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  • 1. Adamczyk, Bartosz
    et al.
    Sietio, Outi-Maaria
    Strakoya, Petra
    Prommer, Judith
    Wild, Birgit
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. University of Vienna, Austria; University of Gothenburg, Sweden.
    Hagner, Marleena
    Pihlatie, Mari
    Fritze, Hannu
    Richter, Andreas
    Heinonsalo, Jussi
    Plant roots increase both decomposition and stable organic matter formation in boreal forest soil2019In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 10, article id 3982Article in journal (Refereed)
    Abstract [en]

    Boreal forests are ecosystems with low nitrogen (N) availability that store globally significant amounts of carbon (C), mainly in plant biomass and soil organic matter (SOM). Although crucial for future climate change predictions, the mechanisms controlling boreal C and N pools are not well understood. Here, using a three-year field experiment, we compare SOM decomposition and stabilization in the presence of roots, with exclusion of roots but presence of fungal hyphae and with exclusion of both roots and fungal hyphae. Roots accelerate SOM decomposition compared to the root exclusion treatments, but also promote a different soil N economy with higher concentrations of organic soil N compared to inorganic soil N accompanied with the build-up of stable SOM-N. In contrast, root exclusion leads to an inorganic soil N economy (i.e., high level of inorganic N) with reduced stable SOM-N buildup. Based on our findings, we provide a framework on how plant roots affect SOM decomposition and stabilization.

  • 2. Fuchslueger, Lucia
    et al.
    Wild, Birgit
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. University of Vienna, Austria.
    Mooshammer, Maria
    Takriti, Mounir
    Kienzl, Sandra
    Knoltsch, Anna
    Hofhansl, Florian
    Bahn, Michael
    Richter, Andreas
    Microbial carbon and nitrogen cycling responses to drought and temperature in differently managed mountain grasslands2019In: Soil Biology and Biochemistry, ISSN 0038-0717, E-ISSN 1879-3428, Vol. 135, p. 144-153Article in journal (Refereed)
    Abstract [en]

    Grassland management can modify soil microbial carbon (C) and nitrogen (N) cycling, affecting the resistance to extreme weather events, which are predicted to increase in frequency and magnitude in the near future. However, effects of grassland management on microbial C and N cycling and their responses to extreme weather events, such as droughts and heatwaves, have rarely been tested in a combined approach. We therefore investigated whether grassland management affects microbial C and N cycling responses to drought and temperature manipulation. We collected soils from in situ drought experiments conducted in an extensively managed and an abandoned mountain grassland and incubated them at two temperature levels. We measured microbial respiration and substrate incorporation, as well as gross rates of organic and inorganic N cycling to estimate microbial C and N use efficiencies (CUE and NUE). The managed grassland was characterized by lower microbial biomass, lower fungi to bacteria ratio, and higher microbial CUE, but only slightly different microbial NUE. At both sites drought induced a shift in microbial community composition driven by an increase in Gram-positive bacterial abundance. Drought significantly reduced C substrate respiration and incorporation by microbes at both sites, while microbial CUE remained constant. In contrast, drought increased gross rates of N mineralization at both sites, whereas gross amino acid uptake rates only marginally changed. We observed a significant direct, as well as interactive effect between land management and drought on microbial NUE. Increased temperatures significantly stimulated microbial respiration and reduced microbial CUE independent of drought or land management. Although microbial N processing rates showed no clear response, microbial NUE significantly decreased at higher temperatures. In summary in our study, microbial CUE, in particular respiration, is more responsive to temperature changes. Although N processing rates were stronger responding to drought than to temperature microbial NUE was affected by both drought and temperature increase. We conclude that direct effects of drought and heatwaves can induce different responses in soil microbial C and N cycling similarly in the studied land management systems.

  • 3. Gentsch, Norman
    et al.
    Wild, Birgit
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. University of Vienna, Austria; Austrian Polar Research Institute, Austria.
    Mikutta, Robert
    Capek, Petr
    Diakova, Katka
    Schrumpf, Marion
    Turner, Stephanie
    Minnich, Cynthia
    Schaarschmidt, Frank
    Shibistova, Olga
    Schnecker, Joerg
    Urich, Tim
    Gittel, Antje
    Santruckova, Hana
    Barta, Jiri
    Lashchinskiy, Nikolay
    Fuss, Roland
    Richter, Andreas
    Guggenberger, Georg
    Temperature response of permafrost soil carbon is attenuated by mineral protection2018In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 24, no 8, p. 3401-3415Article in journal (Refereed)
    Abstract [en]

    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 degrees 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 C-14 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.

  • 4. Landhausser, Simon M.
    et al.
    Chow, Pak S.
    Dickman, L. Turin
    Furze, Morgan E.
    Kuhlman, Iris
    Schmid, Sandra
    Wiesenbauer, Julia
    Wild, Birgit
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. University of Gothenburg, Sweden.
    Gleixner, Gerd
    Hartmann, Henrik
    Hoch, Guenter
    McDowell, Nate G.
    Richardson, Andrew D.
    Richter, Andreas
    Adams, Henry D.
    Standardized protocols and procedures can precisely and accurately quantify non-structural carbohydrates2018In: Tree Physiology, ISSN 0829-318X, E-ISSN 1758-4469, Vol. 38, no 12, p. 1764-1778Article in journal (Refereed)
    Abstract [en]

    Non-structural carbohydrates (NSCs), the stored products of photosynthesis, building blocks for growth and fuel for respiration, are central to plant metabolism, but their measurement is challenging. Differences in methods and procedures among laboratories can cause results to vary widely, limiting our ability to integrate and generalize patterns in plant carbon balance among studies. A recent assessment found that NSC concentrations measured for a common set of samples can vary by an order of magnitude, but sources for this variability were unclear. We measured a common set of nine plant material types, and two synthetic samples with known NSC concentrations, using a common protocol for sugar extraction and starch digestion, and three different sugar quantification methods (ion chromatography, enzyme, acid) in six laboratories. We also tested how sample handling, extraction solvent and centralizing parts of the procedure in one laboratory affected results. Non-structural carbohydrate concentrations measured for synthetic samples were within about 11.5% of known values for all three methods. However, differences among quantification methods were the largest source of variation in NSC measurements for natural plant samples because the three methods quantify different NSCs. The enzyme method quantified only glucose, fructose and sucrose, with ion chromatography we additionally quantified galactose, while the acid method quantified a large range of mono- and oligosaccharides. For some natural samples, sugars quantified with the acid method were two to five times higher than with other methods, demonstrating that trees allocate carbon to a range of sugar molecules. Sample handling had little effect on measurements, while ethanol sugar extraction improved accuracy over water extraction. Our results demonstrate that reasonable accuracy of NSC measurements can be achieved when different methods are used, as long as protocols are robust and standardized. Thus, we provide detailed protocols for the extraction, digestion and quantification of NSCs in plant samples, which should improve the comparability of NSC measurements among laboratories.

  • 5.
    Martens, Jannik
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Wild, Birgit
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Pearce, Christof
    Stockholm University, Faculty of Science, Department of Geological Sciences. Aarhus University, Denmark.
    Tesi, Tommaso
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. National Research Council, Italy.
    Andersson, August
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Bröder, Lisa
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Vrije Universiteit Amsterdam, Netherlands,.
    O'Regan, Matt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Sköld, Martin
    Stockholm University, Faculty of Science, Department of Mathematics.
    Gemery, Laura
    Cronin, Thomas M.
    Semiletov, Igor
    Dudarev, Oleg V.
    Gustafsson, Örjan
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Remobilization of Old Permafrost Carbon to Chukchi Sea Sediments During the End of the Last Deglaciation2019In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 33, no 1, p. 2-14Article in journal (Refereed)
    Abstract [en]

    Climate warming is expected to destabilize permafrost carbon (PF-C) by thaw-erosion and deepening of the seasonally thawed active layer and thereby promote PF-C mineralization to CO2 and CH4. A similar PF-C remobilization might have contributed to the increase in atmospheric CO2 during deglacial warming after the last glacial maximum. Using carbon isotopes and terrestrial biomarkers (Delta C-14, delta C-13, and lignin phenols), this study quantifies deposition of terrestrial carbon originating from permafrost in sediments from the Chukchi Sea (core SWERUS-L2-4-PC1). The sediment core reconstructs remobilization of permafrost carbon during the late Allerod warm period starting at 13,000 cal years before present (BP), the Younger Dryas, and the early Holocene warming until 11,000 cal years BP and compares this period with the late Holocene, from 3,650 years BP until present. Dual-carbon-isotope-based source apportionment demonstrates that Ice Complex Deposit-ice- and carbon-rich permafrost from the late Pleistocene (also referred to as Yedoma)-was the dominant source of organic carbon (66 +/- 8%; mean +/- standard deviation) to sediments during the end of the deglaciation, with fluxes more than twice as high (8.0 +/- 4.6 g.m(-2).year(-1)) as in the late Holocene (3.1 +/- 1.0 g.m(-2).year(-1)). These results are consistent with late deglacial PF-C remobilization observed in a Laptev Sea record, yet in contrast with PF-C sources, which at that location were dominated by active layer material from the Lena River watershed. Release of dormant PF-C from erosion of coastal permafrost during the end of the last deglaciation indicates vulnerability of Ice Complex Deposit in response to future warming and sea level changes.

  • 6. Santruckova, Hana
    et al.
    Kotas, Petr
    Barta, Jiri
    Urich, Tim
    Capek, Petr
    Palmtag, Juri
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Alves, Ricardo J. Eloy
    Biasi, Christina
    Diakova, Katerina
    Gentsch, Norman
    Gittel, Antje
    Guggenberger, Georg
    Hugelius, Gustaf
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Lashchinsky, Nikolaj
    Martikainen, Pertti J.
    Mikutta, Robert
    Schleper, Christa
    Schnecker, Jörg
    Schwab, Clarissa
    Shibistova, Olga
    Wild, Birgit
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. University of Vienna, Austria.
    Richter, Andreas
    Significance of dark CO2 fixation in arctic soils2018In: Soil Biology and Biochemistry, ISSN 0038-0717, E-ISSN 1879-3428, Vol. 119, p. 11-21Article in journal (Refereed)
    Abstract [en]

    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 C limiting conditions. To fill this knowledge gap, we measured dark (CO2)-C-13 incorporation into soil organic matter and conducted a C-13-labelling experiment to follow the C-13 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 C-13 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.

  • 7. Thao, Thi
    et al.
    Gentsch, Norman
    Mikutta, Robert
    Sauheitl, Leopold
    Shibistova, Olga
    Wild, Birgit
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Schnecker, Jörg
    Bárta, Jiri
    Čapek, Petr
    Gittel, Antje
    Lashchinskiy, Nikolay
    Urich, Tim
    Šantrůčková, Hana
    Richter, Andreas
    Guggenberger, Georg
    Fate of carbohydrates and lignin in north-east Siberian permafrost soils2018In: Soil Biology and Biochemistry, ISSN 0038-0717, E-ISSN 1879-3428, Vol. 116, p. 311-322Article in journal (Refereed)
    Abstract [en]

    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 pronotmced 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 HE with soil depth, which may indicate a process of effective recycling of microbial biomass rather than utilizing old plant materials. At the same dine, 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 plant derived 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.

  • 8.
    Wild, Birgit
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. University of Gothenburg, Sweden.
    Alaei, Saeed
    Bengtson, Per
    Bodé, Samuel
    Boeckx, Pascal
    Schnecker, Jörg
    Mayerhofer, Werner
    Rütting, Tobias
    Short-term carbon input increases microbial nitrogen demand, but not microbial nitrogen mining, in a set of boreal forest soils2017In: Biogeochemistry, ISSN 0168-2563, E-ISSN 1573-515X, Vol. 136, no 3, p. 261-278Article in journal (Refereed)
    Abstract [en]

    Rising carbon dioxide (CO2) concentrations and temperatures are expected to stimulate plant productivity and ecosystem C sequestration, but these effects require a concurrent increase in N availability for plants. Plants might indirectly promote N availability as they release organic C into the soil (e.g., by root exudation) that can increase microbial soil organic matter (SOM) decomposition (priming effect), and possibly the enzymatic breakdown of N-rich polymers, such as proteins, into bio-available units (N mining). We tested the adjustment of protein depolymerization to changing soil C and N availability in a laboratory experiment. We added easily available C or N sources to six boreal forest soils, and determined soil organic C mineralization, gross protein depolymerization and gross ammonification rates (using N-15 pool dilution assays), and potential extracellular enzyme activities after 1 week of incubation. Added C sources were C-13-labelled to distinguish substrate from soil derived C mineralization. Observed effects reflect short-term adaptations of non-symbiotic soil microorganisms to increased C or N availability. Although C input promoted microbial growth and N demand, we did not find indicators of increased N mobilization from SOM polymers, given that none of the soils showed a significant increase in protein depolymerization, and only one soil showed a significant increase in N-targeting enzymes. Instead, our findings suggest that microorganisms immobilized the already available N more efficiently, as indicated by decreased ammonification and inorganic N concentrations. Likewise, although N input stimulated ammonification, we found no significant effect on protein depolymerization. Although our findings do not rule out in general that higher plant-soil C allocation can promote microbial N mining, they suggest that such an effect can be counteracted, at least in the short term, by increased microbial N immobilization, further aggravating plant N limitation.

  • 9.
    Wild, Birgit
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. University of Vienna, Austria; Austrian Polar Research Institute, Austria; University of Gothenburg, Sweden; .
    Alves, Ricardo J. Eloy
    Bárta, Jiři
    Čapek, Petr
    Gentsch, Norman
    Guggenberger, Georg
    Hugelius, Gustaf
    Stockholm University, Faculty of Science, Department of Physical Geography. Stanford University, United States of America.
    Knoltsch, Anna
    Kuhry, Peter
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Lashchinskiy, Nikolay
    Mikutta, Robert
    Palmtag, Juri
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Prommer, Judith
    Schnecker, Jörg
    Shibistova, Olga
    Takriti, Mounir
    Urich, Tim
    Richter, Andreas
    Amino acid production exceeds plant nitrogen demand in Siberian tundra2018In: Environmental Research Letters, ISSN 1748-9326, E-ISSN 1748-9326, Vol. 13, no 3, article id 034002Article in journal (Refereed)
    Abstract [en]

    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 N-15 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.

  • 10.
    Wild, Birgit
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. University of Vienna, Austria; University of Gothenburg, Sweden.
    Ambus, Per
    Reinsch, Sabine
    Richter, Andreas
    Resistance of soil protein depolymerization rates to eight years of elevated CO2, warming, and summer drought in a temperate heathland2018In: Biogeochemistry, ISSN 0168-2563, E-ISSN 1573-515X, Vol. 140, no 3, p. 255-267Article in journal (Refereed)
    Abstract [en]

    Soil N availability for plants and microorganisms depends on the breakdown of soil polymers such as proteins into smaller, assimilable units by microbial extracellular enzymes. Changing climatic conditions are expected to alter protein depolymerization rates over the next decades, and thereby affect the potential for plant productivity. We here tested the effect of increased CO2 concentration, temperature, and drought frequency on gross rates of protein depolymerization, N mineralization, microbial amino acid and ammonium uptake using N-15 pool dilution assays. Soils were sampled in fall 2013 from the multifactorial climate change experiment CLIMAITE that simulates increased CO2 concentration, temperature, and drought frequency in a fully factorial design in a temperate heathland. Eight years after treatment initiation, we found no significant effect of any climate manipulation treatment, alone or in combination, on protein depolymerization rates. Nitrogen mineralization, amino acid and ammonium uptake showed no significant individual treatment effects, but significant interactive effects of warming and drought. Combined effects of all three treatments were not significant for any of the measured parameters. Our findings therefore do not suggest an accelerated release of amino acids from soil proteins in a future climate at this site that could sustain higher plant productivity.

  • 11.
    Wild, Birgit
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Andersson, August
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Bröder, Lisa
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. Vrije Universiteit, Amsterdam, The Netherlands.
    Vonk, Jorien
    Hugelius, Gustaf
    Stockholm University, Faculty of Science, Department of Physical Geography.
    McClelland, James W.
    Song, Wenjun
    Raymond, Peter A.
    Gustafsson, Örjan
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Rivers across the Siberian Arctic unearth the patterns of carbon release from thawing permafrost2019In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 116, no 21, p. 10280-10285Article in journal (Refereed)
    Abstract [en]

    Climate warming is expected to mobilize northern permafrost and peat organic carbon (PP-C), yet magnitudes and system specifics of even current releases are poorly constrained. While part of the PP-C will degrade at point of thaw to CO2 and CH4 to directly amplify global warming, another part will enter the fluvial network, potentially providing a window to observe large-scale PP-C remobilization patterns. Here, we employ a decade-long, high-temporal resolution record of C-14 in dissolved and particulate organic carbon (DOC and POC, respectively) to deconvolute PP-C release in the large drainage basins of rivers across Siberia: Ob, Yenisey, Lena, and Kolyma. The C-14-constrained estimate of export specifically from PP-C corresponds to only 17 +/- 8% of total fluvial organic carbon and serves as a benchmark for monitoring changes to fluvial PP-C remobilization in a warming Arctic. Whereas DOC was dominated by recent organic carbon and poorly traced PP-C (12 +/- 8%), POC carried a much stronger signature of PP-C (63 +/- 10%) and represents the best window to detect spatial and temporal dynamics of PP-C release. Distinct seasonal patterns suggest that while DOC primarily stems from gradual leaching of surface soils, POC reflects abrupt collapse of deeper deposits. Higher dissolved PP-C export by Ob and Yenisey aligns with discontinuous permafrost that facilitates leaching, whereas higher particulate PP-C export by Lena and Kolyma likely echoes the thermokarst-induced collapse of Pleistocene deposits. Quantitative C-14-based fingerprinting of fluvial organic carbon thus provides an opportunity to elucidate large-scale dynamics of PP-C remobilization in response to Arctic warming.

  • 12.
    Wild, Birgit
    et al.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry. University of Gothenburg, Sweden.
    Li, Jian
    Pihlblad, Johanna
    Bengtson, Per
    Rütting, Tobias
    Decoupling of priming and microbial N mining during a short-term soil incubation2019In: Soil Biology and Biochemistry, ISSN 0038-0717, E-ISSN 1879-3428, Vol. 129, p. 71-79Article in journal (Refereed)
    Abstract [en]

    Soil carbon (C) and nitrogen (N) availability depend on the breakdown of soil polymers such as lignin, chitin, and protein that represent the major fraction of soil C and N but are too large for immediate uptake by plants and microorganisms. Microorganisms may adjust the production of enzymes targeting different polymers to optimize the balance between C and N availability and demand, and for instance increase the depolymerization of N-rich compounds when C availability is high and N availability low (microbial N mining). Such a mechanism could mitigate plant N limitation but also lie behind a stimulation of soil respiration frequently observed in the vicinity of plant roots (priming effect). We here compared the effect of increased C and N availability on the depolymerization of native bulk soil organic matter (SOM), and of C-13-enriched lignin, chitin, and protein added to the same soil in two complementary ten day microcosm incubation experiments. A significant reduction of chitin depolymerization (described by the recovery of chitin-derived C in the sum of dissolved organic, microbial and respired C) upon N addition indicated that chitin was degraded to serve as a microbial N source under low-N conditions and replaced in the presence of an immediately available alternative. Protein and lignin depolymerization in contrast were not affected by N addition. Carbon addition enhanced microbial N demand and SOM decomposition rates, but significantly reduced lignin, chitin, and protein depolymerization. Our findings contrast the hypothesis of increased microbial N mining as a key driver behind the priming effect and rather suggest that C addition promoted the mobilization of other soil C pools that replaced lignin, chitin, and protein as microbial C sources, for instance by releasing soil compounds from mineral bonds. We conclude that SOM decomposition is interactively controlled by multiple mechanisms including the balance between C vs N availability. Disentangling these controls will be crucial for understanding C and N cycling on an ecosystem scale.

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