Nitrogen accumulation and changes in natural ¹⁵N abundance (δ¹⁵N) in five pea cultivars and two faba bean cultivars were investigated in an alluvial soil in the field during winter and spring. The amounts of nitrogen accumulated in tops were 16 to 20 g m^-2 in pea plants, and 17 and 24 g m^-2 in faba bean at final harvest. About 67~97% of the nitrogen in pea plants and 79 and 86% in faba bean were derived from atmospheric N₂ based on the difference of δ¹⁵N values between N₂-fixing legumes and the reference plant (barley was used).
The nodules in most of the pea, faba bean, and vetch plants were enriched in ¹⁵N (up to +8‰). The ¹⁵N enrichment was more obvious in mature and senescent nodules, and the soluble N fraction of the nodule cytosol was most highly enriched in ¹⁵N. The δ¹⁵N values of ammonia extracted from the nodule bacteroids were higher than those of the organic N in this organelle and of ammonia in the nodule cytosol. The mechanisms of ¹⁵N enrichment in nodules were discussed.