Interactive effects of arbuscular mycorrhizal symbiosis, intraspecific competition and resource availability on Trifolium subterraneum cv. Mt. Barker

We performed two glasshouse experiments to determine whether the presence o f arbuscular mycorrhizal symbiosis reduces the intensity of intraspecific c ompetition at low concentrations of available phosphorus (P), and whether t his effect is modified by a reduction in light intensity. In the first expe riment, Trifolium subterraneum cv. Mt. Barker was grown at different densit ies in controlled conditions of light and temperature, and half of the pots were inoculated with spores of the arbuscular mycorrhizal fungus, Gigaspor a margarita. In the second experiment, the plants were grown in similar con trolled conditions but the light intensity received by half of the pots was reduced by >50%. The biomass and P content of individual mycorrhizal plant s and the biomass response to mycorrhizal infection were drastically reduce d as plant density increased. The effects of density on percentage infectio n, shoot and root P concentrations, and root: shoot ratios were inconsisten t. Generally reduction in light intensity did not alter these effects. Myco rrhizal symbiosis increased intraspecific competition intensity thorough an increase in the availability of soil P. This increase in competition was r eflected in the greater size inequality of low density mycorrhizal treatmen ts. Our results emphasize that the main effects of mycorrhizas at the indiv idual level cannot be expected to be apparent at the population level, beca use they are overridden by density-dependent processes.