Variations in soil organic carbon pool structure in the humus layer influenced by soil nutrient stoichiometry under nitrogen addition treatments: Evidence from a temperate Korean pine plantation in northeast China

Atmospheric nitrogen (N) deposition critically alters the dynamics of forest soil organic carbon (SOC) pools and induces shifts in carbon (C)-N-phosphorus (P) stoichiometric balances. However, comprehensive analyses linking forest SOC pool structure with detailed soil C-N-P stoichiometry remain limited. In this study, varying concentrations of ammonium nitrate (0, 20, 40, and 80 kg N ha-1 yr-1) were applied on northeastern temperate Korean pine plantation in China over nine consecutive years, then the humus layer and topsoil layer (0-5 cm) were collected. The results demonstrated that exogenous N fertilizer application promoted the transformation from readily oxidizable organic C (ROC) to non-readily oxidizable organic C (NROC), enhanced SOC pool stability, and influenced the carbon pool management index (CPMI). Additionally, significant alterations in the C-N-P stoichiometric relationships were observed within both soil horizons. Random forest and structural equation modeling indicated that nutrient stoichiometry, such as the N:P ratio, C:N imbalance, and N:P imbalance, were the primary factors influencing changes in the SOC pool profiles within the humus layer. In contrast, SOC pool profiles in the topsoil layer were strongly associated with pH, total N, and soil cellulase activity. This study contributes to the optimization of C cycle models for terrestrial ecosystems in the context of the continuing increase in global N deposition.