Comparison of B values obtained by two methodologies that use soil as a substrate and their influence on the estimation of the proportion of nitrogen fixed by legumes
The B value is required to quantify the nitrogen derived from the atmosphere (%Ndfa) in the Rhizobium-legume symbiosis using the 15N natural abundance method. When the B value of a particular specie is not known, one possibility Is to use as a proxy the B value of a specie from the same genus, but this can cause the estimate of %Ndfa to be inaccurate. In this work, we compared two methodologies for determining the B value of Crotalaria juncea, C. spectabilis, C. ochroleuca and Cajanus cajan, using soil as the substrate. One method involvedgrowing plants in soil and averaging the lowest {delta}15N values of plant shoots (B-minimum), while the other consisted in adding sucrose to soil to immobilize the mineral nitrogen (N-immobilized), and then averaging the shoot {delta}15N values of all plants. Results showed that B values of C. cajan and C. ochroleuca obtained using the N-immobilized method were up to 1{per thousand} lower than those reported in the literature for these species. Therefore, we propose that, at least in these species, B values determined with the N-immobilized method should be used to estimate the%Ndfa.