For forest and nature policy it is important to know the highest level of atmospheric nitrogen (N) and sulphur (S) deposition below which harmful acidifying effects on forest soils do not occur. Therefore, target loads for acidification were determined for Flemish forests according to the harmonized methodology of the Coordination Center of Effects. Target loads are calculated in a similar way as critical loads, but account for soil buffer mechanims. Critical loads are based on a steady-state mass balance and equal the highest long-term deposition that still respects a pre-defined soil-chemical status. A target load is the deposition for which the chosen status is respected from the target year on. This requires taking into account the buffering capacity of soil processes such as cation exchange and N immobilisation, as these create a time lag in soil recovery when the potentially acidifying deposition declines. Critical loads were calculated for 1438 Flemish forest locations; on the one hand for the protection of roots against acidification using a maximum ratio of aluminum vs. base cation concentration in soil water, on the other hand for eutrophication using a maximum nitrate leaching. For non-calcareous forest soils in Flanders, the median critical load of S (criterion Al:Bc = 1) amounted to 1754 eq ha-1 year-1. The median critical load of acidifying N was 3010 eq ha-1 year-1 including denitrification and 2227 eq ha-1 year-1 assuming no denitrifi-cation. The median critical load for nutrient N with respect to eutrophication (criterion NO3le,acc = 100 eq ha-1 year-1) was 935 eq ha-1 year-1. Target loads for acifidication were determined for 83 non-calcareous Flemish forest stands of the forest soil and forest vitality network (Level I and II plots). The Very Simple Dynamic (VSD) model was used for simulating the chemical composition of soil and soil water throughout time based on soil characteristics and rainfall, deposition and growth data. Target loads were calculated for the target years 2030, 2050 and 2100. To respect the Al:Bc criterion from 2050 on, for example, a N and S deposition reduction was needed in 84% of the plots according to the VSD model. In 12% of the plots no additional deposition reduction was needed compared to the Gothenburg agreements, while for 4% the Al:Bc criterion could not be reached in 2050 even when N and S deposition would be reduced to zero from 2010 on. The median target load of S for target year 2030 amounted to 58% of the median critical load of S; for the years 2050 and 2100 this ratio was 65% and 86%, respectively. For N the difference between target loads and critical loads was smaller than for S due to the time dependent N immobilisation. The critical and target loads of S were lower for deciduous stands than for coniferous stands because of the higher nutrient uptake by deciduous trees. The acceptable acidifying deposition was lower for forests on sandy sails than on loamy or clayey soils due to the lower mineral weathering rate in sandy soils.
According to the sensitivity analysis, the calculated target loads depended mostly on the base cation fluxes mineral weathering, deposition, and net growth uptake. Furthermore, the chosen cation exchange model and the assumed relationship between the soil solution pH and aluminium concentration clearly affected the results. Despite the inherent uncertainty in modelling soil acidification at a regional level, the present research implies that important N and S deposition reductions are needed to allow recovery of Flemish forest soils to a minimal chemical quality
