Microbial Necromass and Extracellular Enzyme Activities Are Associated with Depth-Dependent Soil Carbon Stabilization Along a Wildfire-Severity Gradient

Wildfire can alter soil organic carbon (SOC) pools and microbial pathways of carbon stabilization, but depth-dependent links between microbial necromass and stable carbon pools remain unclear. We investigated a wildfire-severity gradient in a subtropical coniferous forest in Guizhou, China, including four severity classes (unburned, light, moderate, and severe) and two soil layers (0–20 and 20–40 cm). We measured easily oxidizable organic carbon (EOC), recalcitrant organic carbon (ROC), SOC, amino sugars, microbial necromass carbon (MNC), extracellular enzyme activities, and carbohydrate-active enzyme (CAZy) functional indices. MNC peaked under moderate wildfire in both layers, increasing by 73.8% and 85.1% in the topsoil and subsoil, respectively, relative to unburned plots. After accounting for soil physicochemical properties and wildfire severity, MNC was more strongly associated with ROC and SOC in the topsoil than in the subsoil. Extracellular enzyme activities were positively associated with amino sugars and necromass pools, whereas CAZy composite indices showed weaker relationships that did not persist after false discovery rate correction. Exploratory path analysis suggested that the EOC–NAG–MNC–ROC–SOC chain was more pronounced in the topsoil, while the subsoil showed weaker chained associations and stronger direct EOC–MNC and EOC–ROC links. Overall, microbial necromass was associated with depth-dependent post-fire carbon stabilization.