Effects of Erythrophleum fordii–Eucalyptus Mixed Plantation on Soil Nitrogen–Phosphorus Dynamics and Nutrient Uptake in Subtropical Eucalyptus Plantations

To address the limiting effects of soil N-P imbalance in monoculture Eucalyptus plantations, this study reveals how interplanting the leguminous, nitrogen-fixing tree Erythrophleum fordii Oliv. with Eucalyptus synergistically enhances soil N supply via biological fixation and promotes P availability through microbially mediated activation, thereby optimizing soil N-P provision and improving Eucalyptus nutrient uptake. We compared a pure Eucalyptus stand (PP) with an Eucalyptus -E. fordii mixed stand (MP), sampling soil and Eucalyptus leaf and fine-root tissues in both dry and wet seasons. Measured indicators included soil physicochemical properties, total and available N and P fractions, microbial biomass N and P, activities of N- and P-acquiring enzymes, and plant tissue N and P concentrations. Two-way ANOVA and Pearson correlation analyses were used to examine indicator coupling. Compared with PP, MP significantly increased soil moisture, organic carbon, total N and P, available N and P, microbial biomass N and P, and enzyme activities. Eucalyptus foliar N and P concentrations rose markedly, while fine-root P content declined slightly in the dry season. Correlation analysis showed soil total N/total P ratio and organic carbon were highly positively correlated and jointly drove enzyme activity enhancement, and that available N and P were each strongly correlated with foliar N and P, respectively. In summary, mixed planting with E. fordii optimizes soil nutrient dynamics and enhances microbial and enzymatic functions to establish an “N-input-P-activation” mechanism, reshaping the soil available N-P pool and promoting efficient nutrient uptake by Eucalyptus. These findings provide a theoretical basis for near-natural management and precision fertilization of subtropical Eucalyptus plantations.