Pig slurry acidification, separation technology and thermal conversion affect phosphorus availability in soil amended with the derived solid fractions, chars or ashes

Aims The aim was to study effects of slurry acidification, separation technology and thermal processing on the availability of P in soil amended with the solid fraction of pig slurry. Methods Acidified and non-acidified slurry were separated using different technologies: screw press (SCR), decanting centrifuge (DEC) and drainage after chemical pretreatment (CHE). Solids and pyrolysed (400 A degrees C or 600 A degrees C, char) or combusted (625 A degrees C, ash) solids were applied to two soils, with triple superphosphate (TSP) as a reference. Soil P availability was determined over 12 weeks using diffusive gradients in thin films (DGT). Results The initial availability of P from DEC and CHE solids was similar to TSP in both soils. After 6 weeks no significant difference was observed between DEC solids and DEC chars. Acidification did not significantly affect P availability in the solids-amended soils over time, but it did affect P availability with the thermally processed materials. Conclusions Application of separated pig slurry solids generally increased soil P availability initially, but declining with time, indicating soil P fixing reactions. Chars and ashes on the other hand showed lower initial P availability, but remained constant or increased slightly with chars yielding P availability similar to solids after 12 weeks.