Number and effectiveness of native strains of Rhizobium meliloti isolates of soils from the High Valley of Río Negro and Neuquén (Argentina)

Perla Gili S.1, Graciela Marando I. y Marcelo Sagardoy R.

The numbers of naturalized R. meliloti in 86 cultivated soils from the zone of the High Valley of Río Negro and Neuquén were determined. The R. meliloti densities varied between <1 and 2 x 106 bacteria/g of dry soil. Sixty-four per cent of the soil samples contained more than 1000 R. melilotil g. No significant correlation was found between the numbers and eitherthe pH orthe organic matter content of the soils. Symbiotic nitrogen fixation with 83 isolates of R. meliloti was studied under controlled environment with alfalfa cv. Bordenave INTA; plant yield (dry weight) was used as an indirect measurement of nitrogen fixation, and as a criterion for selecting efficient strains of R. meliloti. Eight very stable and efficient strains were selected after 30 months. The effects of three strains of R. meliloti in match inoculants for alfalfa was studied in relationship to its yield. Fifty-six combinations of three strains were used as inoculants. Twelve combinations of those biofertilizers resulted in the highest yields, indicating synergistic effects.

Keywords: Symbiotic nitrogen fixation, inoculants, Medicago sativa, Rhizobium meliloti.
1 Facultad de Ciencias Agrarias, Universidad Nacional del Comahue (UNC), CC 8303, Cinco Saltos, Río Negro, Argentina.