Covered catchment experiment at Gårdsjön: changes in runoff chemistry after four years of experimentally reduced acid deposition

Atmospheric deposition was almost entirely excluded from the forested headwater catchment G1 ROOF, by means of a 7000 m² plastic roof that prevents rain and throughfall from reaching the ground. Under the roof an irrigation system was installed to simulate a natural precipitation regime. The intercepted throughfall was substituted by the same amount of clean pre-industrial throughfall. The experiment started in April 1991. During the four years of the treatment, 2960 mm (18 600 m³) of sprinkled solution was applied under the roof, which is about 15 times the mean water storage of the catchment. After four years of treatment major changes in runoff chemistry were observed. The exclusion of all non-marine sulphate (SO₄ ^2–) input to the catchment (i.e. ca 75 % of total SO₄ ^2– input excluded) resulted in significant decline of sulphate in runoff through all four years of treatment. During the fourth year, annual volume weighted SO₄ ^2– concentration was 46 % lower than the two years prior to the treatment. Concentrations of inorganic aluminium Al³⁺ declined 52 % and Mg²⁺ declined 54 %. No change of H⁺ concentration was detected. As the treatment proceeds there seems to be a trend towards less negative acid neutralising capacity in runoff.     

Magic applied to roof experiments (Risdalsheia,N; Gårdsjön,S; Klosterhede,DK) to evaluate the rate of reversibility of acidification following experimentally reduced acid deposition

Soil and stream water responses to clean rain treatments at three roof covered forest ecosystems in Norway, Sweden and Denmark were modelled by the MAGIC model. The model outputs were compared with measured responses. Over the wide span in site conditions among the three sites MAGIC successfully reproduced the observed effects in the clean rain treatments. Seasalt influence caused single year discrepancies but the long term trends were well reproduced. In particular the significant decreases in SO₄ output were well reproduced. A simultaneous change in base cation leaching was observed, which in the long run will be the basis for the recovery of these acidified systems. The rate of recovery at the 3 studied sites was very slow.     

Dissolved Organic Matter (DOM) in Acid Forest Soils at Gådsjön (Sweden): Natural Variabilities and Effects of Increased Input of Nitrogen and of Reversal of Acidification

Effects of (1) the addition of 35 kg N ha^-1yr^-1(as NH₄NO₃) and (2) interception of throughfall and litterfall by means of a roof on concentrations, chemical characteristics and dynamics of dissolved organic matter (DOM) in acid forest soils (podzols) were studied at Gårdsjön, Sweden. A non-manipulated catchment served as a reference. After 4 yr of treatment no significant effects of either manipulation were found on concentrations and characteristics of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON). The variability of these parameters within catchments appeared to be of the same order of magnitude as the variability between catchments. At all sites, DON contributed more than 70% of the total dissolved nitrogen. In general, the proportion of hydrophilic DOC increased with depth. In contrast, to other studies the C/N ratio of DOM at Gårdsjön did not show a clear pattern with depth. Other studies in SE U.S.A. have suggested that relatively low C/N ratios at depth are due to increased contribution of hydrophilic neutrals to DOC. However, this fraction appears to be small in temperate and boreal spruce forest soils, including Gårdsjön.