Lead in boreal soils and food plants

Abstract

The soils of the boreal zone, characterized by acidic, low-organic-matter sands in uplands and organic deposits in lowlands, represent unique environments for heavy metals. The mobility and plant uptake of Pb can be substantially different than in other soils. A survey of natural levels of Pb in northern Ontario revealed concentrations of 26 mg kg⁻¹ dry soil and 1.3 mg kg⁻¹ dry blueberry leaf, with an apparent plant/soil concentration ratio (CR) of 0.051. In outdoor lysimeters with an acidic sand profile (pH 4.9) and under a boreal climate, 67% of a pulse of Pb, applied as Pb(NO₃)₂, was essentially immobile over 4 yr. The 33% that leached may have been mobilized by soluble organic ligands or the N0₃ ⁻ companion ion. The solid/liquid partition coefficient (K_d) for this soil, using either applied ²¹⁰Pb or stable Pb, was very low: 20 L kg⁻¹ The CR for ²¹⁰Pb in the same soil was correspondingly high: 0.10 for blueberry and 0.059 over all crops studied. In two organic soils, the K_d values were 9 × 10³ L kg⁻¹ (Sphagnum, pH 4.8) and 3 × 10⁴ L kg⁻¹ (sedge, pH 5.5) with corresponding CR values of 8 × 10⁻⁴ and 0.0085 for blueberries (0.0027 overall in the latter soil). The CR was most closely related to soil cation exchange capacity, although organic matter content and pH were undoubtedly important related factors. In combination, the acidic sand and organic soils of boreal settings represent extremes for the mobility of Pb.