Nitrate reductase activity in soils: Effects of trace elements

Abstract

Studies to evaluate the effect of pH and 19 trace elements on nitrate reductase activity in soils showed that using Tris(hydroxymethyl)aminomethane(THAM)-Na acetate buffer, the optimum pH was at pH 7, and that most trace elements inhibited the activity of this enzyme in acid and neutral soils. The relative effectiveness of the trace elements in inhibition of nitrate reductase activity depended upon the type and concentration of the trace element and soil used. When the effect of the trace elements was compared at a concentration of 2.5 μmol g −1 soil, the inhibition of nitrate reductase activity in three soils showed that Ag(I), Cd(II), Hg(II), Se(IV), As(V) and W(VI) were the most effective inhibitors, with percentage inhibition of > 75% and that Mns(II) was the least effective inhibitor, with percentage inhibition ranging from 0 to 8%. Ba(II) either did not inhibit or enhanced the nitrate reductase activity. Other trace elements that inhibited the nitrate reductase activity in acid and neutral soils included Cu(I), Co(II), Cu(II), Fe(II), Ni(II), Pb(II), Zn(II), Al(III), As(III), Cr(III), Fe(III), V(IV), Mo(VI) and Se(VI); their degree of effectiveness varied among the three soils. Of these trace elements, only As(III) and Se(VI) inhibited the activity of this enzyme in the calcareous soil. B(III) enhanced nitrate reductase activity in a neutral soil, had no effect in a calcareous soil, and inhibited the activity of this enzyme in an acid soil. In general, the inhibitory effect of the trace elements increased when the concentration increased by 10-fold. The Eh 7 values decreased significantly after incubation, and they were lower in the presence than in the absence of 2,4-dinitrophenol.