EXPERIMENTAL MANIPULATIONS OF WATER AND NUTRIENT INPUT TO A NORWAY SPRUCE PLANTATION AT KLOSTERHEDE, DENMARK .3. EFFECTS ON THROUGHFALL, SOIL-WATER CHEMISTRY AND DECOMPOSITION

At Klosterhede, Denmark experimental manipulations of the input of wat er and nutrients to a Norway spruce forest stand have been performed. The manipulations included fertilizer application, irrigation and crea tion of artificial dry periods. Further, the acidic input was excluded from the soil by a 1200 m(2) roof. The aim of this study was to quant ify the effects of air pollutants on the internal cycling of nutrients in the forest ecosystem and to assess the influence of the manipulati ons on the canopy interacting processes, the soil water composition an d the turnover of organic matter. The improved nutrient availability a nd the increased soil water content in the fertigation (fertilizer app lication + irrigation) and the irrigation plots under the roof resulte d in an increased decomposition of cotton strips compared to the uncov ered control plot. However, the decomposition of needle material (litt erbags) in the roof covered plots was reduced probably due to altered humidity conditions in the humic layer as a result of the irrigation s ystem. Significant changes to the manipulations were seen in the soil water concentrations of SO42- in the irrigated and the fertigated plot s and of K+ in the irrigated plot which show that the internal cycling and dynamics of sulphur and potassium are very sensitive to interfere nce in the input. The effect of reduced acid input on soil water conce ntrations of Al and H+ was not evident before the 5th year of treatmen t where Al concentration was reduced almost 50% at 55 cm depth on the ''clean rain'' (irrigated) plot. It was not possible to detect any enl arged throughfall deposition on the fertigated and irrigated plots des pite an observed rise in the canopy surface area on these plots or to detect any changes in canopy leaching reflecting changes in nutrient s tatus.