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

Abstract

Sequential fractionation coupled with phosphatase hydrolysis allows a greater understanding of the effects of animal manure on the chemical distribution of soil P. Concentrations of specific soil P fractions were determined after long-term (>10 yr) poultry litter application at rates of 4.5, 6.7, 9.0, 11.2, and 13.4 Mg manure ha−1 yr−1 to watershed-scale plots (cultivated and grazed–ungrazed pasture) on a calcareous Texas Blackland Vertisol. Soil total extractable P (Pt) and inorganic P (Pi) were quantified following sequential extraction with H2O, NaHCO3, NaOH, and HCl. Hydrolyzable organic P (Pe) and non-hydrolyzable organic P (Pne) were determined in the extracted fractions following enzymatic hydrolysis. Litter application increased Pt regardless of land-use type compared with the control. Concentrations of H2O-extractable Pi in litter-amended plots increased by 9 to 34% (cultivated) and 7 to 30% (pasture) over the control, indicating substantial risk of soluble P runoff. Labile organic P (Po) extracted with H2O and NaHCO3 decreased in the order monoester > nucleic acid > phytate > Pne. An average of 68% of Pt was extractable with HCl. Organic P comprised the majority (95%) of HCl-Pt; however, only trace levels of HCl-Po were hydrolyzable, and litter application increased HCl-Pne up to 217%. Thus, litter application increased levels of both soluble Pi and stable Pne, but the specific response varied with application rate and land management. This study increased understanding of P chemical distribution with time in litter-amended soil with high clay and CaCO3 contents under differing land-use scenarios.