Background and aims <p>While nitrogen (N) deposition often alters plant functional traits in N-limited ecosystems, its long-term effects in N-saturated forests across various plant types remain unclear.</p> Method <p>We examined the responses of 15 leaf and 22 root traits in eight species of trees, shrubs, and herbs after 18&#xa0;years of N addition (control: 0; N50: 50; N100: 100; and N150: 150&#xa0;kg N ha⁻¹&#xa0;yr⁻¹) in an N-saturated tropical forest in southern China.</p> Results <p>The results indicated that N addition had negligible effects on leaf and root traits in trees, shrubs, and herbs. The leaf economics spectrum more effectively differentiated among plant types than the root economics spectrum; however, significant shifts in nutrient acquisition strategies were not observed under the N treatments. Although total leaf phenotypic plasticity remained stable across all plant types, herbs uniquely demonstrated increases and decreases in the integration of leaf and root traits, respectively, at higher N levels. Significant negative correlations between phenotypic plasticity and integration were only observed for the root functional traits of herbs.</p> Conclusion <p>The results highlight stabilization of above- and underground plant traits under prolonged N addition in tropical N-saturated forests. These findings are essential for improving trait-based predictions of vegetation dynamics in increasingly N-rich ecosystems.</p>