A field experiment was conducted at the Coconut Research Institute in
Sri Lanka to examine the biological nitrogen fixation potential of thr
ee Gliricidia sepium provenances (OFI 14/84, 17/84, 12/86) and a local
landrace (designated LL), using the N-15 isotope dilution method. The
re was marked variation in dry matter, total N, nodulation and N-15 en
richment among the Gliricidia genotypes (= 0.001), and the dry matter
yield of Cassia siamea (syn. Senna siamea), the non-N-2 fixing referen
ce plant was higher than for G. sepium. In all cases, highest biomass
and total N were aboveground, with roots on average accounting for < 2
0 % of total dry matter or the total N in plants. Atom % N-15 excess w
as 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 o
f which had similar N-15 enrichments), values of % N derived from atmo
spheric N-2 fixation ( % Ndfa) calculated for any individual organ or
for the whole plant were similar. This was because the relative distri
bution of N-15 in the different parts of the fixing plant followed the
same trend as in the reference plant. There were significant differen
ces (p,= 0.01) in N-2 fixation between the Gliricidia genotypes. The v
alues ranged from 17.8 g M tree(-1) (equivalent to 86 kg N ha(-1) at 5
000 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 t
o differences in both % Ndfa and total N in plant, amount of N fixed w
as 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 h
a(-1)) in the 9 months within which N-2 fixation was measured. This re
presents a substantial contribution of N into the soil-plant system.