Effect of pH on the denitrifying enzyme activity in pasture soils in relation to the intrinsic differences in denitrifier communities.

The effects of pH on denitrifying enzyme activity (DEA) and on the ratio of the denitrification products, N(2)O and N(2), were determined in three pasture soils differing in cattle impact. The linkage between intrinsic differences in the denitrifying communities and pH effects on relative N(2)O production was also assessed. Soil pH values were adjusted just before DEA determination to obtain soil slurries with a range of pH values. The intrinsic differences in the denitrifier communities were assessed by measuring the kinetic constants of NO(3)(-) and N(2)O reductions. DEA for all three soils was highest at pH 8.4, regardless of native soil pH. Because DEA has typically been measured at native soil pH, our results suggest that DEA might have been underestimated in many previous studies. Further, relative N(2)O production at different pH values did not differ among the soils, even though the denitrifier communities differed in their intrinsic capability to reduce NO(3)(-) all the way to N(2), suggesting that the ratio of denitrification products (N(2)O and N(2)) is pH-specific rather than soil-specific. This suggests that manipulations of soil pH will alter N(2)O fluxes from agricultural soils.