Nitrogen accumulation in three legumes and two cereals with emphasis on estimation of N2 fixation in the legumes by the natural15 N-abundance techniqueTYoneyamaauthorP T CNambiarauthorK KLeeauthorBSrinivasa RaoauthorJ HWilliamsauthorN accumulation and natural "N ahundance in three legumes (groundnuts, cowpeas, and soybeans) and in two cereals (sorghum and maize) were investigated over two seasons in Alfisols with and without N fertilization. Using the N uptake and natural "N abundance of non-nodulating plants as the indication of N derived from soil and fertilizer, the per cent N derived from atmospheric N2 was calculated for nodulated plants. In the first experiment, the groundnut genotype contained 85% atmospherc-derived N, but the percentage decreased with N application, Estimates of atmosphere-derived N by the N-difference and 15~-abundanctee chniques gave identical results. The percentages of atmosphere-derived N estimated by the two methods at different stages of groundnut growth were also similar. In the second experiment, atmosphere-derived N was estimated in plants grown with 0-200 kg ha-' applied N. The estimated atmosphere-derived N ranged from 42% to 61 % for groundnuts from 33% to 77% for cowpeas, and from 24% to 48% for soybeans, depending on the amount of N applied. Inoculation with a Bradyrhizobium strain increased the percentage of atmospherederived N in soybean plants grown without any fertilizer N. The natural "N abundance of sorghum and maize was very close to that of the non-nodulating groundnut, suggesting that these cereals can be used as reference plants in the estimation of atmosphere-derived N by the natural "N-abundance method.GroundnutSorghumFood Legumes1990Springer VerlagArticle