Yield and biological nitrogen fixation of cowpea varieties in the semi-arid region of Brazil

Cowpea is an important crop in small properties of the Brazilian semi-arid region, where it is cultivated without fertilizer application. In spite of the fundamental role played by biological Nitrogen fixation (BNF), little is known of the symbiosis between cowpea varieties and native rhizobia or recommended rhizobia strains. A field experiment was conducted aiming to estimate BNF and productivities of local varieties, in association with two previously described bradyrhizobial inoculant strains and native rhizobia (no inoculation). The plants received 20 kg ha−1 of enriched 15N fertilizer to allow the use of the isotopic dilution method. After harvest (80 days) straw and grain biomass was determined. The varieties differed in grain and straw productivity and in N and N derived from atmosphere (%Ndfa). Corujinha had the highest grain productivity (1147 kg ha−1), followed by Sempre Verde (920 kg ha−1), Azul (912 kg ha−1) and Cariri (889 kg ha−1). Costela de Vaca had the highest straw productivity (2258 kg ha−1), highest N content in the straw (28 g ha−1) and highest BNF (79 %Ndfa, corresponding to 45 kg ha−1 of N for total aboveground biomass and 39 kg ha−1 for the straw), but the lowest grain productivity (381 kg ha−1) and the lowest harvest index (0.14). The inoculations did not significantly alter productivities, N contents or %Ndfa but there was a tendency of lower grain productivities in the non-inoculated plants, which was reflected in lower total and biologically fixed N quantities, indicating that the native strains may be slightly less efficient.