Effects of long-term nitrogen deposition on soil microbial respiration and metabolic quotient in subtropical natural forests.

Based on a 9-year nitrogen (N) addition experiment in a subtropical evergreen broad-leaved forest at the Fujian Sanming Forest Ecosystem National Field Observation and Research Station, we examined the effects of long-term N addition on soil microbial respiration and metabolic quotient (qCO2). We collected surface soils (0-10 cm) from control (CK, 0 kg N·hm-2·a-1), low nitrogen (LN, 40 kg N·hm-2·a-1), and high nitrogen (HN, 80 kg N·hm-2·a-1) treatments, analyzed soil physicochemical properties, microbial biomass, and the stoichiometric characteristics of extracellular enzymes related to carbon (C), N, and phosphorus (P), and measured the soil microbial respiration through laboratory incubation experiments. The results showed that the HN treatment significantly decreased the soil C/N by 18.8% and increased the soil N/P by 52.9%. The LN and HN treatments significantly decreased the dissolved organic C/N by 31.8% and 34.6%, and increased the microbial N/P by 72.8% and 29.8%, respectively. Microbial respiration in the LN treatment was significantly higher than that in the HN treatment, with no differences observed between either treatment and CK. The qCO2 in the LN and HN treatments was significantly increased by 36.8% and 21.1%, respectively, and the qCO2 in the LN treatment was significantly higher than that in the HN treatment. Stepwise regression analysis indicated that soil C/N and microbial N/P were the primary factors influencing the microbial respiration and qCO2, accounting for 56.2% and 58.4% of the variance, respectively. In conclusion, the effects of long-term N deposition on soil microbial respiration and metabolic quotient in subtropical forests were dose-dependent. High N deposition suppressed microbial respiration by reducing soil C/N, whereas low N deposition increased microbial metabolic quotient by exacerbating microbial N/P imbalance.