Diversity of symbiotic rhizobia resident in Canadian soils

Abstract

The dependency of agriculture on nitrogen fertilizer inputs is associated with adverse effects on the environment and human health. The importance of biological nitrogen fixation by rhizobia in symbiotic association with legumes is underscored by its potential to reduce or replace chemical fertilizer inputs. This paper reviews research on the diversity of the symbiotic rhizobia resident in Canadian soils. Research has focussed on phenotypic and genotypic variation (e.g., nitrogen fixing efficacy, nodulating competitiveness, host range, adaptation to cool climate) within rhizobial species with the objective of selecting efficient strains for use in inoculants for legume crops. The genetic diversity of rhizobia resident in Canadian soils has been reported only for Sinorhizobium meliloti, Rhizobium leguminosarum and Mesorhizobium spp. There is a need for further studies on populations of other rhizobial species, particularly those associated with native legumes. Exploiting the diversity present in natural soil populations via selection and genetic manipulation should permit the development of superior strains for use in legume inoculants. Other rhizobial traits that may be exploited include plant growth-promoting activity and ability to degrade pollutants.