Oxygen isotope (180) as a tool to assess fungal transformations of phosphorus in soil

Abstract

Most soils in the world have low phosphate (Pi) availability, which limits plant growth. Among phosphorus (P) pools in soil, P immobilized by microorganisms stands out for its importance in P cycling in soil as well as its role as a P reserve for plants. In this context, mycorrhizal fungi (MF) have shown beneficial effects on Pi uptake by plants, by promoting P mineralization/solubilization in soil and enabling plants to exploit nutrients in larger volumes of soil. After being taken up by MFs, Pi is incorporated as polyphosphate in hyphae, then can be subsequently hydrolyzed to Pi, and finally taken up by plant roots. However, studies about the contribution of P found in MF hyphae to plant nutrition are still incipient. This is mainly due to methodological difficulties that prevent confirming the precise effect of this P pool. In this context, the use of oxygen stable isotopes in phosphate (18O-P) could enable an accurately determination of the origin of P accumulated in the plant tissue. This is possible because P is always bound to oxygen atoms in Pi, and only biological processes can modify the signature of 18O in phosphate. In this proposal, our aim is to assess whether 18O-P is a viable tool to determine the presence and quantify the polyphosphate pool present in MFs. For this, we will carry out three experiments, as follows: 1- Determine the best 18O-enrichment to study phosphorus accumulation in soil fungi; 2-Assess whether there are differences in 18O isotopic signatures of saprophytic fungi in forest and agricultural soils; and 3- Evaluate whether there are variations in the isotopic signature of 18O-P between ectomycorrhizal fungi and arbuscular mycorrhizal fungi. The results of these experiments will allow obtaining relevant information about the efficiency of the 18O-P tracer to evaluate the polyphosphate transformations in soil-plant systems, and if there are variations in the 18O-P signature between different groups of fungi found in forest and agricultural soils. (AU)