An Alum‐Based Water Treatment Residual Can Reduce Extractable Phosphorus Concentrations in Three Phosphorus‐Enriched Coastal Plain Soils

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Abstract
The accumulation of excess soil phosphorus (P) in watersheds under intensive animal production has been linked to increases in dissolved P concentrations in rivers and streams draining these watersheds. Reductions in water dissolved P concentrations through very strong P sorption reactions may be obtainable after land application of alum‐based drinking water treatment residuals (WTRs). Our objectives were to (i) evaluate the ability of an alum‐based WTR to reduce Mehlich‐3 phosphorus (M3P) and water‐soluble phosphorus (WSP) concentrations in three P‐enriched Coastal Plain soils, (ii) estimate WTR application rates necessary to lower soil M3P levels to a target 150 mg kg−1 soil M3P concentration threshold level, and (iii) determine the effects on soil pH and electrical conductivity (EC). Three soils containing elevated M3P (145–371 mg kg−1) and WSP (12.3–23.5 mg kg−1) concentrations were laboratory incubated with between 0 and 6% WTR (w w−1) for 84 d. Incorporation of WTR into the three soils caused a near linear and significant reduction in soil M3P and WSP concentrations. In two soils, 6% WTR application caused a soil M3P concentration decrease to below the soil P threshold level. An additional incubation on the third soil using higher WTR to soil treatments (10–15%) was required to reduce the mean soil M3P concentration to 178 mg kg−1 After incubation, most treatments had less than a half pH unit decline and a slight increase in soil EC values suggesting a minimal impact on soil quality properties. The results showed that WTR incorporation into soils with high P concentrations caused larger relative reductions in extractable WSP than M3P concentrations. The larger relative reductions in the extractable WSP fraction suggest that WTR can be more effective at reducing potential runoff P losses than usage as an amendment to lower M3P concentrations.