The accumulation of microbially derived carbon under long-term organic matter addition is related to microbial carbon use and acquisition

Soil microbially derived carbon is essential for soil carbon sequestration, yet its dynamics under organic matter addition in croplands remain poorly understood, largely owing to the interactions among microbial carbon, nitrogen, and phosphorus metabolic functions and their divergent responses to soil environmental changes. Here, based on a long-term field experiment, we investigated the effects of combined application of chemical fertilizer with straw or manure on microbially derived carbon accumulation in soils. The 18 O-H 2 O tracer method and soil extracellular enzyme were used to quantify microbial carbon use efficiency and carbon acquisition capacity, and metagenomic sequencing was performed to determine microbial carbon, nitrogen and phosphorus functional genes, aiming to unravel the intrinsic relationship between microbial metabolism and microbially derived carbon. The results showed that, compared to the chemical fertilizer treatment, the chemical fertilizer treatments with straw or manure both significantly increased the accumulation of microbially derived carbon. Furthermore, the microbially derived carbon was significantly positively correlated with microbial carbon use efficiency but negatively correlated with carbon acquisition capacity. Specifically, chemical fertilizer treatments with straw or manure increased microbial carbon use efficiency but reduced carbon acquisition capacity, thereby shifting the microbial metabolic pattern from a “high decomposition” state to a “high utilization” state, which associated with the retention of microbially derived carbon. Correlation analysis showed that microbial carbon use efficiency was significantly positively correlated with nitrate nitrogen, SOC but negatively with microbial carbon and nitrogen functional genes (e.g., those involved in disaccharide metabolism, dissimilatory nitrate reduction, and nitrogen fixation), whereas carbon acquisition capacity exhibited the opposite pattern. Especially, the increased nitrate nitrogen and SOC under chemical fertilizer with straw or manure effectively alleviated microbial resource limitation and were negatively correlated with both nitrogen and carbon functional genes. These findings elucidate the role of nitrogen and carbon availability in regulating the relationships among microbial metabolic functions, carbon acquisition and utilization, improving our understanding of the mechanisms governing microbial-derived carbon accumulation under organic matter addition.