Long-Term Nitrogen Addition and Reduced Precipitation Alter Seasonal Soil N2O Fluxes and N-Transformation Processes in a Temperate Forest

Nitrous oxide (N2O) is a potent greenhouse gas, and forest-soil N2O emissions are highly sensitive to long-term changes in nitrogen deposition and precipitation regimes. However, how long-term nitrogen addition interacts with reduced precipitation to regulate seasonal soil N2O flux and its associated environmental and process-related factors remains poorly understood in temperate forests. We conducted a 14-year field experiment in a temperate broadleaved–Korean pine forest with four treatments: control (CK), nitrogen addition (NA; 50 kg N ha−1 yr−1), precipitation reduction (PR; 30% throughfall exclusion), and their combination (NAPR). Soil N2O flux was monitored continuously throughout May, July, and September 2023; daily mean fluxes were calculated from the continuous measurements and then averaged to obtain monthly means. Soil properties, inorganic N, dissolved organic carbon (DOC) stoichiometry, and N-transformation-related enzyme indicators were measured once per monitoring month. Reduced precipitation markedly decreased monthly mean N2O flux, with the largest treatment divergence occurring in July. Across all three monitoring periods, N2O flux consistently followed the order NA > CK > PR > NAPR. Under ambient precipitation, N addition increased N2O flux, whereas under reduced precipitation it further decreased N2O flux relative to PR. Nitrogen addition substantially increased NH4+–N, NO3−–N, and total inorganic N under both precipitation regimes, but the particularly low N2O flux under NAPR persisted. Redundancy analysis and correlation analyses showed coordinated variation among soil environmental and resource conditions and N-transformation-related indicators, while random forest analysis identified urease (UE) and ammonia monooxygenase (AMO) among the highest-ranked predictors and showed robust predictive performance under grouped cross-validation. Overall, reduced precipitation reversed the stimulatory effect of long-term nitrogen addition on N2O flux, with the treatment response associated with soil moisture, substrate availability, and N-transformation-related indicators rather than attributable to a single measured pathway.