Lead retention in a calcareous soil influenced by calcium and phosphate amendments

•Adding Ca ions with soluble phosphate in a Pb-spiked calcareous soil reduced Pb availability.•Plant growth was also improved by the amendment with Ca.•Ca may help immobilization of Pb by sorbing Pb2+ on newly formed Ca–P compounds.•Adsorption–substitution is an important mechanism for P–Pb reactions in calcareous soil. Phosphate amendments in calcareous lead (Pb)-contaminated soils to immobilize Pb may be hindered due to competition of Pb with calcium (Ca) that may inhibit the retention of Pb as a precipitation mechanism. This study explored the retention of Pb in a calcareous soil spiked and aged with 500mgkg−1 Pb2+ and amended with H2PO4−. In addition, Ca2+ was added immediately or three days before or after phosphate, after which ryegrass (Lolium perenne) was planted. Diethylene triamine pentaacetic acid (DTPA)-extractable Pb of the soils in which Ca was added immediately after phosphate was lower than those only amended with phosphate or when Ca was added three days before or after phosphate. The addition of Ca immediately after phosphate resulted in the greatest reduction of ammonia acetate-exchangeable Ca concentration, Olsen-P concentration, and carbonate phase Pb. Higher plant biomass yields were observed for the simultaneous P and Ca treatment. The results of the study demonstrate that co-added Ca and P may help reduce Pb availability by forming fresh Ca–P sorbent phases to retain Pb through an adsorption–substitution mechanism, which may play an important role in the sequestration of Pb in calcareous soils with soluble phosphate in addition to the mechanism of the direct precipitation as pyromorphite.