Variation in symbiotic effectiveness of cowpea bradyrhizobia indigenous to Ghanaian soils

The potential for improving biological nitrogen fixation through rhizobial inoculation requires knowledge of the abundance and effectiveness of the indigenous rhizobia population in the soil. Cowpea bradyrhizobia in 20 soils in Ghana were enumerated and isolates from nodules of cowpea grown in these soils were examined in sand for symbiotic effectiveness. The 100 isolates examined varied greatly with estimated values for effectiveness, relative to an uninoculated control, ranging from 23.5 to 118%. The isolates thus ranged from ineffective to highly effective, but with most of them (68%) being ranked as moderately effective and with the highly effective ones constituting only 26% of the 100 isolates studied. The results obtained raise an important question, whether the 26% highly effective bradyrhizobia are capable of satisfying optimum N requirements from symbiotic N-2 fixation. Diversity of the isolates in symbiotic effectiveness was again revealed when compared to the fertilized N control or with a standard strain of Bradyrhizobium sp. Some of the isolates had high N-2 fixing capabilities that were comparable to the N fertilized plants with the equivalent of 70 kg/ha of inorganic N, with some of them showing superiority in symbiotic effectiveness relative to the standard strain. These data suggest that native isolates are a potentially useful source of strains for preparing highly effective cowpea inoculants. (C) 2002 Elsevier Science B.V. All rights reserved.