Influence of elevation and applied nitrogen on rhizosphere colonization and competition for nodule occupancy by different rhizobial strains on field-grown soybean and common bean

Abstract

In the absence of indigenous rhizobial populations, the pattern of competition between inoculum strains for nodule occupancy is found to be a stable characteristic, independent of rhizosphere population size, nitrogen application, or elevation. Soybean (Glycine max (L.) Merrill) and common bean (Phaseolus vulgaris L.), inoculated with peat-based rhizobia, were grown with three nitrogen levels at 320- and 150-m sites along an elevational transect on the island of Maui, Hawaii. Rhizosphere soil of 8-day-old plants was examined by immunofluorescence for populations of three strains each of Bradyrhizobium japonicum and Rhizobium leguminosarum bv. phaseoli, which made up the inocula applied to the respective host legumes at planting. Nodules were examined for occupancy by specific strains at two sampling times. Site differences and nitrogen treatment had no significant effect on rhizosphere colonization or nodule occupancy by the three strains. The three inoculum strains colonized their respective host rhizosphere in approximately equal numbers. In soybean, strain TAL 102(USDA 110) occupied most of the nodules, while strain TAL 379 (USDA 1366) was the least competitive in nodule formation in all treatments. In common bean, TAL 182 outcompeted the other two strains in nodule formation, while TAL 1797 (CIAT 899) occupied the least number of nodules. High elevation decreased nodulation by soybean more than that by common bean. Nitrogen application reduced nodule number and mass of both legumes more at the low-elevation than the high-elevation site; the reductions in nodule mass were more pronounced in common bean. The pattern of nodule occupancy remained stable regardless of the differences in nodule number and mass between elevations, N levels, and sampling times. Key words: rhizobial ecology, rhizosphere colonization, interstrain competition, immunofluorescence, legumes.