<i>Stylosanthes</i> cv. Campo Grande response to <i>Bradyrhizobium</i> inoculation in an Amazon Cerrado Oxisol

Abstract Species of the genus Stylosanthes show great potential for use in mixed pastures and as green manure due to their symbiotic potential in symbiosis with diazotrophic bacteria, particularly Bradyrhizobium . These herbaceous legumes can improve pasture quality and contribute to sustainability. This study evaluated performance of Stylosanthes cv. Campo Grande when inoculated with Bradyrhizobium strains isolated from Oxisols in Roraima, Brazil, versus standard strains. A greenhouse experiment tested 18 new Bradyrhizobium strains isolated from soil alongside two recommended strains, including uninoculated and nitrogen‐fertilized controls. Also, two field experiments were conducted using the best‐performing greenhouse strain, the recommended strains, and absolute control and nitrogen fertilization (30 kg ha − 1 of N). Variables measured included nodule number and dry nodule mass, root and shoot biomass, and total nitrogen in shoot biomass. The proportion of nitrogen derived from biological nitrogen fixation (percentage of N derived from the atmosphere) was estimated by the 1 5 N natural abundance method, with Urochloa brizantha as the reference plant. In the greenhouse, strains ERR 917, ERR 922, ERR 942, ERR 1110, and ERR 1173 present great nodulation and dry matter production, identifying ERR 917 as optimal for field testing. Inoculation significantly increased both shoot and root biomass of cv. Campo Grande. Standard strain BR 446 T and ERR 917 enabled plants to obtain 38%–44% of their total nitrogen from biological nitrogen fixation, with inoculated plants accumulating about twice as much nitrogen and biomass versus the uninoculated control (averaging 1900 and 3900 kg ha − 1 of dry biomass for control and inoculated treatments, respectively).