Impact of nitrogen fertilization on nodulation and symbiotic efficiency of indigenous Bradyrhizobium japonicum strains

Nitrogen fixation is a beneficial microbial process that greatly contribute to sustainable agricultural production and environment protection. Soil bacteria, collectively named rhizobia, are characterized by their unique ability to induce the formation of root nodules in which they convert molecular nitrogen into a usable form for plants. Soybean plants require a large amount of nitrogen for their development and achievement of high seed yields. The use of high quality rhizobial inoculants strives to optimize nitrogen nutrition of soybean with minimal environmental impacts. Selection of the most suitable Bradyrhizobium japonicum strains is of great importance for successful soybean inoculation as well as optimal nitrogen fertilization. The aim of the present study was to determine the impact of fertilization with different rates of mineral nitrogen on nodulation and symbiotic efficiency of indigenous B. japonicum strains. In the vegetation pot experiment two indigenous and one reference B. japonicum strain as well as different rates of mineral nitrogen were tested. Application of increased rates of mineral nitrogen reduced number of nodules and nodule dry weight. The highest nitrogen content was determined in plants grown without any mineral nitrogen fertilization but inoculated with indigenous B. japonicum strains.