Copper, zinc, and nickel in soil solution affected by biosolids amendment and soil management

Soil plots on a pasture were amended with biosolids spiked with copper (Cu), nickel (Ni), or zinc (Zn), resulting in maximum concentrations of 181 mg Cu, 58 mg Ni, and 296 mg Zn/kg in soil. Soil solutions from the plots were obtained by centrifugation for chemical analyses, and free metal ion activities (Cu2+, Ni2+, Zn2+) were computed from the Windermere Humic Aqueous Model (WHAM). In the 3 years after biosolids amendment, the concentrations and activities of Cu, Ni, and Zn in soil solution increased with their amounts in biosolids. Copper and Ni concentrations in soil solution were higher than their critical concentrations recently reported in the literature. While Cu in soil solution was dominated by Cu-humic complexes, Ni2+ and Zn2+ were the majority species of the metals. Liming the soil plots to increase pH from 5.5 to ~7 greatly reduced the concentrations of the trace metals, particularly Zn; Cu2+, Zn2+, and Ni2+ were decreased by orders of magnitude 2–3, 2, and 1, respectively. Metal concentrations and activities fluctuated in the next 2 years as soil pH changed slightly and then after the use of elemental sulfur to acidify soil to pH ~6.5. Eight years after application of biosolids and through soil pH adjustment by lime and sulfur, Cu2+ and Zn2+ were very close to, and Ni2+ was a few times higher than, their corresponding baseline values. Maintaining a near neutral pH thus would be the key to keeping bioavailable metal concentrations low in a soil with an organic carbon content of 23.8 g/kg.