Measuring denitrification activity in soils under pasture: Optimizing conditions for the short-term denitrification enzyme assay and effects of soil storage on denitrification activity

Abstract

The short-term denitrification enzyme assay measures the potential denitrification activity of a soil in the field. The technique involves anaerobic incubation of soil samples and the measurement of N 2O emission in the presence of C 2H 2. We examined the effects of incubation conditions and previous soil treatment on the denitrification enzyme activity (DEA) of two soils under permanent pasture. A standardized procedure is recommended. To avoid any effect of growth of denitrifying organisms on the DEA, the incubation period should not exceed 5 h at 20°C. The K m value for substrate NO 3 − concentrations was in the range 21–38 μg NO 3 −N g −1 soil, but NO 3 −N concentrations > 100 μg N g −1 soil inhibited production of N 2O. The optimum concentrations for NO 3 −N and glucose-C were 50 and 300 μg g −1 soil, respectively. Field-moist samples of soil retained their DEA during 5 days' storage at either 2 or 20°C, but thereafter the DEA declined with time. The denitrification activity of air-dried soil increased relative to fresh soil after 1 week's storage, but subsequently declined. The effects were generally consistent with a change in the availability of substrate C in the stored soil (moist or air-dry), but there also appeared to be a decline in the potential activity of the soil denitrifier populations after 5–7 days' storage.