Successful application of microbial biofertilizers, such as phosphorus (P) solubilizing fungi to agroecosystems, is constrained from the lack of knowledge about their ecology; for example in terms of how they respond to an external input of carbon (C) to get established in the soil. In two soil incubation experiments we examined the performance of the P solubilizing fungus Penicillium aculeatum in non-sterile and semi-sterile (γ-irradiated) soil with different C and P sources. Results from the first experiment with C sources showed that starch and cellulose generally improved P solubilization by P. aculeatum measured as water extractable P (P_wep), though only significantly in non-sterile soil. This coincided with an increased population density of P. aculeatum measured with a hygromycin B resistant strain of this fungus. Soil respiration used to measure soil microbial activity was overall much higher in treatments with C compounds than without C in both non-sterile and semi-sterile soil. However, soil respiration was highest with cellulose in semi-sterile soil, especially in combination with P. aculeatum. Hence, for the second experiment with P sources (tricalcium phosphate (TCP) and sewage sludge ash) cellulose was used as a C source for P. aculeatum growth in all treatments. Main results showed that P. aculeatum in combination with cellulose soil amendment increased soil P_wep independent of soil sterilization and P source treatments. Soil resin P (P_res) and microbial P (P_mic), which represents stocks of potentially plant available P, were also affected from P. aculeatum inoculation. Increased soil P_res from TCP and sewage sludge ash was observed with P. aculeatum independent of soil type. On the other hand soil P_mic was higher after P. aculeatum inoculation only in semi-sterile soil. Population density of P. aculeatum measured with qPCR was maintained or increased in non-sterile and semi-sterile soil, respectively, compared to the original inoculum load of P. aculeatum. In conclusion, our results underline the importance of C source addition for P. aculeatum if used as a biofertilizer. For this, cellulose seems to be a promising option promoting P. aculeatum growth and P solubilization also in non-sterilized soil.