Effect of aging biosolids with soils of contrasting pH on subsequent concentrations of Cu and Zn in pore water and on their plant uptake.

PURPOSE: The study examines if a short period of reaction after addition of biosolids to soils can reduce the solubility and potential phytotoxicity of biosolid-borne Zn and Cu. METHODS: The effects of period of aging (zero, 60, and 120 days) of biosolids (applied at 0, 10, 20, and 30 g kg⁻¹) with an acid, neutral, or alkaline soil on pH and concentrations of Zn, Cu, and dissolved organic C in solution over a 60-day growth period of spinach were investigated using Rhizon pore water samplers. RESULTS: In the acid and neutral soils, increasing aging period markedly reduced the concentrations of Zn and Cu in solution and there were concomitant increases in solution pH. The effect was much less pronounced in the alkaline soil. Soluble Zn and Cu concentrations were generally positively correlated with dissolved organic C concentrations, negatively correlated with pH in the alkaline and neutral soils but positively correlated with pH in the acid soil. Spinach yields were lower in the acid than neutral and alkaline soils and tended to increase with increasing rates of biosolids in all three soils. The concentrations of tissue Zn and Cu were notably high in shoots of plants grown in the acid soil. For all biosolid-amended soils, the concentrations of tissue Cu were lower in plants grown after 60 days rather than no aging. CONCLUSIONS: Following biosolids applications to soils, an aging period of only a few months is likely to lower the solubility, and potential phytotoxicity, of biosolid-borne Zn and Cu particularly in acid and neutral soils.