Abstract: Taking a pure Eucalyptus stand and a Eucalyptus−Erythrophleum fordii mixed stand as research subjects, soil physicochemical properties, soil nitrogen and phosphorus fractions, microbial biomass and enzyme activities, and nitrogen and phosphorus concentrations in Eucalyptus leaves and fine roots were measured in both dry and wet seasons. Coupling relationships among the indicators were explored using analysis of variance and correlation analysis. The findings indicated that compared with PP, MP significantly increased soil moisture, organic carbon, total N and P, available N and P, microbial biomass P, and the activities of N−acetyl−β−D−glucosaminidase, leucine aminopeptidase, and acid phosphatas. Eucalyptus foliar N and P concentrations rose markedly, while fine-root P content declined slightly in the dry season. The soil total N/total P ratio and organic carbon were highly positively correlated and jointly drove enzyme activity enhancement, and available N and P were each strongly correlated with foliar N and P, respectively. The mixed planting with E.  fordii optimizes soil nutrient dynamics and enhances microbial and enzymatic functions, thereby establishing an "N-input−P-activation" mechanism that reshapes the available soil N−P pool and promotes efficient nutrient uptake by Eucalyptus.