ASSESSMENT OF CRITICAL LOADS AND THEIR EXCEEDANCE ON DUTCH FORESTS USING A MULTILAYER STEADY-STATE MODEL

Critical acid loads for Dutch forests were derived using a multi-layer steady-state model that includes canopy interactions, nutrient cycling, mineral weathering and N transformations. Values were calculated for combinations of 12 tree species and 23 soil types for a 10 x 10 km grid. Critical acid loads thus derived increased with decreasing soil depth. Nearly 90% of the values varied approximately between 1500 and 4000 mol(c) ha-1 yr-1 at 10 cm soil depth and between 750 and 2000 mol(c) ha-1 yr-1 at the bottom of the rootzone. Separate critical loads calculated for N and S at the bottom of the rootzone varied between approximately 300 and 1000 mol(c) ha-1 yr-1 for N and between 150 and 1250 mol(c) ha-1 yr-1 for S. Using deposition data of 1990, a median reduction of the deposition by approximately 75% was calculated to achieve the critical loads at the bottom of the rootzone. The overall uncertainty in this value was estimated to be about 10%, although it can be much larger for specific soil types such as clay and peat soils. For N a larger reduction deposition percentage was calculated than for S, especially for coniferous forests with a high present N input.