Phosphorus addition accelerates soil organic carbon mineralization by desorbing organic carbon and increasing microbial activity in subtropical forest soils

The magnitude and direction of soil organic carbon (SOC) mineralization are closely regulated by exogenous nutrient addition – and, especially, by limited nutrients such as phosphorus (P) – in tropical and subtropical ecosystems. However, the response of SOC decomposition to P addition and the underlying mechanisms operating in subtropical P-limited forest soils remain unclear. In this study, four subtropical forest soils (sandstone and granite soils planted with the chinkapin Castanopsis carlesii and Cunninghamia lanceolata (Chinese fir)) in Sanming City, Fujian Province, China were selected, and an incubation experiment with three levels (0, 0.1, 0.6 mg P g−1 soil) of P addition was conducted to explore the impact of P addition on SOC mineralization in subtropical forest soils. SOC mineralization, phospholipid fatty acids (PLFAs), enzymatic activities, and soil properties were determined. Results showed that P addition significantly increased the cumulative mineralization of SOC (P < 0.001). Additionally, the microbial biomass (e.g., fungal, bacterial, and total PLFAs) and the activities of C-cycling enzymes significantly increased (by 11.95 %–570.25 %) after P addition (P < 0.001), which suggests that exogenous P input enhanced microbial activity and thus accelerating SOC mineralization. P addition also significantly increased the concentration of dissolved organic C (DOC) (P < 0.001), thereby making soil C more available to microorganisms and allowing it to become quickly decomposed. A variance partitioning analysis (VPA) showed that DOC and fungal biomass acted as the predominant abiotic and biotic factors accelerating SOC mineralization under P addition. Using the results of a partial least squares path analysis (PLM-PM), we conclude that P addition stimulated the decomposition of SOC in these subtropical P-limited forest soils. Two processes help explain these results: (i) the enhancement of microbial activity in response to the alleviation of microbial P limitation and (ii) the fact that P addition desorbed the microbial inaccessible organic C and converting it into DOC, which is easily accessible to microbes, thereby contributing to SOC mineralization. Our study reveals how P input influences organic C mineralization in subtropical forest soils where P is limited. It also provides novel insight into the uncertainty of the global soil C sink, which might be partially attributable to exogenous nutrient inputs, especially in the case of limiting nutrients.