Accumulation of residual soil microbial carbon in Chinese fir plantation soils after nitrogen and phosphorus additions

Nitrogen (N) and phosphorus (P) additions can affect soil microbial carbon (C) accumulation. However, the mechanisms that drive the changes in residual microbial C that occur after N and P additions have not been well-defined for Chinese fir plantations in subtropical China. We set up six different treatments, viz. a control (CK), two N treatments (N1: 50 kg ha−1 a−1; N2: 100 kg ha−1 a−1), one P treatment (P: 50 kg ha−1 a−1), and two combined N and P treatments (N1P: 50 kg ha−1 a−1 of N + 50 kg ha−1 a−1 of P; N2P: 100 kg ha−1 a−1 of N + 50 kg ha−1 a−1 of P). We then investigated the influences of N and P additions on residual microbial C. The results showed that soil pH and microbial biomass decreased after N additions, while microbial biomass increased after P additions. Soil organic carbon (SOC) and residual microbial C contents increased in the N and P treatments but not in the control. Residual microbial C accumulation varied according to treatment and declined in the order: N2P > N1P > N2 > N1 > P > CK. Residual microbial C contents were positively correlated with available N, P, and SOC contents, but were negatively correlated with soil pH. The ratio of residual fungal C to residual bacterial C increased under P additions, but declined under combined N + P additions. The ratio of residual microbial C to SOC increased from 11 to 14% under the N1P and N2P treatments, respectively. Our results suggest that the concentrations of residual microbial C and the stability of SOC would increase under combined applications of N and P fertilizers in subtropical Chinese fir plantation soils.