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

Abstract

AbstractThe 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 δ15N 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 δ15N 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‰ 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.