At Klosterhede, Denmark experimental manipulations of the input of water and nutrients to a Norway spruce forest stand have been performed. The manipulations included fertilizer application, irrigation and creation of artificial dry periods. Further, the acidic input was excluded from the soil by a 1200 m2 roof. The aim of this study was to quantify the effects of air pollutants on the internal cycling of nutrients in the forest ecosystem and to assess the influence of the manipulations on the canopy interacting processes, the soil water composition and the turnover of organic matter.

The improved nutrient availability and the increased soil water content in the fertigation (fertilizer application + irrigation) and the irrigation plots under the roof resulted in an increased decomposition of cotton strips compared to the uncovered control plot. However, the decomposition of needle material (litterbags) in the roof covered plots was reduced probably due to altered humidity conditions in the humic layer as a result of the irrigation system. Significant changes to the manipulations were seen in the soil water concentrations of SO4 2- in the irrigated and the fertigated plots and of K+ in the irrigated plot which show that the internal cycling and dynamics of sulphur and potassium are very sensitive to interference in the input. The effect of reduced acid input on soil water concentrations of Al and H+ was not evident before the 5th year of treatment where Al concentration was reduced almost 50% at 55 cm depth on the “clean rain” (irrigated) plot. It was not possible to detect any enlarged throughfall deposition on the fertigated and irrigated plots despite an observed rise in the canopy surface area on these plots or to detect any changes in canopy leaching reflecting changes in nutrient status.