A nurse plant benefits from facilitative interactions through mycorrhizae

Phosphorus uptake and toxicity are delimited by mycorrhizal symbiosis in P-sensitive Eucalyptus marginata but not in P-tolerant Acacia celastrifolia

Many plant species from regions with ancient, highly weathered nutrient-depleted soils have specialized adaptations for acquiring phosphorus (P) and are sensitive to excess P supply. Mycorrhizal associations may regulate P uptake at high external P concentrations, potentially reducing P toxicity. We predicted that excess P application will negatively impact species from the nutrient-depleted Jarrah forest of Western Australia and that mycorrhizal inoculation will reduce P toxicity by regulating P uptake. For seedlings of the N₂-fixing legume Acacia celastrifolia and the tree species Eucalyptus marginata, we measured growth at P concentrations of 0-90 mg kg^-1 soil and in relation to inoculation with the arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis. Non-inoculated A. celastrifolia maintained leaf P concentrations at <2 mg g^-1 dry mass (DM) across the range of external P concentrations. However, for non-inoculated E. marginata, as external P concentrations increased, leaf P also increased, reaching >9 mg g^-1 DM at 30 mg P kg^-1 soil. Acacia celastrifolia DM increased with increasing external P concentrations, while E. marginata DM was maximal at 15 mg P kg^-1 soil, declining at higher external P concentrations. Neither DM nor leaf P of A. celastrifolia was affected by inoculation with AMF. For E. marginata, even at 90 mg P kg^-1 soil, inoculation with AMF resulted in leaf P remaining <1 mg g^-1 DM, and DM being maintained. These data strengthen the evidence base that AMF may not only facilitate P uptake at low external P concentrations, but are also important for moderating P uptake at elevated external P concentrations and maintaining plant P concentrations within a relatively narrow concentration range.