Population dynamics of indigenous and genetically modified rhizobia in the field

Abstract

Many factors have been shown to affect rhizobial populations in soil. These include: soil fertility; physical properties such as pH and clay content; biotic factors such as distribution of the host plant and the prevalence of predators; and climatic effects including temperature and rainfall. Extremes of soil pH, temperature and moisture are not favourable to rhizobia. In certain circumstances, some rhizobial species seem to survive as part of the saprophytic soil microflora, whereas others cannot be detected unless their leguminous plant host is present. There have been numerous reports over the past century on factors which influence rhizobial survival, reflecting the importance of rhizobial inoculants in agriculture. Many of these results appear to be contradictory, presumably because of the complexity of the interactions between different influences, which this review of the literature attempts to clarify. Results from monitoring population dynamics of different rhizobial species and biovars under various crops at Rothamsted demonstrate that populations of Rhizobium leguminosarum biovars survive in the absence of their host plants at c. 104-105 nodulating cells g-1 soil, and increase only about threefold following cultivation of the host. In contrast, Sinorhizobium meliloti could not be detected before its host was grown, but increased to 106 nodulating cells g-1 soil when lucerne was cultivated. The behaviour of a genetically modified R. leguminosarum bv. viciae strain, RSM2004, following field release as an inoculant, was similar to that of the indigenous population, numbers remaining stable in the absence of the host following an initial decline, with a small but significant numerical advantage being conferred by the presence of the host in the subsequent years. A second genetically modified R. leguminosarum bv. viciae inoculant, CT0370, was found to survive in numbers similar to those of the indigenous population.