The abundance, efficacy, and diversity of Rhizobium leguminosarum bv. viciae populations in southern Manitoba soils

Inoculation of field pea is necessary in the absence of compatible rhizobial strains, when the rhizobial populations are low or symbiotically ineffective. The prevalence and efficacy of Rhizobium leguminosarum bv. viciae (the pea microsymbiont) in southern Manitoba soils is not known. Field experiments were conducted from 1998 to 2000 in multiple sites in southern Manitoba to characterize the abundance, efficacy, and diversity of indigenous R. leguminosarum bv. viciae. Uninoculated and inoculated (with and without N fertilizer) treatments of pea (Pisum sativum L.) were arranged in a randomized complete block design with four replicates in five sites (1999/2000). Pea was established in 21 sites in 1998 to trap indigenous rhizobia. Uninoculated and inoculated plants were evaluated for relative nodulation and growth. Pea and Lathyrus sp. isolates and commercial strains of R. leguminosarum bv. viciae were characterized by plasmid profile and polymerase chain reaction-restriction fragment length polymorphism of 16S-23S rDNA intergenic spacer analyses.;Uninoculated plants were well nodulated in cultivated soils, but not in virgin soils. Inoculation had no effect on nodulation in all sites but one. Inoculated plants accumulated significantly more biomass than uninoculated plants in 1998 and in four of five sites in 2000.;Out of 230 strains of R. leguminosarum bv. viciae typed, 77 distinct plasmid profiles were established. Plasmid profile diversity index (number of distinct profiles/number of isolates typed) varied from 0.2 to 0.83 in sites with 10 or more isolates. Few pea isolates from previously inoculated sites had profiles that matched those of the inoculants. Each site shared at least one profile with 3 to 18 other sites, but the number of profiles shared was independent of proximity.;Variation of the 16S-23S rDNA IGS regions was observed for all the rhizobial strains but two. Strains with the same plasmid profile did not always have similar chromosomal backgrounds and the converse was the case.;This research has shown that R. leguminosarum bv. viciae populations in southern Manitoba are abundant, diverse and competitive in nodulation, but may be less effective nitrogen fixers than commercial strains.