Phosphorus in Poultry Litter and Soil: Enzymatic and Nuclear Magnetic Resonance Characterization

Knowledge of the P forms in poultry litter (PL) and their transformations in soil will help improve our understanding of the long-term role of P in eutrophication. In this study, samples of PL and pasture soils with and without 20 yr of PL application were sequentially extracted to separate P into H₂O, 0.5 mol L⁻¹ NaHCO₃, 0.1 mol L⁻¹ NaOH, and 1 mol L⁻¹ HCl fractions. After appropriate dilution and pH adjustment, the fractions were incubated in the presence of orthophosphate-releasing enzymes. Cross-examination of the solution ³¹P nuclear magnetic resonance spectra of the enzymatically treated and untreated fractions revealed that the peaks of organic P (P₀) species of the enzymatically treated fractions became very weak or disappeared, confirming enzymatic hydrolysis of P₀ in the untreated fractions. Although the majority of P in the NaOH and HCl fractions of PL was in organic forms, these stable P₀ forms could be subjected to enzymatic hydrolysis after being applied to soil, an occurrence that was supported by the soil P data. Compared with soil without litter applied, 20 yr of PL application increased the pools of both labile and stable inorganic P in the soil; however, repeated application of PL did not lead to a significant accumulation of hydrolyzable P₀ in NaOH and HCl fractions, indicating that the stable P₀ must have been converted to other forms. The transformation of stable PL P₀ observed in this study could be an important mechanism for maintaining a balance between labile and immobile P in soils.