Ion concentrations and fluxes of seepage water before and after clear cutting of Norway spruce stands at Ballyhooly, Ireland, and Höglwald, Germany

Ion concentrations and fluxes in seepage water (below the main rooting zone) were compared before and after clear cutting at two similar long-term experimental Norway spruce forest plots. While Ballyhooly (Ireland) was influenced by sea salt deposition, Höglwald (Germany) received high nitrogen (N) deposition. These differences were reflected in seepage water concentrations with sodium (Na⁺) and chloride (Cl^-) dominating at Ballyhooly and high nitrate (NO₃^-) and aluminium concentrations at Höglwald. Following clear cutting of the forest plots, NO₃^- concentrations peaked (Ballyhooly: 2018 μmol_c L^-1, Höglwald: 2595 μmol_c L^-1). Moreover, at Ballyhooly, NO₃^- concentrations and fluxes were continuously elevated for ~1.5 years. At Höglwald, the clear cut plot, which was replanted with spruce and beech saplings, exhibited periodically elevated NO₃^- concentrations with two distinct peaks. However, low concentrations, compared to the control (uncut) plot, were also observed. Further, at Höglwald a plot with a pre-existing dense natural regeneration of Norway spruce exhibited much lower NO₃^- concentrations before and after clear cutting. Nonetheless, NO₃^- concentrations following clear cut at both sites were elevated at least periodically above European drinking water standards (50 mg L^-1). An important prerequisite for NO₃^- leaching is that forests are N saturated or at least not N-limited; consequently chronic elevated N deposition may lead to increased deterioration of seepage water quality across Europe following forest disturbances (harvesting, windthrow, insect attacks). Clear cutting at Ballyhooly was responsible for significant element loss, especially of potassium, N and calcium, while magnesium loss was compensated by high sea salt inputs. At Höglwald the contamination of seepage water with NO₃^- has been the main problem for more than 20 years at the mature stand. A pre-existing regeneration can help to reduce NO₃^- and cation leaching after cutting.