The influence of soil solution properties on phytotoxicity of soil soluble copper in a wide range of soils

• Variation of toxicity thresholds based on soluble Cu was large (> 7) in 17 soils.
• DOC was the most important factor in determining the soluble Cu toxicity.
• pH was less important factor but more complicated in influencing Cu speciation.
• Mg2 + and Ca2 + have less significant effect on soluble Cu toxicity in real soil.

The bioassay of barley root elongation was performed in 17 Chinese soils with added copper (Cu) to investigate the phytotoxicity of soluble Cu based on soil pore water and 0.01 M CaCl2 extraction. For soluble Cu in soil pore water, the soil solution property induced variations of effective concentrations that caused 10% (EC10) and 50% (EC50) inhibition in barley root elongation which were 125 and 18 folds, respectively. Similarly for soluble Cu extracted by 0.01 M CaCl2, the soil solution property induced variation of EC10 and EC50 values was 14 and 7 folds, respectively, indicating that the concentrations of soluble Cu extracted by 0.01 M CaCl2 or in soil pore water were largely affected by soil solution chemistry and properties in a wide range of soils. The multiple regression analysis was applied to derive empirical relationships between soil pore water properties (pH, electrical conductivity (EC), K+, Na+, Ca2 +, Mg2 +, S and dissolved organic matter (DOC)) and soluble Cu toxicity thresholds. When incorporating these parameters into the regression equations, the coefficient of determination (r2) for EC50 or EC10 ranged from 0.78 to 0.87. The results showed that the toxicity thresholds based on both Cu concentrations in soil pore water and 0.01 M CaCl2 extraction were affinitive to soil pore water chemistry. Meanwhile, DOC was constantly significantly correlated to Cu toxicity thresholds (EC50 and EC10) in the pore water and 0.01 M CaCl2 extraction, which could explain 70% of the variance of EC50 for Cu in soil pore water.