Water-extractable soil organic matter inhibits phosphatase activity

We show that water-extractable soil organic matter may inhibit activity of added phosphatase. The inhibition was greater for the topsoils than the corresponding subsoils. The enzyme kinetics in buffer and in soil solutions followed Michaelis–Menten kinetics. Vmax decreased and Km increased with increasing soil water-extractable organic carbon. Both kinetic parameters followed similar trends with the intensities of both fluorescence peaks of the solutions. The effect was more marked at pH below the optimum pH. Inhibition was complex, particularly for topsoil solutions where Vmax was decreased and Km increased. This effect could lead to underestimation of catalytic activity of enzymes extracted from soil and could result in the overestimation of decreased specific activity in the adsorbed phase of enzymes added to soils.