INFLUENCE OF SOIL AND CROP FACTORS ON THE NATIVE RHIZOBIAL POPULATIONS IN SOILS UNDER DRYLAND FARMING

Native rhizobial populations (Bradyrhizobium sp.) in 88 soil samples f rom 13 important legume growing locations in India are estimated and r egressed with mean annual rainfall, soil pH, organic carbon, clay cont ent and crop history. These locations are under dryland farming with A lfisols and Vertisols as predominant soil orders and divergent croppin g histories. Wide variability was found for the abundance of native Br adyrhizobium populations in samples from within a location and among t he locations. The populations showed significant relationship with org anic carbon and previous crop history but clay content, mean annual ra infall and soil pH had no influence. Variation in the solum depth did not have significant influence on the distribution of Bradyrhizobium s p. in the profile (0-25 cm) of a newly cleared Alfisol watershed site which was under long term fallow. The population, however, increased s ignificantly following crop cultivation. The increase was more with pi geonpea and sorghum than with sunflower. Data from 28 inoculation tria ls involving three legumes were used to assess the relationship betwee n population of native Bradyrhizobium at planting, previous crop histo ry and the response of the legumes to inoculation. When castor bean an d sunflower are cultivated in the previous year, there was a significa nt decline in the population of Bradyrhizobium sp. and a positive resp onse to inoculation of legumes in the subsequent year. When pigeonpea, groundnut and sorghum were grown as preceding crops, the population r emained high (> log(10) 3.0 MPN/g soil) at planting and very few trial s showed positive response. While stimulatory effect of legume cultiva tion on rhizobial population is well documented, present studies provi de evidence to the suppression of the population by some non legume cr ops in the cropping system. The overall results revealed that crop rel ated factors have more critical influence on the abundance of native r hizobial populations than soil or climatic factors. (C) 1997 Elsevier Science B.V.