Publications in peer reviewed journals

18 Publications found
  • pH-Dependent Bioavailability, Speciation, and Phytotoxicity of Tungsten (W) in Soil Affect Growth and Molybdoenzyme Activity of Nodulated Soybeans

    Oburger E, Cid CV, Preiner J, Hu J, Hann S, Wanek W, Richter A
    2018 - Environmental Science & Technology, in press


    Increasing use of tungsten (W)-based products opened new pathways for W into environmental systems. Due to its chemical alikeness with molybdenum (Mo), W is expected to behave similarly to its “twin element”, Mo; however, our knowledge of the behavior of W in the plant−soil environment remains inadequate. The aim of this study was to investigate plant growth as well as W and nutrient uptake depending on soil chemical properties such as soil pH and texture. Soybean (Glycine max cv. Primus) was grown on two acidic soils differing in soil texture that were either kept at their natural soil pH (pH of 4.5−5) or limed (pH of ≥7) and amended with increasing concentrations of metallic W (control and 500 and 5000 mg kg−1 ). In addition, the activity of molybdoenzymes involved in N assimilation (nitrate reductase) and symbiotic N2 fixation (nitrogenase) was also investigated. Our results showed that the risk of W entering the food web was significantly greater in high-pH soils due to increased solubility of mainly monomeric W. The effect of soil texture on W solubility and phytoavailability was less pronounced compared to soil pH. Particularly at intermediate W additions (W 500 mg kg−1 ), symbiotic nitrogen fixation was able to compensate for reduced leaf nitrate reductase activity. When W soil solution concentrations became too toxic (W 5000 mg kg−1 ), nodulation was more strongly inhibited than nitrogenase activity in the few nodules formed, suggesting a more-efficient detoxification and compartmentalization mechanism in nodules than in soybean leaves. The increasing presence of polymeric W species observed in low-pH soils spiked with high W concentrations resulted in decreased W uptake. Simultaneously, polymeric W species had an overall negative effect on nutrient assimilation and plant growth, suggesting a greater phytotoxicity of W polymers. Our study demonstrates the importance of accounting for soil pH in risk assessment studies of W in the plant−soil environment, something that has been completely neglected in the past.

  • Traits indicating a conservative resource strategy are weakly related to narrow range size in a group of neotropical trees

    Chacón-Madrigal E, Wanek W, Hietz P, Dullinger S
    2018 - Perspectives in Plant Ecology Evolution and Systematics, 32: 30-37


    Biological traits may co-determine differences in geographical range sizes among closely related species. In plants, trait values linked to a conservative resource-use strategy have been hypothesised to be associated with small range sizes. However, the empirical support is mixed and limited to extra-tropical species so far. Here, we analyse the relationship between range size and eight functional traits linked to the plant economics spectrum in congeneric pairs of neotropical tree species of Costa Rica with contrasting range sizes. In the lowland tropical rainforests of southern Costa Rica, we sampled 345 trees from 35 species in 14 genera and measured leaf thickness, leaf dry matter content, specific leaf area, wood specific gravity (WSG), leaf nitrogen (N), leaf phosphorus, leaf potassium and leaf N:P ratio. For each species, we estimated range size as the extent of occurrence using known localities of occurrence. We correlated range sizes with trait data scaled within-genus and with the principal components of the multivariate trait space. WSG was higher and leaf N was lower in species with small range sizes in univariate regression models, although these traits were only weakly related to range size. None of the other six traits was correlated with range size. Results were similar for a model using the principal components of the multivariate trait space, which explained 36% of the variation in species’ extent of occurrence. Again, the traits most strongly associated with the selected components were WSG and leaf N. Although high WSG and low leaf N can be interpreted as indicators of conservative resource-use, we could not detect strong relationships between the respective trait syndrome and range size in our sample of species. Traits related to conservative resource use may hence be involved in determining the range size of the species analysed, but other factors are apparently more important.

  • Amino acid production exceeds plant nitrogen demand in Siberian tundra

    Wild B, Alves RJE, Barta J, Capek P, Gentsch N, Guggenberger G, Hugelius G, Knoltsch A, Kuhry P, Lashchinskiy N, Mikutta R, Palmtag J, Prommer J, Schnecker J, Shibistova O, Takriti M, Urich T, Richter A
    2018 - Environmental Research Letters, 13: 11


    Arctic plant productivity is often limited by low soil N availability. This has been attributed to slow breakdown of N-containing polymers in litter and soil organic matter (SOM) into smaller, available units, and to shallow plant rooting constrained by permafrost and high soil moisture. Using 15N pool dilution assays, we here quantified gross amino acid and ammonium production rates in 97 active layer samples from four sites across the Siberian Arctic. We found that amino acid production in organic layers alone exceeded literature-based estimates of maximum plant N uptake 17-fold and therefore reject the hypothesis that arctic plant N limitation results from slow SOM breakdown. High microbial N use efficiency in organic layers rather suggests strong competition of microorganisms and plants in the dominant rooting zone. Deeper horizons showed lower amino acid production rates per volume, but also lower microbial N use efficiency. Permafrost thaw together with soil drainage might facilitate deeper plant rooting and uptake of previously inaccessible subsoil N, and thereby promote plant productivity in arctic ecosystems. We conclude that changes in microbial decomposer activity, microbial N utilization and plant root density with soil depth interactively control N availability for plants in the Arctic.

  • Full 15N tracer accounting to revisit major assumptions of 15N isotope pool dilution approaches for gross nitrogen mineralization

    Braun J, Mooshammer M, Wanek W, Prommer J, Walker TWN, Rütting T, Richter A
    2018 - Soil Biology and Biochemistry, 117: 16-26
  • Links among warming, carbon and microbial dynamics mediated by soil mineral weathering

    Doetterl S, Berhe AA, Arnold C, Bodé S, Fiener P, Finke P, Fuchslueger L, Griepentrog M, Harden JW, Nadeu E, Schnecker J, Six J, Trumbore S, Van Oost K, Vogel C, Boeckx P
    2018 - Nature Geoscience, in press


    Quantifying soil carbon dynamics is of utmost relevance in the context of global change because soils play an important role in land–atmosphere gas exchange. Our current understanding of both present and future carbon dynamics is limited because we fail to accurately represent soil processes across temporal and spatial scales, partly because of the paucity of data on the relative importance and hierarchical relationships between microbial, geochemical and climatic controls. Here, using observations from a 3,000-kyr-old soil chronosequence preserved in alluvial terrace deposits of the Merced River, California, we show how soil carbon dynamics are driven by the relationship between short-term biotic responses and long-term mineral weathering. We link temperature sensitivity of heterotrophic respiration to biogeochemical soil properties through their relationship with microbial activity and community composition. We found that soil mineralogy, and in particular changes in mineral reactivity and resulting nutrient availability, impacts the response of heterotrophic soil respiration to warming by altering carbon inputs, carbon stabilization, microbial community composition and extracellular enzyme activity. We demonstrate that biogeochemical alteration of the soil matrix (and not short-term warming) controls the composition of microbial communities and strategies to metabolize nutrients. More specifically, weathering first increases and then reduces nutrient availability and retention, as well as the potential of soils to stabilize carbon.

  • Soil microbial CNP and respiration responses to organic matter and nutrient additions: Evidence from a tropical soil incubation

    Soong J, Marañon-Jimenez S, Cotrufo MF, Boeckx P, Bodé S, Guenet B, Peñuelas J, Richter A, Stahl C, Verbruggen E, Janssens IA
    2018 - Soil Biology and Biochemistry, 122: 141-149


    Soil nutrient availability has a strong influence on the fate of soil carbon (C) during microbial decomposition, contributing to Earth's C balance. While nutrient availability itself can impact microbial physiology and C partitioning between biomass and respiration during soil organic matter decomposition, the availability of labile C inputs may mediate the response of microorganisms to nutrient additions. As soil organic matter is decomposed, microorganisms retain or release C, nitrogen (N) or phosphorus (P) to maintain a stoichiometric balance. Although the concept of a microbial stoichiometric homeostasis has previously been proposed, microbial biomass CNP ratios are not static, and this may have very relevant implications for microbial physiological activities. Here, we tested the hypothesis that N, P and potassium (K) nutrient additions impact C cycling in a tropical soil due to microbial stoichiometric constraints to growth and respiration, and that the availability of energy-rich labile organic matter in the soil (i.e. leaf litter) mediates the response to nutrient addition. We incubated tropical soil from French Guiana with a 13C labeled leaf litter addition and with mineral nutrient additions of +K, +N, +NK, +PK and +NPK for 30 days. We found that litter additions led to a ten-fold increase in microbial respiration and a doubling of microbial biomass C, along with greater microbial N and P content. We found some evidence that P additions increased soil CO2 fluxes. Additionally, we found microbial biomass CP and NP ratios varied more widely than CN in response to nutrient and organic matter additions, with important implications for the role of microorganisms in C cycling. The addition of litter did not prime soil organic matter decomposition, except in combination with +NK fertilization, indicating possible P-mining of soil organic matter in this P-poor tropical soil. Together, these results point toward an ultimate labile organic substrate limitation of soil microorganisms in this tropical soil, but also indicate a complex interaction between C, N, P and K availability. This highlights the difference between microbial C cycling responses to N, P, or K additions in the tropics and explains why coupled C, N and P cycle modeling efforts cannot rely on strict microbial stoichiometric homeostasis as an underlying assumption.

  • Sampling root exudates – Mission impossible?

    Oburger E, Jones DL
    2018 - Rhizosphere, 6: 116-133


    Accurate information about the quantity, quality and spatiotemporal dynamics of metabolite release from plant roots is vital to understanding the functional significance of root exudates in biogeochemical processes occurring at the root-microbe-soil-interface. Significant progress in analytical techniques nowadays allows us to gain a much better picture of the rich diversity of compounds that are present in root exudates, but ultimately the choice of exudation sampling strategy will determine the ecological significance of obtained exudation results. Unfortunately, in the past, little consideration has been given to the experimental strategy used to sample root exudates. To date, our knowledge on root exudation is mainly based on plants grown and sampled in nutrient solution culture (hydroponics). Despite the operational benefit of hydroponic systems, the question remains as to how ecologically relevant exudation results obtained under these artificial conditions are compared to soil environments, particularly in the context of exudate driven rhizosphere processes. The quantitative and qualitative measurement of root exudation in soil, however, is fraught with problems due to: (i) continual removal of exudates from solution by the microbial community; (ii) loss of exudates from solution due to their sorption to the solid phase; and (iii) simultaneous release of compounds from soil organic matter breakdown. While a perfect method for sampling root exudates does not exist, soil based approaches, if appropriately applied and interpreted, may still provide more realistic insights into exudation dynamics in natural soil environments. This review aims to provide an overview of different root exudation sampling approaches and their advantages and limitations to support the selection of the most suitable experimental procedure for any specific research question. We address critical methodological aspects that need to be considered in the choice of experimental approach, like growth and sampling medium (soil, hydroponic), sterility, sampling location (whole root system, individual root segments) as well as plant age, daytime, re-uptake of metabolites affecting duration and timing of the sampling event and data presentation. In addition, we summarize the main analytical approaches to analyze root exudates, ranging from liquid sample analysis to isotope tracking and imaging techniques.

  • Is local trait variation related to total range size of tropical trees?

    Chacón-Madrigal E, Wanek W, Hietz P, Dullinger S
    2018 - PLoS One, 19


    The reasons why the range size of closely related species often varies significantly have intrigued scientists for many years. Among other hypotheses, species with high trait variation were suggested to occupy more diverse environments, have more continuity in their distributions, and consequently have larger range sizes. Here, using 34 tree species of lowlands tropical rainforest in southern Costa Rica, we explored whether inherent trait variability expressed at the local scale in functional traits is related to the species’ total geographical range size. We formed 17 congeneric pairs of one narrow endemic and one widespread species, sampled 335 individuals and measured eight functional traits: leaf area, leaf thickness, leaf dry matter content, specific leaf area, leaf nitrogen content, leaf phosphorus content, leaf nitrogen to phosphorus ratio, and wood specific gravity. We tested whether there are significant differences in the locally expressed variation of individual traits or in multidimensional trait variance between the species in congeneric pairs and whether species’ range size could hence be predicted from local trait variability. However, we could not find such differences between widely distributed and narrow range species. We discuss the possible reasons for these findings including the fact that higher trait variability of widespread species may result from successive local adaptations during range expansion and may hence often be an effect rather than the cause of larger ranges.

  • Significance of dark CO2 fixation in arctic soils

    Santruckova H, Kotas P, Barta J, Urich T, Capek P, Palmtag J, Alves RJE, Biasi C, Diakova K, Gentsch N, Gittel A, Guggenberger G, Hugelius G, Lashchinsky N, Martikainen PJ, Mikutta R, Schleper C, Schnecker J, Schwab C, Shibistova O, Wild B, Richter A
    2018 - Soil Biology and Biochemistry, 119: 11-21


    The occurrence of dark fixation of CO2 by heterotrophic microorganisms in soil is generally accepted, but its importance for microbial metabolism and soil organic carbon (C) sequestration is unknown, especially under Climiting conditions. To fill this knowledge gap, we measured dark 13CO2 incorporation into soil organic matter and conducted a 13C-labelling experiment to follow the 13C incorporation into phospholipid fatty acids as microbial biomass markers across soil profiles of four tundra ecosystems in the northern circumpolar region, where net primary productivity and thus soil C inputs are low. We further determined the abundance of various carboxylase genes and identified their microbial origin with metagenomics. The microbial capacity for heterotrophic CO2 fixation was determined by measuring the abundance of carboxylase genes and the incorporation of 13C into soil C following the augmentation of bioavailable C sources. We demonstrate that dark CO2 fixation occurred ubiquitously in arctic tundra soils, with increasing importance in deeper soil horizons, presumably due to increasing C limitation with soil depth. Dark CO2 fixation accounted on average for 0.4, 1.0, 1.1, and 16% of net respiration in the organic, cryoturbated organic, mineral and permafrost horizons, respectively. Genes encoding anaplerotic enzymes of heterotrophic microorganisms comprised the majority of identified carboxylase genes. The genetic potential for dark CO2 fixation was spread over a broad taxonomic range. The results suggest important regulatory function of CO2 fixation in C limited conditions. The measurements were corroborated by modeling the long-term impact of dark CO2 fixation on soil organic matter. Our results suggest that increasing relative CO2 fixation rates in deeper soil horizons play an important role for soil internal C cycling and can, at least in part, explain the isotopic enrichment with soil depth.

  • In situ observation of localized, sub-mm scale changes of phosphorus biogeochemistry in the rhizosphere

    Kreuzeder A, Santner J, Scharsching V, Oburger E, Hoefer C, Hann S, Wenzel WW
    2018 - Plant and soil, 1-17


    Aims We imaged the sub-mm distribution of labile P and pH in the rhizosphere of three plant species to localize zones and hot spots of P depletion and accumulation along individual root axes and to relate our findings to nutrient acquisition / root exudation strategies in P-limited conditions at different soil pH, and to mobilization pattern of other elements (Al, Fe, Ca, Mg, Mn) in the rhizosphere. Methods Sub-mm distributions of labile elemental patterns were sampled using diffusive gradients in thin films and analysed using laser ablation inductively coupled plasma mass spectrometry. pH images were taken using planar optodes. Results We found distinct patterns of highly localized labile P depletion and accumulation reflecting the complex interaction of plant P acquisition strategies with soil pH, fertilizer treatment, root age, and elements (Al, Fe, Ca) that are involved in P biogeochemistry in soil. We show that the plants respond to P deficiency either by acidification or alkalization, depending on initial bulk soil pH and other factors of P solubility. Conclusions P solubilization activities of roots are highly localized, typically around root apices, but may also extend towards the extension / root hair zone.

  • Recognizing Patterns: Spatial Analysis of Observed Microbial Colonization on Root Surfaces

    Schmidt H, Nunan N, Höck A, Eickhorst T, Kaiser C, Woebken D, Raynaud X
    2018 - Frontiers in Environmental Science, 6: 1-12


    Root surfaces are major sites of interactions between plants and associated microorganisms. Here, plants and microbes communicate via signaling molecules, compete for nutrients, and release substrates that may have beneficial or harmful effects on each other. Whilst the body of knowledge on the abundance and diversity of microbial communities at root-soil interfaces is now substantial, information on their spatial distribution at the microscale is still scarce. In this study, a standardized method for recognizing and analyzing microbial cell distributions on root surfaces is presented. Fluorescence microscopy was combined with automated image analysis and spatial statistics to explore the distribution of bacterial colonization patterns on rhizoplanes of rice roots. To test and evaluate the presented approach, a gnotobiotic experiment was performed using a potential nitrogen-fixing bacterial strain in combination with roots of wetland rice. The automated analysis procedure resulted in reliable spatial data of bacterial cells colonizing the rhizoplane. Among all replicate roots, the analysis revealed an increasing density of bacterial cells from the root tip to the region of root cell maturation. Moreover, bacterial cells showed significant spatial clustering and tended to be located around plant root cell borders. The quantitative data suggest that the structure of the root surface plays a major role in bacterial colonization patterns. Possible adaptations of the presented approach for future studies are discussed along with potential pitfalls such as inaccurate imaging. Our results demonstrate that standardized recognition and statistical evaluation of microbial colonization on root surfaces holds the potential to increase our understanding of microbial associations with roots and of the underlying ecological interactions.

  • Geothermally warmed soils reveal persistent increases in the respiratory costs of soil microbes contributing to substantial C losses

    Marañon-Jimenez S, Soong JL, Leblans NI, Sigurdsson B, Peñuelas J, Richter A, Asensio D, Fransen E, Janssens IA
    2018 - Biogeochemistry, 138: 245-260


    Increasing temperatures can accelerate soil organic matter decomposition and release large amounts of CO2 to the atmosphere, potentially inducing positive warming feedbacks. Alterations to the temperature sensitivity and physiological functioning of soil microorganisms may play a key role in these carbon (C) losses. Geothermally active areas in Iceland provide stable and continuous soil temperature gradients to test this hypothesis, encompassing the full range of warming scenarios projected by the Intergovernmental Panel on Climate Change for the northern region. We took soils from these geothermal sites 7 years after the onset of warming and incubated them at varying temperatures and substrate availability conditions to detect persistent alterations of microbial physiology to long-term warming. Seven years of continuous warming ranging from 1.8 to 15.9 °C triggered a 8.6–58.0% decrease on the C concentrations in the topsoil (0–10 cm) of these sub-arctic silt-loam Andosols. The sensitivity of microbial respiration to temperature (Q10) was not altered. However, soil microbes showed a persistent increase in their microbial metabolic quotients (microbial respiration per unit of microbial biomass) and a subsequent diminished C retention in biomass. After an initial depletion of labile soil C upon soil warming, increasing energy costs of metabolic maintenance and resource acquisition led to a weaker capacity of C stabilization in the microbial biomass of warmer soils. This mechanism contributes to our understanding of the acclimated response of soil respiration to in situ soil warming at the ecosystem level, despite a lack of acclimation at the physiological level. Persistent increases in the respiratory costs of soil microbes in response to warming constitute a fundamental process that should be incorporated into climate change-C cycling models.

  • Spatial variation of soil CO2, CH4 and N2O fluxes across topographical positions in the tropical forests of the Guiana Shield

    Courtois EA, Stahl C, Van den Berge J, Bréchet L, Van Langenhove L, Richter A, Urbina I, Soong JL, Peñuelas J, Janssens IA
    2018 - Ecosystems, In press


    The spatial variation of soil greenhouse gas fluxes (GHG; carbon dioxide—CO2, methane—CH4and nitrous oxide—N2O) remains poorly understood in highly complex ecosystems such as tropical forests. We used 240 individual flux measurements of these three GHGs from different soil types, at three topographical positions and in two extreme hydric conditions in the tropical forests of the Guiana Shield (French Guiana, South America) to (1) test the effect of topographical positions on GHG fluxes and (2) identify the soil characteristics driving flux variation in these nutrient-poor tropical soils. Surprisingly, none of the three GHG flux rates differed with topographical position. CO2 effluxes covaried with soil pH, soil water content (SWC), available nitrogen and total phosphorus. The CH4 fluxes were best explained by variation in SWC, with soils acting as a sink under drier conditions and as a source under wetter conditions. Unexpectedly, our study areas were generally sinks for N2O and N2O fluxes were partly explained by total phosphorus and available nitrogen concentrations. This first study describing the spatial variation of soil fluxes of the three main GHGs measured simultaneously in forests of the Guiana Shield lays the foundation for specific studies of the processes underlying the observed patterns.

  • Fate of carbohydrates and lignin in north-east Siberian permafrost soils

    Dao TT, Gentsch N, Mikutta R, Sauheitl L, Shibistova O, Wild B, Schnecker J, Barta J, Capek P, Gittel A, Lashchinskiy N, Urich T, Santruckova H, Richter A, Guggenberger G
    2018 - Soil Biology and Biochemistry, 116: 311-322


    Permafrost soils preserve huge amounts of organic carbon (OC) prone to decomposition under changing climatic conditions. However, knowledge on the composition of soil organic matter (OM) and its transformation and vulnerability to decomposition in these soils is scarce. We determined neutral sugars and lignin-derived phenols, released by trifluoroacetic acid (TFA) and CuO oxidation, respectively, within plants and soil density fractions from the active layer and the upper permafrost layer at three different tundra types (shrubby grass, shrubby tussock, shrubby lichen) in the Northeast Siberian Arctic. The heavy fraction (HF; > 1.6 g mL−1 ) was characterized by a larger enrichment of microbial sugars (hexoses vs. pentoses) and more pronounced lignin degradation (acids vs. aldehydes) as compared to the light fraction (LF; < 1.6 g mL−1 ), showing the transformation from plant residue-dominated particulate OM to a largely microbial imprint in mineral-associated OM. In contrast to temperate and tropical soils, total neutral sugar contents and galactose plus mannose to arabinose plus xylose ratios (GM/AX) decreased in the HF with soil depth, which may indicate a process of effective recycling of microbial biomass rather than utilizing old plant materials. At the same time, lignin-derived phenols increased and the degree of oxidative decomposition of lignin decreased with soil depth, suggesting a selective preservation of lignin presumably due to anaerobiosis. As large parts of the plant-derived pentoses are incorporated in lignocelluloses and thereby protected against rapid decomposition, this might also explain the relative enrichment of pentoses with soil depth. Hence, our results show a relatively large contribution of plantderived OM, particularly in the buried topsoil and subsoil, which is stabilized by the current soil environmental conditions but may become available to decomposers if permafrost degradation promotes soil drainage and improves the soil oxygen supply.

  • Application of stable-isotope labelling techniques for the detection of active diazotrophs

    Angel R, Panhölzl C, Gabriel R, Herbold C, Wanek W, Richter A, Eichorst SA, Woebken D
    2018 - Environ Microbiol, 20: 44-61


    nvestigating active participants in the fixation of dinitrogen gas is vital as N is often a limiting factor for primary production. Biological nitrogen fixation is performed by a diverse guild of bacteria and archaea (diazotrophs), which can be free-living or symbionts. Free-living diazotrophs are widely distributed in the environment, yet our knowledge about their identity and ecophysiology is still limited. A major challenge in investigating this guild is inferring activity from genetic data as this process is highly regulated. To address this challenge, we evaluated and improved several 15 N-based methods for detecting N2 fixation activity (with a focus on soil samples) and studying active diazotrophs. We compared the acetylene reduction assay and the 15 N2 tracer method and demonstrated that the latter is more sensitive in samples with low activity. Additionally, tracing 15 N into microbial RNA provides much higher sensitivity compared to bulk soil analysis. Active soil diazotrophs were identified with a 15 N-RNA-SIP approach optimized for environmental samples and benchmarked to 15 N-DNA-SIP. Lastly, we investigated the feasibility of using SIP-Raman microspectroscopy for detecting 15 N-labelled cells. Taken together, these tools allow identifying and investigating active free-living diazotrophs in a highly sensitive manner in diverse environments, from bulk to the single-cell level.

  • Soil organic matter quality exerts a stronger control than stoichiometry on microbial substrate use efficiency along a latitudinal transect

    Takriti M, Wild B, Schnecker J, Mooshammera M, Knoltsch A, Lashchinskiy N, Alves RJE, Gentsch N, Gittel A, Mikutta R, Wanek W, Richter A
    2018 - Soil Biology and Biochemistry, 121: 212-220


    A substantial portion of soil organic matter (SOM) is of microbial origin. The efficiency with which soil microorganisms can convert their substrate carbon (C) into biomass, compared to how much is lost as respiration, thus co-determines the carbon storage potential of soils. Despite increasing insight into soil microbial C cycling, empirical measurements of microbial C processing across biomes and across soil horizons remain sparse. The theory of ecological stoichiometry predicts that microbial carbon use efficiency (CUE), i.e. growth over uptake of organic C, strongly depends on the relative availability of C and nutrients, particularly N, as microorganisms will either respire excess C or conserve C while mineralising excess nutrients. Microbial CUE is thus expected to increase from high to low latitudes and from topsoil to subsoil as the soil C:N and the stoichiometric imbalance between SOM and the microbial biomass decrease. To test these hypotheses, we collected soil samples from the organic topsoil, mineral topsoil, and mineral subsoil of seven sites along a 1500-km latitudinal transect in Western Siberia. As a proxy for CUE, we measured the microbial substrate use efficiency (SUE) of added substrates by incubating soil samples with a mixture of 13C labelled sugars, amino sugarsamino acids, and organic acids and tracing 13C into microbial biomass and released CO2. In addition to soil and microbial C:N stoichiometry, we also determined the potential extracellular enzyme activities of cellobiohydrolase (CBH) and phenoloxidase (POX) and used the CBH:POX ratio as an indicator of SOM substrate quality. We found an overall decrease of SUE with latitude, corresponding to a decrease in mean annual temperature, in mineral soil horizons. SUE decreased with decreasing stoichiometric imbalance in the organic and mineral topsoil, while a relationship of SUE with soil C:N was only found in the mineral topsoil. However, contrary to our hypothesis, SUE did not increase with soil depth and mineral subsoils displayed lower average SUE than mineral topsoils. Both within individual horizons and across all horizons SUE was strongly correlated with CBH:POX ratio as well as with climate variables. Since enzyme activities likely reflect the chemical properties of SOM, our results indicate that SOM quality exerts a stronger control on SUE than SOM stoichiometry, particularly in subsoils were SOM has been turned over repeatedly and there is little variation in SOM elemental ratios.

  • Temperature response of permafrost soil carbon is attenuated by mineral protection

    Gentsch N, Wild B, Mikutta R, Capek P, Diakova K, Schrumpf M, Turner S, Minnich C, Schaarschmidt F, Shibistova O, Schnecker J, Urich T, Gittel A, Santruckova H, Barta J, Lashchinskiy N, Fuß R, Richter A, Guggenberger G
    2018 - Global Change Biology, In press


    Climate change in Arctic ecosystems fosters permafrost thaw and makes massive amounts of ancient soil organic carbon (OC) available to microbial breakdown. However, fractions of the organic matter (OM) may be protected from rapid decomposition by their association with minerals. Little is known about the effects of mineral‐organic associations (MOA) on the microbial accessibility of OM in permafrost soils and it is not clear which factors control its temperature sensitivity. In order to investigate if and how permafrost soil OC turnover is affected by mineral controls, the heavy fraction (HF) representing mostly MOA was obtained by density fractionation from 27 permafrost soil profiles of the Siberian Arctic. In parallel laboratory incubations, the unfractionated soils (bulk) and their HF were comparatively incubated for 175 days at 5 and 15°C. The HF was equivalent to 70 ± 9% of the bulk CO2 respiration as compared to a share of 63 ± 1% of bulk OC that was stored in the HF. Significant reduction of OC mineralization was found in all treatments with increasing OC content of the HF (HF‐OC), clay‐size minerals and Fe or Al oxyhydroxides. Temperature sensitivity (Q10) decreased with increasing soil depth from 2.4 to 1.4 in the bulk soil and from 2.9 to 1.5 in the HF. A concurrent increase in the metal‐to‐HF‐OC ratios with soil depth suggests a stronger bonding of OM to minerals in the subsoil. There, the younger 14C signature in CO2 than that of the OC indicates a preferential decomposition of the more recent OM and the existence of a MOA fraction with limited access of OM to decomposers. These results indicate strong mineral controls on the decomposability of OM after permafrost thaw and on its temperature sensitivity. Thus, we here provide evidence that OM temperature sensitivity can be attenuated by MOA in permafrost soils.

  • A multi-isotopic approach to investigate the influence of land use on nitrate removal in a highly saline lake-aquifer system

    Valiente N, Carrey R, Otero N, Soler A, Sanz D, Muñoz-Martín A, Jirsa F, Wanek W, Gómez-Alday JJ
    2018 - Science of The Total Environment, 631: 649-659


    Endorheic or closed drainage basins in arid and semi-arid regions are vulnerable to pollution. Nonetheless, in the freshwater-saltwater interface of endorheic saline lakes, oxidation-reduction (redox) reactions can attenuate pollutants such as nitrate (NO3-). This study traces the ways of nitrogen (N) removal in the Pétrola lake-aquifer system (central Spain), an endorheic basin contaminated with NO3- (up to 99.2mg/L in groundwater). This basin was declared vulnerable to NO3- pollution in 1998 due to the high anthropogenic pressures (mainly agriculture and wastewaters). Hydrochemical, multi-isotopic (δ18ONO3, δ15NNO3, δ13CDIC, δ18OH2O, and δ2HH2O) and geophysical techniques (electrical resistivity tomography) were applied to identify the main redox processes at the freshwater-saltwater interface. The results showed that the geometry of this interface is influenced by land use, causing spatial variability of nitrogen biogeochemical processes over the basin. In the underlying aquifer, NO3- showed an average concentration of 38.5mg/L (n=73) and was mainly derived from agricultural inputs. Natural attenuation of NO3- was observed in dryland farming areas (up to 72%) and in irrigation areas (up to 66%). In the Pétrola Lake, mineralization and organic matter degradation in lake sediment play an important role in NO3- reduction. Our findings are a major step forward in understanding freshwater-saltwater interfaces as reactive zones for NO3- attenuation. We further emphasize the importance of including a land use perspective when studying water quality-environmental relationships in hydrogeological systems dominated by density-driven circulation.

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