Survival of soybean rhizobia in summer and proliferation during monsoon in vertisols of Central India
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High temperatures and desiccation of surface soil layers in semi-arid and arid tropical regions lead to severe decline in soil microbial populations and loss of microbial diversity. Low numbers of rhizobia in soil leads to poor nodulation and nitrogen fixation in legumes. The present study examined the impact of high temperatures of summer on the decline of microbial population in general and soybean rhizobia in particular, and the subsequent revival during monsoon season, at two Vertisol (Typic Eustochrept) sites in Central India, viz., Bhopal in M.P. and Durg in Chhattisgarh. Soil samples from 0 to 5, 5 to 15 and/or 15 to 30 cm depth from cultivated and fallow sites were sampled during peak summer, 2 and 45 or 95 days after onset of monsoon. Plate counts of bacteria, actinomycetes, and fungi revealed lower numbers in peak summer and also a general revival of microbes at both sites during the monsoon. The changes in microbial populations mirrored similar trends in soil biological (dehydrogenase) activity. The activity was lower at site-II due to lower soil organic matter content. Rhizobial numbers at both sites, determined by most probable number (MPN) method using plant infection assay with soybean as host, were lower than 100 cells per gram of soil in summer and while there was a small spurt after monsoon, they remained below the threshold (103g−1 soil) required for optimum nodulation. At site I, where the soils were continuously cultivated with soybean, the rhizobial population increased by 10 to 25 folds during monsoon whereas at site II under cultivation with a heterologous host (mungbean) it increased only by 3 to 8 fold. Decline of population of native rhizobia due to high temperature and low moisture and low populations even after monsoon, reinforced the need to regularly practice rhizobial inoculation of soybean each year to build up populations and to harness the optimum benefits of biological nitrogen fixation.
Biofertilizer, biological nitrogen fixation, desiccation, dehydrogenase, microorganisms, nodulation, rhizobia, soil temperature, soybean.