USE OF DIFFERENT N-15 LABELING TECHNIQUES TO QUANTIFY THE CONTRIBUTION OF BIOLOGICAL N-2 FIXATION TO LEGUMES

The reliable quantification of the contribution of biological N-2 fixa tion (BNF) is an important priority in studies of the N cycle in low-e xternal input farming systems of the tropics where plant-associated BN F if often the largest N input. We have investigated the effect of usi ng more than one control crop, together with different techniques for labelling the soil with N-15, for the application of the isotope dilut ion technique to quantify the BNF contribution to a forage legume (Cen trosema) and French beans (Phaseolus vulgaris). When soil was amended with N-15-labelled organic matter, the N-15 enrichment of the soil min eral N decreased slowly with time but when frequent multiple additions of N-15-labelled (NH4)(2)SO4 were used the N-15 enrichment of this N increased with lime. Despite this different behaviour of soil N-15 enr ichment there were no significant differences between N-15 treatments in the estimates of the plant N derived from BNF (%Ndfa) of the Centro sema calculated using either of the two non-N-2-fixing control crops. This was attributed to the very high %Ndfa of this legume (>84%), a si tuation where differences in N uptake patterns between control crops a re of little importance. In contrast, in the second experiment, the es timates of %Ndfa of P. vulgaris varied from 33 to 62% depending on whi ch N-15 addition treatment or control crop was employed. The two estim ates derived from the use of non-nodulating bean as control in the dif ferent N-15 treatments gave a good agreement, which indicated that thi s crop had a similar soil-N uptake pattern to the nodulated bean crop. The use of several control crops combined with two techniques of N-15 addition which produce contrasting temporal changes in the N-15 enric hment of the soil N, is a powerful technique for the reliable quantifi cation of BNF contributions to plant nutrition, since it permits the i dentification of the most suitable control crop from amongst those uti lized.