While world population is expected to rise to 9.8 billion by 2050, phosphate (P)fertilisers appear to be one of the key factors to maintain food security, as P resources are nonrenewableand being slowly depleted. The expansion of the world population comes along withthe growth of urban areas and accumulation of waste. Sewage sludge, a residual product fromwaste water treatment, is one of the waste materials containing large amount of nutrients thatcan be recycled to land as fertiliser. However, apart from its high P content, sewage sludge mayalso contain organic pollutants, pathogens and sometimes high concentrations of heavy metals,which can prohibit direct field application. Thermal conversion of sewage sludge by incineration,gasification or pyrolysis may be used to reduce the content of potential contaminants withadditional benefits such as energy output. Depending on the thermal conversion method used, aresidual ash or biochar product with a high P concentration is obtained. However, the Pcontained in these products is often poorly available for plants, resulting in rather low plantuptake efficiency when recycled back into the agricultural production chain.FOCUS OF THE PH.D WORKThis PhD study investigated the possibility to use biotic processes to increase Pbioavailability in sewage sludge ashes or biochars, with a focus on P-solubilising microorganisms(PSM). This included testing the possibility to use the ash/biochar as an inoculation carrier aswell as a P-source to solubilise for PSM and investigating if the microbial P-solubilisation couldbe increased by adding nutrients to the ash/biochar substrate. The effect of microbial Psolubilisation was finally compared with other strategies to increase ash P availability, such asnitrogen (N) fertilising approaches, different spatial distribution of the ash/biochar in soil andthe use of plants differing in root exudation.RESULTSThe ash/biochar incubation studies showed that it was possible to grow different Psolubilisingfungi on these materials and at the same time increase their content of soluble P.Moreover, the addition of specific combinations of simple carbon (C) and nitrogen (N) sourcescould enhance the P-solubilisation activity on ash/biochar. The most promising combination offungus, ash and nutrients was tested in a pot trial with semi-sterile soil, but did not show anyclear beneficial effect on plant biomass or P-uptake compared to the untreated ash. A soilincubation followed by a semi-field trial (micro-plot) was carried out in order to verify the effectof the P solubilising fungus on the ash in a plant-less system and in natural environment. Bothsoil incubation and semi-field trial did not show a beneficial effect of the inoculated ash on soil Pavailability.In order to potentially up-scale the inoculated ash production for larger field trials anda possible commercial product, different drying strategies were investigated for the Psolubilising fungi on ash and vermiculite. Drying caused a general decrease of spore survivalwhich could be minimized by applying fluid bed drying using a low desiccation temperature (25°C) and a final water activity above 0.3. The spores on the dried materials could be kept for threemonths without pronounced losses of viability.In a final pot experiment combining the different strategies potentially increasing ash Pavailability, it was confirmed that the ash inoculated with a P-solubilising fungus did not showclear beneficial effects on plant biomass and P-uptake. Using an iso-genic wheat line expressingcitrate excretion on the root tips was also without effect, however, mixing the ash into the soilresulted in a clearly better P fertilising efficacy than ash placement. Furthermore, ammonium(NH4-N) fertilisation caused an increase of ash-P availability in soil due to a greater rhizosphereacidification, but no effect was observed on plant growth, probably due to early stage NH4toxicity.CONCLUSIONSIn conclusion, the results showed that sewage sludge ash/biochar can be used as agrowth substrate and P source to be solubilised for PSM. However, when applied to soil nobeneficial effect of the increased P-solubility was observable. Strategies for rhizosphereacidification such as NH4-N fertilisation may have a greater potential to increase P availabilityfrom sewage sludge ash, but should be further investigated to avoid possible phytotoxic effects.