Abstract Arbuscular mycorrhizal (AM) fungi differ in their response to soil pH. Thus, change in soil pH may in- fluence the relative abundance of mycorrhizal fungi inside roots. Root colonization by two AM fungi was studied in relation to addition of lime (CaCO 3) , quantity of inoculum and inoculum placement. Addition of CaCO 3 to an acid soil decreased the colonization of roots by Acaulospora laevis but increased colonization by Glomus invermaium when both fungi were present. In acid soil (pH 4.7), almost all roots were colonized by A. laevis, while G. invermaium was dominant when soil pH was increased to pH 7.3. This occurred regardless of whether the inoculum was banded or mixed throughout the soil. There was no effect of CaCO 3 on the relative abundance of fungi inside roots at intermediate rates of CaCO 3 application (pH 5.3–6.3) when both fungi were inoculated together. In this experi- ment, both fungi colonized roots at all levels of CaCO 3 when inoculated alone, except for A. laevis at the highest level of CaCO 3 . We conclude that soil pH affects the com- petitive ability of these two AM fungi during mycorrhiza formation primarily by affecting hyphae growth in soil and thus the relative abundance of hyphae at the root surface and subsequently inside the root. Keywords Subterranean clover · pH · Glomus invermaium · Acaulospora laevis · Interactions Introduction Field soils contain diverse communities of arbuscular mycorrhizal (AM) fungi (Smith and Read 1997). Al- though root systems usually become colonized by more than one AM species (Daft and Nicolson 1974; Abbott and Robson 1982; Merryweather and Fitter 1998; Jacquot et al. 2000), little is known of the interactions between species during mycorrhiza formation (Juniper et al. 1998). The quantity of mycorrhizas formed by spe- cies of AM fungi in field soils will depend on both the amount of infective hyphae and their relative competi- tive ability (Abbott and Robson 1984). The length of root colonized by AM fungi at early stages of root growth is related to inoculum density (Carling et al. 1979; Abbott and Robson 1984; Wilson 1984; Giovannetti and Avio 1986) and species of AM fungi differ in their competitive ability (Wilson and Trinick 1983; Abbott and Robson 1984; Wilson 1984; Lopez- Aguillon and Mosse 1987; Smith et al. 2000). The dif- ferences in competitive ability to form mycorrhizas may be based on antagonism (Hepper et al. 1988), de- pletion of nutrients within roots (Wilson and Trinick 1983) and carbon demand (Pearson et al. 1993). The compounding effects on colonization of both inoculum level (Abbott and Robson 1984) and rate of hyphae growth from inoculum to root surface have not been clarified. AM fungi clearly differ in their response to soil pH (see review by Robson and Abbott 1989). Increasing the pH of an acid soil may result in marked changes in the species of AM fungi present as assessed by spores in the soil (Siqueira et al. 1990). Acaulospora laevis, for exam- ple, is generally found more in acid than in neutral soils (Davis et al. 1985; Porter et al. 1987a; Young et al. 1985) and germination of spores was limited at >pH 6 (Hepper 1984; Porter 1987b). On the other hand, an isolate of Glomus invermaium appeared less affected by soil pH, colonizing roots of subterranean clover (Trifolium sub- terraneum L.) over the soil pH range of 5.3 to 7.5 (Abbott and Robson 1985). We investigated the role of soil pH and inoculum quantity on the relative competitive ability of A. laevis and G. invermaium. In one experiment, we varied soil pH by adding CaCO 3 and the soil was inoculated throughout with propagules of A. laevis. Subsequently, S.M. Sano · L.K. Abbott ( ✉ ) · M.Z. Solaiman · A.D. Robson Soil Science and Plant Nutrition, Faculty of Natural and Agricultural Sciences, The University of Western Australia, Crawley, WA 6009, Australia e-mail: labbott@cyllene.uwa.edu.au Tel.: +61-8-93802499, Fax: +61-8-93801050 Present address: S.M. Sano, Embrapa CPAC, Caixa Postal 08223, Planaltina-DF, CEP 73301-970 Brazil Mycorrhiza (2002) 12:285–290 DOI 10.1007/s00572-002-0185-6 ORIGINAL PAPER S. M. Sano · L. K. Abbott · M. Z. Solaiman A. D. Robson Influence of liming, inoculum level and inoculum placement on root colonization of subterranean clover Received: 4 December 2001 / Accepted: 21 May 2002 / Published online: 20 July 2002 © Springer-Verlag 2002