Nitrogen-fixation dynamics in a cut-and-carry silvopastoral system in the subhumid conditions of Guadeloupe, French Antilles

This paper summarizes several studies on N recycling in a tropical silvopastoral system for assessing the ability of the system to increase soil fertility and insure sustainability. We analyzed the N-2 fixation pattern of the woody legume component (Gliricidia sepium), estimated the recycling rate of the fixed N in the soil, and measured N outputs in tree pruning and cut grass (Dichanthium aristatum). With this information, we estimated the N balance of the silvopastoral system at the plot scale. The studies were conducted in an 11-year-old silvopastoral plot established by planting G. sepium cuttings at 0.3 m x 2 m spacing in natural grassland. The plot was managed as a cut-and-carry system where all the tree pruning residues (every 2-4 months) and cut grass (every 40-50 days) were removed and animals were excluded. No N fertilizer was applied. Dinitrogen fixation, as estimated by the 15N natural abundance method, ranged from 60-90% of the total N in aboveground tree biomass depending on season. On average, 76% of the N exports from the plot in tree pruning (194 kg [N] ha(-1) yr(-1)) originated from N-2 fixation. Grass production averaged 13 Mg ha(-1) yr(-1) and N export in cut grass was 195 kg [N] ha(-1) yr(-1). The total N fixed by G. sepium, as estimated from the tree and grass N exports and the increase in soil N content, was about 555 kg [N] ha(-1) yr(-1). Carbon sequestration averaged 1.9 Mg [C] ha(-1) yr(-1) and soil organic N in the 0-0.2 m layer increased at a rate of 166 kg [N] ha(-1) yr(-1), corresponding to 30% of N-2 fixation by the tree. Nitrogen released in nodule turnover ( < 10 kg [N] ha(-1) yr(-1)) and litter decomposition (< 40 kg [N] ha(-1) yr(-1)) contributed slightly to this increase, and most of the recycled N came from the turnover or the activity of other below-ground tree biomass than nodules.