Permafrost degradation creates hydrologically distinct landscapes, including dry soils with enhanced drainage and wet soils with poor drainage. In Fairbanks (Alaska, USA), we compared the active layer of dry and wet degraded permafrost landscapes with that of a non-degraded intact permafrost landscape to assess how hydrological conditions shape microbial community composition, extracellular enzyme activities, and microbial extracellular polymeric substances (EPS). Based on 16S rRNA sequencing, the alpha diversity of the prokaryotic community was significantly lower in the dry site compared to the wet and intact sites, with soil organic carbon (SOC) and pH identified as the primary environmental drivers. The active layer of the dry site was dominated by Thermoleophilia and Acidobacteriae, whereas the wet site was dominated by Gammaproteobacteria and Gemmatimonadetes. The relative proportions of most bacterial classes differed significantly across depths in the dry and wet sites, while the intact site exhibited less vertical variation. Hydrolytic enzyme activities were significantly higher in the topsoil across all sites, whereas oxidative enzyme activities showed relatively uniform patterns with depth but generally exceeded hydrolytic activities. EPS content varied among sites and depths, with the highest EPS-sugar content observed in the wet site overall, whereas topsoil EPS-sugar and EPS-protein contents were highest in the wet site, followed by the dry and intact sites, and subsoil EPS content was highest in the intact site. This study demonstrates that hydrological shifts in degraded permafrost soils shape microbial community structure, enzymatic activity patterns, and EPS content, with potential implications for SOC loss and stabilization.

The methane-cycling microbiomes play crucial roles in methane dynamics. However, little is known about their distributions on a pan-Arctic scale as well as their responses to the widespread permafrost degradation. Based on 621 datasets of 16S rRNA gene amplicons from intact permafrost soils across the pan-Arctic, we identified only 22 methanogen and 26 methanotroph phylotypes. Their relative abundances varied significantly between sites and soil horizons. Only four methanogen phylotypes were detected at all locations. Remarkably, the permafrost soil methane filter was almost exclusively dominated by some obligate methanotroph (Methylobacter-like) phylotypes. However, a case study in Alaska suggests that atmospheric methane oxidizing bacteria (Methylocapsa-like phylotypes) dominated methanotrophs in a drier condition after permafrost degradation. These findings point towards a few key microbes particularly relevant for future studies on Arctic methane dynamics in a warming climate and that under future dry conditions, increased atmospheric methane uptake in Arctic upland soils may occur.

The redox status of permafrost soils is a decisive factor for their nutrient cycling, organic matter decomposition, and greenhouse gas emissions. Although being associated with a variety of processes, data availability of continuous redox measurements in permafrost soils is scarce. Here, we provide a unique dataset covering three years of soil redox potential measurements, obtained from a monitoring approach at three research sites near Fairbanks, Alaska. Redox potential pattern in the permafrost soil active layer showed large seasonal differences, with reducing conditions in the short summer/autumn to largely oxidizing conditions in winter and spring. However, conditions for methane production were at no time recorded in the three years. Especially the freezing and thawing had substantial impact on the redox status, highlighting that assessment of redox conditions in permafrost soils should be extended beyond the typical summer observation periods.

Permafrost degradation, freezing and thawing processes, and poor drainage due to underlain frozen ground have far-reaching consequences on soil hydrology and biology and, thus, on the redox dynamic in soils of the Arctic. Assessing the redox status of these soils is essential for understanding soil organic matter decomposition processes and can be done by temporal measurements in the field, analyses of redox-sensitive elements, or identification of microbial species or enzymes in redox process chains. While such approaches provide snippets of the complex redox dynamic, publications reporting long-term in-situ redox potential (EH) measurements in arctic permafrost soils are scarce. Limited accessibility to study sites and technical limitations in measuring the redox potential in a frozen environment may be two reasons for this research gap.But how does the redox potential develop in permafrost soils at different depths in the active layer during the summer? What happens during freezing and thawing? Finally, do thawing/degrading permafrost soils show different patterns compared to intact permafrost?We approached these research questions by installation of a unique soil monitoring setup at 3 sites near Fairbanks, Alaska, in August 2021. An intact permafrost soil (active layer depth about 50 cm) was equipped with 3 redox electrodes (for EH) and 3 hydra probes (for water content and soil temperature) in the topsoil and subsoil, respectively, and connected to a logger unit allowing continuous measurement of these parameters in both depths every 15 minutes. In addition, two sites with advanced permafrost degradation (permafrost level below 100 cm) were equipped in the same way. One degraded site featured large water contents, representing a wet thaw scenario, while the other site was well-drained, representing a dry thaw scenario, thus representing different endmembers of the ongoing climate-change induced permafrost thaw.Here, we present the first 2 years of soil monitoring in a discontinuous permafrost area in Interior Alaska from 09/2021 to 09/2023. Overall, pH values of all soils varied between 4.5-6.3. The dry thaw scenario showed oxic conditions (i.e., EH >600 mV) in top- and subsoil, while water contents were low. The wet thaw scenario exhibited high topsoil redox potentials (i.e., EH >500 mV), while subsoil redox potential was lower (i.e., EH 400 mV in the summer period (August to October), suggesting less oxygen consumption in this recently thawed permafrost subsoil.

Bacterial exopolysaccharides (EPS) act as natural biopolymers that bind soil particles together, promoting structural stability and creating protective microenvironments for microbial survival. This study aimed to isolate and characterize potential EPS-producing bacteria from the active layer of two degraded permafrost soils with different hydrological landscapes, and from non-degraded permafrost soil. A total of 64 bacterial isolates were obtained, representing three phyla: Firmicutes, Actinomycetota, and Pseudomonadota. EPS production was assessed by determining the polysaccharide content measured as glucose equivalent, and 26 bacterial isolates were identified as potential EPS producers. Among the bacterial isolates, Curtobacterium oceanosedimentum, Frigoribacterium faeni, Streptomyces strains, Neobacillus bataviensis, and Mesobacillus subterraneus exhibited high polysaccharide concentrations. Uronic acids were present in EPS produced by C. oceanosedimentum and N. bataviensis, while amino sugars were identified in EPS from isolates of Bacillus, Streptomyces, Luteimonas, and Phyllobacterium. Based on 16S rRNA gene sequence similarities, the relative proportion of taxa associated with EPS-producing bacterial isolates such as Bacillus, Peribacillus, and Streptomyces was higher in the dry site, while Neobacillus, Pseudarthrobacter, and Microbacterium were more abundant in the wet and intact sites. This study suggests that EPS production with diverse carbohydrate compositions primarily promotes structural stability in degrading permafrost soils by binding soil particles together and forming protective microenvironments. Additionally, EPS may contribute to nutrient retention and microbial protection under fluctuating environmental conditions, complementing their primary role in soil stability.

Forest fires are among the most influential disturbances in ecosystems and have varying effects on the soil depending on fire intensity and biomass consumption. The significant decline in biodiversity in European forests due to centuries of non-sustainable forest management, combined with worsening drought from climate change, has greatly increased vulnerability to wildfires. Incomplete combustion during fires leads to the formation of black carbon (BC), a group of substances known for their persistence in soil. However, studies suggest that medium-condensed BC species may have lower chemical and spatial stability and are therefore potentially more mobile and consequently only serve as temporary carbon sinks. In order to assess the mobilization of BC, we investigate short-term changes in BC under field conditions, particularly of the low-condensed BC, and call into question the established concept of the general stability of BC pools. We investigated the dynamics of BC alterations during the post-fire period within one winter, following a late summer forest fire. We selected two comparable sites featuring spruce-dominated forest stands with different geologic parent material and weather conditions, particularly with respect to the amount of precipitation during the observation period. We sampled soil profiles down to 40 cm depth shortly after the fire event in late summer and after a 6-month period in late spring. After performing density fractionation to separate the mineral associated organic matter (MAOM) from particulate organic matter (POM), we analysed the BC content in the MAOM fraction using benzene polycarboxylic acids (BPCA) analysis. The results show a high content of low to medium condensed BPCAs directly after fire, which decreased, especially the medium condensed BPCA marker, during the observation period. Taking into account the fast change in medium BPCA values in the MAOM fraction, we conclude that the general assumption that BC is in principle a stable, long-term carbon sink needs to be addressed more carefully.

Soil organic matter (SOM) is a highly heterogeneous component of soils and its composition differs strongly between sites, depending on the specific environmental conditions. Thawing of permafrost leads to the exposure of large amounts of SOM to decomposition, resulting in the release of greenhouse gases. Moreover, SOM might be mobilized as dissolved organic matter (DOM), possibly contributing to losses of carbon from soils. Detailed knowledge of SOM composition is key for understanding mineralisation processes and for the quantification of greenhouse gas emissions from thawing permafrost soils of different moisture, thaw depth, parent material and slope position.We sampled permafrost soils along two transects on Disko Island, West Greenland, to characterize SOM from soils with different characteristics. Installation of suction cups allowed pore water sampling to determine the amount and composition of DOM. We measured emissions of CO2 and CH4 with a manual chamber system to quantify greenhouse gas fluxes at the different sites. To determine the degree of SOM decomposition and the potential impact of site characteristics on greenhouse gas emissions and SOM leaching, we fractionated SOM and subsequently analysed lignin components, amino sugars, and stable isotopes (δ13C and δ15N). Molecular microbial analyses were carried out to understand the underlying biological processes that control SOM cycling and greenhouse gas production.Lignin components and derived molecular ratios matched with the recent vegetation. Sites are characterized by woody angiosperms in the well aerated and drained soils at the top of the slopes and by herbaceous plants in the wetland area at the lower end of the transects. The data indicated weak decomposition at the wet sites and stronger decomposition at the dry sites, which correlated with the proportion of particulate OM within the total SOM. Stable isotopes showed according patterns, becoming more positive with depth within the soil profile but becoming more negative along the transects. Leaching of DOM showed a more complex pattern with the lowest C contents in the wettest areas and the highest C contents in the intermediate slope positions but increasing C contents within the soil profiles. Only the wettest sites emitted CH4, while the drier locations were neutral in terms of CH4 or acted as CH4 sinks. We observed decreasing CO2 emissions along the transects during the day, with the driest sites being sources of CO2. The observations of CH4 fluxes were supported by higher abundances of methanogenic microorganisms in the wetter areas.The results underline the susceptibility of SOM to decomposition in thawing permafrost. While topsoils and litter layers contain larger amounts of SOM than subsoils, results suggest that C is transported downwards along the soil profile with infiltrating water, possibly buffering decomposition. DOM appears to be transported down the hillslopes until it is either drained into waterbodies or emitted as CH4. Concerning scenarios of soil moisture changes, and daily and seasonal variations in CO2 uptake, the observed soils might therefore turn from C sinks to sources. However, the extend of this C-relocation by lateral DOM transport has not yet been quantified and needs further observation.

With promising methods such as ultrahigh-resolution mass spectrometry (FT ICR MS), soil scientists have more opportunity than ever to gain a comprehensive picture of the composition and transformation of soil organic matter (SOM). With soils as central mediators for carbon capture and storage this understanding is key when it comes to tackling major challenges, such as climate change and soil health. However, as novel techniques are often imported from other scientific fields, the evaluation and interpretation of data with regard to the heterogeneous pedosphere often remains a major challenge. For FT ICR MS, several commonly used indices were developed from empirical observations of the deep ocean. While these indices seem statistically transferable from ocean DOM to the terrestrial realm, there is legitimate concern that no causality ultimately ensures this applicability. Indices are needed that allow interpretation of the data from a conceptual perspective. Viewing SOM as a thermodynamically driven mediator of energy fluxes in the soil food web provides an opportunity to put a foot on the ground of data analysis with more general applicability. We aim to show that bioenergetic and thermodynamic molecular indices allow a better understanding of soil organic matter transformation by comparing FT ICR MS samples from complex, mixed sources and single source endmembers.We investigated the molecular composition of stream DOM and soil leachates along a biome gradient between alpine meadow and alpine steppe, including a chronosequence of degradation in the southern Tibetan Nam Co watershed. Our results suggest a certain match of commonly used DOM molecular indices, such as the ‘island of stability‘, the ‘degradation index‘ and the ‘terrestrial index‘, applied to marine settings for terrestrial DOM and SOM. However, when comparing SOM and DOM phase transitions within endmember sources, we noted inconsistencies. In contrast, indicators representing the bioenergetics of organic matter composition, such as the ‘nominal oxidation state of carbon’ and the ‘Gibbs free energy for carbon oxidation’, show good agreement for the key phase transition between SOM and DOM. These results provide reasonable evidence in line with conceptual understanding, such as more oxidised DOM and SOM in degraded areas and generally less oxidised molecular formulae in mainly allochthonous stream DOM compared to extracted SOM. Our data support the notion that bioenergetic and thermodynamic indicators may be a way forward to better understand the complex nature of organic matter transformation in soils with FT ICR MS. These indicators can serve as important building blocks for molecular fingerprinting.

Arctic warming increases the thaw depth of permafrost affected soils, altering the local water cycle and accelerating soil-forming processes and decomposition of soil organic matter. The consequences vary greatly across the circumpolar region due to strong heterogeneity in soil-forming factors and soil properties. Despite numerous studies have been carried out in the tundra of West Greenland, detailed descriptions of soil development under contrasting soil-hydrological conditions are lacking, which impairs the accuracy of soil maps and Earth-system models. We address this knowledge gap by analysing soil profiles from three field transects located at slopes of a glacially shaped valley and on a moraine within the same valley, representing typical environmental and geomorphological settings of West Greenland. Dryer soils dominated at the top of the slopes, with solifluction and cryoturbation shaping soil properties, while water logging and accumulation of organic matter characterized the lower end of the slopes. In the vicinity of a braided-river, the terrain was flat but well-drained and soils were shallow but organic-rich, overlying coarse rubble. We show that soil development depends strongly on slope dynamics and hydrological conditions. We also show that soil organic carbon stocks are highly heterogeneous with 4 +/- 6 Mg C ha-1 in shallow and poorly developed soils and 451 +/- 160 Mg C ha-1 in the upper meter of peat-rich wetlands. The results highlight the great heterogeneity in soil moisture and vegetation types, driving marked differences in soil development and carbon stocks across typical West Greenlandic tundra.

The methane-cycling microbiomes in Arctic permafrost-affected soils play crucial roles in the production and consumption of this important greenhouse gas. However, little is known about the distributions of Arctic methanogens and methanotrophs across the regional scale and along the vertical soil profile, as well as their responses to the widespread permafrost thaw. Using a unique sample set from nine different locations across the pan-Arctic, we identified methanogen and methanotroph phylotypes in 729 datasets of 16S rRNA gene amplicons. In 621 samples of intact permafrost soils across the pan-Arctic, only 22 methanogen and 26 methanotroph phylotypes were identified. Relative abundances of both functional groups varied significantly between sites and soil horizons. Only four methanogen phylotypes were detected at all locations, with the hydrogenotrophic Methanobacterium lacus dominating. Remarkably, the permafrost soil methane filter was almost exclusively comprised of a few phylotypes closely related to the obligate methanotrophic species Methylobacter tundripaludum. In degraded permafrost sites in Alaska, M. tundripaludum also dominated the methanotroph microbiome in the wet site. However, in dry, water-drained former permafrost site, Methylocapsa phylotypes, closely related with the atmospheric methane oxidizing bacteria, were exclusively found and dominant, indicating a massive restructuring of the methanotroph guild that consequently resulted in functional changes from a soil methane filter to an atmospheric methane sink. This study provides first insights into the identity and intricate spatial distribution of methanotrophs and methanogens in permafrost soils at a pan-Arctic scale and their responses to different water status after permafrost degradation. These findings point towards a few key microbes particularly relevant for future studies on Arctic CH4 dynamics in a warming climate and that under future dry conditions more atmospheric CH4 uptake in Arctic upland soils might happen. ### Competing Interest Statement The authors have declared no competing interest.

Permafrost soils in the northern hemisphere are known to harbor large amounts of soil organic matter (SOM). Global climate warming endangers this stable soil organic carbon (SOC) pool by triggering permafrost thaw and deepening the active layer, while at the same time progressing soil formation. But depending, e.g., on ice content or drainage, conditions in the degraded permafrost can range from water-saturated/anoxic to dry/oxic, with concomitant shifts in SOM stabilizing mechanisms. In this field study in Interior Alaska, we investigated two sites featuring degraded permafrost, one water-saturated and the other well-drained, alongside a third site with intact permafrost. Soil aggregate- and density fractions highlighted that permafrost thaw promoted macroaggregate formation, amplified by the incorporation of particulate organic matter, in topsoils of both degradation sites, thus potentially counteracting a decrease in topsoil SOC induced by the permafrost thawing. However, the subsoils were found to store notably less SOC than the intact permafrost in all fractions of both degradation sites. Our investigations revealed up to net 75

Permafrost degradation leads to the formation of contrasting hydrological conditions such as water-saturated anoxic and dry oxic soils, which significantly influence the bacterial community structure and abundance. To investigate the bacterial abundance and diversity under these hydrological conditions, we collected soil samples from different horizons of both dry and wet degraded permafrost soils and non-degraded intact permafrost soil for comparison. The bacterial alpha diversity, measured by the Observed and Chao1 indices, was significantly greater in wet degraded permafrost soil (wet site) and intact permafrost soil (intact site) than in dry degraded permafrost soils (dry site). Notably, the wet and intact sites exhibited similar levels of alpha diversity as well as shared greater number of zOTUs. The relative proportion of most bacterial taxa was significantly differed among the sites. At the class level, dry site was dominated mainly by K-strategic bacteria like aliphatic degraders (Thermoleophilia), acidophilic and cellulolytic (Acidobacteriae), while wet site was dominated by mainly r-strategic bacteria from class Gammaproteobacteria and anoxygenic aerobic phototrophs (Gemmatimonadetes). According to Spearman correlation analysis, the relative proportion of bacterial taxa in dry and wet sites showed significant correlation with soil physicochemical parameters, whereas fewer correlations were found in intact site. The relative proportion of Pseudomonadota classes and Acidobacteriota were positively correlated with SOC, N, and C:N ratio, but were negatively correlated with pH in both dry and wet sites. In contrast, anaerobic methylotrophs (Methylomirabilota), anoxygenic photoheterotrops (Chloroflexota), Gemmatimonadota and filamentous Actinobacteriota were negatively correlated with SOC, N, and C:N. Additionally, strong correlations were observed between bacterial taxa and extracellular enzyme activities, where Alphaproteobacteria, Gammaproteobacteria, and Acidobacteriota showed positive correlations with both hydrolytic and oxidative enzymes in dry and wet sites, except for PerOx in wet site where they showed negative correlation. Conversely, Gemmatimonadota and Actinobacteriota displayed negative correlations with enzymes in dry and wet site, except for the PerOx in wet site, where they showed positive correlation. ### Competing Interest Statement The authors have declared no competing interest.

Large amounts of terrestrial organic carbon (OC) are stored in Arctic permafrost-affected soils. Through processes of cryoturbation and solifluction, the subsoils can contain subducted topsoil material, which largely contribute to the large OC storage in these soils. While the bacterial, archaeal, and fungal communities in such soils have been studied to some degree, information about protists and meso- and macrofauna is scarce, although these groups might substantially contribute to OC processing, through e.g., food web interactions. Different organic and mineral horizons, including subducted topsoil material, of Arctic soils were investigated using a metatranscriptomics three-domain community profiling approach. Soil horizons were compared in regards to their total microbial community composition including all three domains of life. Furthermore, abundances of different pro- and eukaryotic micropredators were examined and a variety of functional groups involved in the carbon (C) and the nitrogen (N) cycle were analyzed in relation to specific taxonomic groups and abiotic soil parameters. Our study showed that RNA yields positively correlated with the OC content of the horizon and that the composition of the microbial community in subducted topsoil material rather matched that of mineral subsoils instead of organic top horizons. Horizon-resolved profiling revealed heterogeneity in the associated microbiomes and showed major differences in microbiomes of topsoil and subducted topsoil. The abundance of protist and nematode micropredators decreased in subducted topsoil, while predatory myxobacteria remained remarkably constant and comprised high proportions of the total communities in all horizons. Correlations analysis between functional guilds and biotic and abiotic parameters suggest a major impact of predatory myxobacteria on carbon and nitrogen cycles of subducted topsoils. The study adds urgently needed information about the total biota structure in permafrost soils and first insights into the associated soil microbial food webs.

Permafrost thaw can result in two distinct environmental landscapes (i.e. wet condition soils rich in ice wedges and low drainage, and dry condition well-drained areas with higher evapotranspiration). These different conditions influence the bacterial communities and may impact their capacity to produce exopolysaccharides (EPSs), which are crucial for bacterial survival and adaptation. This study aimed to isolate and characterize EPSs-producing bacteria from the active layer of two permafrost degrading landscapes and from undisturbed permafrost soil. A total of 54 bacterial isolates were obtained, representing three phyla: Firmicutes, Actinomycetota, and Pseudomonadota. EPSs production was assessed by determining the polysaccharide content measured as glucose equivalent, and 26 isolates were identified as potential EPSs-producers. Among the isolates, Curtobacterium oceanosedimentum, Frigoribacterium faeni, Streptomyces strains, Neobacillus bataviensis and Mesobacillus subterraneus had the highest polysaccharide yield. They were also found inhabiting in the different horizons of degraded permafrost soil and undisturbed permafrost soil, by determining their relative proportion within the total bacterial community based on 16S rRNA gene sequences similarities. ### Competing Interest Statement The authors have declared no competing interest.

Preferential flow paths (PFPs) are intertwined soil regions that link top and subsoil and through which water and consequently nutrients flow across the soil profile. PFPs enable newly available carbon sources to reach deeper soil layers, enabling soil microorganisms to flourish in an otherwise substrate-poor subsoil. A reliable assessment of organic carbon (OC) translocation into the subsurface requires an understanding of the small scale variability of dissolved organic carbon (DOC) concentrations and fluxes into the subsoil.Using segmented suction plates over a 5-year period, we measured DOC and water fluxes, and subsequently OC translocation, at three depths in three soil profiles down to 1.5 m in a sandy Dystric Cambisol in Lower Saxony (Germany). DOC fluxes and water fluxes were correlated and decreased with depth. Overall fluxes were dependent on seasonal fluctuations of precipitation, with the winter and spring months bearing the highest water fluxes. We found significant flux variability between suction plates and soil depths. Rank analysis showed stable regions of high and low water and DOC fluxes, suggesting stable subsoil PFPs over these five years. Furthermore, the significance of small scale spatial heterogeneity as estimated by intraclass correlation was higher than the seasonal variability in each hydrological year, strengthening the idea that PFPs in a soil profile persist over years. In addition, SUVA analysis showed a decrease in OM aromaticity with depth in all three profiles and it was moderately correlated with water fluxes, indicating selective retention of complex organic matter along the soil profile.These findings highlight the potential for long-term stability of PFPs in subsoils and their significance for the development and maintenance of biogeochemical subsoil C hotspots, and that small scale soil heterogeneity plays a major role in controlling water and nutrient movements across the soil profile.

Soil density fractionation is a common tool to separate organic matter of different function and turnover. But it has not been tested so far how much soil material is necessary to obtain reproducible results. A reduction of chemicals like polytungstate would further save valuable resources. Here, we show that soil weight reduction from 25 to 5 g was not significantly affecting fractionation results. Compared to the commonly used 10-25 g, this corresponds to a saving of resources of up to 80%.

<p>Fires belong to the most intensive disturbances in ecosystems, but do have different effects on the soil depending on their intensity and fuel materials. Taiga ecosystems contain significant reserves of potentially fire-prone materials, and as temperatures rise in the circumpolar region and precipitation patterns change, an increase in the frequency and intensity of fires is observed. In these fires, incomplete combustion processes result in the formation of black carbon (BC), which is known as a long-term carbon sink due to its chemical properties. As the majority of forest fires are ground fires burning at a rather low intensity in terms of duration and temperature, it is discussed that the BC species formed under these circumstances are chemically less stable than those formed at high temperatures and should therefore only be considered as temporary carbon sinks.</p><p>Here we studied the effects of low intensity ground fire shortly after the event and tracked changes in BC within the first four years after the fire event at the southern edge of the boreal forest. We analysed a fire transect running through the two main forest types of this region, focusing on the BC species that we could quantify using the BPCA method. Our results indicate a decline in BC after the fire within the four years of observations, which mainly mainly occurred for the low condensed BPCAs. This finding is independent of the forest typ. Since the precipitation within the experimental period was also negligible and only occurred in very small amounts, we exclude leaching as well as a possible significant aeolian losses, since the trees remained unaffected by the fire and covered the soil against strong wind. We therefore deduce that <em>in situ</em> degradation of the BC must have occurred. <br>Concluding, the general assumption that BC is a stable, long-term carbon sink needs to be questioned more critically. Together with other studies, our results show a quite fast decrease in the concentration of low-condensed BC species in soil over time, indicating a potential for degradation.</p>

Background Soils are important carbon (C) sinks or sources and thus of utmost importance for global carbon cycling. Particularly, subsoils are considered to have a high potential for additional C storage due to mineral surfaces still available for sorptive stabilization. Aims Little information exists about the extent to which additional litter-derived C is transferred to and stabilized in subsoils. This study aimed at evaluating the role of litter-derived dissolved organic matter (DOM) inputs for the formation of stable mineral-associated C in subsoils. Methods We carried out a multiple-method approach including field labeling with C-13-enriched litter, exposure of C-13-loaded reactive minerals to top- and subsoils, and laboratory sorption experiments. Results For temperate forest soils, we found that the laboratory-based C sink capacity of subsoils is unlikely to be reached under field conditions. Surface C inputs via litter leachates are little conducive to the subsoil C pool. Only 0.5% of litter-derived C entered the subsoil as DOM within nearly 2 years and most of the recently sorbed C is prone to fast microbial mineralization rather than long-term mineral retention. Desorption to the soil solution and an adapted microbial community re-mobilize organic matter in subsoils faster than considered so far. Conclusions We conclude that the factors controlling the current mineral retention and stabilization of C within temperate forest subsoils will likewise limit additional C uptake. Thus, in contrast to their widely debated potential to accrue more C, the role of forest subsoils as future C sink is likely overestimated and needs further reconsideration.

Permafrost affected soils are highly vulnerable to climate change. These soils store huge amounts of organic carbon (C), and a significant proportion of this carbon is stored in subsoil horizons where it might become available to microbial decomposition under global warming. An important factor in understanding and quantifying the C release from soils include the limitation of resources for microbes. Microbes decompose soil organic matter (SOM) by secreting extracellular enzymes into the soil, thus enzyme activity and their ratios are considered important indicators of soil nutrient availability and microbial substrate limitation. To evaluate nutrient limitation and the limitation of microbial substrate utilization, we investigated the potential enzyme activity from whole soil profiles, including topsoil, cryoturbated organic matter, mineral subsoil, and permafrost of Herschel Island (Canada) and Disko Island (Greenland). We included seven enzymes (five hydrolytic and two oxidative) and related them to bacterial and fungal gene abundance. The results showed hydrolytic enzymatic activity was strongly influenced by soil type, whereas oxidative enzymes varied between different localities. The enzyme ratios indicated that the topsoil microbial communities were C and phosphorus (P) co-limited in both localities, whereas the subsoil communities were nitrogen (N) limited from HI and C, P limited from DI. A strong positive correlation between all measured enzymes and bacterial gene abundance compared to that of fungi suggested that bacteria might play a more important role in SOM decomposition in permafrost soil horizons. This study suggests that Arctic permafrost microbial communities were not only limited by N, but also by C, P, and their co-limitation under specific conditions (i.e., higher abundance of bacteria and lower abundance of fungi).

Formation of mineral-associated organic matter (MAOM) is a decisive process in the stabilization of OM against rapid microbial decomposition and thus in the soils’ role as global carbon (C) sink. Sorption experiments of dissolved OM (DOM) repeatedly showed that particularly mineral subsoils have a large sorption capacity to retain more C. However, there is also an increasing body of literature, revealing an increasing output of dissolved organic C (DOC) from soils. Here, we investigated into this paradox in forest soil under beech by a combination of a field labelling experiment with 13C-enriched litter with a unique DO13C and 13CO2 monitoring, an in-situ C exchange experiment with 13C-coated minerals, and batch sorption experiments. Within two years of 13C monitoring, only 0.5% of litter-derived DO13C entered the subsoil, where it was only short-term stabilized by formation of MAOM but prone to fast microbial mineralization. The 13C monitoring, sorption/desorption experiments in the laboratory, and also the in-situ C exchange on buried soil minerals revealed that there is a frequent exchange of DOM with native OM and a preferential desorption of recently retained OM. Hence, there appeared to be a steady-state equilibrium between C input and output, facilitated by exchange and microbial mineralization of an adopted microbial community. The remobilized OM was also richer in less sorptive carbohydrates. Along with transport of most of DOM along preferential paths, this further increased the discrepancy between laboratory-measured sorption capacities of subsoil and the actual C loading of minerals. Finally, the 13C labeling experiments revealed that input of fresh litter-derived OM into subsoil may even mobilize old-soil derived OM. Hence, in the field different biogeochemical constraints are acting that prevent that the laboratory-based C sink can be reached in the field. We conclude, that forest subsoils can hardly be considered as additional C sink, even at management options that increase DOC input to subsoil.