Slow and fast-growing soybean rhizobial population, their symbiotic efficiency and soil nitrogen behavior under different cropping systems in Vertisols of Madhya Pradesh, India

An attempt was made in this study to assess the indigenous composition (slow and fast-growers) of soybean-rhizobia, their symbiotic efficiency and nitrogen fixation in Vertisols of Madhya Pradesh under different cropping systems. The soils were collected from soybean-based (soybean-wheat and soybean-chickpea) and cereal-based (maize-wheat, rice-wheat and maize-chickpea) systems during 2013-15 from two agro climatic zones of the state. One of the sampling sites was under Long Term Fertilizer Experiment with soybean-wheat system where three treatments - absolute control, recommended dose of fertilizers (RDF) and RDF+FYM were considered. Ratio of fast-growing soybean rhizobia was more in those cropping systems where soybean was one of the crops while population of slow-growers was more with cereal-cereal or maize-chickpea cropping systems. Nodulation and N symbiotic efficiency (N content and uptake) was better with slow-growing rhizobia as compared to fast-growing. Maximum SOC was found at maximum vegetative growth stage in soybean-wheat rotation (5.7 g kg-1 soil) under RDF+FYM and minimum (3.2 g kg-1 soil) in absolute control. Available soil N was more in kharif season with soybean. Legume based systems and rabi season reflected better conversion of NH4+ to NO3--N.