In sustainable agriculture, biological N2 fixation is an important pathway of nitrogen input into agricultural soils besides the application of organic and mineral fertilizers. In this respect, cultivation of legumes is of great importance due to the symbiosis with nitrogen-fixing bacteria. The perennial legume Medicago sativa (alfalfa), a major herbal protein source for livestock, is frequently grown in ecologically based agriculture being able to fix up to 180 kg N ha-1 per season. Effective symbiosis between legumes and rhizobia does not only depend on the capacity of nitrogen fixation but also on the entire nitrogen turnover in the rhizosphere. In the present study, a greenhouse experiment was set up to test the influence of inoculation with two indigenous Sinorhizobium meliloti strains, with different efficiency in terms of plant growth promotion, on nitrogen turnover processes in the rhizosphere during the growth of alfalfa. Quantification of six target genes (bacterial amoA, nirK, nirS, nosZ, nifH and archaeal amoA) was performed by real-time PCR in rhizosphere samples that have been taken before nodules formation, at the bud development stage as well as at the late flowering stage. The results indicated that efficiency of rhizobial inocula was highly related to abundance of nifH genes in the late flowering phase of alfalfa. At the same plant development stage, low copy numbers of genes involved in ammonia oxidation as well as in denitrification indicated that the ammonia derived from nitrogen fixation was mostly used by the plant and wasn’t transferred to the rhizosphere. However, the respective gene abundances differed overall to a greater extent between the three plant development stages than between the inoculation variants.

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