Microbial activity of soil during the inoculation of soya bean with symbiotic and free-living nitrogen-fixing bacteria

Soya bean has a great importance in the organic production due to its ability to coexist with nitrogen-fixing bacteria of the genus Bradyrhizobium. The soya bean seed inoculated with the symbiotic nitrogen-fixing bacteria Bradyrhizobium japonicum and with a mixture of nitrogen-fixing bacteria was observed in this study. The following highly efficient strains of free nitrogen-fixing bacteria were in the liquid inoculum:Azotobacter chroococcum, Azospirillum lipoferum, Beijerinckia Derx and Klebsiella planticola. Beside bacterization, different rates of pure nitrogen per hectare were applied (0, 40, 60, 80 kg N/ha). The effect of nitrogen-fixing bacteria at different levels of mineral nitrogen on the abundance of those groups of soil microorganisms and on the amounts of (NO3+NH4)- N/ha at the end of the growing season was determined. Three-year bacterization resulted in the significant increase of the observed parameters. The highest values when inoculation was done with mixed nitrogen-fixing bacteria were obtained with the application of 40 kg N/ha. The application of both types of nitrogen-fixing bacteria in the soya bean production is a very beneficial activity from ecological and economical aspects. Therefore, this measure should be accepted, especially in relation to soils contaminated with excess use of mineral nitrogen fertilisers. Key words: Fertilising, nitrate and ammonium nitrogen, nitrogen-fixing bacteria, soya bean.