Influence of soil and crop factors on the native rhizobial populations in soils under dryland farming

Abstract

Native rhizobial populations ( Bradyrhizobium sp.) in 88 soil samples from 13 important legume growing locations in India are estimated and regressed with mean annual rainfall, soil pH, organic carbon, clay content and crop history. These locations are under dryland farming with Alfisols and Vertisols as predominant soil orders and divergent cropping histories. Wide variability was found for the abundance of native Bradyrhizobium populations in samples from within a location and among the locations. The populations showed significant relationship with organic carbon and previous crop history but clay content, mean annual rainfall and soil pH had no influence. Variation in the solum depth did not have significant influence on the distribution of Bradyrhizobium sp. in the profile (0–25 cm) of a newly cleared Alfisol watershed site which was under long term fallow. The population, however, increased significantly following crop cultivation. The increase was more with pigeonpea and sorghum than with sunflower. Data from 28 inoculation trials involving three legumes were used to assess the relationship between population of native Bradyrhizobium at planting, previous crop history and the response of the legumes to inoculation. When castor bean and sunflower are cultivated in the previous year, there was a significant decline in the population of Bradyrhizobium sp. and a positive response to inoculation of legumes in the subsequent year. When pigeonpea, groundnut and sorghum were grown as preceding crops, the population remained high (> log 10 3.0 MPN/g soil) at planting and very few trials showed positive response. While stimulatory effect of legume cultivation on rhizobial population is well documented, present studies provide evidence to the suppression of the population by some non legume crops in the cropping system. The overall results revealed that crop related factors have more critical influence on the abundance of native rhizobial populations than soil or climatic factors.