Nitrogen accumulation and changes in natural15N abundance in the tissues of legumes with emphasis on N2fixation by stem-nodulating plants in upland and paddy fields

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Abstract
Several legumes were grown during the summer season, with particular interest in the growth and N2 fixation by two stem-nodulating legumes (Sesbania rostrata and Aeschynomene indica) under upland and paddy conditions. The variations of natural 15N abundances in plant parts were investigated, and from the differences in the values in N2-fixing legumes and non-fixing plants (non-nodulating soybeans and paddy weeds), the % of fixed N in N2 -fixing plants was calculated. In upland fields, soybean, mungbean, crotalaria, and two stem-nodulating plants gained by N2 fixation as much as 46-100% N in total N or 8-43 g N.m-2 after 116-137 days of growth. In paddy fields, A. indica and S. rostrata gained by N2 fixation as much as 93 and 78% N, or 12.7 and 28.1 g N-m-2, respectively. In IRRI paddy fields (Philippines), a low natural abundance of 15N was also detected in S. rostrata as compared to rice plants, suggesting the large contribution of N2 fixation. In both stem-nodulating legumes, the stem and root nodules were enriched in 15N and bacteroids were the site of 15N enrichment as in the case of the root nodules of Crotalaria juncea and Sesbania sesban.