Spatial distributions and key drivers of microbial necromass carbon in permafrost deposits across the Tibetan Plateau

Permafrost region stores 1014–1035 Pg (1 Pg=10¹⁵ 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 ~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.