Extremely High Soil Copper Content, yet Low Phytotoxicity: A Unique Case of Monometallic Soil Pollution at Kargaly, Russia

Ecotoxicological studies on soil metal toxicity often rely on artificially contaminated soils. A major difficulty in using soils contaminated by anthropogenic activities (e.g., mining and agriculture) is the presence of multiple metals, which can make it impossible to distinguish the toxic effects of a particular metal. Therefore, sites with monometallic pollution have great potential for ecotoxicological research. One such site is an agricultural field in Kargaly, Orenburg region, Russia, where copper mining and smelting activities were carried out during the 18th–20th centuries. Samples of Mollisols (chernozems) were collected in the studied field. At several sampling points there were copper ore rocks on the surface, containing malachite (CuCO₃ · Cu(OH)₂). The soil samples had a high copper content, up to approximately 10 g kg⁻¹, compared with 75 mg kg⁻¹ in the background soil. Importantly, the content of other elements in all soil samples was similar to that in the background soil, highlighting the uniqueness of the monometallic contamination in the study area. Despite the extremely high total copper content, exchangeable copper was relatively low, with a maximum of approximately 0.5 mg kg⁻¹. We performed a short-term (21-day) ecotoxicity assessment using perennial ryegrass as an indicator of copper toxicity. Contrary to expectations, plant growth was not affected by the high copper content in the studied soils. The low copper phytotoxicity may be explained by the low solubility of malachite. However, future long-term experiments may be warranted to determine copper toxicity thresholds for plants under field conditions. The site discovered in the present study could potentially acquire the same significance as the Danish Hygum site for the study of monometallic soil contamination. Environ Toxicol Chem 2023;42:707–713.