ALBIOS: A Comparison of Aluminum Biogeochemistry in Forested Watersheds Exposed to Acidic Deposition
This chapter presents a case study of the broad interregional patterns of aluminum biogeochemistry and aluminum toxicity in the forest landscapes of North America and northern Europe. Sulfur deposition at the 14 ALBIOS study catchments ranged 20-fold from approximately 4 kg S ha-1yr-1 at the Experimental Lakes Area, Ontario, to >80 kg S ha-1yr-1 at Soiling, West Germany. On a regional basis, the lowest total monomelic aluminum (MAL) concentrations (0–10 μM) were found in watersheds of the southeastern and midwestern USA, which contain soils characterized by high soil percent base saturation and/or a large sulfate adsorption capacity. Intermediate MAL concentrations (15–80 (μM) were found in soil solutions of northeastern North America and northern Europe, where soils are characterized by low percent base saturation and low sulfate adsorption capacity. The highest MAL concentrations (up to 240 (μM) were observed in the soil solutions of a West German spruce stand. Headwater streams in the study catchments contained MAL concentrations that ranged from less than 1 μM to peak values of approximately 55 μM in one West German stream.
Two dominant geochemical patterns were observed in most watersheds: (1) upper soil horizon and wetland zones characterized by aluminum adsorption-desorption reactions on solid-phase humic materials; and (2) mineral soil horizon and groundwater zones dominated by aluminum solubility relationships with some form of Al(OH)3. Much of the overall variation in aquo aluminum ion activity could be explained on the basis of relatively simple equilibrium pH-solubility and adsorption models.
The ALBIOS evidence indicated that the relationship between watershed inputs of H2SO4 or HNO3 and outputs of soluble aluminum is not necessarily simple and straightforward. However, for those watersheds characterized by aluminum-saturated soils and low retention of strong acid anions, increased concentrations and fluxes of sulfate and nitrate in soil water were accompanied by increased concentrations and fluxes of soluble aluminum, both on a broad geographic basis and on a single catchment basis.
Experimental plant response studies showed that honey locust (Gleditsia triacanthos), red spruce (Picea rubens), sugar maple (Acer saccharum), American beech (Fagus grandifolia), red oak (Quereus rubra), and loblolly pine (Pinus taeda) could be grouped into sensitive, moderately sensitive, and insensitive classes on the basis of growth and nutritional responses to soluble aluminum. The toxicity thresholds for the sensitive and moderately sensitive tree species were within range of the peak concentrations of soluble aluminum observed in soil solutions at some of the northern and European watersheds. Likewise, soluble aluminum concentrations in many headwater streams were in excess of toxicity thresholds for fish species like brown trout (Salmo trutta L.) and brook trout (Salvelinus fontinalis). As such, it is likely that aluminum toxicity serves as a contributing stress factor in these kinds of northern watersheds.
KeywordsSoil Solution Acidic Deposition Headwater Stream Aluminum Toxicity Ingrowth Core
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