Manure–Straw Substitution Promotes Microbial Necromass Carbon Accumulation Alongside Soil Stoichiometric and Microbial Community Shifts in Open-Field Vegetable Soils

Microbial necromass C (MNC) represents a substantial pool of soil organic C (SOC), but its long-term response to organic N substitution remains poorly resolved in open-field vegetable soils. We examined endpoint soils from a 13-year equal-N field trial on a calcareous cinnamon soil in North China. The four regimes were mineral N alone (N100), 25% N substitution with manure (N75M25), 25% with straw (N75S25), and 50% with equal manure and straw contributions (N50M25S25). Fungal necromass C (FNC), bacterial necromass C (BNC), and total MNC were quantified and related to soil nutrient stoichiometry, extracellular enzyme stoichiometry, and PLFA-based microbial community structure. All organic substitution treatments increased FNC, BNC, and MNC relative to N100. N50M25S25 showed the highest FNC, BNC, and total MNC contents, which were 100.28%, 88.68%, and 97.68% higher than those under N100, respectively. FNC comprised 77.32–80.19% of total MNC. N75S25 had the highest FNC/BNC ratio and increased FNC/SOC and MNC/SOC by 25.26% and 21.25%, respectively. Organic substitution also increased SOC and total N, altered nutrient and enzyme stoichiometry, and raised the fungi-to-bacteria and Gram-positive-to-Gram-negative bacterial ratios. Random Forest ranked EEC:EEN highest for FNC and total MNC, whereas soil N:P ranked highest for BNC. PLS-PM indicated that MNC variation was most strongly associated with microbial community structure within a network linked to soil nutrient and enzyme stoichiometry. Overall, combined manure–straw substitution was associated with the highest bulk-soil MNC content, together with coordinated shifts in nutrient stoichiometry, enzyme allocation, and microbial community structure, highlighting its potential to enhance microbial-derived C accumulation in open-field vegetable soils.