Phosphorus in urban sewage sludge is a potential source of P for agriculture if it is plant-available. In that case it could be an alternative to mineral P fertilizers known as non renewable source. However, to manage TCSS it is necessary to have an accurate assessment of its P fertilizing value. This work aimed to assess and compare the plant availability of P in TCSS and triple superphosphate (TSP). A pot experiment with a mixture of ryegrass and fescue was carried out for 2 months on a P-deficient and slightly acidic soil in which 50 mgP.kg–1 was applied as TSP and TCSS. The 32P–labeling of soil P was used to accurately quantify the P taken up by plants from the different P sources and to evaluate interactions after applying P-TCSS on P taken up from soil. In parallel, soil incubations were conducted on the same treatments to analyze soil respiration and soil microbial dynamics. In comparison to the control (no P applied), shoot and root biomass productions, and plant P nutrition increased with TSP and TCSS application. The contribution of TCSS and TSP to plant nutrition was 55.2% and 55.7%, respectively and the relative agronomic effectiveness of TCSS was 64%. This decrease is explained by the carbon associated with the 50 mgP.kg-1 applied to soil. This carbon initiates a priming-effect with a high microbial activity marked by high soil respiration and high P content in microbial biomass which competed for plant-available P and delayed the root and shoot growth. As a consequence, P taken up from soil and TCSS decreased both. In conclusion, this study shows that it is possible to substitute TSP by TCSS.Read less <