Pastures often experience a pulse of phosphorus (P) when fertilised. We examined the role of arbuscular mycorrhizal fungi (AMF) in uptake of P from a pulse. Five legumes (Kennedia prostrata, Cullen australasicum, Bituminaria bituminosa, Medicago sativa, Trifolium subterraneum) were grown in a moderate P, sterilised field soil, either with (+AMF) or without (-AMF) addition of unsterilised field soil. After 9 to 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 colonised 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 uncolonised and colonised plants, but shoot P concentration increased by 143% in uncolonsied plants and 53% in colonised 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.
Via Jean-Michel Ané