BIOLOGICAL NITROGEN-FIXATION IN 4 GLIRICIDIA-SEPIUM GENOTYPES

A field experiment was conducted at the Coconut Research Institute in Sri Lanka to examine the biological nitrogen fixation potential of three Gliricidia sepium provenances (OFI 14/84, 17/84, 12/86) and a local landrace (designated LL), using the N-15 isotope dilution method. There was marked variation in dry matter, total N, nodulation and N-15 enrichment among the Gliricidia genotypes (= 0.001), and the dry matter yield of Cassia siamea (syn. Senna siamea), the non-N-2 fixing reference plant was higher than for G. sepium. In all cases, highest biomass and total N were aboveground, with roots on average accounting for < 20 % of total dry matter or the total N in plants. Atom % N-15 excess was highest in C. siamea, and lowest in OFI 14/84. Although atom % N-15 excess was lower in Gliricidia leaves than in the other organs (all of which had similar N-15 enrichments), values of % N derived from atmospheric N-2 fixation ( % Ndfa) calculated for any individual organ or for the whole plant were similar. This was because the relative distribution of N-15 in the different parts of the fixing plant followed the same trend as in the reference plant. There were significant differences (p,= 0.01) in N-2 fixation between the Gliricidia genotypes. The values ranged from 17.8 g M tree(-1) (equivalent to 86 kg N ha(-1) at 5000 trees ha(-1)) in OFI 12/86 to 61.7g N tree(-1) (equivalent to 309 kg N ha(-1)) in OFI 14/84. Although most of this variability was due to differences in both % Ndfa and total N in plant, amount of N fixed was more correlated with total N in plant (r = 0.935) than with % Ndfa (r = 0.707). On average, % Ndfa in all three G. sepium provenances and LL was about 55 % or 34.6 g N tree(-1) (equivalent to some 166 kg N ha(-1)) in the 9 months within which N-2 fixation was measured. This represents a substantial contribution of N into the soil-plant system.