NITROGEN ADDITIONS (NANO3 AND NH4NO3) AT ABER FOREST, WALES .2. RESPONSE OF TREES AND SOIL-NITROGEN TRANSFORMATIONS

A manipulation experiment was carried out to investigate the impacts of increased nitrogen deposition on a mature Sitka stand which has symptoms of nitrogen saturation. Sodium nitrate at 35 or 75 kg N ha(-1) year(-1) and ammonium nitrate at 35 kg N ha(-1) year(-1) were applied weekly to the forest floor as a spray. A water-only control (or zero-N) and no-spray control were included in the design to enable water and nitrogen effects to be differentiated. Data from in situ polythene bag incubations indicate that there was no significant impact on annual soil net N-mineralisation or nitrate accumulation rates in the forest floor material following 2.5 years of treatment. Net nitrogen mineralisation in the forest floor was estimated to be 68-90 kg N ha(-1) year(-1) and nitrate accumulation 6-22 kg N ha(-1) year(-1), with large within-treatment variability. No changes in litter decomposition rates in response to treatments was observed. The main response of the trees appeared to be increased water availability in the sprayed plots, indicating that the trees at this site are normally drought stressed. There was a trend towards increased mean needle weight and %N in the current year foliage in all of the sprayed plots, with no relationship to nitrogen loading. Increased water availability also increased the standing crop of roots (diameter less than 5 mm) in the forest floor in the water-only control. Increases in nitrate loading, however, reversed this effect in the sodium nitrate treatment plots, as a decrease in root mass in the sodium nitrate treatment plots relative to the water-only control treatment was observed. Increased above-ground litterfall in the third winter appeared to be positively associated with nitrate loading with no change in litter chemistry observed. More long-term monitoring is needed to determine the future impacts of these changes on tree vitality and the long term impact of ammonium on soil nitrogen dynamics.