Low survival of Rhizobium japonicum inoculant leads to reduced nodulation, nitrogen fixation and yield of soybean in the current crop but not in the subsequent crop

Abstract

Reports of nodulation failures in first-year crops of soybean, resulting in nitrogen deficiency and low yield, have been common in Australia. In experiments simulating commercial practice, successive crops (1982-83, 1983-84) of soybean were sown into soil containing either a very small population of Rhizobium japonicum (rhizobia-low; 2 g-1 soil, 3-9 cm), or a large population (rhizobia-high; 270 000 g-1). The soil was a vertisol at Breeza, N.S.W. Each crop was inoculated with R. japonicum at a normal rate (n = 25 x 1010 organisms per 100 kg of seed) or at 100 x normal (100n) or was not inoculated (o). The 1982-83 crop was sown in hot (maximum air temperature 3g�C), dry weather, and < 1% of the inoculant survived the first day. This did not affect nodulation in rhizobia-high plots and production of foliage and seed nitrogen was satisfactory, irrespective of rate of inoculation. In contrast, nodulation was inadequate in all rhizobia-low soil (although 100n > n > o) and the N content of foliage and seed was correspondingly reduced. In spite of poor nodulation in these rhizobia-low treatments, a substantial population of R. japonicum (18 000 g-1soil) developed between seasons. When the plots were re-sown in 1983-84, nodulation, growth and yield of the crops, irrespective of the initial rhizobial status of the soil, were equally good, and there was no response to further inoculation. Poor nodulation of first-year crops of soybean is a likely consequence of failure to establish adequate numbers of R. japonicum in the soil at sowing.