DENITRIFICATION IN A SITKA SPRUCE PLANTATION AND THE EFFECT OF CLEAR-FELLING

To determine the rate of denitrification from an upland plantation forest of Picea sitchensis, a field coring and laboratory incubation technique was developed. The method was tested in the laboratory to ensure that the C2H2 block worked in the acid, organic, poorly drained peaty gley soil under investigation. Similarly, the relationship between denitrification rate and temperature was determined, so that field estimates could be made based on the laboratory incubations. The technique was used to monitor denitrification rates in standing and felled forest areas, and to evaluate annual N losses. The total loss of N from the standing forest through denitrification was estimated to be 3.2 kg N ha-1 over the year studied and, of this loss, approximately 80% was as N2O. Denitrification increased as a consequence of clear-felling, and an estimated 9-40 kg N ha-1 year-1 were lost in the first 2 years after felling. Denitrification can be a significant component of the forest N cycle, yet has been frequently assumed to be negligible. Although the absolute amounts of N lost from forests in this way may be small, they can be of comparable magnitude to pollutant inputs, and contribute significant quantities of N2O to the atmosphere.