Nitrogen fixation during vegetative and reproductive growth of irrigated soybeans in the field: application of δ 15N methods

The dry mass, total nitrogen and the natural abundance of "N were measured in stems plus leaves, pods and seeds during all stages of growth of a crop of irrigated Forrest soybeans. The sources of nitrogen accumulated in the seed and the magnitude of N2 fixation were calculated. During early growth, the proportion (PN2) of plant nitrogen arising from N2 fixation reflected residual effects of different rates of inoculation imposed two years previously and increased from 26-42% at 58 days to 49-57010 at 71 days. Growth and accumulation of nitrogen in stems plus leaves ceased at about 108 days; growth of and nitrogen accumulation by seeds continued from 100 days to 142 days. During the most rapid phase of seed growth (100-125 days), 7545% of seed nitrogen arose from N2 fixation. After net accumulation of plant nitrogen ceased (125 days), continued accumulation of seed nitrogen was derived mainly by relocation from pods and leaves but some Nz fixation continued. During ripening (142-167 days), there was loss of seed mass and nitrogen. The amounts of N2 fixed during seed development were substantial. After attainment of the maximum levels of nitrogen in the supporting shoots, N2 fixation totalling almost twice that achieved in the first 108 days of growth was directed exclusively to developing seeds during a period of only 34 days.