Field-scale ‘clean rain’ treatments to two Norway spruce stands within the EXMAN project—effects on soil solution chemistry, foliar nutrition and tree growth

‘Clean rain’ treatments were performed in spruce forests at Klosterhede, Denmark, and Solling, Germany, by replacement of ambient throughfall water with artificial throughfall reducing the load of S (55–95%), N (70–90%) and acidity (ca. 90%). The induced strong reductions in SO 4 2− input caused corresponding strong reductions in soil solution concentrations at Klosterhede and to a minor extent at Solling. The difference in response to the reduced input is suggested to be largely a consequence of differences in storage capacity between the two sites. Reduced input of N at Solling caused strong and immediate reduction in the soil solution concentration. At Klosterhede, N concentrations in the soil solution were already low prior to the treatment and no effects were recorded. The effects on soil acidification reflected by Al and H concentrations were slow and small although a slight improvement in acid neutralising capacity (ANC) was observed at Klosterhede, mainly caused by a strong reduction in strong acid anions. The input reductions decreased the general ion activity in the soil solution and the accumulation of base cations at both sites were consequently increased. Within the 6 yr of treatment at Klosterhede and the 2 yr at Solling, the effects on the soil solution chemistry were generally moderate. Accordingly, no significant changes in vegetation were observed apart from a water induced increase in growth rate at Klosterhede.