Phosphorus uptake and toxicity are delimited by mycorrhizal symbiosis in P-sensitive but not in P-tolerant .

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 N-fixing legume and the tree species , we measured growth at P concentrations of 0-90 mg kg soil and in relation to inoculation with the arbuscular mycorrhizal fungus (AMF) . Non-inoculated maintained leaf P concentrations at <2 mg g dry mass (DM) across the range of external P concentrations. However, for non-inoculated , as external P concentrations increased, leaf P also increased, reaching >9 mg g DM at 30 mg P kg soil. DM increased with increasing external P concentrations, while DM was maximal at 15 mg P kg soil, declining at higher external P concentrations. Neither DM nor leaf P of was affected by inoculation with AMF. For , even at 90 mg P kg soil, inoculation with AMF resulted in leaf P remaining <1 mg g 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.