Legacy Phosphorus in Calcareous Soils: Effects of Long-Term Poultry Litter Application.

The effect of manure application on soil phosphorus (P) has been intensively studied with modifications of the Hedley sequential fractionation procedure, X-ray absorption near-edge structure spectroscopy, and ³¹P-nuclear magnetic resonance. Modern sequential fractionation techniques, coupled with phosphatase hydrolysis, have recently allowed for greater understanding of manure effects on soil P distribution. We evaluated the effect of long-term (> 10 years) poultry litter (broiler and turkey litter) application (4.5, 6.7, 9.0, 11.2, and 13.4 Mg manure ha^-1) on P in watershed-scale plots (cultivated and grazed/ungrazed pasture) near Riesel, Texas. Soil inorganic P (P_i) and organic P (P_o) were quantified by sequential extraction (H₂O, NaHCO₃, NaOH, and HCl) and hydrolysis with acid phosphomonoesterase, nuclease P1, and 3-phytase (P_e). Litter application increased the total extractable P (P_t) in all plots, regardless of land-use type; application at 13.4 Mg ha^-1 increased P_t in cropland and pasture by 23- and 8-fold, respectively. Compared to control, H₂O-P_i from litter-amended plots increased by 9 to 34-fold (cultivated), and 7- to 30-fold (pasture), indicating substantial risk of soluble P runoff. Most labile P_o (extracted with H₂O and NaHCO₃) was monoester-like, and decreased in the order of monoester > nucleic acid > phytate > non-hydrolysable organic P (P_ne). Litter application did not affect NaOH-P_i or NaOH-P_e (P > 0.05), but increased NaOH-P_ne: the general composition was P_i > monoester > phytate > nucleic acid > P_ne. An average of 68% of P_t was extractable with HCl. The HCl-P composition was predominantly (95%) P_o, and increased 2- to 3-fold by high rates of litter application in cultivated and grassed plots, respectively. Only trace levels of HCl-P_e were found; however, litter application increased HCl-P_ne by 1- to 3-fold. Thus, repeated litter application increased levels of both soluble P_i and stable, non-hydrolysable P_o, but specific response varied with application rate and land management.