Nitrogen accumulation and changes in natural ¹⁵N abundance in the tissues of legumes with emphasis on N₂ fixation by stem-nodulating plants in upland and paddy fields

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.

Key Words:

• legumes
• N₂ fixation
• natural ¹⁵N abundance
• root nodule
• stem nodule