Potential to improve root access to phosphorus: the role of non-symbiotic microbial inoculants in the rhizosphere

Phosphate anions in soil solution are extremely reactive and may be rapidly immobilised in the soil through precipitation and adsorption reactions, resulting in sparingly soluble forms of phosphorus (P) that are essentially unavailable to plants. This low P-fertiliser efficiency is often offset through high application rates, which are economically and environmentally unsustainable and not an available option for organic producers. Microorganisms play a fundamental role in the biogeochemical cycling of inorganic and organic P in the rhizosphere and detritusphere. Free-living rhizosphere microbes can directly increase the availability of phosphate to plant roots via mechanisms associated with solubilisation and mineralisation of P from inorganic and organic forms of total soil P. These include releasing organic anions, H+ ions, phosphatases, and cation chelating compounds into the rhizosphere. Many soil-borne microbes also increase P availability indirectly by producing phytohormones that increase root density and function. There is increasing interest worldwide in the use of rhizosphere microorganisms as inoculants to increase P availability for agricultural production. Recent research has focussed on developing actively sporulating Penicillium fungi known to express mechanisms to enhance P mobilisation and therefore, considered to be a key component of the mycoflora involved in P cycling in soils. Penicillium species do not exhibit specific plant or soil associations and have a broad agro-ecological range, indicating their potential to be developed as inoculants for a range of plant production systems. Successful adoption of microbial inoculants requires a thorough understanding of their rhizosphere ecology, genetic stability, and the mechanisms associated with enhancing P availability in soils and plant-growth promotion. This will provide a better understanding of which inoculants to use under particular agro-ecological conditions for increased efficacy and consistent performance.