Distribution of heavy metals in a sandy forest soil repeatedly amended with biosolids

The suitability for land application of biosolids can be limited by the presence of heavy metals. We investigated the distribution of heavy metals in the soil of a research trial within a Pinus radiata (D. Don) plantation following repeated applications of aerobically digested liquid biosolids. The trial is located on a sandy soil at Rabbit Island near Nelson, New Zealand. Biosolids were applied in 1997, 2000, and 2003, at 3 application rates: 0 (control), 300 (standard), and 600 kg N/ha (high). Litter layer and soil samples (down to 1 m) were taken from the trial site and analysed to assess the accumulation of heavy metals (Cu, Zn, Cd, Cr, Pb, and Ni) within the soil profile and the effect of repeated biosolids application on metal availability and mobility. Sequential fractionation methods were used to evaluate the distribution of exchangeable, specifically sorbed, oxide bound, organic bound, and residual fractions of the metals. Concentrations of Cr, Cu, Pb, and Zn in the litter layer were significantly (P < 0.05) higher in the high biosolids-treated plots than the control, indicating that the litter layer had retained considerable amounts of biosolids-derived metals. Significantly elevated concentrations of environmentally available Cu in the topsoil (0–0.1 m) were observed in plots receiving the high biosolids treatment and residual Cu concentration was significantly higher in the biosolids treatments, but apart from an accumulation of oxide-bound Zn in the 0.1–0.25 m layer, no accumulation of other metals in the soil was observed. The residual fraction was the most abundant pool for all metals examined, indicating low bioavailability. Total concentrations of the metals in both biosolids-treated and untreated control soils were very low (e.g. <7 mg Cu/kg, <30 mg Zn/kg). Although there were no significant changes in concentrations below the topsoil, mass balance calculation implied that a proportion of biosolids-derived Zn in the high biosolids treatment may have been leached through the soil profile.