Mechanisms underlying the impacts of subtropical natural secondary succession on soil nitrogen mineralization

To explore the changes in soil nitrogen (N) mineralization and underlying mechanisms during the succession of subtropical natural secondary forests, we employed a space-for-time substitution approach to study five forests at different developmental stages (i.e., 5, 8, 21, 27, and 40 years), with the primary forests over 100 years old as a control. We measured soil net N mineralization rate under microcosm cultivation, as well as soil physi-cochemical properties, microbial biomass, and the richness of understory. The results showed that soil net N mine-ralization rates significantly increased with the succession of natural secondary forests, which varied from (-0.04±0.01) to (0.39±0.04) mg·kg^-1·d^-1. The net N mineralization rate in primary forests (0.73±0.12 mg·kg^-1·d^-1) was significantly higher than that in secondary forests. Results of random forest analysis showed that soil total N and carbon-to-nitrogen ratios were significant predictors of soil net N mineralization rates. Regression analysis showed that soil microbial biomass carbon significantly and positively correlated with soil net N mineralization rates. The understory shrub layer richness was significantly correlated with net soil N mineralization rate. Structural equation models showed that forest ages had a direct positive effect on soil microbial biomass carbon, and an indirect one via reducing soil pH. Soil microbial biomass carbon and soil carbon-to-nitrogen ratios had significantly positive and negative effects on soil net N mineralization rates, respectively. In conclusion, soil carbon-to-nitrogen ratio and microbial biomass carbon were important factors regulating the impacts of natural secondary forest succession on soil N mineralization.