Water Availability Modulates Leaf Functional Trait Responses of Robinia pseudoacacia L. Seedlings to Nitrogen–Phosphorus Supply

Water availability strongly influences nutrient utilization and plant adaptive strategies, yet how nitrogen (N) and phosphorus (P) supply interact with water conditions to regulate leaf functional traits remains insufficiently understood. In this study, Robinia pseudoacacia L. seedlings were subjected to contrasting water regimes combined with N and P additions to examine leaf morphological, structural, physiological, and stoichiometric traits. The results showed that water availability significantly modulated seedling responses to N and P supply. Under well-watered conditions, nutrient addition promoted leaf growth and biomass accumulation, whereas under water deficit, seedlings enhanced osmotic adjustment and altered resource allocation. Significant interactions among water, N, and P were observed for leaf morphological, physiological, and stoichiometric traits. Soil organic carbon (SOC), total nitrogen (TN), and available phosphorus (AP) in rhizosphere soil were the soil factors most strongly associated with the variation in leaf functional traits, among which SOC exhibited the highest explanatory power. Functional trait network analysis revealed that specific leaf area (SLA) and leaf nitrogen-to-phosphorus ratio (N:P) were identified as central hubs of the leaf trait network. Overall, drought shifted R. pseudoacacia seedlings from a resource-acquisitive strategy toward a resource-conservative and stress-tolerant strategy. This study elucidates the short-term response patterns of R. pseudoacacia seedlings to combined water and nutrient stress, providing preliminary insights for water and fertilizer management during seedling establishment in arid regions.