Phosphate reactivity in long-term poultry litter-amended southern Delaware sandy soils

Eutrophication caused by dissolved P from poultry litter (PL)-amended agricultural soils has been a serious environmental concern in the Delaware-Maryland-Virginia Peninsula (Delmarva), USA. To evaluate state and federal nutrient management strategies for reducing the environmental impact of soluble P from long-term PL-amended Delaware (DE) soils, we investigated (i) inorganic P speciation; (ii) P adsorption capacity; and (iii) the extent of P desorption. Although the electron microprobe (EMP) analyses showed a strong correlation between P and Al/Fe, crystalline Al/Fe-P precipitates were not detected by x-ray diffraction (XRD). Instead, the inorganic P fractionation analyses showed high levels of oxalate extractable P, Al, and Fe fractions (615-858, 1215-1478, and 337-752 mg kg^-1, respectively), which were susceptible to slow release during the long-term (30-d) P desorption experiments at a moderately acidic soil pH_water. The labile P in the short-term (24-h) desorption studies was significantly associated with oxalate and F extractable Fe and Al, respectively. This was evident in an 80% reduction maximum in total desorbable P from NH₄ oxalate/F pretreated soils. In the adsorption experiments, P was strongly retained in soils at near targeted pH of lime (≈6.0), but P adsorption gradually decreased with decreasing pH near the soil pH_water (≈5.0). The overall findings suggest that P losses from the can be suppressed by an increase in the P retention capacity of soils via (i) an increase in the number of lime applications to maintain soil pH_water at near targeted pH values, and/or (ii) alum/iron sulfate amendments to provide additional Al- and Fe-based adsorbents.