Shifts in microbial metabolic strategies underlie differential microbial necromass accumulation across soil layers in a subalpine forest

Regulation of microbial necromass carbon (MNC) dynamics by microbial metabolic traits is fundamental to understanding forest soil carbon (C) sequestration and its carbon sink function. However, in the soil profile with heterogeneous resource environments, it remains unclear how the trade-offs in different microbial traits regulate MNC accumulation. Herein, we sampled topsoil (0–10 cm) and subsoil (20–30 cm) from a subalpine spruce ( Picea asperata ) forest across three sampling times. We quantified MNC using biomarker analysis and measured microbial metabolic parameters, including growth, biomass, turnover rate, and extracellular enzyme activity. The results showed that the MNC concentrations in the topsoil were 57.5–106.6% higher than those in the subsoil across the three sampling times. MNC concentrations were positively correlated with microbial growth traits (microbial growth rate and carbon use efficiency) in the topsoil, whereas MNC concentrations were negatively correlated with microbial investment traits (extracellular enzyme activity) in the subsoil. The path analysis results suggested that soil nutrients and stoichiometric characteristics jointly changed the microbial metabolic trade-offs, i.e., from prioritizing growth yield in the nutrient-rich topsoil to emphasizing resource investment in the nutrient-poor subsoil, thereby influencing MNC accumulation across soil layers. Overall, our findings underscore the importance of shifts in microbial metabolic strategies in underlying MNC dynamics across the soil profile, which is instrumental for modeling applications to improve the evaluation and prediction of soil C sequestration in forests.