Critical loads and exceedances for eutrophication and acidification for Flemish forests

Critical loads were estimated for over more than 1400 receptors supporting forest vegetation in northern Belgium using simple mass balance method. Necessary data were derived from old historical soil database, recent data from forest surveys, meteo data, level I and II plots and regional studies concerning elemental sequestration in woody biomass. Deposition estimates were performed with the OPS–model, which had been validated with deposition measurements of N and S in 6 level II plots over the period 1994–1998. In addition, an edge enhancement factor was calculated to account for enhanced deposition in plots, situated in forest edges.

Magnitude of the critical loads depended on the criteria used for acceptable leaching of acidity and nitrogen. For eutrophying nitrogen, critical loads were related to protection of shallow groundwater for nitrate leaching or to conservation of floral composition. For acidifying nitrogen and sulphur, critical loads were related to criteria preventing root damage from Al toxicity (mol Ca/ Al = 1) and further acidification of the soil profile (ANC = 0).

The study revealed that demands to conserve species composition and prevent further soil acidification are the sternest, followed by requirements to prevent nitrate leaching to contaminating groundwater and impairing root systems. Deciduous forest ecosystems were less vulnerable to nitrate leaching than coniferous sites. Emission reduction strategies must especially be aimed towards reducing ammonia emissions being the main cause of acidification and eutrophication in Flemish forests.