Nutrient inputs reshape microbial processing of straw and native soil organic carbon by modulating microbial anabolic strategies

Crop straw is an important carbon input in agroecosystems, which can be converted into soil organic carbon (SOC) through microbial anabolism. Nevertheless, it remains unclear how much straw carbon can be converted into microbial necromass (MNC), and how nutrient addition regulates this anabolic process. A 300-day incubation experiment with 13 C-labelled rice straw was conducted under three nutrient levels. Straw carbon flow into microbial biomass and necromass was traced using 13 C phospholipid fatty acids (PLFAs) and amino sugar analysis. Microbial carbon use efficiency based on 13 C-PLFAs (CUE′) and necromass accumulation efficiency (CAE) were further quantified. Our results showed that 10.3%–12.1% of straw carbon was transformed into microbial necromass after incubation, accounting for 25.9%–31.4% of straw-derived SOC, confirming the critical role of microbial anabolism in SOC formation. Nutrient (nitrogen and phosphorous) addition regulated microbial straw carbon transfer in a time-dependent manner: initially stimulated microbial utilization of straw carbon (higher 13 C-PLFAs and CUE′), but later accelerated turnover, as shown by a pronounced decline in 13 C-necromass and CAE. A concurrent shift of 13 C-assimilating communities toward fungi and Gram-positive bacteria reflects nutrient-triggered alterations of microbial anabolic strategies. Our findings highlight a biological trade-off among microbial growth, metabolism, and mortality to cope with varied nutrient conditions. Notably, straw and nutrient addition stimulates microbial conversion of native SOC into necromass, uncovering a previously overlooked pathway through which straw return reshapes SOC composition and stabilization. Our work demonstrates nutrient regulation of microbial metabolic strategies for fresh and native carbon, providing mechanistic insights into SOC dynamics in straw return agroecosystems.