Effects of Inoculation of Rhizobium on Nodulation and Nitrogen Accumulation in Cowpea Subjected to Water Availabilities

With high protein content, the cowpea is one of the most widely consumed legumes in northeastern Brazil. This study was aimed to evaluate the rhizobia inoculation effect on nitrogen accumulation and nodulation in cowpea subjected to water availability. The experiment was conducted in a greenhouse, in a completely randomized design, with six water availabilities (40%, 60%, 80%, 100%, 120% and 140% of maximum soil water retention capacity), with four replications. Soil moisture maintenance was performed by gravimetric method. The BR3267 strain (Bradyrhizobium japonicum) recommended for cowpea grown in Brazil was used. At thirty-five days after treatments implementation, the nitrogen nutritional status was assessed through SPAD reading, shoot nitrogen concentration and total accumulation. Nodulation was assessed by nodules number and dry matter. The increased water availability provided negative effect on SPAD reading and shoots nitrogen concentration. Shoot nitrogen accumulation was favored to up to 78.8% water availability, with subsequent decline. Nodulation was favored when soil water availability was between 85% and 87%. Water availability affects nodulation and cowpea nutritional status, although negative effects are more pronounced when there is soil water excess.