Rhizobial ecology as affected by the soil environment

In this paper we review the influence of various soil factors on the legume–Rhizobium symbiotic relationship. Abiotic factors such as extremes in soil pH (highly acidic or alkaline soils), salinity, tillage, high soil temperature and chemical residues, all of which can occur in crop and pasture systems in southern Australia, generally reduce populations of Rhizobium in the soil. Naturally occurring Rhizobium populations, although often found in high numbers, are generally poor in their ability to fix nitrogen and can compete strongly with introduced Rhizobium inoculant. The introduction of new legume genera as a continuing and essential part of change in farming systems usually requires the need to identify new and specific inoculant Rhizobium strains not found in the soil, but necessary for optimum nitrogen fixation. It is therefore necessary to characterise the specific requirements or limitations in the soil for establishing Rhizobium populations to ensure optimal nitrogen fixation following inoculation of legumes. The ability of the introduced Rhizobium to form effective nodules is rarely linked to a single soil attribute; therefore the study of rhizobial ecology requires an understanding of many soil and environmental factors. This paper reviews current knowledge of the influence of soil factors on rhizobial survival, the nodulation process, and nitrogen fixation by legumes.