ECOSYSTEM RESPONSES TO REDUCED NITROGEN AND SULFUR INPUTS INTO 2 CONIFEROUS FOREST STANDS IN THE NETHERLANDS

Atmospheric inputs of nitrogen and sulphur were reduced to pre-industrial levels in a nitrogen and sulphur saturated Scots pine (Pinus sylvestris) and Douglas fir (Pseudotsuga menziesii) stand in the Netherlands. Starting in 1989, throughfall water was intercepted by means of a roof and replaced by simulated, clean throughfall water. Underneath the roof two plots were designed to receive either clean water (roof clean) or ambient throughfall (roof control). Outside the roof a second control plot was established (ambient control). Until 1992 a significant roof effect was found owing to differences in water application, but automation of the watering regime significantly reduced this problem. Throughfall chemistry showed a dominance of ammonium to nitrate, whereas the reverse was observed in the soil solution. In the roof clean plots a quick response of soil solution chemistry was observed. The sulphur and nitrogen concentrations in the upper soil layers strongly decreased, as did the fluxes of these elements through the soil profile. As a result, leaching of base cations and ratios of ammonium to various cations decreased. Decomposition studies in the Scots pine stand showed a positive effect of nitrogen deposition on the decomposition rate in the roof control plot compared with the roof clean plot, whereas in the Douglas fir stand no differences between these plots were found. A reduction of atmospheric nitrogen and sulphur deposition in the Scots pine stand increased the species diversity of microarthropods, owing to a decreased dominance of some species at a constant species number. In the Scots pine stand fine root biomass and the number of root tips increased as nitrogen deposition decreased, indicating an increased nutrient uptake capacity. As a result potassium and magnesium concentrations and their ratios to nitrogen in the needles increased. After 4 years of treatment, nitrogen concentrations in the needles of the roof dean plot remained high, but were significantly lower than in the needles of the control plots. In the fourth year of the experiment nitrogen concentrations in the older needles of the clean plot became lower than in the current needles, which is typical for a nitrogen limited forest ecosystem. This is in agreement with the nitrogen flux via litterfall, which was lower onto the roof clean plot than onto the control plots. Until now, no significant changes in nutrient concentrations in the needles of the Douglas fir stand have been observed.