H.P.S. JAYASUNDARA¹, M.D. DENNETT² and U.R. SANGAKKARA³

¹Coconut Research Institute, Lunuwila, Sri Lanka
²School of Plant Sciences, University of Reading, Reading, United Kingdom
³Department of Crop Science, Faculty of Agriculture, University of Peradeniya, Sri Lanka

Abstract

A field experiment was conducted at the Coconut Research Institute, Sri Lanka to examine the N₂-fixing potential of Gliricidia sepium and Leucaena leucocephala and transfer of fixed N to Setaria sphacelata var. splendida in tree legume-grass mixtures using the ¹⁵N isotope dilution method. A non-N-fixing tree legume, Senna siamea, was used as the reference plant. Gliricidia, leucaena and senna were monocropped or mixcropped with grass (50 : 50 replacement density) in April 1992. Percent N derived from fixation (%Ndfa) in gliricidia and leucaena was measured during 4 periods of regrowth during August 1992–May 1993. Percent N derived from transfer (%Ndft) in grass grown with gliricidia and leucaena was calculated using the ¹⁵N enrichment of grass grown with senna as the reference. Nitrogen yield was higher (P < 0.05) in gliricidia than in leucaena and senna (which had similar N) monocultures. Mixcropped tree legumes produced similar N yields. In 3 tree legume species, a major percentage of the total N was in above-ground plant parts with roots accounting for around 22% at the final harvest. The ¹⁵N enrichments were significantly higher (P < 0.001) in senna than in gliricidia and leucaena at all harvests. Within a species, the ¹⁵N enrichments in different plant parts were not significantly different. %Ndfa varied little in monoculture gliricidia and leucaena, ranging from 61–72% in different regrowth periods. This increased to an average of 81% in mixcropped gliricidia and leucaena at the last 2 harvests. In both species, %Ndfa values calculated for different plant parts were not significantly different from the %Ndfa based on the weighted average ¹⁵N enrichment for the whole plant. N yield of grass grown with gliricidia and leucaena was 16% higher than that of grass monoculture. The ¹⁵N enrichments were significantly lower (P < 0.05) in grass grown with gliricidia and leucaena than in grass grown with senna and grass monoculture at the last 2 harvests. On average, up to 21% of N in grass growing with gliricidia and leucaena could be derived from transfer of biologically fixed nitrogen.