Factors regulating denitrification in a soil under pasture

Experiments were conducted to obtain insights into factors regulating denitrification rate in a silt loam soil under permanent pasture in New Zealand, by removing possible limitations to denitrification during the incubation for the denitrification measurement. Soil temperature in the field was found to limit denitrification rate in all seasons relative to the denitrification rate measured at 25°C in the laboratory. This temperature effect was greatest in the cool–wet season. Additions of nitrate solution to soil cores stimulated denitrification rates in all seasons. This increase in denitrification rate suggests the availability of NO3− may have limited denitrification in this pasture soil. Denitrification rates also increased when soluble-carbon was added to the soil cores, but the magnitude of the effect depended on other edaphic factors. A large increase in denitrification rate was obtained by saturating the soil cores collected in most seasons, but particularly during the warm–dry period. However, little enhanced effect on denitrification rate by anaerobic incubation of soil cores was observed. These results suggest that the observed effect of water addition on denitrification rate may have been due to the easier diffusive movement of NO3−, or possibly soluble-C, to the microsites where denitrification was occurring in this pasture, and the creation of anaerobic sites in the soil may not have been as important to the increase of denitrification rate.