COMPETITION BETWEEN INOCULATED AND INDIGENOUS RHIZOBIUM BRADYRHIZOBIUM SPP STRAINS FOR NODULATION OF GRAIN AND FODDER LEGUMES IN PAKISTAN

The competitive ability of inoculated and indigenous Rhizobium/Bradyrhizobium spp. to nodulate and fix N2 in grain legumes (Glycine max, Vigna unguiculata, Phaseolus vulgaris) and fodder legumes (Vicia sativa, Medicago sativa, and Trifolium subterraneum) was studied in pots with two local soils collected from two different fields on the basis of cropping history. The native population was estimated by a most-probable-number plant infectivity test in growth pouches and culture tubes. The indigenous rhizobial/bradyrhizobial population ranged from 3 to 2 x 10(4) and 0 to 4.4 x 1O(3) cells g-1 in the two soils (the first with, the second without a history of legume cropping). Inoculated G. max, P vulgaris, and T. subterraneum plants had significantly more nodules with a greater nodule mass than uninoculated plants, but N2 fixation was increased only in G. max and P. vulgaris. A significant response to inoculation was observed in the grain legume P. vulgaris in the soil not previously used to grow legumes, even in the presence of higher indigenous population (> 10(3) cells g-1 soil of Rhizobium leguminosarum bv phaseoli). No difference in yield was observed with the fodder legumes in response to inoculation, even with the indigenous Rhizobium sp. as low as < 14 cells g-1 soil and although the number and weight of nodules were significantly increased by the inoculation in T. subterraneum. Overall recovery of the inoculated strains was 38-100%, as determined by a fluorescent antibody technique. In general, the inoculation increased N2 fixation only in 3 out of 12 legume species-soil combinations in the presence of an indigenous population of rhizobial/bradyrhizobial strains.