Evaluation of arsenic pollution in field-contaminated soil at the soil's actual pH

•Total As, As fractions and phosphatase activity reached a significant level.•Vmax and Vmax/Km of phosphatase decreased with elevated As pollution.•Vmax and Km were significantly negatively correlated with soil pH.•Phosphatase is more sensitive to As contamination at the soil's pH.•Vmax/Km as an index of pollution degree in As field contaminated soil. The pollution and ecological risks posed by arsenic (As) entering the soil are the major environmental challenges faced by human beings. Soil phosphatase can serve as a useful indicator for assessing As contamination under specific soil pH conditions. However, the study of phosphatase kinetics in long-term field As-contaminated soil remains unclear, presenting a significant obstacle to the monitoring and evaluation of As pollution and toxicity. The purpose of this study was to determine phosphatase activity and explore enzyme kinetics in soils subjected to long-term field As contamination. Results revealed that the soil phosphatase activity varied among the tested soil samples, depending on the concentrations of As. The relationship between total As, As fractions and phosphatase activity was found to be significant through negative exponential function fitting. Kinetic parameters, including maximum reaction velocity (Vmax), Michaelis constant (Km) and catalytic efficiency (Vmax/Km), ranged from 3.14 × 10−2–53.88 × 10−2 mmol/(L·hr), 0.61–7.92 mmol/L, and 0.46 × 10−2–11.20 × 10−2 hr−1, respectively. Vmax and Vmax/Km of phosphatase decreased with increasing As pollution, while Km was less affected. Interestingly, Vmax/Km showed a significant negative correlation with all As fractions and total As. The ecological doses (ED10) for the complete inhibition and partial inhibition models ranged from 0.22–70.33 mg/kg and 0.001–55.27 mg/kg, respectively, indicating that Vmax/Km can be used as an index for assessing As pollution in field-contaminated soil. This study demonstrated that the phosphatase kinetics parameters in the soil's pH system were better indicators than the optimal pH for evaluating the field ecotoxicity of As. [Display omitted]