Rapid nitrogen fixation contributes to a similar growth and photosynthetic rate of Robinia pseudoacacia supplied with different levels of nitrogen

Robinia pseudoacacia is a legume species that is widely used in afforestation which has high N2-fixation capacity and rapid growth rate. Both nitrogen (N) supply and phenology affect plant growth, photosynthesis and leaf senescence. The aim of this study was to determine how N supply affects N2 fixation, leaf photosynthesis and senescence of R. pseudoacacia at different phenological stages. Seedlings of R. pseudoacacia were supplied with different levels of 15N-labeled NH4NO3 solution, with seedlings of Sophora japonica as reference plants to calculate the percentage of N derived from the atmospheric N2 (%Ndfa). Compared with plants supplied with a high N level, those with a low N supply had a higher %Ndfa at an early developmental stage. Nitrogen fixation compensated the effect of a low N supply on plant growth in R. pseudoacacia. A high N supply decreased biomass allocation to lateral roots and nodules, and increased the relative growth rate of plant height as well as specific leaf area. The eighth mature compound leaf of R. pseudoacacia tended to have a higher net photosynthetic rate than the fourth leaf, and the leaves still maintained a moderate photosynthetic rate in early autumn. Plants tended to allocate more biomass to leaves at an early developmental stage and to stems and roots at a later developmental stage (three months old). The N level did not affect leaf photosynthesis at different phenological stages, primarily due to 1) a high %Ndfa under low N supply at early growing stage, and a similar high %Ndfa under all N supplies at a late growing stage, and 2) the delayed greening phenotype of expanding leaves to save nutrients for mature leaves.