Acidification and anaerobic digestion change the phosphorus forms and distribution in particle fractions of cattle slurry and phosphorus dynamics in soil after application

Advanced slurry processing (anaerobic digestion, acidification and separation) is being promoted to generate renewable energy, reduce ammonia emissions and optimise storage and transport, respectively. However, these processes could affect slurry phosphorus (P) forms and bioavailability, and hence P dynamics in soil after application. This study investigated the effect of slurry processing, viz. separation, acidification and anaerobic digestion on P forms and distribution of cattle slurry, as well as P dynamics in soil following application of mechanically separated solid and liquid cattle slurry. The results showed that acidification and anaerobic digestion had little impact on TP distribution among particle fractions of slurry, but had substantial influence on P speciation and distribution. Acidification increased the dissolved inorganic-P (IP–H2O) proportion in cattle slurry, by reducing the mineral surface adsorbed (IP–NaHCO3) and precipitated inorganic P (IP–HCl) proportion. Anaerobic digestion decreased OP-NaHCO3 proportion by degradation and increased the IP-NaHCO3 and IP-HCl proportion by adsorption and precipitation. Particles <63 μm accounted for over 65% of P present in cattle slurry. The above changes in whole slurry made by acidification and anaerobic digestion were dominated by the <63 μm fraction. Slurry-derived soluble inorganic-P in soil decreased with time, due to adsorption and microbial immobilisation. This decrease was slower in the soil amended with anaerobically digested slurry. Results suggest that anaerobic digestion and acidification change P speciation of separated solid and liquid slurry and the subsequent P dynamics in soil after application.