Impact of acid atmosphere deposition on soils : field monitoring and aluminum chemistry

The effect of acid atmospheric deposition on concentrations and transfer of major solutes in acid, sandy soils was studied. Emphasis was given to mobilization and transport of potentially toxic aluminum. Data on solute concentrations and fluxes in meteoric water as well as soil solutions were obtained from intensive monitoring programmes conducted at a number of sites in northwestern Europe and North-America. Specific hypotheses were tested in laboratory experiments.Atmospheric acid inputs dominated total acid loads in nearly all soils studied. This was particularly true for forest soils in the Netherlands, due to an extreme dry deposition of (NH _{<font size="-2">4</font>} ) _{<font size="-2">2</font>} SO _{<font size="-2">4</font>} in the forest canopy. Input of (NH _{<font size="-2">4</font>} ) _{<font size="-2">2</font>} SO _{<font size="-2">4</font>} may acidify the soil, due to NH _{<font size="-2">4</font>} assimilation, or nitrification of NH _{<font size="-2">4</font>} and subsequent leaching of NO _{<font size="-2">3</font>} .In soils rich in weatherable minerals, mobilization of base cations dominated acid neutralization, so that percolation water was buffered at relatively high pH values and the mobility of aluminum was low. In these soils the mobilization and transport of aluminum was mainly driven by organic acids, which is consistent with the podzolization theory. However, in soils depleted in weatherable minerals (e.g. most of the acid, sandy soils in the Netherlands), mobilization rates of base cations were too low for a substantial neutralization of atmospheric acidity, so that pH values of the percolation water were often low enough to cause a significant mobilization of aluminum. In these soils the atmospherically derived strong acid load was almost fully neutralized by aluminum dissolution. Much of this dissolved inorganic aluminum is currently lost to the groundwater.Although aluminum is abundant in soils only a minor fraction, largely consisting of non-silicate organically bound aluminum, formed in the course of soil development (podzolization), was readily dissolved. The current rapid and irreversible depletion of this fraction in many acid, sandy soils of the Netherlands constitutes a drastic change in soil genesis. Depletion may eventually also result in reduced acid neutralization, due to decreased dissolution of aluminum.The aluminum solubility control varied with soil depth. Surface soil solutions were highly undersaturated with respect to gibbsite solubility, whereas saturation was approached in the subsoil. Possibly, the low aluminum solubility in the surface soil was controlled by an exchange type equilibrium with organic soil aluminum. Insight in the aluminum solubility control in soils is not only relevant for improving our understanding of the aluminum chemistry in soils per se, but it may also help explaining the aluminum chemistry of surface water in mountainous catchments, where soil solutions may reach the stream via different flowpaths through the soil, depending on hydrological conditions.