Distribution and Symbiotic Effectiveness of Rhizobium Meliloti in Rangeland Soils of the Intermountain West

Abstract

Although the interaction between the host plant and its associated Rhizobium is a major factor in determining biological nitrogen fixation, competitive interactions among naturally occurring rhizobial populations can also greatly influence the success of nodulation and subsequent nitrogen fixation. To determine the influence of naturally occurring rhizobial populations on the symbiotic relationship between alfalfa (Medicago sativa L.) and Rhizobium meliloti, the distribution of naturalized populations of R. meliloti forming effective nodules on alfalfa was determined in representative rangeland soils of the Intermountain West of the United States. A total of 256 sites were sampled within 10 vegetation types where alfalfa is a potential legume for farming, wildlife, or revegetation use. R. meliloti capable of forming effective nodules on alfalfa were detected in 4 of the vegetation types (juniper-pinyon woodland, mountain mahogany-oak scrub, sagebrush steppe, and wheatgrass-needlegrass shrubsteppe). However, these rhizobia were detected in only 30% or less of those sites having a natural vegetative cover. Only where the sagebrush steppe vegetation type had been converted to wheat or alfalfa-grass mixtures were rhizobia detected in the majority of sites (81 and 100%, respectively). Populations of R. meliloti within the individual vegetation types ranged from 6 to greater than 1.7× 105 per g of soil. Isolates of these naturally occurring R. meliloti exhibited a wide range of symbiotic N-fixation effectiveness on alfalfa (cv Spredor 2) with the mean value for the 360 isolates tested being 69% of the check inoculant strains. Of the isolates tested, 48% were classified as inferior N-fixers with significantly lower plant yields than control plants inoculated with check rhizobia strains. The relatively limited distribution of detectable populations of R. meliloti in the majority of Intermountain West rangeland soils and the high percentage of these that were inferior in N-fixing capability underscores the need to inoculate alfalfa with highly effective and competitive strains of R. meliloti.