Phosphorus availability from the solid fraction of pig slurry is altered by composting or thermal treatment

The alteration of easily available phosphorus (P) from the separated solid fraction of pig slurry by composting and thermal processing (pyrolysis or combustion at 300-1000 °C) was investigated by water and acidic extractions and the diffusive gradients in thin films (DGT) technique. Temporal changes in P availability were monitored by repeated DGT application in three amended temperate soils over 16 weeks. P availability was found to decrease in the order: drying > composting > pyrolysis > combustion with increasing degree of processing. Water extractions suggested that no P would be available after pyrolysis above 700 °C or combustion above 400 °C, respectively, but during soil incubation, even char and ash, processed at 800 °C, increased P availability. Low-temperature pyrolysis vs. combustion was found to favor P availability as did application to acidic vs. neutral soil. Composting and thermal treatment produced a slow-release P fertilizer, with P availability being governed by abiotic and biotic mechanisms.