Effect of rates and sources of soluble phosphorus on the behavior of cowpea plants inoculated with rhizobacteria from soils of the north region of Brazil

Cowpea can obtain N through biological nitrogen fixation (BNF) through symbiosis with rhizobacteria. However, nodulation and BNF are influenced by edaphoclimatic factors that may bring about benefits or damages to the process, and the availability of nutrients is among the main factors that affect BNF and phosphorus (P). Thus, the present study aimed to determine the effectiveness of doses and sources of soluble P on nodulation, accumulation of nutrients, N and P absorption and use efficiency in cowpea plants inoculated with or without rhizobacteria. The experiment was conducted in a greenhouse at the Federal Institute of Education, Science and Technology, Rondonia, Colorado do Oeste-RO Campus, Brazil. The experimental design was randomized blocks in a 5 x 2 x 2 factorial arrangement, corresponding to five P rates (0, 20,  40, 80 and 160 kg ha-1 of P2O5), two soluble sources of P2O5 [single superphosphate (SSP) and thermophosphate], absence and presence of inoculation, with four replication. The findings of this study show that inoculation with Bradyrhizobium sp. promotes increment in the dry matter production and increases N and P absorption efficiency in cowpea plants. The single superphosphate led to higher N and P absorption efficiency, production of shoot dry matter and production of nodules, when compared with thermophosphate. Inoculation with Bradyrhizobium sp. associated with SSP fertilization promotes higher P absorption efficiency in cowpea plants. Therefore, increase in P rates promoted increments in P concentrations in cowpea leaves. Key words: Vigna unguiculata (L.), Bradyrhizobium, phosphorus, fertilization, nutrition.