Mechanism of microbial necromass formation during decomposition of Stipa bungeana above-ground residues

Soil microorganisms modulate the formation of soil organic carbon (SOC) by catalyzing residue decomposition. Further research is required to fully understand how residue-derived carbon (C) flows through the microbial portion of anabolism-forming compounds, primarily referred to as microbial necromass in the soil food web. To examine the effects of residue decomposition on the microbial contribution to SOC, and how microbial groups (¹³C-phospholipid fatty acids (PLFAs)) and soil enzymes regulate necromass accumulation (¹³C-amino sugars) and SOC, a 163-d decomposition experiment with ¹³C-enriched aboveground residue of Stipa bungeana was conducted. The δ¹³C value of above-ground residue decreased whereas that of soil increased, thus providing direct evidence for the contribution of residue-derived C to SOC. Residue-derived ¹³C in SOC, microbial biomass C, and dissolved organic C increased during decomposition. The soil microbial groups shifted from gram-negative to gram-positive bacteria and actinobacteria upon decomposition. C-cycling enzyme activity increased as decomposition proceeded. Bacterial necromass dominated in the early decomposition stage, fungal necromass dominated thereafter, and the total necromass increased with decomposition. The SOC and necromass were significantly correlated with residual mass, the residue C to nitrogen (N) ratio, and ¹³C-arbuscular mycorrhizal fungal (AMF) PLFAs. By incorporating aboveground grassland residues into the soil, microorganisms regulate soil enzyme activity, control residue-derived C through the soil food chain and facilitate the transformation of microbial products into SOC through microbial anabolism. Our findings underscore how variations in the residue decomposition stage shape the primary microbial groups, influencing enzyme activity that, in turn, determines necromass turnover, and thus SOC formation.