Critical Loads and a Dynamic Assessment of Ecosystem Recovery

A dynamic soil acidification model (SMART) is used to assess the impact of the so-called 50%-gap-closure scenario on the state of forest soils in Europe. This scenario aims to reduce the excess deposition of sulfur over the critical loads by at least 50% everywhere in Europe by the year 2000 at minimal emission reduction costs and is currently under discussion as a basis for a new UN/ECE sulfur protocol. The concentration of aluminum in soil solution is used as an indicator for potential damage due to acidifying deposition. The time required for soils to recover, i.e. to reach an aluminum concentration of less than 0.2 eq/m3, is computed and mapped on a 150×150 km2 grid covering Europe. Results show that the implementation of the 50%-gap-closure scenario will “protect” an additional 10% of the European forest soils; however, they also indicate that deposition of acidifying nitrogen compounds have to be reduced as well in order not to create new areas with an elevated risk to ecosystem damage.