Copper Phytotoxicity Thresholds for Sunflower: A Field Experiment at a Site with Unique Monometallic Soil Contamination

Ecotoxicological research often employs artificially contaminated soils, while studies using real-world contaminated soils remain limited. The presence of multiple metals in real-world soils can introduce confounding factors and hinder the interpretation of results. This makes monometallic contaminated sites (i.e., contaminated predominantly with a single metal) particularly valuable. One such site is a chernozem (Mollisol) agricultural field in Kargaly, Orenburg Region, Russia, where copper mining activities occurred during the Bronze Age. Sunflowers (Helianthus annuus L.) were strategically sampled along a visible gradient of copper toxicity in the field, and their corresponding rhizospheric soil was also collected. The affected soils exhibited a total copper content reaching up to ~5500 mg/kg. Notably, the contents of other elements in all soil samples closely resembled those in background soils, highlighting the distinctiveness of the monometallic contamination in the study area. The effective concentrations at 25 and 50% (EC25 and EC50) of shoot copper content in sunflowers were determined to be 24 and 27 mg/kg, respectively. These values align with previous reports, reinforcing the credibility of our findings. Conversely, the EC25 and EC50 values of total soil copper were found to be 1130 and 1747 mg/kg, respectively. These values surpass previous reports for sunflowers in contaminated soils and are attributed to low solubility of copper-containing phases (malachite, azurite, and chalcocite) in the soils. Specifically, EC50 values obtained across the studies followed the trend in the partitioning coefficient (Kd) values for copper. Furthermore, the EC25 and EC50 values of soluble soil copper (extracted by 0.01 M CaCl2 solution) were found to be 48 and 100 µg/L, respectively, whereas previous reports tend to be highly variable. This study’s findings are novel, considering that only seven studies have estimated copper phytotoxicity thresholds using real-world contaminated soils.