Abstract

Amending poultry litter (PL) with aluminum sulfate (alum) has proven effective in reducing water-soluble phosphorus (P) in the litter and in run-off from fields that have received PL applications, and it has therefore been suggested as a best management practice. Although its effectiveness has been demonstrated on a macroscopic scale in the field, little is known about P-speciation in either alum-amended or unamended litter. This knowledge is important for the evaluation of long-term stability and bioavailability of P, which is a necessary prerequisite for the assessment of the sustainability of intensive poultry operations.

Solid-state MAS and CP-MAS ³¹P-NMR as well as ³¹P{²⁷Al}TRAPDOR was used to investigate the P speciation in alum-amended and unamended PL. The results indicate the presence of a complex mixture of organic and inorganic orthophosphate phases. A calcium phosphate phase, probably a surface precipitate on calcium carbonate, could be identified in both unamended and alum-amended PL, as well as phosphate bound by hydrogen bonds to adsorbed water molecules or organic functional groups. Phosphate associated with Al was found in the alum-amended PL, present either as a poorly ordered Wavellite or as phosphate surface complexes on aluminum hydroxide that had been formed by the hydrolysis of alum. However, a complex mixture of organic and inorganic phosphate species could not be resolved.

Preliminary data about the effects of storage of the samples indicated that more phosphate is present as calcium phosphate and in association with aluminum hydroxide after seven months of storage, while the quantity of the phosphate pool, which could not be resolved by solid-state NMR, decreased and the physical bound phosphate completely disappeared.

Combining solid-state NMR-spectroscopy with a chemical extraction sequence confirmed that calcium phosphate phases present in the litter are removed only by extraction with hydrochloric acid. However, phosphate associated with aluminum as surface complexes on aluminum hydroxide or in an uncondensed aluminum hydroxyphosphate, was not exclusively extracted by sodium hydroxide, as has been proposed in the literature, but also by hydrochloric acid.