Soybean can be a viable alternative crop for lowlands. However, soybean relies on biological nitrogen fixation, a process that is normally affected by soil flooding, depending on the macrosymbiont genotype. Moreover, the behavior of different bradyrhizobia strains in flooded soil is yet unknown. The present research aimed to evaluate the efficiency of Bradyrhizobium elkanii and Bradyrhizobium japonicum strains recommended for soybean in comparison to mineral nitrogen fertilization, in flooded soil. The experiment was carried out in a greenhouse, where soybean cultivar CEP 20 was sown in pots with soil. The treatments applied were either inoculation with one of the Bradyrhizobia strains: SEMIA 587, SEMIA 5019, SEMIA 5079 or SEMIA 5080, or the application of 100 mg kg-1 mineral nitrogen at two soil moisture levels - 100% of field capacity or flooded with a 1 cm-water layer on the soil surface. The soil was flooded 14 days after plant emergence and it was maintained under such conditions for 30 days. The nodule number and dry matter, and shoot dry matter were evaluated at 46 and 75 days after plant emergence. The amount of accumulated nitrogen in the soybean shoot was determined 75 days after plant emergence. Soil flooding reduced the nodule number and dry mass in both evaluation periods. There was no difference in the soybean shoot dry matter among the plants inoculated with distinct bradyrhizobia strains, which accumulated less dry matter than the treatment with mineral nitrogen. However, in the second evaluation period the treatment with mineral nitrogen accumulated less dry matter in the flooded soil as compared to the same treatment in soil at field capacity. The amount of nitrogen accumulated in the shoot was also lower in the flooded soil. Results show that the process of nitrogen fixation is affected by soil flooding, hobut, when waterlogging cease nodulation and biological nitrogen fixation is recovered. Plants fertilized with mineral nitrogen are also affected by soil flooding.
nodulation; biological nitrogen fixation; lowland
