ORIGINAL ARTICLE Large genotypic variation but small variation in N 2 fixation among rhizobia nodulating red clover in soils of northern Scandinavia S. Duodu 1 , G. Carlsson 2 , K. Huss-Danell 2 and M.M. Svenning 1 1 Department of Biology, Faculty of Science, University of Tromsø, Tromso, Norway 2 Crop Science Section, Department of Agricultural Research for Northern Sweden, Swedish University of Agricultural Sciences, Umea, Sweden Introduction Rhizobia are well known for their contribution to the soil nitrogen pool through N 2 fixation in symbioses with legu- minous plants. In many areas where legumes are cultiva- ted or make up a part of the native flora, indigenous or naturalized populations of rhizobia are abundant in the soil. Studies of these populations have been motivated by ecological and agricultural considerations. In terms of agriculture, high abundance of rhizobia in the soil facili- tates cultivation of legumes as they can be grown without inoculation. On the other hand, soils containing indigen- ous rhizobia are problematic as they create a barrier to the establishment of introduced efficient inoculants in nodules of target host plants (Vlassak and Vanderleyden 1997). Also, there is evidence of widespread suboptimal efficiency of native strains with agricultural legumes (Brockwell et al. 1995). However, it is generally argued that indigenous populations are highly adapted to their local soil environments and may form comparable or more effective symbioses than commercial inoculants iso- lated from a distant and unrelated soil environment (Gandee et al. 1999). Thus, selection of indigenous strains with high nitrogen-fixing capacity, adapted to a range of environmental conditions at a specific site, remains an important strategy to maximize legume production. Keywords: DNA fingerprinting, genetic diversity, N 2 fixation, 15 N isotope dilution, Rhizobium leguminosarum bv. trifolii, seasonal changes, subarctic, Trifolium pratense. Correspondence Mette M. Svenning, Department of Biology, Faculty of Science, University of Tromsø, N-9037 Tromsø, Norway. E-mail mette.svenning@ib.uit.no 2006/0549: received 19 April 2006, revised 10 August 2006 and accepted 1 September 2006 doi:10.1111/j.1365-2672.2006.03196.x Abstract Aims: To analyse the symbiotic variations within indigenous populations of rhizobia nodulating red clover (Trifolium pratense L.) in soils of northern Norway and Sweden at different times of the growing season. Methods and Results: A total of 431 nodule isolates sampled under field condi- tions in summer and autumn, were characterized genetically by targeting both chromosomal and symbiotic genes. The Enterobacterial Repetitive Intergenic Consensus polymerase chain reaction (PCR) fingerprinting of chromosomal DNA revealed considerable variation within the isolated populations that was more influenced by geographical origin than sampling time. Analysis of PCR amplified nodEF gene on the symbiotic plasmid by restriction fragment length polymorphism revealed a high proportion of nod types common to the two studied sites. The symbiotic efficiency of the isolates, representing both domin- ating and rare nodEF genotypes, showed high N 2 fixation rates in symbiosis with the host plant in a greenhouse experiment using the 15 N isotope dilution method. Conclusions: Effective N 2 -fixing strains of Rhizobium leguminosarum bv. trifolii nodulating red clover are common and genetically diverse in these northern Scandinavia soils. Significance and Impact of the Study: This study provides information on the variability, stability and dynamics of resident populations of rhizobia nodulat- ing red clover in Scandinavian soils which has practical implications for apply- ing biological nitrogen fixation in subarctic plant production. Journal of Applied Microbiology ISSN 1364-5072 ª 2006 The Authors Journal compilation ª 2006 The Society for Applied Microbiology, Journal of Applied Microbiology 102 (2007) 1625–1635 1625