Effect of soybean inoculation with<i>Bradyrhizobium</i>and wheat inoculation with<i>Azotobacter</i>on their productivity and N turnover in a Vertisol

Abstract

Abstract A long-term field experiment was conducted for 8 years on a Vertisol in central India to assess quantitatively the direct and residual N effects of soybean inoculation with Bradyrhizobium and wheat inoculation with Azotobacter in a soybean–wheat rotation. After cultivation of soybean each year, its aerial residues were removed before growing wheat in the same plots using four N levels (120, 90, 60 and 30 kg ha−1) and Azotobacter inoculation. Inoculation of soybean increased grain yield by 10.1% (180 kg ha−1), but the increase in wheat yields with inoculation was only marginal (5.6%; 278 kg ha−1). There was always a positive balance of soil N after soybean harvest; an average of +28 kg N ha−1 yr−1 in control (nodulated by native rhizobia) plots compared with +41 kg N ha−1 yr−1 in Rhizobium-inoculated plots. Residual and direct effects of Rhizobium and Azotobacter inoculants caused a fertilizer N credit of 30 kg ha−1 in wheat. Application of fertilizers or microbial inoculation favoured the proliferation of rhizobia in crop rhizosphere due to better plant growth. Additional N uptake by inoculation was 14.9 kg N ha−1 by soybean and 20.9 kg N ha−1 by wheat crop, and a gain of +38.0 kg N ha−1 yr−1 to the 0–15 cm soil layer was measured after harvest of wheat. So, total N contribution to crops and soil due to the inoculants was 73.8 kg N ha−1 yr−1 after one soybean–wheat rotation. There was a total N benefit of 13.8 kg N ha−1 yr−1 to the soil due to regular long-term use of microbial inoculants in soybean–wheat rotation. Keywords: biofertilizerbiological nitrogen fixation (BNF) Bradyrhizobium Azotobacter soil carbon Acknowledgements We are grateful to the Indian Council of Agricultural Research for financial support through the All India Network Project on Soil Biodiversity-Biofertilizers, Indian Institute of Soil Science, Bhopal, India.