Relationship between N(2)-Fixing Efficiency and Natural N Enrichment of Soybean Nodules

Non-nodular tissue of soybean (Glycine max L. Merrill) plants grown hydroponically in the absence of added N have a (15)N abundance close to that of atmospheric N(2). In contrast, nodules are usually enriched in (15)N. In this paper, we report measurements of the (15)N 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 (15)N enrichment of nodules was closely correlated with N(2)-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 (N)N enrichment of soybean nodules and N(2)-fixing efficiency is consistent with the hypothesis that new nodule tissue is synthesized from a pool of recently fixed N within the same nodule.