Enhancing microbial carbon use efficiency via exogenous carbon inputs: Implications for soil carbon sequestration and phosphorus availability

Exogenous carbon (C) inputs have been shown to enhance microbial C use efficiency (CUE), thereby promoting soil C sequestration and fertility. However, the underlying mechanisms through which microbial CUE influences the phosphorus (P) availability remain poorly understood. This study aimed to elucidate the impact of carbon inputs—specifically, organic fertilizer and straw—on microbial CUE and its subsequent effects on soil organic C (SOC) pools and P availability. Over a five-year field experiment, we observed that substituting 30 % of chemical P fertilizer with organic fertilizer (OM) and incorporating straw into the soil (ST) significantly enhanced microbial CUE. This led to increases of 45.15 % and 95.2 % in microbial biomass C (MBC), and 51.8 % and 72.8 % in microbial biomass P (MBP), respectively, compared to treatment with chemical P fertilizer alone (CF). SOC content increased by 21.3 % and 23.4 %, with corresponding increases in particulate organic C (POC) of 1.4 g kg⁻¹ and 1.2 g kg⁻¹, and in mineral-associated organic C (MAOC) of 1.8 g kg⁻¹ and 2.2 g kg⁻¹. Among the various P pools, labile organic P (LP_o) exhibited the most significant response to carbon inputs, with increases of 72.6 % to 84.7 %, while available P (AP) increased by 17.6 % and 18.5 %. Partial least squares path modeling indicated that carbon inputs directly enhance microbial CUE, which in turn influences P availability via two primary metabolic pathways. First, increased CUE promotes C anabolism, leading to the formation of microbial residues (MBC and MBP) and by-products (LP_o). Second, enhanced CUE improves C catabolism, stimulating the production of enzymes (β-1,4-glucosidase and alkaline phosphatase) and mineralization of LP_o and MBP, ultimately increasing AP. Our findings highlight the dual role of enhanced microbial CUE in advancing C sequestration and improving P availability, providing new insights into the beneficial effects of exogenous carbon inputs on overall soil health.