Nitrogen fertilization induces microbial carbon limitation in subtropical soils: new evidence from microbial necromass decomposition

Microbial necromass is an important contributor to soil organic carbon (C) in subtropical forests. However, how microbial necromass decomposition responds to nitrogen (N) fertilization and whether this process depends on phosphorus (P) availability remain poorly understood. This study assessed the impact of long-term N fertilization and P addition on microbial necromass decomposition using soils collected from a subtropical Cunninghamia lanceolata forest. We combined a seven-year field N fertilization experiment with a laboratory incubation using 13C-labeled microbial necromass. We found that N fertilization accelerated decomposition of the fast degradable pool by 54.3% while reducing its proportional size by 66.9%, indicating intensified microbial processing of readily available C. This shift was mechanistically linked to N-induced relative microbial C limitation, evidenced by depleted dissolved organic C, elevated oxidase activities, and microbial communities shifted toward taxa favoring labile substrates (e.g. increased Rozellomycota and decreased Basidiomycota). Moreover, P addition moderated N effects on stable pool decomposition, resulting in more necromass-13C retention in N + P than N-only treatments at the end of incubation. These findings demonstrate that N enrichment accelerates the turnover of the fast degradable necromass pool through inducing microbial C limitation. Our study reveals that microbial necromass constitutes a vulnerable C sink under N enrichment, suggesting that N-driven acceleration of decomposition may reduce soil C sequestration capacity in subtropical forests, which could be mitigated by concurrent P addition.