soil enzyme activity and P concerntraion

Activities of phosphatase were expressed as mg ρNP kg soil-1 h-1 using modified pH (6.5) for the determination of albic soil, black soil and brown soil. The enzyme kinetic parameters Vmax (maximum reaction rate) and Km (Michaelis-Menten constant) for APase and PD were determined using a range of substrate concentrations, following the same procedure as described in Section 2.5. The kinetic parameters for AcP and AlP were measured with seven substrate concentrations (i.e. 10 mM, 20 mM, 30 mM, 40 mM, 50 mM, 60 mM and 70 mM), while PD was measured with six substrate concentrations (10 mM, 20 mM, 30 mM, 40 mM, 50 mM and 60 mM).

Each substrate concentration included a negative control. The kinetic parameters Vmax and Km were calculated using the Hanes method (Moscatelli et al., 2012). A straight-line graph can be generated (Fig. S1), where the slope corresponds to 1/Vmax and the intercept is Km/Vmax, facilitating the determination of Vmax and Km.