Soil properties influence kinetics of soil acid phosphatase in response to arsenic toxicity

- pH and amorphous Fe explained 50.8-66.1% and 29.8-43.8% of soil ACP Vmax, Km and Vmax/Km variations across soils;
- The type of As inhibition on soil ACP was noncompetitive or mixed inhibition mainly depend on soil types;
- Soil organic matter and cation exchange capacity were major controlling factors affecting As inhibitory effect on soil ACP.
- Catalytic efficiency (Vmax/Km) could sensitively indicate the toxicity of As to soil ACP.

Soil phosphatase, which plays an important role in phosphorus cycling, is strongly inhibited by Arsenic (As). However, the inhibition mechanism in kinetics is not adequately investigated. In this study, we investigated the kinetic characteristics of soil acid phosphatase (ACP) in 14 soils with varied properties, and also explored how kinetic properties of soil ACP changed with different spiked As concentrations. The results showed that the Michaelis constant (Km) and maximum reaction velocity (Vmax) values of soil ACP ranged from 1.18 to 3.77 mM and 0.025-0.133 mM h−1 in uncontaminated soils. The kinetic parameters of soil ACP in different soils changed differently with As contamination. The Km remained unchanged and Vmax decreased with increase of As concentration in most acid and neutral soils, indicating a noncompetitive inhibition mechanism. However, in alkaline soils, the Km increased linearly and Vmax decreased with increase of As concentration, indicating a mixed inhibition mechanism that include competitive and noncompetitive. The competitive inhibition constant (Kic) and noncompetitive inhibition constant (Kiu) varied among soils and ranged from 0.38 to 3.65 mM and 0.84-7.43 mM respectively. The inhibitory effect of As on soil ACP was mostly affected by soil organic matter and cation exchange capacity. Those factors influenced the combination of As with enzyme, which resulted in a difference of As toxicity to soil ACP. Catalytic efficiency (Vmax/Km) of soil ACP was a sensitive kinetic parameter to assess the ecological risks of soil As contamination.