Alfalfa (Medicago sativa L.) and Sinorhizobium meliloti: prospects of using rhizobial inoculants in Serbia

It is considered that the symbiotic association between alfalfa (Medicago sativa L.) and Sinorhizobium meliloti is one of the most efficient interactions between N2-fixing bacteria and legumes. But even greater efficiency of symbiotic nitrogen fixation can be achieved by selecting the best combinations of host genotypes and nodule bacteria. The Serbian K-28 cultivar has the potential capacity to be a good parental genotype in breeding programs for improved N2 fixation capability through inoculation with particular compatible strains. On the other hand, S. meliloti strain L5 is a broadly effective inoculant with most alfalfa cultivars, with a mean value of fixed N2 of 35% over a two-year period. Serbian soils are well supplied with autochthonous rhizobial strains which specifically nodulate alfalfa, and this is why the use of rhizobial inoculants in alfalfa production in Serbia is not a common practice. However, the number and effectiveness of such strains depend on soil type, the land use system (alfalfa stand, arable land and lea) and the alfalfa host plant. For this reason, use of rhizobial inoculants with highly effective strains is needed in alfalfa production. Our two-year field experiment showed that shoot dry weight increased by 42-77% in inoculated alfalfa, while the amount of fixed nitrogen increased by 35%. Alfalfa inoculation with effective strains is an alternative approach to improving the long-term productivity of alfalfa. In our five-year trial, the inoculated K-28 alfalfa cultivar showed maximum nitrogen fixation (293 kg N ha-1) in the fourth year of utilisation. During this period, symbiotic nitrogen fixation of alfalfa, determined from fixed N2 in the second cutting, varied between 32 and 44%. The most effective strain promoted an increase of shoot dry weight by 34-60% in relation to the control in the third and fourth years of utilisation. In addition, Sinorhizobium spp. have the potential to be used as plant growth-promoting rhizobacteria (PGPR). Inoculation of Italian ryegrass seeds with a particular rhizobial strain the year before alfalfa growing ensured abundant nodulation and better growth of the alfalfa. In Serbia there are rhizobial inoculants of alfalfa with consistently high quality which permit reduction of mineral N fertiliser use in production of this crop, with consequent economic and ecological benefits