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

This article focuses on ecotoxicological assessment of monometallic copper contamination in soils from an agricultural field in Kargaly, Orenburg region, Russia, a site historically impacted by copper mining and smelting. Soil samples showed extremely high total copper concentrations (up to ~10 g/kg) with minimal contamination by other metals, highlighting the site's unique monometallic pollution. Despite high total copper, exchangeable (bioavailable) copper levels were low due to the low solubility of malachite (CuCO3·Cu(OH)2), and a 21-day bioassay using perennial ryegrass indicated no significant phytotoxic effects on plant growth. The study suggests that exchangeable copper concentration, rather than total copper content, is a more relevant indicator of copper toxicity in soils and recommends long-term experiments to better define copper toxicity thresholds under field conditions. The Kargaly site is proposed as a valuable natural laboratory for studying copper ecotoxicity, complementing other monometallic contamination sites such as the Danish Hygum site.