Journal of Conference Abstracts

Integrated Assessment of Soil Chemical Status in Switzerland. 2. Application of a Regionalised Model to
622 Forested Sites

D. Kurz¹ (GeoSci@aol.com), M. Alveteg² (Mattias.Alveteg@chemeng.lth.se) &
H. Sverdrup² (Harald.Sverdrup@chemeng.lth.se)

¹ EKG Geo-Science, Bern, Switzerland.

² Department of Chemical Engineering II, Lund, Sweden.

The dynamic, process-oriented, multi-layer soil chemistry model SAFE reconstructs acidification of terrestrial ecosystems by calculating the development of different chemical state variables. A regionalised version of SAFE was applied to 622 equally distributed forest sites in Switzerland. The objective of the exercise was to clarify if and when the chemical status of Swiss forest soils will improve, given the current emission reduction plans.

The dynamic perspective of acidification completes and extends static modeling approaches used so far for the evaluation of emission abatement scenarios in Europe. Static calculations evaluate threshold loads below which no adverse changes in ecosystem structure and function occurs in the long-term perspective according to present knowledge. Emission reductions recently established for S will, however, not lead to acidifying pollutant deposition below critical loads of acidity everywhere in Switzerland, nor in the rest of Europe (Posch et al., 1995). Persisting exceedances of critical loads have already, and will continue to lead to adverse effects on forest ecosystems. To investigate temporal aspects of this adverse evolution and to evaluate the damage time lag, a dynamic approach is needed. A dynamic approach can also address questions regarding whether or not a temporary exceedance will lead to violations of the chemical criteria used in the calculation of the critical load.

The exercise was produced from a comprehensive revision of the general model approach particularly of the input data generation necessary for a regional application (Alveteg et al., this volume). Simulations imply that the present day chemical status of Swiss forest soils is a result of the last 50 years' acid deposition. Indicative soil parameters, such as soil solution pH, total aluminum concentration, acid neutralising capacity (ANC), base cation to total aluminum molar ratio (BC/Al) and the base saturation, consistently deteriorate since the beginning of the 1950s, when acid loads start to increase. Between 1945 and today, modeled pH drops by 0.6 average units in the organic layers and by 1.1, 0.8 and 0.6 units in the model A/E, B, and C layers, respectively. Average total aluminum concentration increases by a factor of 10 to 40 in the four soil layers starting at values below 15 µeq L^-1 during the same time period. By the late 1970's, 50 % of the sites' upper soil layers have BC/Al molar ratios below 3.4, whereas median BC/Al in the lower soil layers drops to 1.8 by the end of this century. Median ANC consistently decreases from around, or slightly above 0 to -240, -230, -150 and -140 µeq L^-1 in the respective layers by the late 1970's. Exchangeable base cations, currently, are depleted markedly at approximately 50 % of the sites.

The adverse trend of deteriorating soil chemistry stops around 1975 in the upper layers and around 2010 in the deeper horizons, indicating a damage time lag of up to 35 years. The adapted current reduction plans of S and N emissions will lead to an improvement in upper soil conditions and to a halt of acidification of the lower soil horizons. Simulated solution pH, total aluminum concentrations and ANC in the upper horizon recover to levels simulated for the 1950's and 60's for more than 80 % of the sites. At 45 % of the sites, however, the lower soil solutions key parameter values compare with values of much later years. Slightly more than 10 % never have lower pH, ANC, and higher total aluminum concentrations than by 2050 (the end of the simulation period). Considerable recovery time lags are predicted for BC/Al and base saturation. In 2050, 50 % of the sites have BC/Al and base saturation values in the lower soil layers comparable to values modeled for the time after the peak of acid deposition in the late 1970's, and 20 % reach their all-time low in 2050.

References

Posch, M., de Smet, P.A.M., Hettelingh, J-P. & Downing, R.J., RIVM Rep. No. 259101004 CCE/RIVM, Bilthoven (1995).