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

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Abstract
Effects of four N sources and two defoliation treatments on growth and nitrogenase activity of Gliricidia sepium (Jacq.) Walp seedlings were studied in a greenhouse. All nutrients were supplied in irrigation water to the sterile growing medium. The N sources were: (1) 100 mg l−1 of N supplied as NO3− (high-NO3−), (2) 50 mg l−1 of N supplied as NO3− and inoculation with Rhizobium spp. medium-NO3−), (3)100 mg l−1 of N supplied as NH4NO3, and (4) inoculation with Rhizobium spp without mineral N (N2). At 35 weeks after sowing, mean total biomass was 130.5, 50.5, 22.9 and 17.4 g seedling−1 in the NH4NO3, N2, medium-NO3− and high-NO3− treatments, respectively. The root/shoot ratio was high in all of the N treatments (1.73–2.77) because the seedlings had big taproots. The medium-NO3− treatment completely inhibited nodulation, whereas seedlings in the N2 treatment were profusely nodulated. At 32 weeks after sowing, groups of seedlings in the N2 and high-NO3− treatments were subjected to 50 or 100% defoliation. Closed-chamber acetylene reduction assays of intact root systems were conducted to compare nitrogenase 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 nitrogenase activity was strongly correlated with foliage regrowth in the completely defoliated seedlings, but not in the partially defoliated seedlings. Abundant belowground C and N reserves in the large taproot probably contributed to the rapid recovery from defoliation. Accumulation of belowground biomass may also improve defoliation tolerance of mature trees.

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