The correlation between Pb species formation and bioaccessibility in alkaline, smelter-impacted soil co-contaminated with other toxic trace elements after treatment with phosphorus-containing amendments was investigated. The soil was collected near a former copper smelter, El Paso, Texas. It contained Pb (3200 +/- 142 mg kg-1), As (254 +/- 14 mg kg-1), and Cd (110 +/- 8 mg kg-1). The soil was mixed with two organic and two inorganic P fertilizers, and incubated for up to 24 weeks. Bioaccessible concentrations were measured periodically using a modified physiologically based extraction test. Speciation of Pb was performed using synchrotron techniques and compared with the sequential extraction test data. Results indicated a significant decrease in bioaccessible-Pb,-As, and-Cd and pH over time in treated soils compared to untreated soil. Synchrotron analysis and sequential extraction confirmed the formation of Pb-associated P and Fe minerals, including pyromorphite, Pb bound to Fe, and organic complexes. Class A Fe-rich biosolids outperformed all other treatments in terms of reducing bioaccessible-Pb and transforming more Pb to residual fractions. Biosolids, triple superphosphate, and monoammonium phosphate effectively diminished exchangeable Pb fractions and formed stable fractions in the studied alkaline soil. These findings have practical implications for in situ soil remediation strategies in arid regions soils via reduction of Pb bioavailability.