Faculty of Agriculture

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In terms of production, rice is the fourth most important cereal (after sorghum, maize and millet) in sub-Saharan Africa (SSA). It occupies 10 percent of the total land under cereal production and accounts for 15 percent of total cereal production (FAOSTAT, 2006). Approximately 20 million farmers in SSA grow rice and about 100 million people depend on it for their livelihoods (Nwanze et al., 2006). Rice is the staple food of a growing number of people in SSA: from 1961 to 2003 consumption increased at a rate of 4.4 percent per year (Kormawa, Keya and Touré, 2004). Among the major cereals cultivated, rice is the most rapidly growing food source in Africa: between 1985 and 2003, the annual increase in rice production was 4 percent, while production growth for maize and sorghum was only about 2.4 and 2.5 percent, respectively (Kormawa, Keya and Touré, 2004). The most widely grown rice species, Oryza sativa, is originally from Asia and was introduced into Africa only about 450 years ago. Another less well-known rice species, O. glaberrima (Steud), is originally from Africa and was domesticated in the Niger River Delta over 3 500 years ago (Viguier, 1939; Carpenter, 1978). As a result of their evolution, domestication and breeding history, both species have distinct and complementary advantages and disadvantages for use in African farming systems. The Asian rice (O. sativa) is characterized by good yields, absence of lodging and grain shattering, and high fertilizer returns – unlike its African counterpart (O. glaberrima). However, in contrast to Asian rice types, landraces of O. glaberrima often have good weed competitiveness and resilience against major African biotic and abiotic stresses (Koffi, 1980; Jones et al., 1997a). Dalton and Guei (2003) concluded that research into genetic enhancement of rice generated approximately US$360 million in 1998, compared with a total investment of just US$5.6 million. This is evidence that rice variety improvement has a potentially enormous impact on the economic development of SSA. Numerous conventional breeding efforts have been made to improve the performance of upland rice (O. sativa) for use in African farming systems. These efforts have had only limited success, partly because the Asian rice, O. sativa, lacks resistance or tolerance to many of the typical African stresses (Jones et al., 1997a).

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Nitrogen compounds comprise from 40 to 50 percent of the dry matter of protoplasm, the living substance of plant cells. For this reason, nitrogen is required in large quantities by growing plants and is indeed the key to soil fertility. Non-nitrogen-fixing plants, for example cereals, obtain all the nitrogen they need from the soil. In Senegalese conditions this uptake was estimated to be as follows: 79-132 kg N ha/crop for pearl millet; 74-84 kg N ha/crop for rice; 134 kg N hdcrop for sorghum; and 121-139 kg N ha/crop for maize. Nitrogen-fixing plants, essentially legumes, take a part of the nitrogen they require from the atmosphere, the other part being provided by the soil."

Application of P (150 kg P/ha) increased nodulation, dry matter yield, P uptake, tissue N yield, dinitrogen fixation and seed yield of the three bean cultivars (Rose Coco, Canadian Wonder and Mwezi moja) at both N levels (10 and 100 kg N/ha). A high dose of N severely reduced nodulation only where P was not applied but severely reduced dinitrogen fixation at both P levels. Where P was applied cultivars fixed comparable quantities of dinitrogen. At no P + 10 kg N/ha cultivar Rose Coco nodulated well early in the growth stages and fixed substantial dinitrogen

A global network of Microbiological Resources Centres (MIRCENs) , set up by Unesco, includes five centres concerned with biological nitrogen fixation. These centres are Kenya, Brazil, the United States and Senegal-train experts in the methods of Rhizobium inoculation, which when applied to certain leguminous plants can often avert the need to use expensive chemical nitrogen fertilizers in food crop production. The MIRCENs constitute an excellent example of international co-operation in science.

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N fixation was studied by the 15N method in Vigna unguiculata cv. ERI-2, Vita 4 and Machakos 74 grown in a field which had been fallow for 3 years and given 20 or 100 kg N/ha. The higher N rate reduced nodulation in all cv., with effects differing somewhat between cv., and increased DM and N yield/plant and uptake of non-fertilizer soil N, with least and greatest effect in cv. ERI-2 and Machakos 74, resp. The amount of N fixed was 50.5, 73.7 and 60.7 kg/ha at the low N rate and was 69.7, 74.5 and 100% lower at the high N rate in the 3 cv., resp.

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In field experiments at Katumani in 1982, cowpea cv. Katumani 80 and Vita 4 were grown in chromic luvisol soils and treated with 15N-labelled ammonium sulphate at 20 kg N/ha, with or without 70 kg P/ha. Differences in nodule DW, DM yield, P uptake and tissue N yield were detected between cv. at maturity, but P rate had no effect. Av. seed yields of Katumani 80 and Vita 4 were 1.16 and 1.05 t/ha, resp., and were unaffected by P rate.

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Microbial populations estimated in termite-affected soils show that bacteria and actinomycetes are most abundant during the wet season. The highest density of bacteria recorded was 106 and, of actinomycetes, 105g dry soil. In contrast, fungi, which dominate only during dry periods, numbered 104 and declined to 102 cells/g dry soil during the wet period. Fungi, actinomycetes, bacteria and Protozoa were higher in 'dead' than in 'live' mounds. Counts of denitrifiers, ammonifiers, cellulose decomposers, nitrifiers and Protozoa were in the order of 103/g dry soil. The evolution of CO, was also related to microbial activities. This is the first time such information has been recorded for Kenyan soils. The study provides evidence that 'live' termite mounds differ from 'dead' ones in respect of the microorganisms associated with them.

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Rhizobium strains of the cowpea group did not lose viability readily when added to soil, but Bdellovibrio acting on these rhizobia were found in 32 of 90 soils examined. Bdellovibrio did not initiate replication in liquid media at low host densities, but it did multiply once the Rhizohium numbers increased through growth to about 108 ml−1. From about 104 to 6 × 105 ml−1Rhizohium cells survived attack by the parasites in liquid media. In nutrient-free buffer, no significant increase in vibrio abundance was evident if the rhizobial frequency was low. whereas Rhizobium populations containing 6 × 108 cells ml−1 were lysed rapidly. Bdellovibrio did not multiply when introduced into sterile soil with small numbers of the host, but it replicated when the rhizobia were abundant because of the latter's use of soil organic matter for growth or because of the deliberate addition of 108Rhizohium g−1. Nevertheless, the host persisted in such vibrio-rich soil samples. The abundance of indigenous bdellovibrios increased appreciably in nonsterile soil if the rhizobia were introduced in large but not small numbers. It is suggested that a major reason for the lack of elimination of the host population in soil by its parasites is the need for a critical host cell frequency, large Rhizobium numbers being required for Btiellovibrio to initiate replication and low numbers of surviving hosts no longer being able to support the parasite.

A fall in Rhizobium abundance occurred in nonsterile soil inoculated with large numbers of the root-nodule bacteria but many of the rhizobia still survirved. No such decline was evident in sterile soil. Protozoa feeding on these bacteria were isolated from soil and other environments. As the abundance of Rhizobium meliloti and a cowpea Rhizobium strain in soil decreased, the protozoan density increased. The inability of the predators to eliminate their prey,from soil was not the result of the presence of organisms feeding on the protozoa because many rhizobia survived in sterile soil inoculated with the prey and cultures of individual protozoa nor was it the result of the rapid multiplication of the bacteria to replace those consumed because survivors were still numerous in essentially organic matter free soil in which the hactena did not grow appreciably. The lack of elimination also was not associated with a protective effect of soil particles because survivors were still abundant in solutions inocuiated with protozoa and bacteria.It is suggested that the size of the prey population diminishes until a density is attained at which the energy used by the predator in hunting for the survivors equals that obtained from the feeding.