Effects of varied soil nitrogen and phosphorus concentrations on the growth and biomass allocation of three leguminous tree seedlings

We selected three different functional groups of leguminous tree seedlings from a 60 hm² sample plot of tropical mountain rainforest in Jianfengling, Hainan, China, as the study objects. The three leguminous trees included Ormosia balansae Drake, Ormosia semicastrata Hance f. litchifolia How, and Archidendron clypearia (Jack) I. C. Nielsen, with low, medium, and high nitrogen (N) demands, respectively. We conducted a soil nutrient control experiment in a greenhouse and compared growth performance of the three leguminous plant seedlings under five N and two phosphorus (P) addition concentration gradients. Results showed that:(1) Under N enrichment, the sensitivities of the five indicators, i.e., seedling height, total leaf area, total root length, root surface area, and biomass, to medium and high N addition were in the order of O. balansae > A. clypearia > O. semicastrata f. litchifolia. Additionally, total leaf area was the most sensitive index to changes in soil N concentration, followed by total root length and root surface area of O. semicastrata f. litchifolia seedlings as well as total root length of A. clypearia seedlings. (2) The root shoot ratio of O. semicastrata f. litchifolia and A. clypearia seedlings was relatively unaffected by N addition. In contrast, the root shoot ratio of O. semicastrata f. litchifolia seedlings increased markedly with the increase in N concentration, implying that O.semicastrata f. litchifolia had a greater tolerance to medium and high soil N concentrations. (3) Under P enrichment, O. balansae and A. clypearia seedlings showed a greater growth rate under low than high P addition, indicating low N and low P demand for these two species at the seedling stage. However, O. semicastrata f. litchifolia seedlings grew faster with high P and low N treatment, thus suggesting low N and high P demand at the seedling stage.