In the Selling experimental forest in central Germany a manipulation study with three roof constructions of 300 m(2) area each is underway in a 60-year-old Norway spruce (Picea abies (L.) Karst.) forest. The experimental treatments are: (1) 'clean rain' roof with simulated pre-industrial throughfall; (2) control roof for roof effects alone, without manipulation; (3) drought/rewetting roof, for simulation of extreme drought; (4) control without roof. The clean rain roof is part of the NITREX project; we investigate the time scales and biological reactions of the system which lead to reversal of nitrogen saturation. The data include soil water chemistry for 2 years prior to treatment and 1.5 years of treatment. The response to reduced inputs of N and S was lower concentrations in soil solution of N ions, sulfate and Al down to 40 cm depth. Nitrogen ions virtually disappeared from the soil water in the rooting zone (less than 70 cm soil depth) during the 1.5 years of clean rain application. If this trend continues, nitrogen leaching will cease within a relatively short period of time, and N-saturation will be reversed. Future studies will focus on the reaction of the forest to the significantly reduced N levels in soil solution.
