Competitive ligand control of Cu bioavailability: Decoupling anion (Cl^-/SO₄^2-/PO₄^3-) versus fulvic acid regulation in...

The interactions between anions and dissolved organic matter play a crucial role in determining copper (Cu) bioavailability in soil environment. This study examined the impact of chloride (Cl^-), sulfate (SO₄^2-), phosphate (PO₄^3-), and fulvic acid (FA) on soluble Cu speciation distribution, and phytotoxicity in barley root elongation. FA had a superior Cu²⁺-complexing ability, reducing the free Cu²⁺ content to 0.042 %-2.36 % through organic complexation (FA-Cu), which accounted for >97.4 % of the total Cu. In contrast, anion treatments without FA primarily maintained soluble Cu as free ions with limited formation of inorganic complexes, with FA having the most significant effect, followed by PO₄^3-, SO₄^2-, and Cl^-. The coexistence of FA and anions led to the emergence of competitive ligand interactions, with particularly pronounced variations in Cu speciation and complexation patterns observed. At low Cu, FA-Cu was the dominant soluble species, with free Cu²⁺ ≤ 31.50 %. Elevated Cu levels shifted speciation toward free Cu²⁺ and FA-Cu, except in systems with high PO₄^3--FA concentrations where PO₄^3- precipitates dominated. Furthermore, the single FA and co-coexistence of FA and anions of FA all increased the toxicity threshold (50 % inhibition, EC50) within a range of 2.8-12.4 folds, generally in the order of FA + Cl^- < FA + SO₄^2- < FA + PO₄^3-, FA. It indicated that the protective effect of FA against Cu phytotoxicity was gradually weakened with the increasing addition of Cl^- and SO₄^2-. This study provides important insights into the role of both organic and inorganic ligands in assessing Cu speciation and phytotoxicity in Cu-contaminated agroecosystems.