Effect of rhizobia inoculation and starter-nitrogen application on nodulation, biomass and grain yield of food legumes
A field experiment was conducted at the University of Nairobi's Faculty of Agriculture farm in 2004 long rains (LR) and short rains (SR) to determine the effect of rhizobia inoculation and starter-N dose on the performance of grain legumes. Common bean (Phaseolus vulgaris L.), lima bean (Phaseolus lunatus L.), cowpea (Vigna unguiculata L), green gram (Vigna radiata L.), pigeon pea (Cajanus cajan L.) and lablab (Lab lab purpureus L.) were the tested in the field experiment. The test crops were either uninoculated, inoculated with rhizobia, or supplied with 26 kg N ha·1 • The treatments were laid out in a randomized complete block design (RCBD) in a split plot arrangement with three replicates. Nodule numbers, nodule biomass, shoot biomass, root biomass and grain yield were determined. Rhizobia inoculation had no significant effect on nodule numbers plant" in both seasons at 4 weeks after emergence (WAE) and 6 WAE. In most cases, common bean had significantly higher nodule numbers and nodule biomass than most of the other legumes while lima bean generally registered the fewest nodules. At 6 WAE, rhizobia inoculation improved nodule biomass in the short rains (SR) but not in the long rains (LR). Starter-N dose had no significant effect on nodule biomass in both seasons. Inoculation and starter-N had no significant effect on shoot biomass accumulation and grain yield in both seasons. In the LR, lima bean produced the highest grain yield followed by common bean while the converse was the case in SR. Pigeon pea and green gram performed the poorest in the SR. Mean grain yield varied from 148 kg ha" to 1472 kg ha' in LR and from 213 to 4398 kg ha" in SR. It was concluded that inoculation and starter-N application was not necessary for the purpose of improving grain yield under the prevailing field experimental conditions. A follow-up experiment was conducted in a greenhouse to determine the abundance of indigenous soil rhizobia nodulating common bean (Phaseolus vulgaris L.) and cowpea (Vigna unguiculata L.) in soils sampled from Kabete (cultivated and uncultivated land), Machakos, Nyeri and Kajiado sites. The population size of indigenous rhizobia was determined using the most probable number technique. Cowpea and common bean were used as "trap" hosts for Bradyrhizobium spp and Rhizobium spp, respectively. Nodule numbers, nodule biomass, shoot biomass, plant tissue-N and the number of rhizobial bacteria cells g-l dry soil were determined. The soil samples varied in chemical characteristics with the Nyeri site having a low pH. Indigenous rhizobia nodulating cowpea in the sites ranged from 78.5 to more than 900 bacterial cells per gram of dry soil. Common bean nodulating rhizobia had more than 900 bacterial cells per gram of dry soil at each of the sites. The Nyeri site had the lowest rhizobia population. In most sites, common bean produced significantly more nodules per plant than cowpea. A similar trend was observed with respect to nodule biomass, though this was only significant with Kabete soils. Inoculation with commercial inocula produced more nodule and shoot biomass than inoculation with soil inocula. It was concluded that indigenous rhizobia were widespread in central Kenya.