Influence of forage harvesting regimes on dynamics of biological dinitrogen fixation of a tropical woody legume

Effects of three forage harvesting regimes-total removal of foliage and bra nches once (T-12) or twice a year (T-6) and 50% removal every 2 months (P-2 )-on growth and biological dinitrogen fixation of Gliricidia sepium (Jacq.) Walp were studied under subhumid tropical conditions in Guadeloupe, French Antilles. Gliricidia sepium was grown in association with the perennial C- 4 grass Dichantium aristatum (Poir) C.E. Hubbard in a two-storied fodder pr oduction system. The medium-term effects of pruning on N-2 fixation were as sessed by the N-15 natural abundance method. Gmelina arborea Roxb. was used as the non-fixing reference. The trees in the T-12 regime followed the nat ural phenological cycle, and flowering and podfilling at the beginning of t he dry season reduced both foliage and nodule biomass. The T-6 regime imped ed flowering, and only a few flowers, on older branches, were produced in t he P-2 regime. In trees in the T-12, T-6, and P-2 regimes, fixed N comprise d 54-87, 54-92, and 60-87%, respectively, of the total N in aboveground bio mass, depending on sampling date. Total annual accumulation of N in harvest able aboveground biomass was highest in trees in the T-6 regime at 313 kg h a(-1), of which 204 kg ha(-1) of N was fixed from the atmosphere. In all tr eatments, about 70% of the N exported per year from the plot in the fodder harvest came from N-2 fixation. Thus, N-2 fixation makes an important contr ibution to the N economy of the G. sepium-D. aristatum forage production sy stem, and greatly reduces the need for fertilizer application.