Soil phosphatase assay to evaluate arsenic toxicity should be performed at the soil's actual pH

Soil phosphatase is considered an indicator to assess soil arsenic (As) pollution. In the phosphatase activity determination, a fixed buffer value (pH 5-10) is commonly used for all soils, ignoring the soil's actual pH. Here, we determined the soil phosphatase activity of 20 soils under As stress at the soils' pH, and the As inhibition mechanism was also explored by the enzyme kinetics. Our results show that soil phosphatase activity was significantly inhibited under As stress. The inhibition rate in acid soils (39.2 %) was considerably higher than in alkaline soils (25.4 %) when As concentration was 600 mg kg(-1). For alkaline soils, As inhibited phosphatase by competitive inhibition or linearmixed inhibition, while for acid soils, it was more complex, including linear mixed inhibition, non-competitive inhibition, and anti-competitive inhibition. Simultaneously, our results showed that the ecological dose (ED10) described by the partial inhibition model was far below than the complete inhibition model. According to the partial inhibition model, the ED10 of As ranged from 2.66 to 164.07 mg kg(-1) for alkaline soils and 0.11 to 89.95 mg kg(-1) for acid soils. Moreover, V-max/K-m of phosphatase is a more sensitive index for evaluating As contamination than Vmax in partial inhibition models. The ED10 obtained based on the relationship between Vmax/Km and As concentration was 0.64-34.75 mg kg(-1) for acid soils and 8.48 to 20.16 mg kg(-1) for alkaline soils. This also confirms V-max/K-m as a sensitive and ideal index for assessing As pollution under soils' actual pH. Furthermore, soil pH and cation exchange capacity are dominant factors affecting As inhibition on soil phosphatase. The above kinetic studies indicate that performing the assay by adjusting the buffer pH to the soil pH is essential for more accurately evaluating arsenic toxicity.

Automated summary

Soil phosphatase is inhibited under arsenic stress, with greater inhibition observed in acid soils compared to alkaline soils. The inhibition mechanism includes competitive inhibition, linear mixed inhibition, non-competitive inhibition, and anti-competitive inhibition. The ecological dose (ED10) for arsenic ranged from 0.11 to 164.07 mg/kg depending on soil type. The V-max/K-m of phosphatase is a more sensitive index for assessing arsenic contamination, and soil pH and cation exchange capacity are important factors affecting arsenic inhibition on soil phosphatase. Adjusting the assay buffer pH to the soil pH is crucial for accurate evaluation of arsenic toxicity.