Establishment and expression of soybean symbiosis in a soil previously free of Rhizobium japonicum

Abstract

An irrigated crop of Chaffey soybeans [Glycine max (L.) Merrill] was grown at Leeton, N.S.W., on a soil that was initially free of Rhizobium japonicum, with six rates of seed bed inoculation, three plant spacing treatments (4.5, 9.0, 18 cm), and two soil pretreatments (cropped, fallow). Rhizosphere populations were larger, nodulation throughout growth was better, and increases in amount of N in the plants between 78 and 114 days were greater on previously cropped than on previously fallow land because there was more plant-available soil N in the fallow than in the previously cropped treatments. Especially in the previously cropped treatments, there were h~ghly significant correlations, In sequence, between rate of inoculation, number of R. japonicum in the rhizosphere, extent of early nodulation, and progressive increase in nodulation. Lower, but still significant, correlations were found between extent of nodulation, shoot N, seed yield, and seed N. Plants compensated for wider spacing by increased growth. However, plants grown 18 cm apart were unable to compensate fully and yielded less seed and less seed N per hectare than plants grown at 4.5 and 9.0 cm. Plant spacing had little effect on the sequential development of the symbiosis. Rate of inoculation and seed yield were highly correlated and the correlation coefficients were higher in previously cropped treatments than treatments on previously fallow land. The amount of seed N in the treatments on previously cropped land with the highest rate of inoculation was 207 kg N ha-1, whereas shoot N in that treatment 47 days before final harvest was 355 kg N ha-1. This substantial quantity of N taken up by the well-nodulated soybeans in response to high levels of inoculation was greatly in excess of the requirements for seed production and might contribute to the nitrogen nutrition of subsequent crops.