Contrasting responses of two Sitka spruce forest plots in Ireland to reductions in sulphur emissions: results of 20 years of monitoring

Abstract

Long-term trends in ion concentrations of bulk precipitation, throughfall, forest floor leachate (humus water) and shallow and deep soil water were assessed at two Sitka spruce (Picea sitchensis) stands—one on an Atlantic peat bog in the west of Ireland (Cloosh), the other on the east coast on a peaty podzol (Roundwood). Deposition at Cloosh was dominated by marine ions (sodium, [Na+], chloride [Cl−], and magnesium [Mg2+]), whereas bulk precipitation and throughfall at Roundwood was characterized by inputs of non-marine sulphate (nmSO42−), acidity and inorganic nitrogen (NH4+, NO3−). Significant declines in concentrations of nmSO42− and acidity in bulk precipitation and throughfall were observed at both sites. The decline in throughfall nmSO42− was significantly related to reductions in European sulphur dioxide (SO2) emissions. At Roundwood, SO42− declined significantly in humus, shallow and deep soil water. In deep soil water this was accompanied by a long-term increase in pH and a reduction in total aluminum (Altot). The recovery from acidification was delayed by high concentrations of NO3−, which strongly influenced acidity and Altot concentrations. At Cloosh, there was a significant decline in SO42− in humus water but long-term trends were not evident in shallow or deep soil water; SO42− concentrations at these depths fluctuated in response to drought-events. Marine ions strongly influenced soil water chemistry at both sites; at Cloosh soil water acidity was strongly related to Na+ and Cl−, while at Roundwood, Na+, Cl− and Mg2+ influenced Altot concentrations. Dissolved organic carbon increased significantly in humus and soil water at Roundwood, where it was associated with declining acidity. Soil water at both sites was influenced by a combination of anthropogenic sulphur (S) and nitrogen (N) deposition, drought and sea-salt events. The study highlights the value of long-term monitoring in assessing the response of forest soils to S and N deposition against a background of climate influences on soil water through drought and sea-salt events.