Moderating mycorrhizas: arbuscular mycorrhizas modify rhizosphere chemistry and maintain plant phosphorus status within narrow boundaries

Pastures often experience a pulse of phosphorus (P) when fertilized. We examined the role of arbuscular mycorrhizal fungi (AMF) in the uptake of P from a pulse. Five legumes (Kennedia prostrata, Cullen australasicum, Bituminaria bituminosa, Medicago sativa and Trifolium subterraneum) were grown in a moderate P, sterilized field soil, either with (+AMF) or without (−AMF) addition of unsterilized field soil. After 9–10 weeks, half the pots received 15 mg P kg−1 of soil. One week later, we measured: shoot and root dry weights; percentage of root length colonized by AMF; plant P, nitrogen and manganese (Mn) concentrations; and rhizosphere carboxylates, pH and plant‐available P. The P pulse raised root P concentration by a similar amount in uncolonized and colonized plants, but shoot P concentration increased by 143% in uncolonized plants and 53% in colonized plants. Inoculation with AMF decreased the amount of rhizosphere carboxylates by 52%, raised rhizosphere pH by ∼0.2–0.7 pH units and lowered shoot Mn concentration by 38%. We conclude that AMF are not simply a means for plants to enhance P uptake when P is limiting, but also act to maintain shoot P within narrow boundaries and can affect nutrient uptake through their influence on rhizosphere chemistry. Pastures often receive a pulse of phosphorus (P) following application of P fertiliser. Response of colonised plants by arbuscular mycorrhizal fungi (AMF) to a P pulse has not previously been addressed in the literature. In this study we investigated three hypotheses associated with this scenario for plants grown in a soil with moderate P levels representative of agricultural systems: 1) AMF moderate shoot P concentration following a P pulse through reducing transport of P from the roots to the shoots; 2) a P pulse causes a decrease in rhizosphere carboxylates; and, 3) mycorrhizal plants have lower amounts of rhizosphere carboxylates. Hypotheses 1) and 3) were supported.