Relationship between N2 fixing efficiency and natural 15N enrichment of soybean nodules

Non-nodular tissue of soybean (Glycine max L. Merrill) plants grown hydroponically in the absence of added N have a 15N abundance close to that of atmospheric N2. In contrast, nodules are usually enriched in 15N. In this paper, we report measurements of the 15N abundance of foliar tissue and nodules of soybeans inoculated with 11 variably efficient strains of Rhizobum japonicum and grown hydroponically with no added N. The efficiency of the 11 symbioses varied over a wide range as judged by a 16-fold difference in N content. The degree of 15N enrichment of nodules was closely correlated with N2-fixing efficiency (milligrams N fixed per milligram N in the nodules). These results confirm prior preliminary data based on six variably efficient R. japonicum strains. The strong correlation between 15N enrichment of soybean nodules and N2-fixing efficiency is consistent with the hypothesis that new nodule tissue is synthesized from a pool of recently fixed N within the same nodule.