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Rhizobial Inoculants for Legume Crops

Summary Legumes are an important source of protein for humans and livestock. Legumes have also been used for soil improvement for centuries because of their N and non-N rotational benefits to non-legume crops. The N benefits include N2 fixation and mineralization, sparing of soil inorganic N, and reduced immobilization of soil inorganic N. The non-N benefits include breaking pest cycles, improvement of soil structure, and the nutritional and disease-control effects of endophytic rhizobia. Therefore, optimizing the legume-Rhizobium symbiosis is important, and it can be done by selecting or modifying either (or both) symbiotic partner(s) for desirable traits related to N2 fixation. Rhizobium strains can be selected or genetically modified for traits like N2 fixation potential, nodulation competitiveness, persistence in soil, compatibility with inoculant carriers, and tolerance to environmental stress factors. Legume genotypes can also be selected, bred or genetically modified for N2 fixation potential, restricted or preferential nodulation, and tolerance to nitrate and environmental stress factors. When choosing prospective strains or legume genotypes for a particular environment, time and resources can be saved by realizing that the most adaptable rhizobia or legume genotypes are usually those isolated from similar environments. Inoculant delivery methods also affect N2 fixation. Soil inoculation, particularly with granular inoculants, seems to be often better and never worse than seed inoculation for initiating nodulation and N2 fixation. Use of pre-inoculated seeds eliminates the seed inoculation operation, but Rhizobium numbers in pre-inoculated seeds tend to be lower than those in traditional inoculant products. Therefore, the time saved by using pre-inoculated seeds should be weighed against the possibility that crop yields may be lower if insufficient Rhizobium numbers are delivered. Until tools for genetic modification of rhizobia or legumes to suit specific requirements are commonly used, N2 fixation can be enhanced by adopting practices like choosing the best combinations of Rhizobium strains and legume genotypes, the best inoculant formulation and delivery methods, optimum inoculation rates, and providing favourable growing conditions for the crop.