Agricultural production in East African rangelands is limited by rainfall variability and declining soil fertility. The problem of water scarcity for crop production can be addressed through irrigation, and in current times of rising cost of mineral fertilizers, an alternative method of improving soil fertility can be found through biological nitrogen fixation. Adequate population of compatible rhizobia is required in the soil to achieve sufficient nitrogen fixation to support crop growth. However, rhizobia population tend to vary considerable amongst the natural range, and with cover crops in the cultivated rangeland. The purpose of this study was to establish the baseline rhizobia population in the natural grassland and to determine the effects of crop rotation on rhizobia assemblages in the cultivated rangeland areas. We postulated that rhizobia population in the soils can be enhanced by repeated cultivation of similar legume symbionts. We used the Most Probable Number plant infection method (MPN) to compare rhizobia population in samples of soil randomly collected from natural grassland and plots under different rotational regimes in cultivated rangeland at Isinya District, in southern Kenya during the first quarter of 2011. The MPN technique was carried out in laboratory condition using seedlings of French beans (Phaseolus sp). Rhizobia assemblages were found in varying densities in all the sampled plots. The natural grasslands had low rhizobia concentration (575 cells/g soil). The rhizobia population declined with repeated cultivation of non-legume crops, but increased sharply (30,000 cells/g soil) with the introduction grain legumes. With repeated cultivation of French beans, the abundance of rhizobia increased by over 200 times the estimated population in the natural grasslands. Soils of the unmanaged rangeland at Isinya contain nitrogen fixing native plants species, which maintain rhizobia assemblages in free living state. Vegetation clearing and cultivation of cereal crops can lead to reduction in the rhizobia abundance and depletion of soil nitrogen. The introduction and repeated cultivation of symbiotic grain legumes can lead to rapid build-up of rhizobia population and significant improvement in soil nitrogen fixation. Rhizobia population remained high in the soil following cereals and beans intercropping. Regular crop rotation with grain legumes can replenish soil fertility through biological nitrogen fixation and consequently significantly improve crop yields and food security in East African rangelands.

• published: 10 Feb 2012
• views: 391