Fate of biosolids Cu and Zn in a semi-arid grassland

Biosolids land application applies varying trace metal amounts to soils. Measuring total soil metals is typically performed to ensure environmental protection, yet this technique does not quantify which soil phases play important metal release or attenuation roles. We assessed the distribution of biosolids-borne Cu and Zn associated with soluble/exchangeable, specifically adsorbed/carbonate-bound, amorphous and crystalline Mn/Fe oxyhydroxide-bound, residual organic, and residual inorganic phases. Biosolids were surface-applied (no incorporation) to experimental plots, at the Meadow Springs Ranch (40 53′46″N, 104 52′28″W) which is owned by the city of Fort Collins, CO, USA, in 1991 at rates of 0, 2.5, 5, 10, 21, and 30 Mg ha−1. Plots were split in half in 2002, with one-half receiving biosolids at rates identical to 1991 rates. In 2003, 0–8, 8–15, and 15–30-cm soil depths were collected and subjected to 4 M HNO3 digestion and sequential fractionation. The 4 M HNO3 extraction suggested downward Cu transport, while Zn was immobilized in the soil surface. The sequential extraction procedure, more sensitive to changes in soil metal pools, suggested that repeated biosolids application did not affect vertical Zn movement, but did increase the downward transport potential of organically complexed Cu. In the given time, organically complexed Cu was likely mineralized and subsequently associated with soil mineral oxide phases. Because bioavailability of Cu is associated with dissolved phases, and soluble/exchangeable Cu concentrations were below detection limits in the subsoil, a reduction in environmental quality should be minimal. Still, we advocate that on coarse-textured semi-arid soils, biosolids application rates should match the plant N needs to avoid potential downward trace metal transport.