Mixing high rates of monoammonium phosphate (MAP) or monocalcium phosphate fertilizers with orchard soil often stimulates growth of newly planted pome fruit trees. This practice may increase As solubility in soils contaminated with lead arsenate pesticide residues because of competitive PO4‐AsO4 exchange. Laboratory experiments were conducted examining As release in batch suspensions of five lead arsenate‐contaminated soils. Dissolved Pb concentrations were <5 µmol/L in all soil suspensions. Redox potential exceeded 500 mV in all suspensions, suggesting that the primary form of dissolved As was probably AsO4. Phosphate‐induced As release was dependent on soil type, occurring in four nonvolcanic soils but not in a pumiceous soil. Arsenic release was positively related to added PO4 concentration but was not significantly influenced by PO4 source. Ammonium sulfate did not enhance As release. Arsenic solubility was consistent with a mechanism of specific PO4‐AsO4 exchange, while PO4 solubility was consistent with control by metastable PO4 minerals. Phosphate‐released As was not appreciably resorbed after multiple wetting and drying cycles. These results explain enhanced As mobility, phytoavailability, and phytotoxicity observed in lead arsenate‐contaminated soils amended with MAP.

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Abstract. Past use of lead arsenate insecticides has resulted in elevated concentrations of lead (Pb) and arsenic (As) in topsoils of many existing and former deciduous tree fruit orchard sites throughout the world. Application of phosphate (PO4)-containing fertilizers to these soils can increase soil As solubility, phytoavailability and downward mobility. A laboratory soil column experiment was conducted to determine if As released by phosphate additions to a topsoil artificially contaminated with lead arsenate (1.65 mmol total Pb/kg; 1.10 mmol total As/kg) would be appreciably resorbed by the underlying uncontaminated subsoil. Treatments were a factorial combination of topsoil amendment with monoammonium phosphate (MAP, 0 or 16.67 mmol POjkg), and amount of leaching ( 1, 5 and I0 pore volume displacements (PVD) with distilled water under saturated flow conditions). Soil As decreased in the topsoil with increasing amount of leaching and increased in the subsoil. Addition of MAP substantially increased loss of topsoil As, promoted As transport into and through the subsoil, and increased dissolved As concentrations in the column leachates. After 10 PVDs, 95% of the initial soil As remained in the -MAP columns, while 56% of the initial soil As remained the +MAP columns. Dissolved Pb concentrations were <0.05 itmol/L in all column leachates. The data are consistent with a mechanism of PO4-enhanced release of As in the topsoil and subsequent promotion of As movement through the subsoil by continuing competition of dissolved As and PO4 for ion adsorption sites. The experimental results indicate that use of PO4-containing fertilizers on lead arsenate-contaminated soils has the potential to greatly enhance downward movement of soil As.