Abstract: 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.

Key words: nitrogen mineralization, soil microbial biomass, natural secondary forest, stand age, forest restoration