Permafrost region stores 1014-1035 Pg (1 Pg=10(15) g) carbon (C) in the upper 3 m of soils, approximately twice of the atmosphere C pool. Over the past few decades, climate warming has caused substantial permafrost thaw. Consequently, a proportion of permafrost C becomes available for microbial utilization and can be decomposed as carbon dioxide (CO2) and methane (CH4) into the atmosphere, thus triggering potential C-climate feedback. However, the magnitude of this feedback remains highly uncertain, partly due to limited understanding of the formation and stabilization mechanisms of permafrost organic C. As an important component of soil stable C pool, microbial necromass C could make up more than 50% of soil organic carbon (SOC). Therefore, our knowledge of spatial distributions and key drivers of microbial necromass C in permafrost deposits is crucial for accurately predicting permafrost C dynamics under the context of global warming. Based on large-scale permafrost sampling along a similar to 1000 km transect on the Tibetan Plateau and biomarker analysis of amino sugars, we determined microbial necromass C content in permafrost deposits across 24 sampling sites. We then compared the contribution of microbial necromass C to SOC between permafrost deposits and active layer. To investigate key determinants of microbial necromass C content in permafrost deposits, we obtained climatic factors (e.g., mean annual temperature, mean annual precipitation) and measured soil variables (e.g., active layer thickness, soil moisture, soil texture), as well as microbial properties (e.g., fungal and bacterial biomass on the basis of phospholipid fatty acids analysis). Our results showed that total microbial necromass C, fungal and bacterial necromass C content in permafrost deposits increased from the west to the east of the study area. The average content of microbial necromass C in permafrost deposits was 2741.0 +/- 815.3 (values were reported as mean +/- standard error) mg kg(-1), and its contribution to SOC was 13.2%+/- 1.1%. The fungal necromass C and its contribution to SOC were significantly higher than that of bacterial necromass C. Our results also indicated that the contribution of fungal necromass C to SOC in the permafrost deposits was significantly lower than that in the active layer, however, there were no significant differences in the contribution of bacterial necromass C to SOC between these two layers. Regression analyses showed that total microbial necromass C, fungal and bacterial necromass C content in permafrost deposits increased with mean annual precipitation, soil moisture and their corresponding microbial biomasses, but decreased with mean annual temperature and active layer thickness. Structural equation modeling analyses further revealed that soil moisture and microbial biomass were the direct drivers of microbial necromass C content in permafrost deposits, and climatic factors indirectly affected microbial necromass C content. Overall, this study offers the first attempt to analyze the spatial distribution and dominant drivers of permafrost microbial necromass C on the Tibetan Plateau. The contribution of microbial necromass C to SOC observed in permafrost deposits was lower than those reported in temperate and global grassland soils. Moreover, the key factors of microbial necromass C detected in permafrost deposits were distinct from those reported in other ecosystems, where plant C input and mineral protection are dominant factors affecting soil microbial necromass C content. These findings illustrate the unique characteristics of C formation and accumulation in permafrost soils, suggesting that C formation processes and mechanisms obtained in other ecosystems cannot be simply generalized to permafrost ecosystems. More importantly, despite the relatively lower contribution of microbial necromass C to SOC, microbial necromass C is a non-negligible source of permafrost C, and its dynamics may affect the positive feedback between permafrost C cycle and climate warming.

Thaw slumps can lead to considerable carbon loss in permafrost regions, while the loss of components from two major origins, i.e., microbial and plant-derived carbon, during this process remains poorly understood. Here, we provide direct evidence that microbial necromass carbon is a major component of lost carbon in a retrogressive permafrost thaw slump by analyzing soil organic carbon (SOC), biomarkers (amino sugars and lignin phenols), and soil environmental variables in a typical permafrost thaw slump in the Tibetan Plateau. The retrogressive thaw slump led to a similar to 61% decrease in SOC and a similar to 25% SOC stock loss. As evident in the levels of amino sugars (average of 55.92 +/- 18.79 mg g-1 of organic carbon, OC) and lignin phenols (average of 15.00 +/- 8.05 mg g-1 OC), microbial-derived carbon (microbial necromass carbon) was the major component of the SOC loss, accounting for similar to 54% of the SOC loss in the permafrost thaw slump. The variation of amino sugars was mainly related to the changes in soil moisture, pH, and plant input, while changes in lignin phenols were mainly related to the changes in soil moisture and soil bulk density.

Climate warming is causing rapid permafrost degradation, including thaw-induced subsidence, potentially resulting in heightened carbon release. Nevertheless, our understanding of the levels and variations of carbon components in permafrost, particularly during the degradation process, remains limited. The uncertainties arising from this process lead to inaccurate assessments of the climate effects during permafrost degradation. With vast expanses of permafrost in the Tibetan Plateau, there is limited research available on SOC components, particularly in the central Tibetan Plateau. Given remarkable variations in hydrothermal conditions across different areas of the Tibetan Plateau, the existing limited studies make it challenging to assess the overall SOC components in the permafrost across the Tibetan Plateau and simulate their future changes. In this study, we examined the properties of soil organic carbon (SOC) and microbial necromass carbon (MicrobialNC) in a representative permafrost thaw-subsidence area at the southern edge of continuous permafrost in the central Tibetan Plateau. The results indicate that prior to the thaw-subsidence, the permafrost had a SOC content of 72.68 +/- 18.53 mg g(-1), with MicrobialNC accounting for 49.6%. The thaw-subsidence of permafrost led to a 56.4% reduction in SOC, with MicrobialNC accounting for 70.0% of the lost SOC. MicrobialNC constitutes the primary component of permafrost SOC, and it is the main component that is lost during thaw-subsidence formation. Changes in MicrobialNC are primarily correlated with factors pH, plant input, and microbial properties. The present study holds crucial implications for both the ecological and biogeochemical processes associated with carbon release from permafrost, and it furnishes essential data necessary for modeling the global response of permafrost to climate warming. Based on this study and previous research, permafrost thawing in the Tibetan Plateau causes substantial loss of SOC. However, there's remarkable heterogeneity in SOC component changes across different regions, warranting further in-depth investigation.