A comparison of denitrification in felled and unfelled plots in a Sitka spruce plantation
There has been little work done to investigate the importance of denitrification in forest soils. This has been caused by difficulties associated with measurement of the denitrification process and from the assumption that nitrification, and hence also denitrification, was insignificant in acid environments. Nitrification can, however, occur even in the acid conditions found in coniferous forest soils, and is especially important after clear-felling when levels of nitrate in soil and drainage waters are often observed to increase. A potential exists, therefore, for gaseous losses of N via denitrification from such soils. This thesis describes the establishment of a suitable method for measurement of denitrification using the acetylene block technique. This method was used to monitor denitrification losses of N, both as N2 and N2O, from a peaty-gley soil at Kershope Forest. The total loss of N from the standing forest through denitrification was estimated to be 3.2 kg ha-1 over the year studied. Of this loss, approximately 80% was produced as H2O. Gaseous loss of N through denitrification represents approximately the same order of magnitude as the N lost from the site via leaching. An adjacent site, clear-felled four years previously, was also monitored for denitrification losses. Although this site was denitrifying at only a slightly greater rate than the standing forest, examination of past records from the site revealed that an estimated 9-40 kg N ha-1y-1 had been lost in the two years immediately after felling. To assess the factors which controlled denitrification in the field, sub-samples of the soils used for denitrification measurements were analysed for water content, extractable nitrate, and available carbon. None of these factors, however, were found to correlate clearly with the denitrification rate. Further experiments, using laboratory amendments of soil cores, indicated that nitrate concentrations had the greatest effect on denitrification rates, although both the availability of carbon and the aeration status of the soil also affected the rates measured.