Enhancing the phosphorus bioavailability of thermally converted sewage sludge by phosphate-solubilising fungi

•Different fungal strains solubilised phosphorus from sewage sludge thermal residues.•The highest rate of P solubilisation was found in sewage sludge incineration ash.•P. bilaiae P solubilisation increased with the addition of specific C and N sources.•P. bilaiae amended ash did not promote ash fertilising effect in a pot trial. Biochars and ashes from sewage sludge have a high phosphorus (P) content, but plant P availability is typically rather low. Phosphate-solubilising microorganisms (PSM) have been shown to have the ability to solubilise P from different compounds. The aim of this study was to explore the P-solubilisation potential of different PSM on various biochars and ashes, and the effect of the addition of different carbon (C) and nitrogen (N) sources on their P-solubilisation activity. The most promising combination of PSM, thermal residue and nutrients was then tested for its effect on plant growth and P uptake in a pot trial. Six PSM strains (four Penicillium bilaiae (Pb), one Penicillium aculeatum (Pa) and one Aspergillus niger (An)) were tested on two sewage sludge ashes and one biochar. Pb.4 and An showed the highest P-solubilisation rates on fluid-bed incineration (FB-I) ash. Pb.4 solubilised higher amounts of P when it was supplied with fructose in combination with NH4-N, while An performed equally well with fructose, maltose, mannose and xylose in combination with NH4-N. Increasing the concentration of the C source generally also increased the P solubilisation. However, when FB-I ash was inoculated with Pb.4 plus xylose/NH4-N and applied to spring wheat in a pot trial with γ-irradiated soil, the inoculation did not significantly affect plant shoot biomass or P uptake. The results indicate that the amount and temporal availability of P solubilised by the fungal strain from the ash did not match plant requirements, suggesting that further work is required that focuses on further increasing solubilisation efficiency.