Evaluation of nitrous oxide emission by soybean inoculated with Bradyrhizobium strains commonly used as inoculants in South America

Abstract

Aims: The purpose of this work was to analyze the agronomic and environmental performance of soybean plants inoculated with the Bradyrhizobium strains widely used as soybean biofertilizers in South America and to determine if these strains possess any functional or taxonomic trait associated with the N2O emission. Methods: Bradyrhizobium japonicum E109 and CPAC 15, B. diazoefficiens USDA 110 and CPAC 7, and B. elkanii SEMIA 5019 and SEMIA 587 were used to inoculate soybean seeds. The field experiment was carried out in a soil without history of soybean cultivation in the Argentinian Humid Pampa. The natural 15N abundance method was applied to estimate N2-fixation, and N2O production was evaluated using gas chromatography. Among other physiological parameters, shoot dry weight, shoot N content, and crop yield were estimated after harvest. Results: B. japonicum inoculation produced the greatest increases in soybean growth and crop yield but also led to higher N2O emissions compared to all other inoculated treatments. Plants inoculated with B. diazoefficiens released the lowest amount of N2O, and their growth and yield were the least affected. Inoculation with B. elkanii resulted in intermediate N2O emission fluxes and crop yield compared with B. japonicum and B. diazoefficiens. Conclusions: We found that soybean inoculation with strains of B. japonicum and B. elkanii that lack the nosZ gene led to the highest N2O emissions under field conditions, but also to the highest crop yield, while inoculation with strains that carry out complete denitrification, nosZ-containing B. diazoefficiens, showed lower N2O emission and lower crop yield.