driving processes associated with methane emissions at two scales and explain diurnal patterns in emissions measured by the tower. The mean methane emission rates from flux chambers were greater than the daytime, daily mean rates measured by the tower, but the nighttime, daily mean emissions from the tower were often an order of magnitude greater than emissions recorded during the daytime. Thus, daytime measurements from either the tower or chambers would lead to a biased estimate of total methane emissions from the wetland. We found the timing of nighttime emission events was coincident with the cooling and convective mixing within hollows, which occurred regularly during the growing season. We propose that diurnal thermal stratification in shallow pools traps methane by limiting turbulent transport. This methane stored during daytime heating is later released during evening cooling due to convective turbulent mixing.