Microbial necromass carbon links soil carbon sequestration to heavy metal stabilization under long-term fertilization

Long-term fertilization sustains soil productivity but concurrently introduces heavy metals (HMs) into agroecosystems. The mechanisms by which soil carbon pools regulate HM speciation, however, remain inadequately understood. Based on a 15-year field experiment, this study investigated the influence of fertilization on HM accumulation and speciation in relation to microbial necromass carbon (MNC), soil aggregation, and microbial community reorganization. Fertilization increased soil carbon pools, soil organic carbon increased by 44% under mineral fertilization and by 101-140% under organic amendments, while MNC rose by 92% and 304-342%, respectively. Sequential extraction revealed a consistent redistribution of HMs toward oxidizable fractions, particularly under organic fertilization where the proportion of this fraction increased by 7-26% relative to the unfertilized control. Shifts in microbial community composition toward copiotrophic taxa accelerated microbial turnover and promoted necromass accumulation. Aggregate-scale analyses further demonstrated that MNC was preferentially enriched in macroaggregates > 0.25 mm, whereas HMs were preferentially enriched in the 0.25-2 mm fraction. Random forest highlighted MNC as a primary predictor of Cu and Cd partitioning, whereas PLS-PM indicated that necromass accumulation and direct HM inputs jointly regulate HM speciation. These findings identify MNC as a previously underappreciated biogeochemical interface linking soil carbon stabilization with long-term HM retention in fertilized agricultural soils.