Differential adsorption of Al, Ca, and Mg by roots of red spruce (Picea rubens Sarg.).

The surface adsorption characteristics of red spruce (Picea rubens) roots were examined as a function of changes in external acidity and cation concentrations. Root cation exchange capacity varied significantly with changes in pH, increasing from 110 micromol(c) g(-1) at pH 3.5 to 155 micromol(c) g(-1) at pH 4.5, and reaching 250 micromol(c) g(-1) at pH 7.0. In general, Al adsorption by spruce roots was much greater than either Ca or Mg adsorption under the same initial conditions. However, root affinity for the divalent cations was proportionately much more sensitive to pH changes than was root affinity for Al. The fractions of adsorbed Ca and Mg increased by 50 to 100% as pH increased from 3.5 to 4.5, whereas the fraction of adsorbed Al remained relatively constant at both initial pH conditions. Competition experiments at pH 3.5 and 4.5 indicated that Al adsorption was strongly favored over Ca adsorption, except at low Al concentrations (~10 micromol l(-1)), high solution Ca(2+)/Al(3+) ion activity ratios (> 1.5 to 5.0), and at the higher pH. These results suggest that cell wall exchange sites in red spruce roots will tend to become progressively saturated with Al under the prevailing conditions of many acidic forest soils. To the extent that root adsorbed Al interferes with the active uptake of Ca and Mg, this process of competitive cation adsorption can contribute to impaired mineral nutrition in the spruce forest community.

Canopy processing of acidic precipitation by coniferous and hardwood forests in New England.

There are several important factors that may influence how forest canopies interact with acidic deposition, including forest community species composition, phenological status, and differences in atmospheric loading of strong acids. Results from comparative throughfall chemistry studies in New Hampshire, where precipitation pH is 4.1, indicate that northern hardwood canopies produce a throughfall solution chemistry that is less acid and higher in basic cations than either direct precipitation or throughfall solutions derived from nearby subalpine balsam fir forests. Neutralization of acid precipitation in the hardwood canopy appears to occur through two major processes: ion exchange removal of free H+ by the foliage, and Brønsted base leaching from the plant canopy. Data obtained during the period of senescence preceding leaf-drop suggest a strong link between alkalinity release and potassium leaching in the hardwood canopy. Compared with the hardwood canopy, the coniferous forest canopy exhibits several distinct quantitative differences in canopy processing.

Aluminum leaching response to Acid precipitation: effects on high-elevation watersheds in the northeast.

Atmospheric inputs of sulfuric acid and nitric acid to noncalcareous higher-elevation watersheds in the White Mountain and Adirondack regions lead to comparatively high concentrations of dissolved aluminum in surface and ground waters. This phenomenon appears to result from modern increases in soil aluminum leaching. Transport of this aluminum to acidified lakes can lead to fish mortality. Combined results from areas of silicate bedrock in the United States and Europe suggest that aluminum represents an important biogeochemical linkage between terrestrial and aquatic environments exposed to acid precipitation.

Forest floor leaching: contributions from mineral, organic, and carbonic acids in new hampshire subalpine forests.

Analyses of soil water and groundwater samples from a high-elevation coniferous ecosystem in New England indicate that sulfate anions supply 76 percent of the electrical charge balance in the leaching solution. This result implies that atmospheric inputs of sulfuric acid provide the dominant source of both H(+) for cation replacement and mobile anions for cation transport in subalpine soils of the northeastern region affected by acid precipitation. In soils of relatively unpolluted regions, carbonic and organic acids dominate the leaching processes.