Abstract:
Biological nitrogen fixing capacity of Gliricidia sepium and Callianndra caiothyrsus was investigated under
field conditions in two contrasting locations in tlie Mid- country Intermediate and Mid-eountry Wet Zones
of Sri Lanka at Nillambe and Matale respectively using tine ISN isotope dilution method. Two non
nodulating tree legume species Senna siamea and Senna spectabilis were used as reference crops. Under
Matale conditions, all the species performed well. The higlaest above ground biomass and total N content
were found in S. spectabilis ( 5622g and 126.97gN plant'1). C. caiothyrsus produced a higher biomass and
contained more N than G. sepium. However, under Nillambe conditions, total biomass production was
extremely low in comparison to Matale and the highest biomass recorded, by C. calothrsus was only 197g
plant'1. Low biomass production under these conditions is attributed to the moisture stress and strong windy
conditions prevailed during the growth period. Under Matale conditions no significant-( P=0.01) difference
was found between the two reference crops and estimated average Pfix values for G. sepium and C.
calothysus were 56 and 42%, respectively. Total amount of N2-fixed by above ground parts of C. caiothyrsus
(33g N plant'1) was greater than G. sepium ( 26g plant'1). The values are equivalent to 110 and 88kg N ha'1
(3333 plants ha'1). Under Nillambe conditions Pfix values for both fixing species were higher than at Matale
but total N fixed were limited to the maximum of 6.0 and 9.2 kg N ha'1 yr'1 for G. sepium and C. caiothyrsus,
respectively. Performance of two reference species was also significantly (p=0.01) different at Nillambe.
Overall results suggest that the performance of N2 fixing trees varies with the agroclimatic conditions and
S. siamea as well as S. spectabilis appear to be equally suitable as reference plants for estimation of BNF
potential of G. sepium and C. caiothyrsus under the conditions of this study.
