Phosphorus uptake and toxicity is 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 specialised adaptations for acquiring 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 to 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 9 mg g-1 DM at 30 mg P kg-1 soil. A. 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 were 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.