Establishment and Expression of Soybean Symbiosis in a Soil Previously Free of Rhizobium japonicum

摘要

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