Nitrogen saturation at Gårdsjön, southwest Sweden, induced by experimental addition of ammonium nitrate

Abstract

To investigate the risk and consequences of nitrogen saturation in coniferous forests typical of southern Scandinavia, a whole-catchment experiment is presently being conducted whereby nitrogen is added to throughfall at Gårdsjön, southwest Sweden. The Gårdsjön experiment is part of the European NITREX project (nitrogen saturation experiments). From April 1991, about 35 kg NH 4NO 3-N ha −1 year −1 in 5% extra water has been added in weekly doses to the ambient 12 kg NH 4NO 3-N ha −1 year −1 in throughfall. Elevated concentrations of nitrate in runoff occurred during the first 2 weeks of treatment, but then were very low during the growing season. Nitrate leakage appeared again in mid-November 1991 and continued during the winter and throughout the second year of treatment. The nitrate lost during the first and second years was about 0.6% and 1.1%, respectively, of the total inorganic nitrogen input. Of the other ions analysed in runoff, sulphate, inorganic aluminium and total phosphorus showed statistically significant changes, with lower concentrations during the treatment period relative to the untreated control catchment. The nitrogen input-output data indicate that the forested catchment ecosystem G2 NITREX is proceeding rapidly through several stages of ‘nitrogen saturation’. Both the frequency and the magnitude of nitrate peaks in runoff increased during the non-growing season in the first year (stage 1) and throughout the second year (stage 2). The results from NITREX Gårdsjön demonstrate that nitrogen saturation can be induced over a relatively short time by increasing atmospheric deposition of nitrogen. The rate of response suggests that at ambient nitrogen deposition of 13 kg inorganic N ha −1 year −1, the ecosystem is near the threshold at which additional N inputs cause significant nitrate leaching.