Effects of acid rain and enhanced nitrogen deposition on soil water chemistry and growth of Scots pine seedlings
The effects of wet-deposited acid and ammonium-N on the soil solution chemistry and growth of conifer seedling were investigated in two pot experiments run concurrently outdoors in Aberdeen. Seedlings of Scots pine growth in a range of four Scottish upland mineral soils, covering a range of expected sensitivity to acidification, were exposed to simulated acid rain (H2SO4) and nitrogen ((NH4)2SO4) treatments for 2-3 growing seasons. At the end of the main experiments, germination and early growth of Scots pine and an understorey grass species were examined in one of the polluted soils. Both acid and ammonium deposition caused significant changes in soil solution chemistry, including acidification and short-term cation mobilisation. It was shown that ammonium sulphate may cause more acidification than a corresponding amount of sulphuric acid in soils such as the limestone in the present study, due to a high nitrification rate. The direct and rapid pH reduction induced by mobile anion effects was strongly evident from the experiments, and this eventually will lead to long-term soil acidification. It was clear from the experiments that ammonium at concentrations exceeding critical load could produce similar cation displacement effects to those of more direct acid inputs. These need to be considered when setting critical loads for deposition of reduced N species. Study of the effects on plant growth revealed that acid or ammonium deposition may have site-specific effects on plants, depending on soil parent material and evolutionary state, as reflected by enhanced growth in the nutrient-rich limestone soil but poor growth in the acidic base-poor soils. This flags the need to somehow encompass overall soil nutrient status when setting critical loads. The two types of pollutant used in the experiments had similar effects on most of the studies aspects of growth of Scots pine because the overriding effect of both was that of soil acidification, and no persistent fertilizer effect on the excess N deposition on growth was evident.