Rapid climate change in the Northern Hemisphere cryosphere threatens ancient permafrost carbon. Once thawed, permafrost carbon may migrate to surface waters. However, the magnitude of permafrost carbon processed by northern freshwater remains uncertain. Here, we compiled '1800 radiocarbon data of aquatic dissolved organic carbon (DOC) and particulate organic carbon (POC) in the Arctic and Qinghai -Tibet Plateau (QTP) to explore the fate of permafrost carbon under climate warming over the past 30 years. We showed that the contribution of aged carbon has significantly increased since 2015. Approximately 70 % of DOC and POC was derived from aged carbon for QTP rivers. In Arctic waters, an average of '67 % of POC was derived from aged carbon, however, '75 % of DOC was derived from modern carbon, mainly due to low temperatures and protection by vegetation limiting the export of aged DOC. For both regions, DOC 14 C age was positively correlated with the active layer thickness, whereas the POC 14 C age was positively correlated with the mean annual ground temperature, suggesting that gradual thaw accelerated the mobilization of aged DOC while abrupt thaw facilitated the export of aged POC. Furthermore, POC 14 C age was positively correlated with the soil organic carbon density, which was attributed to well-developed pore networks facilitated aged carbon output. This study suggests that permafrost carbon release is affected by both permafrost thermal properties and soil organic carbon density, which should be considered in evaluation of permafrost carbon -climate feedback.

Soil organic matter (SOM) stability in Arctic soils is a key factor influencing carbon sequestration and greenhouse gas emissions, particularly in the context of climate change. Despite numerous studies on carbon stocks in the Arctic, a significant knowledge gap remains regarding the mechanisms of SOM stabilization and their impact on the quantity and quality of SOM across different tundra vegetation types. The main aim of this study was to determine SOM characteristics in surface horizons of permafrost-affected soils covered with different tundra vegetation types (pioneer tundra, arctic meadow, moss tundra, and heath tundra) in the central part of Spitsbergen (Svalbard). Physical fractionation was used to separate SOM into POM (particulate organic matter) and MAOM (mineral-associated organic matter) fractions, while particle-size fractionation was applied to evaluate SOM distribution and composition in sand, silt, and clay fractions. The results indicate that in topsoils under heath tundra POM fractions dominate the carbon and nitrogen pools, whereas in pioneer tundra topsoils, the majority of the carbon and nitrogen are stored in MAOM fractions. Moreover, a substantial proportion of SOM is occluded within macro-and microaggregates. Furthermore, the results obtained from FTIR analysis revealed substantial differences in the chemical properties of individual soil fractions, both concerning the degree of occlusion in aggregates and across particle size fractions. This study provides clear evidence that tundra vegetation types significantly influence both the spatial distribution and chemical composition of SOM in the topsoils of central Spitsbergen.

Permafrost thawing is mobilizing dissolved organic carbon (DOC) stored in Arctic frozen soils into rivers, but vertical transport mechanisms within soil columns remain unclear, hindering accurate estimation of soil-derived DOC export. Through leaching experiments on active-layer organic soils and underlying mineral permafrost, this study reveals that mineral permafrost exhibits high soil carbon loss as DOC (3.27%-11.42%). However, 11.17%-46.42% of active-layer DOC is retained by mineral permafrost during vertical transport, forming an internal soil carbon sink. The sink selectively retains aromatic compounds, acting as a molecular filter that alters DOC composition and bioavailability. This internal retention complicates interpretations of active-layer DOC transport dynamics and alters the chemistry of both thawed permafrost and exported DOC. The findings emphasize the critical role of intra-soil DOC transformations in Arctic carbon cycling.

Massive stores of ancient soil organic carbon (SOC) in permafrost can decompose with Arctic warming and accelerate global climate change. Declining SOC stocks are central to the permafrost carbon feedback, but direct measures of SOC loss are extremely rare due to methodological challenges related to subsidence in the Arctic. To fully capture changing SOC dynamics during thaw, we directly measured SOC stock and bulk soil radiocarbon (C-14) changes, while accounting for subsidence, during 13 years of permafrost thaw in a warming experiment in Interior Alaska. We found significant declines in SOC stocks: 14% (+/- 6%) in ambient plots that experienced regional warming and 23% (+/- 5%) in snow fence warmed plots, entirely in deep, mineral soil layers. Losses were largely driven by winter soil warming but were mediated by changing soil moisture and vegetation conditions. Plots with low shrub biomass had greater SOC losses, suggesting that vegetation community composition may play an important role in SOC storage. Surface soil C-14 measurements suggest that carbon inputs were three times greater in warming plots compared to ambient plots, but that decomposition increased proportionally leading to no detectable change in surface organic layers. We observed significant SOC losses of 5.2-8.1 kg C m(-2) from deeper soil layers where carbon was sequestered similar to 2400 to similar to 4500 years ago. Our findings indicate that warmer soils in the winter will accelerate SOC losses, but that increasing density of shrub species through shrub expansion could help to mitigate SOC losses in deep soils. The significant loss of SOC from deep, mineral soils observed over just 13 years of ambient and experimental permafrost thaw highlights the vulnerability of this old C pool as it enters the active global carbon cycle.

Glacier shrinkage, a notable consequence of climate change, is expected to intensify, particularly in high-elevation areas. While plant diversity and soil microbial communities have been studied, research on soil organic matter (SOM) and soil protein function dynamics in glacier forefields is limited. This limited understanding, especially regarding the link between microbial protein functions and biogeochemical functions, hampers our knowledge of soil-ecosystem processes along chronosequences. This study aims to elucidate the mechanistic relationships among soil bacterial protein functions, SOM decomposition, and environmental factors such as plant density and soil pH to advance understanding of the processes driving ecosystem succession in glacier forefields over time. Proteomic analysis showed that as ecosystems matured, the dominant protein functions transition from primarily managing cellular and physiological processes (biological controllers) to orchestrating broader ecological processes (ecosystem regulators) and increasingly include proteins involved in the degradation and utilization of OM. This shift was driven by plant density and pH, leading to increased ecosystem complexity and stability. Our confirmatory path analysis findings indicate that plant density is the main driver of soil process evolution, with plant colonization directly affecting pH, which in turn influenced nutrient metabolizing protein abundance, and SOM decomposition rate. Nutrient availability was primarily influenced by plant density, nutrient metabolizing proteins, and SOM decomposition, with SOM decomposition increasing with site age. These results underscore the critical role of plant colonization and pH in guiding soil ecosystem trajectories, revealing complex mechanisms and emphasizing the need for ongoing research to understand long-term ecosystem resilience and carbon sequestration.

Understanding soil organic carbon (SOC) distribution and its environmental controls in permafrost regions is essential for achieving carbon neutrality and mitigating climate change. This study examines the spatial pattern of SOC and its drivers in the Headwater Area of the Yellow River (HAYR), northeastern Qinghai-Xizang Plateau (QXP), a region highly susceptible to permafrost degradation. Field investigations at topsoils of 86 sites over three summers (2021-2023) provided data on SOC, vegetation structure, and soil properties. Moreover, the spatial distribution of key permafrost parameters was simulated: temperature at the top of permafrost (TTOP), active layer thickness (ALT), and maximum seasonal freezing depth (MSFD) using the TTOP model and Stefan Equation. Results reveal a distinct latitudinal SOC gradient (high south, low north), primarily mediated by vegetation structure, soil properties, and permafrost parameters. Vegetation coverage and above-ground biomass showed positive correlation with SOC, while soil bulk density (SBD) exhibited a negative correlation. Climate warming trends resulted in increased ALT and TTOP. Random Forest analysis identified SBD as the most important predictor of SOC variability, which explains 38.20% of the variance, followed by ALT and vegetation coverage. These findings likely enhance the understanding of carbon storage controls in vulnerable alpine permafrost ecosystems and provide insights to mitigate carbon release under climate change.

With global warming and the intensification of human activities, frozen soils continue to melt, leading to the formation of thermokarst collapses and thermokarst lakes. The thawing of permafrost results in the microbial decomposition of large amounts of frozen organic carbon (C), releasing greenhouse gases such as carbon dioxide (CO2) and methane (CH4). However, little research has been done on the thermo-water-vapor-carbon coupling process in permafrost, and the interactions among hydrothermal transport, organic matter decomposition, and CO2 transport processes in permafrost remain unclear. We considered the decomposition and release of organic C and established a coupled thermo-water-vapor-carbon model for permafrost based on the study area located in the Beiluhe region of the Qingzang Plateau, China. The model established accurately reflected changes in permafrost temperature, moisture, and C fluxes. Dramatic changes in temperature and precipitation in the warm season led to significant soil water and heat transport, CO2 transport, and organic matter decomposition. During the cold season, however, the soil froze, which weakened organic matter decomposition and CO2 transport. The sensitivity of soil layers to changes in the external environment varied with depth. Fluctuations in energy, water, and CO2 fluxes were greater in shallow soil layers than in deeper ones. The latent heat of water-vapor and water-ice phase changes played a crucial role in regulating the temperature of frozen soil. The low content of soil organic matter in the study area resulted in a smaller influence of the decomposition heat of soil organic matter on soil temperature, compared to the high organic matter content in other soil types (such as peatlands).

Permafrost degradation, driven by rising temperatures in high-latitude regions, destabilizes previously sequestered soil organic carbon (OC), increasing greenhouse gas emissions and amplifying global warming. In these ecosystems, interactions with mineral surfaces and metal oxides, particularly iron (Fe), stabilize up to 80% of soil OC. This study investigates the mechanisms of Fe solubilization and OC release across a permafrost thaw gradient in Stordalen, Abisko, Sweden, including palsa, intermediate, and highly degraded permafrost stages. By integrating geophysical measurements-including relative elevation, thaw depth, soil water content, and soil temperature with redox potential and soil pore water chemistry, we identify the environmental conditions driving iron and organic carbon release into soil pore waters with permafrost degradation. Our results show that combining relative elevation, thaw depth, soil water content, soil pore water pH, and soil pore water conductivity with shifts in vegetation species enables very-high-resolution detection of permafrost degradation at submeter scales, distinguishing intact from degraded permafrost soils. We show that small-scale changes in thaw depth and water content alter soil pH and redox conditions, driving the release of Fe and dissolved organic carbon (DOC) and promoting the formation of Fe-DOC complexes in soil pore water. The amount of exported Fe-DOC complexes from thawed soils varies with the stage of permafrost degradation, and the fate of Fe-DOC complexes is likely to evolve along the soil-stream continuum. This study highlights how environmental conditions upon thaw control the type of Fe-DOC association in soil pore waters, a parameter to consider when quantifying what DOC is available for microbial and photo-degradation in aquatic systems which are significant sources of greenhouse gas emissions across Arctic landscapes.

Simple Summary The enzymic latch and iron gate theories represent two prevailing and contrasting mechanisms governing ecosystem carbon stability: the former via a phenolics accumulation mediated biochemical cascade that suppresses hydrolytic enzyme activity, and the latter via an abiotic pathway where ferrous iron oxidation suppresses phenol oxidase activity and promotes iron-bound soil organic carbon formation. Therefore, deciphering the stabilization mechanisms for the vast carbon stocks in permafrost peatlands represents a central challenge for climate change projections. In this study, we assessed the spatial distribution and interrelationships of peatland soil extracellular enzyme activities, iron phases, and iron-bound soil organic carbon across three permafrost zones in the Great Hing'an Mountains. Contrary to the enzymic latch mechanism, our data revealed that hydrolytic enzyme activities (beta-glucosidase, cellobiohydrolase, and beta-N-acetylglucosaminidase) were neither negatively correlated with phenolics nor positively correlated with phenol oxidase activity. Instead, iron emerged as the central regulator, with a positive correlation between ferrous iron and phenol oxidase activity and with ferric iron stabilizing soil organic carbon through co-precipitation. Our results highlighted that permafrost degradation could poses a threat to the dominant iron gate carbon sequestration mechanism in peatlands, potentially triggering a positive climate feedback.Abstract Distinct paradigms, such as the enzymic latch and iron gate theories, have been proposed to elucidate SOC loss or accumulation, but their relative significance and whether they are mutually exclusive in permafrost peatlands remain unclear. To address this, we evaluated their relative importance and identified the dominant factors controlling SOC stability. Therefore, we employed a space-for-time substitution approach across a permafrost gradient (continuous, discontinuous, and isolated) by systematically quantifying extracellular enzyme activities, iron (Fe) phases, and iron-bound soil organic carbon (Fe-SOC) at various depths (0-10, 10-30, and 30-50 cm) in peatlands. Our results did not support the enzymic latch theory, with hydrolytic enzyme activities (beta-glucosidase (BG), cellobiohydrolase (CBH), and beta-N-acetylglucosaminidase (NAG)) showing positive correlations with phenolics but negative correlations with phenol oxidase (PHO) activity. However, ferrous iron (Fe(II)) was significantly positively correlated with PHO activity, and ferric iron (Fe(III)) stabilized SOC through co-precipitation with it to form Fe-SOC, supporting the iron gate theory. Moreover, Fe-SOC decreased from the continuous to the isolated permafrost zone, and with soil depth from 0-10 cm to 30-50 cm. Partial least squares path modeling (PLS-PM) analysis indicated that Fe(III) directly and indirectly (via Fe-SOC and phenolics) affected SOC. Our study demonstrated the primacy of the iron gate mechanism in controlling carbon stability in the Great Hing'an Mountains permafrost peatlands, providing new insights for projecting carbon-climate feedback.

Soil organic carbon (SOC) plays a critical role in global carbon cycling and climate regulation, particularly in high-altitude permafrost regions. However, the impact of altitudinal gradients of alpine shrubs on SOC fractions remains poorly understood. In this study, we evaluated the rhizosphere SOC fractions and microbial biomass of Potentilla parvifolia along an altitudinal gradient (3,204, 3,350, 3,550, and 3,650 m). Our findings revealed that P. parvifolia significantly increased gram-positive bacterial and fungal biomass at medium and low altitudes (3,204, 3,350, and 3,550 m), enhancing the contribution of mineral-associated organic carbon (MAOC) to total SOC compared to bare soil. Moreover, SOC accumulation was primarily driven by the buildup of microbial necromass carbon, particularly fungal necromass carbon, within the MAOC fraction. These results improve our understanding of how altitudinal gradients influence SOC dynamics and microbial mechanisms, providing a scientific basis for developing effective bioprotection strategies to conserve high-altitude ecosystems under global climate change.IMPORTANCEThis study addresses critical knowledge gaps in understanding how altitudinal variation of shrubs affects soil carbon dynamics in the Qilian Mountains' seasonal permafrost. Investigating the redistribution between particulate organic carbon and mineral-associated organic carbon, along with microbial necromass (fungal vs bacterial), is vital for predicting alpine carbon-climate feedbacks. Shrub encroachment into higher elevations may alter vegetation-derived carbon inputs and decomposition pathways, potentially destabilizing historically protected permafrost carbon stocks. The unique freeze-thaw cycles in seasonal permafrost likely modulate microbial processing of necromass into stable carbon pools, a mechanism poorly understood in cold biomes. By elucidating altitude-dependent shifts in carbon fractions and microbial legacy effects, this research provides mechanistic insights into vegetation-mediated carbon sequestration under climate change. Findings will inform models predicting permafrost carbon vulnerability and guide alpine ecosystem management strategies in this climate-sensitive headwater region critical for downstream water security.