Common bean (Phaseolus vulgaris L.) may benefit from biological N2fixation, but inconsistent responses of the crop to rhizobium inoculation indicate the need for supplemental mineral N fertilization. In this study, we aimed to evaluate the response of the common bean crop to rhizobium inoculation in association with supplemental mineral N in the Cerrado (Brazilian tropical savanna) and Atlantic Forest biomes. Four field experiments were carried out, two in Santo Antônio de Goiás (State of Goiás), one in Valença (State of Rio de Janeiro), and another in Macaé (State of Rio de Janeiro), all in Brazil. Inoculation with commercial rhizobium strains was compared to inoculation with strain BR 923 of Sinorhizobium sp., to mineral N fertilization, and to supplementation with N at sowing and in topdressing. Evaluation of the native rhizobium population indicated 105 cells g-1 in the soil of the experimental area in Goiás, previously cultivated with common bean, and 102 cells g1 in the soil in Valença, previously kept in pasture. In both experiments in Goiás, grain yields of around 2,100 kg ha-1 did not differ among the control treatments, rhizobia inoculation, or the application of 120 kg ha-1 of N. In Valença, inoculation with commercial strains provided yields from the cultivar Ouro Negro higher than the absolute control, in the lack of topdressing N fertilization. With 40 kg ha-1 of N in topdressing, rhizobium inoculation led to 3,420 kg ha-1 of grain yield, higher than the other treatments. In the mean of different N sources at sowing, topdressing fertilization increased grain yield from 2,367 to 2,542 kg ha-1. In Macaé, in a soil with high organic matter content, the highest yields were obtained with inoculation of commercial strains associated with 40 kg ha-1 of N in topdressing, and there were deleterious effects from application of 80 kg ha-1 of N at sowing. We conclude that in areas without a previous common bean crop, inoculation with commercial rhizobium strains increases grain yield, particularly when associated with topdressing N fertilization.
biological nitrogen fixation; Phaseolus vulgaris; topdressing fertilization
