Improved soybean production after inoculation with Bradyrhizobium japonicum

摘要

Inoculation of legumes with Rhizobium or Bradyrhizobium spp. is a common agricultural practice used to increase the yields of legumes without adding nitrogen (N) fertilizers. Rhizobial populations in the soil exist as saprophytes and are able to use a diverse range of energy sources including almost all organic compounds. The presence of the legume host plant stimulates the Rhizobium populations in the rhizosphere, where the inoculants have to compete for nutrients with the existent microbial community, including the native Rhizobium populations (McDermott et al., 1991). Therefore, it is not surprising that population densities have been positively correlated not only with the presence of legumes and vigour of legume growth but also with other biotic factors such as soil organic matter. Abiotic factors such as clay content, soil pH, base saturation, mineral N status, mean annual rainfall and irradiance also affect population densities (Bezdicek, 1972; Hitbolt et al., 1985; Hurse & Date, 1992; Bushby, 1993). Other studies, however, have not shown any simple relationship between population density and specific environmental parameters (George et al., 1987; Bottomley, 1992). In order to investigate whether naturalized soil strains of Bradyrhizobium japonicum in Argentine soils can compete successfully for nodulation and make a better contribution to the crops than inoculated strains, a field experiment was set up using strains of B. japonicum of different origins.