The shallow East Siberian Arctic Shelf (ESAS) shows high methane concentration in seawater; however, methane oxidation processes for the ESAS are poorly constrained. Here, we examined porewater geochemical profiles, microbial lipids, and their carbon isotope compositions in multiple sediment cores to explore molecular-isotopic diagnostics of microbial methane oxidation in the outer Laptev Sea. There was covariation between methane and sulfate profiles in only one core in the studied upper 20 cm. Across all cores, concentrations of microbial fatty acids (C-12-C-18) decreased sharply below the surface sediment with carbon isotope compositions (delta C-13) ranging from -31 to -20 parts per thousand. Notably, anaerobic methane oxidizing archaea (ANME)-derived lipid biomarkers, e.g., 2,6,10,15,19-pentamethylicosane, archaeal and hydroxyarchaeol, were absent or below detection limits, indicating that anaerobic methane oxidation (AOM) was not a dominant process in the studied sediment. However, the tetraether-based Methane Index increased from 0.03 to 0.8 in the core where sulfate and methane covaried, implying either a developing AOM microbial community with presently low activity or an episodic AOM event occurring within the past decades to a century. In contrast, hop-17(21)-ene was present throughout the cores, exhibiting strong C-13 depletion, with values ranging from -66.1 to -44.0 parts per thousand. The concentration and delta C-13 values of hop-17(21)-ene were distinct from the terrestrially-derived nC(31) alkane, which showed relatively constant delta C-13 values of -34.9 +/- 1.1 parts per thousand. This suggests that hop-17(21)-ene in the sediments is primarily derived from marine microbes. Given the oxic seawater in this system (< 60 m), the C-13-depleted hop-17(21)-ene likely reflects aerobic methane oxidation (AeOM) occurring in the water column and/or in surface sediments. AeOM may play a significant role in regulating methane in this subsea permafrost seepage system.

Elevated methane concentrations are observed in the shallow water column above subsea permafrost on the East Siberian Arctic Shelf, including the inner Laptev Sea. The subsea source of this methane is poorly understood, yet crucial for predicting future methane emissions. Here, we combine analyses of dissolved methane concentrations, triple-isotopic fingerprinting, and Bayesian Markov Chain Monte Carlo statistics to constrain the source of high-concentration methane sampled during four expeditions, 2016-2020. In contrast to earlier findings of predominantly thermogenic methane release in the outer Laptev Sea, our results for the major methane release region of the inner Laptev Sea suggest that old microbial methane (radiocarbon age >48000 y BP) is being released from preformed methane pools stored within the subsea permafrost system. This observation reveals that several sources contribute to the elevated methane on the shelf and thus the necessity to consider a diversity of sources when estimating future methane release trajectories. In the inner Laptev Sea, observed methane releases dominantly stem from microbial methane pools within the subsea permafrost system, according to triple-isotopic analyses of dissolved methane and statistical source apportionment.

Arctic permafrost faces multiple interactive changes in thaw, drainage, and vegetation shift due to climate warming. Thaw-induced waterlogging can shift vegetation from shrubs to graminoids, altering greenhouse gas emissions. This study aims to quantify and mechanistically explain how vegetation-specific root exudates amplify or dampen greenhouse gas emissions from Arctic permafrost during thaw and drainage transitions. Using field observations and soil incubations, we show that environmentally relevant concentrations of root organic exudates (15% of dissolved organic carbon), obtained from thaw stage-specific plants, altered greenhouse gas fluxes under site-realistic redox conditions. Graminoid exudates were richer in sugars and carboxylates, whereas shrub exudates were richer in amino acids. In thawed, anoxic permafrost soil incubations, graminoid exudates stimulated the emission of 51% more CO2 and 83% more CH4 compared to the absence of exudates. In intact, drained permafrost soil, shrub exudates stimulated 14% more CO2 and negligible CH4 compared to untreated soils. Geochemical and microbial analyses revealed that soil, including their hydrology, and exudate differences drove exudate and soil organic matter decomposition. These soil incubation findings were supported by field measurements: Bare locations per soil-habitat provided baseline greenhouse gas fluxes in relation to each soil's properties of moisture and geochemistry. Vegetation by graminoids increased greenhouse gas emissions from thawed permafrost soils significantly while shrubs barely affected greenhouse gas emissions from drained permafrost soils. Collectively, this study shows that thaw-specific vegetation shapes greenhouse gas fluxes, indicating that vegetation shifts can intensify radiative forcing beyond the known direct effect of permafrost thaw and associated hydrological transitions. Clarifying the context-dependence and mechanisms underlying these distinct exudate effects may improve projections of Arctic terrestrial climate feedback.

Elevated methane concentrations in seawater have been reported over extensive areas of the East Siberian Arctic Seas, overlying thawing subsea permafrost. However, observed methane concentrations of the ephemeral seawater are highly variable across both space and time, compromised by both the timing of rare measurements and storm-driven exchanges to the atmosphere. Here, we applied time-integrated signals of the delta 13C-composition of specific C30 hopanoids (diploptene, hop-17(21)-ene, neohop-13(18)-ene and diplopterol) in surface sediments to trace aerobic methane oxidation as a proxy for enhanced methane cycling. Interpretations of hopanoids and possible sources were further assessed by 16S-rRNA analyses in the surface sediments. The consistently low delta 13C-C30 hopenes signals, ranging between -57.5 parts per thousand to -37.1 parts per thousand (n=23) across the Laptev Sea shelf indicated aerobic methane oxidation. This suggests ubiquitous methane cycling with the most pronounced intensities in the outer shelf region, broadly consistent with the observed methane concentrations. Notably, depleted delta 13C-C30 hopenes were also found in the mid-shelf region of the Laptev Sea, earlier thought to be an area of comparatively low methane cycling. High methane concentrations were also observed in the vicinity of the Lena River delta, yet the isotopically heavier delta 13C-C30 hopenes may here reflect a combination of lower aerobic methane oxidation, a greater relative abundance of type II methanotrophs (lower isotope fractionation during hopanoid production) and isotope dilution from non-methanotrophic sources. While this complicates the biomarker interpretation in the unique setting near the Lena River delta, the delta 13C-C30 hopenes were still much lower than delta 13C-organic carbon, indicating aerobic methane oxidation and a clear methane cycling signal also in this regime. Taken together, the results unravel the wider cross-shelf patterns of enhanced methane cycling in the Laptev Sea through probing of methane fossilised in membrane lipids of aerobic methanotrophs, with the molecular-isotopic pattern being preserved in the sedimentary archive.

Amplified Arctic warming can induce strong ecosystem changes with adverse climate feedbacks through greenhouse gas (GHG) release. Shifting plant species and traits with permafrost thaw may contribute to the permafrost carbon feedback. How vegetation dynamics in thawing permafrost systems affect GHG release and how this varies with season, plant species, and soil conditions is poorly understood. Here, we assessed GHG emissions, redox potentials, and geochemical signatures as well as the carbon input in the form of root exudation along a vegetation density gradient and a permafrost thaw gradient over a growing season in Stordalen mire, Sweden. Ecosystem respiration and CH4 emissions increased along the thaw gradient from bog to fen, possibly due to high graminoid root carbon release rates into an anoxic soil, fuelling fast organic matter oxidation and lowering redox potentials to enhance methanogenesis. CH4 emissions increased seven-fold with increasing graminoid cover compared to non-vascular plant controls in the thawed soil. Plants may mediate CH4 transport, which was responsible for 80% of the graminoid-induced increase in CH4 emissions in the bog environment. In the fen environment, graminoid root carbon release stimulated CH4 formation, which dominated by contributing 70% of the graminoid-induced increase. Overall, photosynthesis-related CO2 fixation was substantial in the early and peak growing season, but when expressed as CO2 equivalents, CH4 release offset this uptake, resulting in net positive radiative forcings from graminoid-vegetated thawed soils throughout the growing season. Graminoids increased the net CO2-equivalent flux up to 8.9-fold compared to non-vascular plant locations with the strongest forcing toward late season in graminoid-vegetated fens. Our study showcases how fine-scaled, plant-mediated processes differently contribute to GHG emissions across a thawed bog and fen soil and how the time of growing season can overprint these effects to determine whether the system is a net GHG source or sink.

Permafrost soils constitute a large part of the terrestrial carbon pool that is vulnerable to future climate warming. Continued warming of the low Arctic is also leading to the encroachment of large shrubs and trees into tundra ecosystems with effects on microbial community composition, organic matter cycling and physical soil parameters. To date it is still largely unknown how such vegetation shifts affect soil organic matter cycling in permafrost soils on short and long timescales. Here, we investigated differences in soil organic matter properties under graminoid tussock (Eriophorum vaginatum), birch shrub (Betula glandulosa), spruce tree (Picea mariana) and alder shrub (Alnus viridis) vegetation by density fractionation and subsequent measurements of organic carbon, total nitrogen, δ13C, and lignin phenol biomarker contents. Particulate organic matter constituted 1.3–11.3

Rapid expansion of deciduous shrubs and evergreen trees on the Arctic tundra could induce large losses of soil carbon stocks through increased rhizosphere priming. Through the use of isotopic and molecular techniques, we investigated whether the belowground carbon cycling differed between three plant species that are encroaching Canadian tundra. 13CO2 pulse chase labelling showed that dwarf shrubs (Betula glandulosa) had faster turnover of recent 13C-photosynthates belowground than tall shrubs (Alnus viridis) and black spruces (Picea Mariana). Depth-resolved 13C flux estimations and partial 13C source isolation, both from field and lab measurements, elucidated multiple drivers of the differences in belowground carbon cycling. Turnover rates were strongly dependent on relative belowground carbon allocation, source of respiration and soil depth. Carbon cycling data will be compared with microbial community composition in bulk and rhizosphere soil to disentangle the specific interactions between encroaching plants and their soils. Overall, both plant and soil characteristics were key influences on the fate of recently assimilated carbon belowground. Our work suggests that changing plant communities will influence the belowground carbon cycling of the Arctic tundra. Our data pinpoints towards multiple factors influencing the feedback from northern ecosystems to on-going climate change, which further complicates accurate predictions of soil carbon losses in the northern hemisphere.

Global warming increases the vegetation cover and leads to shifts in vegetation types in the Arctic. An increase in the vegetation cover might substantially enhance carbon dioxide (CO2) emissions from northern permafrost soils, since root exudation of labile carbon and nitrogen can stimulate soil organic matter (SOM) decomposition via the rhizosphere priming effect. The current understanding of Arctic rhizosphere priming largely rests on soil incubation studies that simulate root exudation by adding various organic substrates in varying concentrations to soils. How the specific exudates of different plants influence rhizosphere priming is unclear as Arctic plant root exudate release rates and composition are largely unknown. Using targeted and non-targeted liquid chromatography-mass spectrometry, we compared the exudate composition and exudation rates of total organic carbon, 7 organic acids, 14 amino acids and 9 carbohydrates from three abundant and functionally different tundra plants (Betula glandulosa, Alnus viridis and Eriophorum vaginatum). While organic carbon and primary metabolites exudation were similar among the studied plants despite their different nitrogen acquisition strategies, distinct differences between the plant species were found in the overall root exudate composition. Between 80 and 94 % of the root exudate metabolome was not shared among the three plants. Our findings indicate that a change in vegetation types across the Arctic will primarily alter the release of secondary plant metabolites into the soil and thereby could alter soil microbial processes. Our observations further suggest that previous laboratory experiments studying priming frequently oversaturated microorganisms with labile substrates compared to natural conditions; this highlights the need for more realistic priming studies. Our data on root exudation provide critical background information for improving laboratory experiments.

The Arctic is warming rapidly, causing permafrost thaw and vegetation shifts. As a result, shrubs and trees from lower latitudes are encroaching into the tundra, altering biomass distribution above and below ground. These changes impact greenhouse gas (GHG) emissions by influencing litter input, root distribution, and microbial activity. A key mechanism in GHG production in soils is the rhizosphere priming effect, where labile carbon inputs from plants into the soil stimulate microorganisms to produce enzymes that decompose both labile and recalcitrant soil organic matter (SOM). However, the effects of rhizosphere priming on SOM decomposition and its influence on greenhouse gas emissions under natural conditions remain poorly understood. To address this, we simulated sub-Arctic vegetation changes in a controlled environment using tundra soil and plants sampled from the Northwest Territories, Canada. The soil was processed, homogenized, and placed into macrocosm chambers while preserving the original horizon sequence. The experiment included four vegetation types and one control, with plant species that are characteristic for the transition from sub-Arctic to lower Arctic bioclimate zones and included a small tree (Picea mariana), deciduous shrubs (Betula glandulosa, Alnus viridis) and graminoids (Eriophorum vaginatum, Carex sp.). Over three months, representing one growing season, weekly soil pore gas samples were taken at different depths, and surface efflux was measured additionally every three weeks. Preliminary results indicate that soil pore gas concentrations of CO2 increased with depth and over the experiment's duration across all vegetation groups and the control, and showed variability among vegetation types. Soil pore gas concentrations will be compared with soil efflux, dissolved organic carbon, microbial carbon contents, extracellular enzyme activity, and other parameters currently under evaluation. These data will help us to elucidate the role of woody plant species for permafrost soil processes and their contribution to GHG production in Arctic tundra ecosystems.

The permafrost in the Northern Hemisphere holds approximately 50% of the global soil organic carbon, constituting a reservoir that is twice the size of atmospheric carbon storage. Permafrost carbon plays a vital role in governing the global carbon cycle through its reactivity and accessibility to microbial respiration to release greenhouse gases. Existing studies have revealed spatial variability of degradation of coastally-exported organic matter across regimes in the East Siberian Arctic Shelf Seas. While the degradation patterns and ambient rates are reasonably constrained for the Laptev Sea, these aspects are less well understood for the Kara Sea. Here, we quantified carbon isotopes (13C and 14C), TOC, specific surface area, lipid biomarkers, and lignin phenols along a Kara Sea cross-shelf transect to assess terrigenous organic matter degradation, and compare patterns with three East Siberian cross-shelf transects (Kara Sea, Laptev Sea, Western East Siberian Sea, and Eastern East Siberian Sea). The data demonstrate the highest degradation rate constant of 2.5 kyr-1 in the Eastern East Siberian Sea, a moderate value of 2.0 kyr-1 in the Laptev Sea, and the lowest value of 1.2 kyr-1 in the Western East Siberian Sea. Intriguingly, no statistical trend in degradation was observed across the Kara Sea. The recalcitrant fractions of terrestrial organic carbon are determined to be largest (50%) in the Western East Siberian Sea, moderate (31%) in the Eastern East Siberian Sea, and smallest (11%) in the Laptev Sea. The spatial variabilities in degradation rate constants and recalcitrant fractions are likely attributed to different organic carbon speciation across regimes on land, e.g., from fibrous plant residues to mineral-associated organic carbon, and potential biological controls (e.g., priming effect) during transport.

Thawing of permafrost soils results in drastic changes in soil biogeochemistry and plant community composition. Specifically, the thawing process in subarctic regions can transform previously stable permafrost soils, home to slow growing, shallow-rooted shrubs into water-saturated, oxygen-depleted soils with fast-growing, deep-rooted graminoids. This change in soil biogeochemistry, along with the distinct characteristics and requirements of these contrasting plant types, leads to the hypothesis that the way these plants interact with the soil may impact biogeochemical cycles. Consequently, this could result in changes in the amounts and ratios of released greenhouse gases, influencing climate-relevant processes. On the one hand, tall graminoids may increase CO2 fixation. On the other hand, root exudation might prime the formation of CH4 and the root internal CH4 transport protecting it from oxidation outweighing increased CO2 fixation in thawed permafrost soils as compared to intact permafrost soil.To explore this idea, we conducted a study in Stordalen, Abisko, Sweden, at a permafrost site with three different thawing stages. Sampling locations in intact, intermediately, and fully thawed permafrost soil were selected, each with varying densities of shrubs and graminoids. Representative plants were sampled to analyze the quantity and composition of root exudates. Data on soil redox potential at different depths were combined with porewater geochemical parameters like the amount and speciation of dissolved iron, dissolved organic carbon, inorganic nitrogen species, dissolved porewater gases, and soil microbial functional genes. Net emissions of CO2, CH4, and N2O were tracked using static gas flux chambers. Most reducing redox conditions were observed in fully thawed soils compared to intact and intermediately thawed permafrost soils. Additionally, redox potentials decreased at greater depth in the soil and with higher graminoid density. At the same time graminoid roots exuded larger amounts of organic carbon than shrub roots with a high fraction of easily available organic molecules. We relate the decreasing redox potentials with increasing graminoid density to the rapid depletion of available electron acceptors such as iron(III) caused by an increased supply of easily available organic molecules through root exudation. This, in turn, might prime CH4 production, indicated by increased porewater CH4 at depth. Given the net CH4 flux increase at an increased porewater CH4 at depth, we suggest that this is partly from CH4-priming and partly from aerenchyma transport of CH4 from the soil to the atmosphere (Ström et al. 2005). Since thawing permafrost areas are rapidly expanding and contribute to climate change, the plant-specific alterations of these contrasting rhizosphere biogeochemical systems are important to consider altering greenhouse gas fluxes and warming potentials.Ström, L. et al. Species-specific Effects of Vascular Plants on Carbon Turnover and Methane Emissions from Wetlands. Biogeochemistry 75, 65–82 (2005).

Land permafrost thaw transfers increasing amounts of organic matter and nutrients to the Arctic Ocean. These nutrients could stimulate primary production directly, or indirectly following remineralization in sediments. Projections of this effect are limited by scarce observations and poor understanding of the underlying controls. Here, we focus on the Kara, Laptev, and East Siberian Sea shelves that receive strong input from large rivers and coastal erosion, linking ship‐board measurements of sediment–water nutrient fluxes to environmental parameters associated with land input. Ammonium and nitrite releases were positively related to high concentration and low decomposition state of terrigenous organic matter, based on biomarkers. Nitrate release was related to O 2 penetration depth. Phosphate and silicate release were highest at stations with strong marine influence. Our findings suggest that changes in environmental conditions, such as land input, might alter the nutrient balance in the Siberian Arctic Ocean, with implications for ecological and biogeochemical processes.

Abstract. Elevated methane concentrations in seawater have been reported over extensive areas of the East Siberian Arctic Seas, overlying thawing subsea permafrost. However, observed methane concentrations of the ephemeral seawater are highly variable across both space and time, compromised by both the timing of rare measurements and storm-driven exchanges to the atmosphere. Here, we applied time-integrated signals of the δ13C-composition of specific C30 hopanoids (diploptene, hop-17(21)-ene, neohop-13(18)-ene and diplopterol) in surface sediments to trace aerobic methane oxidation and thereby provide a proxy for methane release. Interpretations of hopanoids and possible sources were further assessed by 16S-rRNA analyses in the surface sediments. The consistently low δ13C-C30 hopenes signals, ranging between −57.5 to −37.1 ‰ (n=23) across the Laptev Sea shelf indicated aerobic methane oxidation. This suggests ubiquitous methane release with the most pronounced intensities in the outer shelf region, broadly consistent with the observed methane concentrations. Notably, depleted δ13C-C30 hopenes were also found in the mid-shelf region of the Laptev Sea, earlier thought to be an area of comparatively low methane emissions. High methane concentrations were also observed in the vicinity of the Lena River delta, yet the isotopically heavier δ13C-C30 hopenes may here reflect a combination of lower aerobic methane oxidation, a greater relative abundance of type II methanotrophs (lower isotope fractionation during hopanoid production) and isotope dilution from non-methanotrophic sources. While this complicates the biomarker interpretation in the unique setting near the Lena River delta, the δ13C-C30 hopenes were still much lower than δ13C-OC, indicating aerobic methane oxidation and a clear methane release signal also in this regime. Taken together, the results unravel the wider cross-shelf patterns of methane releases in the Laptev Sea through probing of methane fossilised in membrane lipids of aerobic methanotrophs with the molecular-isotopic pattern being preserved in the sedimentary archive.

Arctic climate warming is causing permafrost thaw and erosion, which may lead to enhanced inputs of terrestrial organic matter into Arctic Ocean shelf sediments. Degradation of terrestrial organic matter in sediments might contribute to carbon dioxide production and bottom water acidification. Yet, the degradability of organic matter in shallow Arctic Ocean sediments, as well as the contribution of terrestrial input, is poorly quantified. Here, potential organic matter degradation rates were investigated for 16 surface sediments from the Kara Sea, Laptev Sea, and the western East Siberian Sea and compared with physicochemical sediment properties including molecular biomarkers, stable and radioactive carbon isotopes, and grain size. Aerobic oxygen and carbon dioxide fluxes, measured in laboratory incubations of sediment slurry, showed high spatial variability and correlated significantly with organic carbon content as well as with the amount and degradation state of terrestrial organic matter. The dependency on terrestrial organic matter declined with increasing distance from land, indicating that the presence of terrestrial organic matter is likely a constraining factor for organic matter degradation in shallow shelf seas. However, sediment oxygen consumption rates, measured in incubations of intact sediment cores, also exhibited substantial spatial variability but were not related to organic carbon content or terrestrial influence. Oxygen consumption of intact sediments may be more strongly influenced by in situ redox conditions. Together with previous observations, our findings support that terrestrial organic matter is easily degradable in shelf sea sediments and might substantially contribute to aerobic carbon dioxide production and oxygen consumption.

The Arctic Ocean is currently changing at a high rate, and projections over the next decades include an increase in water temperature and retreat of sea ice, as well as increased input of freshwater and land-derived material released by permafrost thaw. These changes might substantially alter marine biogeochemical cycles and primary production, with repercussions for the Arctic Ocean greenhouse gas balance as well as ocean acidification. The large and shallow continental shelf seas north of Siberia are particularly affected by these changes as they receive land-derived material from strong coastal erosion and large rivers such as Ob, Yenisey and Lena. In recent studies, we have shown a transition from predominantly land- to marine-derived organic matter in sediments from the coast to the shelf break based on isotopes and biomarkers, an increased decomposition state of land-derived organic matter, as well as a decrease in sediment and water column nitrogen concentrations. We here combine this understanding with incubation experiments to assess the impact of these gradients on benthic CO2 production and nutrient remineralization. We found that fresh, land-derived organic matter typical for near-shore environments showed highest decomposability to CO2 in controlled, aerobic laboratory incubations, as indicated by correlations of CO2 production with concentrations of terrigenous biomarkers (lignin, high molecular weight n-alkanes), and biomarker proxies indicating the decomposition state of these compounds. Fresh, land-derived organic matter was also associated with highest ammonium and nitrite release to the water column, measured during on-board incubation of intact sediment cores. The opposite pattern was observed for phosphate and silicate fluxes that were highest in more marine-influenced settings. Our data suggest that increased input of land-derived organic matter to the Siberian Arctic Ocean shelves could promote benthic decomposition processes near the coast, including CO2 release that might contribute to the strong ocean acidification already observed in the region. Furthermore, the different controls on nutrient fluxes indicate a de-coupling of nitrogen, phosphorus and silicon remineralization, with implications for nutrient limitation and primary production in the Arctic Ocean.

Meiofauna (all invertebrates smaller than 1 mm) are not only sensitive to environmental changes but also contribute significantly to nutrient cycling and energy transfer to higher trophic levels. Despite their importance, meiofauna distribution and ecology in the Siberian seas remain understudied. Here, we employ sediment environmental DNA metabarcoding to characterize meiofauna diversity across the unexplored Siberian seas. We show that meiofauna community structure is primarily driven by river discharge and coastal erosion, which are heavily influenced by climate change, rather than geographical distinctions between the seas. We observed higher meiofauna diversity in nearshore areas where river plumes promoted colonizer nematode communities that are resilient to disturbances. Yet, their dominance may lead to decreased ecosystem stability in the future. This study provides a valuable baseline for meiofauna diversity in remote Siberian seas undergoing rapid environmental change, which will be useful for assessing the future direction and pace of benthic ecological trajectories.

Arctic warming is facilitating the encroachment of trees into tundra landscapes. Trees at the forest-tundra ecotone are typically small, slow-growing and show high mortality rates. Tree necromass enters the soil as root, leaf and stem litter. This material can be decomposed or contribute to long-term soil organic matter stocks, as well as change decomposition of native soil organic matter (priming). We here tested whether decomposition processes change under dying spruce trees in tundra soils in a controlled laboratory experiment. The opportunity for addressing this question came up within a laboratory macrocosm experiment on the effect of various living tundra plants on carbon and nitrogen cycling in a tundra soil. For this experiment, root-picked soil was homogenized and filled into macrocosms, reconstructing the original horizon sequence. Plants with washed roots were placed in the soil and macrocosms watered regularly from the top. The small (ca. 50 cm) spruce trees died early in the experiment, and we kept the experiment running to assess changes in carbon and nutrient cycling resulting from the decomposition of spruce necromass compared to the plant-free control soil. Pore gas CO2 concentrations at 7 cm depth were significantly higher in spruce than in the control soils. We further observed significantly lower pH values, as well as significantly, ca. 25% lower potential activities of hydrolytic (leucine-aminopeptidase, cellobiohydrolase, N-acetyl-beta-D-glucosaminidase), but not oxidative extracellular enzymes. These findings suggest that the input of root and needle-leaf litter altered the functioning of the soil decomposer community. These differences extended into the deeper soil below the rooting zone of spruce plants, pointing at an important role of leaching. These first observations will be compared with surface CO2 fluxes, dissolved organic and microbial carbon concentrations to dissect the decomposition dynamics of spruce necromass at the forest-tundra ecotone.

The availability of silicon (Si) in the ocean plays an important role in regulating biogeochemical and ecological processes. The Si budget of the Arctic Ocean appears balanced, with inputs equivalent to outputs, though it is unclear how a changing climate might aggravate this balance. In this study, we focus on Si cycling in Arctic coastal areas and continental shelf sediments to better constrain the Arctic Ocean Si budget. We provide the first estimate of amorphous Si (ASi) loading from erosion of coastal Yedoma deposits (30-90 Gmol yr-1), demonstrating comparable rates to particulate Si loading from rivers (10-90 Gmol yr-1). We found a positive relationship between surface sediment ASi and organic matter content on continental shelves. Combining these values with published Arctic shelf sediment properties and burial rates we estimate 70 Gmol Si yr-1 is buried on Arctic continental shelves, equivalent to 4.5% of all Si inputs to the Arctic Ocean. Sediment dissolved Si fluxes increased with distance from river mouths along cruise transects of shelf regions influenced by major rivers in the Laptev and East Siberian seas. On an annual basis, we estimate that Arctic shelf sediments recycle approximately up to twice as much DSi (680 Gmol Si) as is loaded from rivers (340-500 Gmol Si). Coastal erosion loads 30-90 Gmol Si yr-1 to the Arctic Ocean in the form of amorphous siliconContinental shelf sediments in the Arctic Ocean recycle more silicon than is loaded from riversApproximately 4.5% of silicon loaded on the Arctic Ocean is buried in continental shelf sediments

The abrupt warming events punctuating the Termination 1 (about 11.7-18 ka Before Present, BP) were marked by sharp rises in the concentration of atmospheric methane (CH4). The role of permafrost organic carbon (OC) in these rises is still debated, with studies based on top-down measurements of radiocarbon (14C) content of CH(4 )trapped in ice cores suggesting minimum contributions from old and strongly C-14-depleted permafrost OC. However, organic matter from permafrost can exhibit a continuum of C-14 ages (contemporaneous to >50 ky). Here, we investigate the large-scale permafrost remobilization at the Younger Dryas-Preboreal transition (ca. 11.6 ka BP) using the sedimentary record deposited at the Lena River paleo-outlet (Arctic Ocean) to reflect permafrost destabilization in this vast drainage basin. Terrestrial OC was isolated from sediments and characterized geochemically measuring delta C-13, Delta C-14, and lignin phenol molecular fossils. Results indicate massive remobilization of relatively young (about 2,600 years) permafrost OC from inland Siberia after abrupt warming triggered severe active layer deepening. Methane emissions from this young fraction of permafrost OC contributed to the deglacial CH4 rise. This study stresses that underestimating permafrost complexities may affect our comprehension of the deglacial permafrost OC-climate feedback and helps understand how modern permafrost systems may react to rapid warming events, including enhanced CH4 emissions that would amplify anthropogenic climate change.