Effects of nitrogen source and defoliation on growth and biological dinitrogen fixation of Gliricidia sepium seedlings

Effects of four N sources and two defoliation treatments on growth and nitr ogenase activity of Gliricidia sepium (Jacq.) Walp seedlings were studied i n a greenhouse. All nutrients were supplied in irrigation water to the ster ile growing medium. The N sources were: (1) 100 mg l(-1) of N supplied as N O; (high-NOT), (2) 50 mg l(-1) of N supplied as NO3- and inoculation with R hizobium spp. (medium-NO3-), (3) 100 mg l(-1) of N supplied as NH4NO3, and (4) inoculation with Rhizobium spp without mineral N (N-2) At 35 weeks afte r sowing, mean total biomass was 130.5, 50.5, 22.9 and 17.4 g seedling(-1) in the NH4NO3, N-2, medium-NO3- and high-NO3- treatments, respectively. The root/shoot ratio was high in all of the N treatments (1.73-2.77) because t he seedlings had big taproots. The medium-NO3- treatment completely inhibit ed nodulation, whereas seedlings in the N-2 treatment were profusely nodula ted. At 3 2 weeks after sowing, groups of seedlings in the N-2 and high-NO3 - treatments were subjected to 50 or 100% defoliation. Closed-chamber acety lene reduction assays of intact root systems were conducted to compare nitr ogenase activity at 7, 14 and 28 days after defoliation (DAD). At 7 and 14 DAD, nitrogenase activity of completely and partially defoliated seedlings was about 10 and 60%, respectively, of that of undefoliated controls. At 28 DAD, nitrogenase activity of completely defoliated seedlings was twice the predefoliation value, whereas nitrogenase activity of partially defoliated seedlings was only 87% of the predefoliation value. Recovery of nitrogenas e activity was strongly correlated with foliage regrowth in the completely defoliated seedlings, but not in the partially defoliated seedlings. Abunda nt belowground C and N reserves in the large taproot probably contributed t o the rapid recovery from defoliation, Accumulation of belowground biomass may also improve defoliation tolerance of mature trees.