Influence of liming, inoculum level and inoculum placement on root colonization of subterranean clover

Abstract.

Arbuscular mycorrhizal (AM) fungi differ in their response to soil pH. Thus, change in soil pH may influence the relative abundance of mycorrhizal fungi inside roots. Root colonization by two AM fungi was studied in relation to addition of lime (CaCO₃₎, quantity of inoculum and inoculum placement. Addition of CaCO₃ 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 CaCO₃ on the relative abundance of fungi inside roots at intermediate rates of CaCO₃ application (pH 5.3–6.3) when both fungi were inoculated together. In this experiment, both fungi colonized roots at all levels of CaCO₃ when inoculated alone, except for A. laevis at the highest level of CaCO₃. We conclude that soil pH affects the competitive 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.