Diversity and Efficiency of Cowpea Rhizobial Isolates from Semiarid Soil of Pernambuco State, NE Brazil.

Biological nitrogen fixation (BNF) by the legume-rhizobia symbiosis is one of the main nitrogen sources on agricultural systems. The rhizobial component of the symbiosis is highly diverse, especially in tropical soils, and strongly affected by soil and climate variability. We evaluated the effects of soil and climate factors on rhizobial diversity, as a basis to select strains more efficient on BNF with cowpea (Vigna unguiculata (L.) Walp). Soil samples were collected at eight municipalities of Pernambuco State´s semiarid region and used as inoculants for strain isolation. Four to five nodules were collected from each jar, and used for standard isolation and characterization procedures. We estimated rhizobial diversity through diversity, dominance, equitability and richness indexes, and determined a linear correlation matrix between those indexes and soil physical and chemical characteristics, sampling municipality average climactic data, and plants nodulation and biomass data. Rhizobial efficiency was studied through an experiment on sterile substrate evaluating rhizobial isolates, a strain currently used for inoculant production (BR3267), and three uninoculated treatments, with and without added nitrogen. Shoot, root and nodule dry masses (SDM, RDM and NDM), nodule number (NN), total shoot nitrogen (TSN) and relative efficiency (RE) were determined and used for a correlation analysis. 109 rhizobial isolates were obtained most being fast growers (70%). Shannon´s diversity index was highest for Santa Cruz do Capibaribe (2.83) and lowest for Parnamirim (1.09), and significant correlations were found among it and clay content (positive) and P content (negative). Several of the isolates were as efficient as BR3267 for all variables, and did not differ significantly from uninoculated plants receiving the equivalent to 90 kg of N.ha^-1. These results confirm the effect of soil characteristics on rhizobial diversity, and the potential of unexplored native rhizobia from the semiarid region for further studies and possible use on inoculant production.