Interactions between rhizosphere microorganisms and plants governing iron and phosphorus availability

Abstract

Because Fe availability is low in most aerobic soil, microorganisms and plants release low molecular-weight compounds (chelators) which increase Fe availability. Microorganisms appear to be far more competitive than plants: they can utilise Fe bound to plant-derived chelators and decompose them, whereas microbial chelators are poor Fe sources for plants. However, some plants, such as grasses, grow well in Fe-deficient soils, which may be explained by the spatially and temporarily concentrated release of phytosiderophores. Plants and microorganisms have developed a number of strategies to increase soil P availability. Microorganisms can increase plant P uptake by mobilising more P than they require and by stimulating root growth and mycorrhizal colonisation. However, microorganisms may also decrease P availability by (i) net P immobilisation in their biomass, (ii) decomposition of P-mobilising root exudates and (iii) decreasing root growth or mycorrhizal colonisation. Depending on the availability of carbon, the microbial biomass can influence Fe and P availability to plants by acting as either a source or a sink. We propose the following hypothesis: at high availability of carbon such as in the zone immediately behind the root tip, Fe and P immobilisation dominates, whereas in the mature root zones with decreased C availability, mineralisation is dominant. While net Fe and P immobilisation behind the root tip is likely to directly decrease plant uptake, net mineralisation along the mature root zones that have a low capacity for nutrient uptake may have a relatively small effect on plant uptake.