Microbial activity mediates the contrasting effects of biochar combined with nitrogen or phosphorus on soil respiration and easily oxidizable organic carbon across soil depths in Cunninghamia lanceolata plantations

Biochar can alter soil organic carbon (SOC) dynamics by changing nutrient availability and microbial activity, but whether its effects differ when combined with nitrogen or phosphorus and how these responses vary over time and soil depth remain unclear. We conducted a field experiment in Cunninghamia lanceolata plantations with five treatments: control (CK), biochar alone (B), biochar with nitrogen (BN), biochar with phosphorus (BP), and biochar with both nitrogen and phosphorus (BNP). Soil samples were collected from the 0-10, 10-20, and 20-40 cm layers at six and eighteen months after treatment application to determine SOC fractions, soil nutrients, microbial phospholipid fatty acids (PLFAs), enzyme activities, and soil respiration. After six months, SOC content was significantly lower under B and BNP than under CK in the 0-10 cm soil layer and significantly lower under BP than under CK in the 10-20 cm soil layer. These differences were no longer significant after eighteen months, suggesting that the initial SOC losses were transient and may have been associated with enhanced apparent carbon mineralization and nutrient immobilization following biochar application. Easily oxidizable organic carbon (EOC) was the carbon fraction most responsive to biochar application and was positively associated with total nitrogen, indicating that nitrogen availability was an important regulator of labile carbon dynamics. Compared with the six-month sampling, bacterial, fungal, and Gram-negative bacterial PLFAs in the 10-20 cm layer was significantly lower after eighteen months, reflecting a temporal adjustment in the microbial community. Compared with CK, BN suppressed soil respiration, whereas BP enhanced it, possibly because nitrogen and phosphorus addition exerted contrasting effects on microbial nutrient limitation and carbon utilization. Overall, biochar effects on SOC depended on nutrient co-application, soil depth, and application duration, with temporal changes in nutrient availability and microbial activity contributing to the disappearance of the initial SOC decline.