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INHIBITION OF PYROPHOSPHATASE ACTIVITY IN SOILS BY TRACE ELEMENTS1

Pyrophosphatase catalyzes the hydrolysis of pyrophosphatase to orthophosphate. Information on the control of pyrophosphatase activity in soil is desired. Thus, we conducted this study to evaluate the effect of 21 trace elements on the activity of this enzyme in soils. Results showed that 19 of these elements inhibited pyrophosphatase activity and that the relative effectiveness of the trace elements in inhibiting this enzyme depended on the soil. When the trace elements were compared by using 25 μmol/ g soil, the average inhibition in three soils showed that Hg(II), As(V), Mo(VI), and W(VI) were the most effective inhibitors (average inhibition >50%). Moderately effective inhibitors were: Cd(II), Cu(II), Fe(II), B(III), and V(IV) (average inhibition between 30 and 50%). Other trace elements that inhibited pyrophosphatase activity in soils were: As(I), Cu(I), Mn(II), Ni(II), Pb(II), Sn(II), Zn(II), Al(III), As(III), Cr(III), Fe(III), Se(IV), and Ti(IV). Pyrophosphatase activity was not inhibited by Ba(II) or Co(II). The inhibitory effect of trace elements decreased when the amount of trace elements added per gram of soil was decreased fivefold (from 25 to 5 μmol/g soil). The inhibition by the divalent cations Hg, Cd, Cu, Fe, and Sn showed noncompetitive kinetics, but BO2−3, MoO2−4, WO2−4, As3−4, and VO2+ exhibited competitive kinetics. Among the trace elements studied, MoO2−4, and WO2−4 are the most effective inhibitors of pyrophosphatase activity in soils.