Mycorrhizal symbiosis and phosphorus supply determine interactions among plants with contrasting nutrient‐acquisition strategies

Highly diverse plant communities growing on nutrient-impoverished soils are test beds for theories on species coexistence. Here, neighbouring mycorrhizal and non-mycorrhizal plants compete for limited phosphorus. The impact of below-ground interactions on community dynamics is underexplored. We used an experimental approach to investigate effects of inoculation with arbuscular mycorrhizal (AM) fungi and a phosphorus supply gradient on competitive and facilitative interactions among mixed assemblages of woody plants in microcosms. The plant species, one cluster root-forming (CR) species and four AM species, are native to jarrah forest that grows on nutrient-impoverished soils in south-western Australia. We measured plant growth in microcosms, with and without inoculation with the AM fungus Rhizophagus irregularis, and across a gradient of P supply: 0, 9, 27 and 243 mg P per kg of soil. Our data show evidence of plant–plant facilitation at low P supply and competition at high P supply. Growth of the CR species, Hakea undulata, was highest in microcosms with 0P and without AM inoculation. One AM species, Bossiaea aquifolium, also performed better at lower P levels, possibly benefitting from P mobilised by H. undulata. The other three AM species, one strongly obligates, performed better at higher P levels. Data for Acacia celastrifolia suggested it was facultatively mycotropic, and because there was no correlation between AM colonisation and the relative inoculum effect, we suggest positive effects of AM inoculation at 9P might be due to benefits other than P acquisition, such as pathogen defence. Benefit of AM inoculation diminished for three of four mycorrhizal species at the highest P level as we had predicted. The fourth species, Eucalyptus marginata (jarrah), had higher growth in microcosms that were not inoculated with AM, perhaps because the species benefits more from ectomycorrhizas. Synthesis. Our experimental data suggest spatial heterogeneity of soil P, coupled with a diversity of nutrient-acquisition strategies, and plasticity among plant–plant and plant–AM fungi interactions, contributes to plant species coexistence in the nutrient-impoverished jarrah forest. Our research highlights the importance of below-ground mechanisms for understanding factors determining community structure including a potential role of AM fungi in plant pathogen defence