Advances on the determination of thresholds of Cu phytotoxicity in field-contaminated soils in central Chile

To better determine phytotoxicity thresholds for metals in the soil, studies should use actual field-contaminated soil samples rather than metal-spiked soil preparations. However, there are surprisingly few such data available for Cu phytotoxicity in field-contaminated soils. Moreover, these studies differ from each other with regards to soil characteristics and experimental setups. This study aimed at more accurately estimating Cu phytotoxicity thresholds using field-collected agricultural soils (Entisols) from areas exposed to contamination from Cu mining. For this purpose, the exposure to Cu was assessed by measuring total soil Cu, soluble Cu, free Cu²⁺ activity, and Cu in the plant aerial tissues. On the other hand, two bioassay durations (short-term and long-term), three plant species (Avena sativa L., Brassica rapa CrGC syn. Rbr, and Lolium perenne L.), and five biometric endpoints (shoot length and weight, root length and weight, and number of seed pods) were considered. Overall plant growth was best predicted by total Cu content of the soil. Despite some confounding factors, it was possible to determine EC₁₀, EC₂₅ and EC₅₀ of total Cu in the soil. Brassica rapa was more sensitive than Avena sativa for all endpoints, while Lolium perenne was of intermediate sensitivity. For the short-term bioassay (21 days for all three species), the averaged EC₁₀, EC₂₅ and EC₅₀ values of total soil Cu (in mg kg⁻¹) were 356, 621, and 904, respectively. For the long-term bioassay (62 days for oat and 42 days for turnip), the averaged EC₁₀, EC₂₅ and EC₅₀ values of total soil Cu (in mg kg⁻¹) were 355, 513, and 688, respectively. The obtained results indicate that chronic test is a suitable method for assessing Cu phytotoxicity in field-contaminated soils. - Highlights: • Total soil Cu best predicted biometric plant responses to Cu excess. • Brassica rapa was more sensitive to Cu than Avena sativa. • Lolium perenne was of intermediate sensitivity to Cu. • Long-term bioassays are more suitable, in comparison with short-term ones. • Chronic test is a suitable method for Cu phytotoxicity assessment. - This study presents threshold values for Cu phytotoxicity in field-contaminated soils, considering two bioassays durations, three plant species, and five biometric endpoints.