Population ecology of soybean-rhizobia in diverse crop rotations in Central India

Ensuring adequate populations of effective rhizobia in soil is essential to guarantee optimum nodulation and nitrogen fixation in legumes. The population of soybean-rhizobia and the proportion of slow and fast growers were enumerated in Vertisols of central India in different crop rotations for three years during 2013-15. Soils were from soybean-based (soybean-wheat and soybean-chickpea) and cereal-based rotations (maize-wheat, rice-wheat and maize-chickpea) rotations. Soybean-based rotations stimulated the rhizobial population by 22-fold compared to cereal rotations. Lowest populations were in the summer but increased in soybean rotations by 13.3-fold after the rainy season to 2743 cells g−1 soil at maximum vegetative growth. In the cereal rotations, the increase was by 1.7-fold to 61 cells g−1 soil. Long-term integrated nutrient management, by annual application of farmyard manure in soybean along with chemical fertilizers, improved the rhizobial numbers by 1.5-fold over chemical fertilization alone and by 2.9-fold over unfertilized treatments. Increased crop growth by chemical fertilizers also stimulated rhizobial populations by 1.9-fold over unfertilized soybean. The proportion of slow-growing soybean rhizobia was lower in soybean-based rotations (38%) compared to cereal-based rotations (62%), showing that continued soybean growth led to a greater proliferation of fast-growing rhizobia. Application of farmyard manure did not change the proportion of slow growers. The proportion of slow growers was ∼15% more in winter compared with the summer populations. The slow growers were symbiotically superior, producing greater dry matter and nodule mass (+12%), with more biologically fixed nitrogen (+18%) than the fast growers. Results highlight the importance of crop rotation with cereals, application of organic amendments along with chemical fertilizers and selection of slow-growing rhizobia as inoculants.