Transformation of phosphorus in sewage sludge biochar mediated by a phosphate-solubilizing microorganism

Producing sewage sludge biochar followed by applying it as a P fertilizer is considered to be a competitive way to recover and reuse P resources. To apply sewage sludge biochar as a P fertilizer, it is essential to understand the transformation pathway of different P species in biochar mediated by soil microorganisms, as soil microorganisms are closely related to the mineralization, immobilization, redox reaction, and solubilization of various nutrients in the soil. However, this process has largely been ignored by previous studies. Here, two sludge biochars produced at different temperature (400 and 700 °C, denoted as B400 and B700) were incubated with a phosphate-solubilizing microorganism (i.e., Pseudomonas putida (P. putida)) that is closely involved in soil P turnover. The release pattern and transformation of biochar-P were investigated in both abiotic and biotic conditions. The results demonstrate that the release of biochar-P could be enhanced by P. putida. The P species in B400 were more vulnerable to P. putida than those in B700, as the P species in B400 had lower polymerization degree and poorer crystal structure than those in B700 did. The Pyro-P released from biochar could be easily transformed into Ortho-P by P. putida, which can further benefit other species in soil. This study provides insights on the release and transformation of sludge biochar-P mediated by PSM and reveals the effect of the pyrolysis temperature on biochar-P release and transformation which is essential for the production and application of sewage sludge biochar.