ORIGINAL ARTICLE Trifolium species associate with a similar richness of soil-borne mutualists in their introduced and native ranges Kevin J. McGinn 1 *, Wim H. van der Putten 2,3 , Richard P. Duncan 4 , Natasha Shelby 1 , Carolin Weser 1,2 and Philip E. Hulme 1 1 Bio-Protection Research Centre, Lincoln University, Lincoln 7647, New Zealand, 2 Department of Terrestrial Ecology, Netherlands Institute of Ecology (NIOO- KNAW), Wageningen 6708 PB, The Netherlands, 3 Laboratory of Nematology, Wageningen University, 6700 ES Wageningen, The Netherlands, 4 Institute for Applied Ecology, University of Canberra, Canberra ACT 2601, Australia *Correspondence: Kevin J. McGinn, Bio-Protection Research Centre, P.O. Box 85084, Lincoln University, Lincoln 7647, New Zealand. E-mail: kevinmcginn@outlook.com ABSTRACT Aim While plant species introduced to new regions may benefit from escaping natural enemies, their success may be impaired by losing key mutualists. We aimed to elucidate whether a selection of annual and perennial Trifolium (clo- ver) species have lost associations with nitrogen-fixing bacteria (rhizobia) and arbuscular mycorrhizal fungi (AMF) in their introduced range. Location Introduced range in New Zealand (NZ) and native range in the Uni- ted Kingdom (UK). Methods We compared the strain richness of rhizobia associated with five Trifolium species in both ranges using genetic fingerprinting (rep-PCR with ERIC primers). Phylogenetic analysis of the nodD gene was conducted to test for between-range differences in rhizobia genotypes associated with seven Trifolium species. We also used TRFLP to compare the richness of AMF associ- ated with three Trifolium species in both ranges. Results Genetic fingerprinting indicated that Trifolium associate with a similar richness of rhizobia strains in NZ as they do in the UK. According to variation in the nodD gene, genotypes of rhizobia were indistinguishable between NZ and UK provenances. A total of 17 AMF operational taxonomic units were detected but there were no significant between-range differences in richness or in community structure. Main conclusions Contrary to general expectations regarding the loss of mutualists following species introduction, our findings suggest that alien plants, including those accidentally introduced, can have access to rich communities of soil-borne mutualists that are likely to facilitate successful naturalization. Keywords below-ground mutualism, biological invasion, exotic, naturalization, plant–soil interactions, soil biota, Trifolium, weed INTRODUCTION Plant species anthropogenically introduced to new regions (hereafter referred to as alien plant species) can benefit from escaping the inhibitory effects of below-ground pathogens, parasites and herbivores (e.g. Reinhart et al., 2003), yet their performance can be impaired by leaving behind key below- ground mutualists such as nitrogen-fixing bacteria (rhizobia) or mycorrhizal fungi (e.g. Richardson et al., 1994; Parker et al., 2006). To overcome rhizobial constraints, alien legumes may need to be co-introduced with coevolved rhizo- bia, either intentionally or unintentionally, or acquire novel indigenous rhizobia strains. While the establishment and spread of many alien legume species has been facilitated by encountering compatible rhizobia (Stepkowski et al., 2005; Wei et al., 2009; Rodr  ıguez-Echeverr  ıa et al., 2011; Cris o- stomo et al., 2013; Ndlovu et al., 2013), introduced rhizobia populations can exhibit limited strain richness as a result of having few founding individuals (e.g. Junier et al., 2014). Rhizobia strains can, in addition, vary greatly in the degree to which they promote plant growth, and can even become parasitic on non-target hosts (Thrall et al., 2000; Denison & Kiers, 2004). Thus, even if compatible rhizobia are present in the introduced range, differences in the strains of rhizobia ª 2016 John Wiley & Sons Ltd http://wileyonlinelibrary.com/journal/jbi 1 doi:10.1111/jbi.12690 Journal of Biogeography (J. Biogeogr.) (2016)