Populations of Rhizobium Meliloti in Areas with Rangeland Alfalfa

Abstract

The perpetuation of legumes in rangeland pasture plantings is dependent upon natural seeding and nodulation of the seedlings by naturalized soil rhizobia. This study determined the distribution and effectiveness of Rhizobium meliloti Dang. isolates in mature stands of rangeland alfalfa (Medicago sativa L.). Five sites in northern Utah and southern Idaho were studied. At each site, soil samples were taken from beneath the crown area of 10 plants and also at 50 and 100-cm intervals downslope and at right angles to the slope. Populations of R. meliloti ranged from 1.7 X 10^2 to more than 1.7× 105 per g of soil in all soil samples taken from within the crown area of established plants. The distribution of R. meliloti in soil adjacent to established plants differed with site and sampling position. R. meliloti were present in all the interplant spaces sampled on 1 site while the percentage of interplant samples with R. meliloti varied from 20% to almost 100% on the other 4 sites. Movement of R. meliloti downslope was detected only on 1 site. It was concluded that failure of R. meliloti to spread into the interplant spaces in rangeland alfalfa stands may limit natural reseeding.