Inoculation with diazotrophic bacteria is well documented as a means to enhance growth and increase yields of various crops, especially when used as an alternative or a supplement to the use of nitrogenous fertilizers and agrochemicals for sustainable agriculture. Nitrogen is the most limiting nutrient for increasing crop productivity, and the use of chemical sources of N fertilizers is expensive, and may contribute to environmental pollution. Therefore, there is a need to identify diazotrophic inoculants as an alternative or supplement to N-fertilizers for sustainable agriculture. The search for effective diazotrophic bacterial strains for formulation as biofertilizers has been going on for over 40 years and a number of inoculant biofertilizers have been developed and are commercially available
1.1 Background of the study
By 2050 the global human population is projected to increase by 50%, and the global grain demand is projected to double (Alexandratos, 1999). Poor soil fertility is one of the major constraints for crop production (Ouédraogo et al., 2001). Millions of people in the world are fed by modern agriculture, benefiting from increased yields resulting from greater inputs of fertilizer, pesticides and other technologies (Cassman, 1999). However, ensuring sustainability of agriculture, enhancing crop growth and improving crop yields, all without compromising environmental integrity or human health are major challenges (Tilman et al., 2002). Moreover, continuous use of agrochemicals may impact negatively on the environment (Poudel et al., 2001; Wilson and Tisdell, 2001). The high cost of fertilizers also inflates the cost of crop production. The use of microorganisms in agriculture has therefore been identified as a cheaper and more environmentally friendly alternative or supplementary mechanism to improve crop production and reduce production costs (Parr et al., 1994; Wu et al., 2005; Berg, 2009).
The first major groups of biofertilizers identified were Rhizobium spp., that fix nitrogen from the atmosphere in root nodules on legumes. They have been used commercially as inoculants for legumes for over 100 years (Boonkerd and Singleton, 2002). Research in the field of biofertilizers has resulted in the development of different kinds of microbial inoculants or biofertilizers including nitrogen fixing bacteria, phosphate solubilizing microorganisms, vesicular–arbuscular mycorrhizae (VAM) and plant growth promoting rhizobacteria (PGPR). Several free-living bacteria genera have been reported to enhance plant growth, subsequently increasing yields of crops (Kloepper et al., 1989; Glick, 1995; Kennedy et al., 2004; Lucy et al., 2004). Improvements in growth parameters resulting from the use of microbial inoculants, combined with reduced rates of chemical fertilizers, have been also reported in previous research (Chen, 2006; Jilani et al., 2007; Adesemoye et al., 2009; Kumar et al., 2009). Research on the use of microbial inoculants to enhance growth and increase yields of crops has been the focus of many studies (Okon and Vanderleyden, 1997; Dobbelaere et al., 2001; Riggs et al., 2001; Matiru and Dakora, 2004; Mehnaz et al., 2010). Typically these beneficial microorganisms have been isolated from the rhizosphere of plants and formulated into microbial inoculants.