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Highlights • The toxicity of OPs was slightly increased with the decrease of soil pH. • Mechanistic models were developed to interpret the phenomena. • Bioavailability and toxicodynamics are key factors for toxicity variation. • Bioavailability was related to hydrolysis and biodegradation characteristics. • Toxicodynamics were influenced by the interaction between AchE with OPs. Abstract Organophosphorus pesticides (OPs) have been widely used to control agricultural insects. Soil acidification is a major problem in soil of intensive agricultural systems, especially in red soil with a low pH buffer capacity. However, the effects of soil acidification on the toxicity of pesticides are still unclear. In the present study, the toxicity of three OPs on E. fetida was determined at individual (14-day lethal test) and biochemical levels (antioxidative defence enzymes) by using acidified soils (pH = 5.5, 4.3 and 3.1). The results showed that the toxicity of tested OPs was slightly increased with the decrease of soil pH. To interpret the phenomena, an optimum Quantitative Structure Activity Relationship (QSAR) model was developed based on the toxicity mechanism and the partial least squares regression (PLS) method. The model indicated bioavailability and toxicodynamics are key factors of soil acidification affecting the toxicity of the OPs. Further results revealed the bioavailability of the OPs was strongly related to their hydrolysis and biodegradation character, whereas the effects of soil acidification on toxicodynamics were mainly caused by the interaction between the acetylcholinesterase (AchE) and the OPs. Results will increase understanding of the effects of soil acidification on the toxicity of pesticides and its mechanism. [ABSTRACT FROM AUTHOR]

Hormesis is an intriguing phenomenon that is characterized by low dose stimulation and high dose inhibition. Several traditional parameters, such as the concentration of the zero equivalent point ( ZEP ) and the maximal stimulatory effect ( Y max ), have been used to characterize the zone of hormesis or the extent of the stimulatory effect. However, the characteristics of hormesis for chemicals cannot be quantified completely by one parameter, which is important to accurately compare the hormetic effects of chemicals and to describe the combined effects of chemical mixtures at low doses. In the present study, a novel parameter, termed the relative standard area of hormetic zone ( H o r A r e a R ), was developed and proposed to quantify the hormetic effects (24 h exposure) of nine metal ions (Cr 3+ , Cu 2+ , Mg 2+ , Cd 2+ , Fe 3+ , Ni 2+ , Zn 2+ , Co 2+ , Cs + ) towards Vibrio fischeri , both individually and as binary mixtures. The results indicate that H o r A r e a R can be used not only to accurately assess the hormetic effects and its relationship with structural characteristics but also to conveniently describe the combined effects of interactive mixtures at low dose. Thus, the H o r A r e a R parameter can quantitatively assess the hormetic effects and can offer a useful approach to perform environmental risk assessments of chemicals at low doses. [ABSTRACT FROM AUTHOR]