Slurry acidification and anaerobic digestion affects the speciation and vertical movement of particulate and nanoparticulate phosphorus in soil after cattle slurry application

Livestock slurry contains a large amount of particulate organic matter. When applied to soil these particles can act as carriers transporting phosphorus (P) through and over soils to watercourses. Little is known, however, about how slurry treatment (e.g. acidification and anaerobic digestion) may affect slurry particle size distribution and P forms within the particulate fraction. To characterize the small slurry particles, we separated untreated, acidified and anaerobically digested cattle slurry into 0.45–63 μm, <0.45 μm, <100 kDa and <3 kDa fractions and determined their P speciation. To understand the role of slurry particles on P leaching/translocation in soil, we applied the <63 μm, <0.45 μm and <100 kDa fractions of untreated and processed slurries to the surface of an unplanted agricultural, sand-textured soil in laboratory microcosms and studied P movement under a simulated rainfall regime. Results showed that the 0.45–63 μm particulate fraction accounted for >60% of total P content of the untreated and anaerobically digested cattle slurries. Acidification reduced the total P quantity of the 0.45–63 μm fraction by 26% through dissolution and desorption, resulting in a greater P in the <0.45 μm fraction. Anaerobic digestion increased the total P content of the 0.45–63 μm fraction, as the decomposition and breakdown of coarser particles transferred more P to this fraction. After application to soil, 3.3% of the added inorganic P and 1.7% of the added organic P was found in the leachate in the untreated <63 μm treatment. The 0.45–63 μm particulate fraction largely contributed to this P leaching, while the nanoparticulate fraction (3 kDa-0.45 μm) contained little P and had no appreciable effect on P leaching. Overall, acidification had no impact on P leaching. In contrast, anaerobic digestion increased inorganic P leaching by 67% and organic P leaching by 127%. In conclusion, our findings highlight the importance of the 0.45–63 μm fraction in P transport through soil, and the enhanced bioavailability and mobility of P following anaerobic digestion. These need greater consideration in future studies promoting the sustainable use of livestock waste and P recovery technologies.