REVIEW OF ACID-DEPOSITION CATCHMENT INTERACTION AND COMMENTS ON FUTURE-RESEARCH NEEDS

This review of acid-deposition-catchment interaction follows from the Journal of Hydrology's Special Issue of August, 1990 (Volume 116). For some years acid deposition research has laboured under the constraint of a 'politically correct' paradigm. Nevertheless, this review documents appreciable advances in the state of the science. These advances have led to the point where a paradigm shift is possible. Atmospheric acid deposition contributes to the acidity of catchments. It necessarily interacts with all catchment materials: organic, biological, and mineral. However, acid-deposition-catchment interactions need to be critically revisited and put into perspective with the fundamental knowledge of catchment biogeochemistry, geography and the effects of disturbance history possessed by other disciplines. Each natural acid-neutralizing and acid-producing mechanism ignored by popular acidification theory contributes to a cumulative overestimate of the importance of acidic deposition in catchment acidity. Previous research on catchment sulphur sources has been deficient to the degree it has accepted the paradigm that such catchments have negligible natural sources of sulphur. This cannot be so given the need ecosystems have for sulphur as an essential macronutrient, their adaptation to excessive amounts of it (i.e. ecosystems are leaky in respect to sulphur), the sulphur content of bedrock, and weatherability of sulphur-bearing minerals. Research needs to be initiated on all aspects of catchment retention of atmospherically deposited sulphur, especially enhanced retention by imperfectly drained terrestrial soils. Deficiencies in catchment sulphur cycle research are not random; they err cumulatively on the side of overestimating the importance of atmospheric sulphur deposition.