Abstract
The objectives of this work were to describe the distribution, density and seasonal variation of the indigenous populations of Rhizobium loti in different Uruguayan soils and to determine the symbiotic effectiveness and stress tolerance factors of different isolates, both with the aim of obtaining selected strains to re-introduce as inoculants in Lotus pastures. R. loti was present in ten soils studied and their densities varied from year to year and within each soil. All the isolates nodulated Lotus corniculatus effectively. The nodules in Lotus pedunculatus and Lotus subbiflorus were small, red on the surface and ineffective in nitrogen fixation. The study of 50 isolates from the ten soils showed high variability in their symbiotic efficiency and tolerance to pH. The indigenous population was acid tolerant in culture medium (pH 4.5), 83% of them could grow at pH 4.5 in 3 days. This work showed that there was a great diversity between the strains of R. loti isolated from Uruguayan soils and supports the importance of selecting among them the most efficient and resistant strains to be included in the inoculants.
Highlights
The activity of native Rhizobium population is of great importance, either when a new strain is intro-Montevideo, Uruguay. 3 Chemistry, Dr, Dep. de Ciências Biológicas, Facultad de duced from an inoculant or as a factor conditioning the persistence of the legume year by year
All the ten soils chosen to be representative of the Uruguayan soils (Table 1) contained R. loti
A higher R. loti population in relation to R. leguminosarum bv. trifolii was found in the analized soils
Summary
The activity of native Rhizobium population is of great importance, either when a new strain is intro-Montevideo, Uruguay. 3 Chemistry, Dr, Dep. de Ciências Biológicas, Facultad de duced from an inoculant or as a factor conditioning the persistence of the legume year by year. The activity of native Rhizobium population is of great importance, either when a new strain is intro-. Inability of the inoculant strains to successfully compete with established Rhizobium populations in soil has been frequently observed (Van Rensburg & Strijdom, 1985; Triplett & Sadowsky, 1992). As concluded by Roughley et al (1976) and Vance & Graham (1995), the success of an inoculant decreases with increase of native rhizobia population in soil. Lotus corniculatus was introduced in Uruguay many decades ago, especially for production on soils. In farming Lotus is inoculated with strain U-226. L. corniculatus and L. tenuis are nodulated by fast growing Rhizobium loti (Jarvis et al, 1982) and L. pedunculatus and L. subbiflorus form nodules with Bradyrhizobium sp. Specificity is not yet well defined in the genus Lotus
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