Which soil Cu pool governs phytotoxicity in field-collected soils contaminated by copper smelting activities in central Chile?

Several studies have attempted to predict the so-called “phytoavailable” fraction by correlating plant responses with different soil metal pools. Most of the data derived from these studies tend to be inconsistent, making interpretations difficult. Thus, the main objective of this study was to determine which soil Cu pool (free Cu²⁺, salt-exchangeable Cu or total Cu) controls Cu phytotoxicity in soils near a Cu smelter in central Chile. We studied the following traits of the local plant community grown spontaneously on the study site: species richness, shoot biomass, and plant cover. The site was dominated by four early plant colonizers: Eschscholzia californica Cham., Hirschfeldia incana (L.) Lagr.-Fossat, Lolium perenne L., and Vulpia bromoides (L.) Gray. We determined exchangeable soil Cu and activity of free Cu²⁺ in 0.1 M KNO₃ extracts using soil/solution ratio of 1/2.5. The effect of total soil Cu on plant responses was not significant (p > 0.05). In our field-collected soil series, exchangeable Cu was a better indicator of soil phytotoxicity than either total soil Cu or free Cu²⁺ in the soil solution. We determined upper critical threshold values for Cu exposure using the three plant traits cited above. The mean values of EC₁₀, EC₂₅, and EC₅₀ (effective concentration at 10%, 25%, and 50%, respectively) of exchangeable soil Cu (in μg L⁻¹) were 255, 391, and 533, respectively. The mean EC₁₀, EC₂₅ and EC₅₀ values of pCu²⁺ were 7.5, 6.8, and 5.9, respectively. We highlight the importance of further studies on Cu phytotoxicity using actual field-contaminated soils.