Contributions of fixed nitrogen and soil nitrate to the nitrogen economy of irrigated soybean

Effects of soil nitrate and numbers of Bradyrhizobium japonicum on the development and functioning of a soybean symbiosis and on crop production were studied in a field experiment at Breeza, New South Wales, Bragg soybean was grown with irrigation on soil, initially free of B. japonicum, with four rates of fertilizer-N (0, 100, 200, 300 kg N ha −1 as ammonium nitrate applied 6 weeks before sowing to provide four concentrations of soil nitrate) and four rates of inoculation [nil, normal (n). 100n, 1000n]. The inoculant strain was B. japonicum CB1809. Observations were made on nodulation, the relative abundance of ureidcs in xylem exudates as an index of N 2 fixation, dry matter and seed yield, and total nitrogen in shoots and seed. Results showed clearly that soil nitrate repressed nodulation, that the effect was magnified as soil nitrate concentrations increased, but that inhibition was substantially ameliorated by increased numbers of rhizobia. The relative abundance of ureides in xylem exudates responded similiarly. The highest yields of dry matter and of N in shoots and in seed occurred at the highest rates of inoculation (100n, 1000n) at intermediate and high soil nitrate (N100, N200, N300); at low soil nitrate (N0), yields were increased by inoculation per se but not by the rate used. Uninoculated plants did not nodulate and yields in these plots reflected concentrations of soil nitrate. Data suggested that soil nitrate and N 2 fixation were not always complementary in meeting the N requirements of the growing crop. Absence of rhizobia. except at the highest rate of nitrate, and repression of nodulation at the normal rate of inoculation by intermediate concentrations of nitrate resulted in reduced N yields because of insufficient N supply to the crop during the final stages of growth.