Extractability of Pb in urban gardens and orchards linked to soil properties

The hypothesis that lability and bioavailability of lead (Pb) in strongly contaminated soil is limited by interaction with phosphate and organic matter was tested on soil from urban gardens and orchards by extracting available Pb and P with the modified Morgan's extractant (ammonium acetate, pH 4.8) and measuring organic matter by loss‐on‐ignition. The extractable fraction of total Pb was larger in general in the urban garden than orchard soil, which indicates greater lability in urban garden soil than was expected if insoluble Pb phosphates such as pyromorphite were limiting Pb extractability. The extractable Pb fraction was generally larger in soil with greater total Pb, an indication that soil with large Pb concentrations contained Pb in more labile forms than soil with small concentrations. Larger organic matter contents in garden soil reduced the extractable fraction of Pb, but large available phosphate concentrations did not have this effect. The results indicate that soil organic matter has an important role in limiting Pb extractability in strongly contaminated soil, whereas a large soil phosphate concentration does not, despite the expectation that formation of insoluble Pb phosphates would reduce Pb lability. Nevertheless, a spatial association of P with Pb in Pb‐rich particles identified in urban garden soil with the electron microprobe suggests that Pb phosphates have formed in such soil from more reactive forms of Pb. The hypothesis is that a small fraction only of the total Pb in the urban soil occurs in a phosphate phase, and more acid‐labile forms of Pb co‐exist with insoluble phosphates for long time periods. HIGHLIGHTS: Lead bioavailability in soil can potentially be reduced with phosphate. Hypothesis is that soil Pb availability is limited by available phosphate or total organic matter. Large soil organic matter content correlated with reduced Pb extractability, but not large available phosphorus content. Lead‐phosphate phases did not appear to limit soil Pb extractability.