Cowpea (Vigna unguiculata (L.) Walp.) can obtain sufficient N for its own supply from the atmosphere, by the process of biological nitrogen fixation (FBN), when associated with specific and effective rhizobia bacteria. The purpose of this study was to evaluate the effect of different rhizobia inoculum rates on nodulation and biological N fixation by cowpea (cv. BRS Pujante). A complete randomized block design with seven treatments and four replicates was used. The trial was conducted in a greenhouse using 2 kg of soil as the growth medium. The treatments consisted of five rates of viable cells of Bradyrhizobium sp. (strain BR 3267), a control treatment (without rhizobia and no mineral N) and a treatment without inoculation but with mineral nitrogen (control + 20 kg ha-1 N). Nodulation (nodule number and biomass), shoot biomass and total N uptake were determined. Higher rhizobia inoculum rates (viable cells per seed) increased the nodulation and N fixation values. In the treatment with rhizobia (strain BR 3267) at the highest inoculum rate (6,65x10(7) viable cells per seed), the shoot dry matter was equivalent to the treatment with mineral N application (20 kg ha-1) and promoted an increase of up to 27% compared with the control treatment. The inoculum rate (strain BR 3267) up to 8x10(5) (viable cells seed-1) contributed to accumulate a higher amount of N in cowpea compared to the control and the control + N treatments. Cowpea (cv. BRS Pujante) was benefited by the BNF when inoculated with strain BR 3267, proportionally to the inoculum rate.
symbiotic effectiveness; inoculant; native rhizobia population
