Abstract: We examined the effects of nitrogen addition on the seedlings of three leguminous species with different nitrogen requirements in tropical rainforest of Hainan, including Ormosia balancae with low nitrogen requirement, Ormosia semicastrata f. litchifolia with medium nitrogen requirement, and Archidendron clypearia with high nitrogen requirement. A pot experiment with five nitrogen addition levels (0, 75, 150, 300 and 600 mg·kg^-1) was conducted. Responses of root morphological traits and nutrient content of root and leaf were investigated. The results showed that: (1) Increasing nitrogen addition levels significantly reduced total root length, total root surface area, and root dry weight of the leguminous species with low nitrogen demand (O. balansae) and medium nitrogen demand (O. semicastrata f. litchifolia), but did not affect that of leguminous species with high nitrogen demand (A. clypearia). (2) The correlation analysis showed that total root length, total root surface area, and root dry weight of O. balansae and O. semicastrata were significantly negatively correlated with nitrate and ammonium concentrations in rhizosphere soil. (3) Nitrogen addition had positive effects on the nutrient content of roots and leaves of leguminous species with low and medium nitrogen demands, but did not affect that of high nitrogen demand species. Specifically, nitrogen addition significantly increased the total nitrogen content of leaves and roots of leguminous species with low nitrogen demand (O. balancae), while nitrogen and phosphorus content in the leaves and roots of legumes with medium nitrogen demand (O. semicastrata f. litchifolia) were promoted. In summary, the effects of nitrogen addition on leguminous plants depended on plant nitrogen demands and nitrogen addition levels. Nitrogen addition significantly inhibited root growth of legumes with low and medium nitrogen demands, and enhanced N and P contents of roots and leaves. In contrast, nitrogen addition had no effects on root morphological traits of legumes with high nitrogen demand, and only promoted root N content. It is necessary to fully consider the characteristics of leguminous plants when examining the effects of nitrogen deposition on stoichiometric characteristics of legume species and community dynamics.

Key words: tropical rainforest, legume, soil inorganic nitrogen, root morphology trait, nutrient status