INSOLUBLE FE-ASSOCIATED INORGANIC AND ORGANIC PHOSPHATES IN ANIMAL MANURE AND SOIL.

Identifying and quantifying insoluble and reducible phosphorus (P) species in animal manure and soil are important issues in environmental P chemistry. We applied a joint dithionite-3-phytase incubation at pH 5.0 (100 mM acetate buffer) to investigate reducible P in animal manure and soil. After removal of soluble phosphate by 100 mM acetate (pH 5.0), poultry litter and dairy manure solids were incubated with the reducing agent sodium dithionite and/or 3-phytase to identify insoluble manure P species. Poultry litter contained most (spontaneous) labile P (9.7% insoluble P), reducible inorganic P (3.2% insoluble P), non-Fe-associated (reducibleirrelevant) organic P (5.7% insoluble P), and Fe-associated (reducible) organic P (16.2% insoluble P). In dairy manure, 51.5% insoluble P was most labile P, 28.1% non-Fe-associated organic P, and 20.4% Fe-associated organic P, but no reducible inorganic P. In a sandy loam soil, only Fe-associated inorganic P (682 mg kg-1 of dry soil) was observed compared to the original 2 mg soluble P kg-1 of dry soil). These data indicate that changes in the chemical and biological environments of manure and soil, such as anaerobic conditions and enzymatic activity, can influence manure P solubility and therefore bioavailability.