The objective of this work was to evaluate the biological nitrogen fixation (BNF) in five cowpea cultivars: BR 17 Gurguéia, BRS Guariba, BRS Mazagão, UFRR Grão Verde and Pretinho Precoce 1. In 2007, one greenhouse and one field experiment were performed using a factorial design with five cowpea cultivars and four nitrogen sources: urea fertilization (50 kg ha-1 of N), inoculation with BR 3262 or BR 3267 Bradyrhizobium strains, and absolute control. Thirty-five days after plant emergency, the number and dry weight of nodules, the dry matter weight and the total N of the shoots, the nodule efficiency in greenhouse plants and the grain yield for the field experiment plants were evaluated. In the greenhouse, high nodulation and nodule efficiency were observed for both strains in all cultivars. In the field, the nodulation and total N were lower for all cultivars in comparison with the greenhouse experiment, which indicates the interference of edaphoclimatic factors in the BNF. An increase in grain yield was observed for all cultivars as a consequence of the inoculation, especially for the BR 3262 strain.
Bradyrhizobium; Vigna unguiculata; biological nitrogen fixation
