Extractability of P b in urban gardens and orchards linked to soil properties

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