Isotopic fractionation during N2 fixation by four tropical legumes

To quantify the contribution of biological nitrogen fixation (BNF) to legume crops using the (15) N natural abundance technique, it is necessary to determine the 15 N abundance of the N derived from BNF-the B value. In this study, we used a technique to determine B whereby both legume and non-N-2-fixing reference plants were grown under the same conditions in two similar soils, one artificially labelled with N-15, and the other not. The proportion of N derived from BNF (%Ndfa) was determined from the plants grown in the N-15-labelled soil and it was assumed that the %Ndfa values of the legumes grown in the two soils were the same, hence the B value of the legumes could be calculated. The legumes used were velvet bean (Mucuna pruriens), sunnhemp (Crotalaria juncea), groundnut (Arachis hypogaea) and soybean (Glycine max) inoculated, or not, with different strains of rhizobium. The values of %Ndfa were all over 89%, and all the legumes grown in unlabelled soil showed negative 815 N values even though the plant-available N in this soil was found to be approximately + 6.0parts per thousand. The B values for the shoot tissue (B-s) were calculated and ranged from approximately- 1.4parts per thousand for inoculated sunnhemp, and groundnut to - 2.4 and - 4.5parts per thousand for soybean inoculated with Bradyrhizobium japonicum strain CPAC 7 and Bradyrhizobium elkanii strain 29W, respectively. The B (B-wp) values for the whole plants including roots, nodules and the original seed N were still significantly different between the soybean plants inoculated with CPAC 7 (- 1.33parts per thousand) and 29W (- 2.25parts per thousand). In a parallel experiment conducted in monoxenic culture using the same soybean variety and Bradyrhizobium strains, the plants accumulated less N from BNF and the values were less negative, but still significantly different for soybean inoculated with the two different Bradyrhizobium strains. The results suggest that the technique utilized in this study to determine B with legume plants grown in soil in the open air, yields B values that are more appropriate for use under field conditions. (C) 2004 Elsevier Ltd. All rights reserved.