Impact of acid atmospheric deposition on soils : quantification of chemical and hydrologic processes

Atmospheric deposition of SO _x , NOx and NHx will cause major changes in the chemical composition of solutions in acid soils, which may affect the biological functions of the soil. This thesis deals with quantification of soil acidification by means of chemical budgets, kinetics of mineral weathering of aluminum and base cations, and simulation of soil acidification. Most results apply to an acid forest soil on the Hackfort estate, for which monitoring data were available from 1981 to 1987.Spatial variability of soil solution composition was found to be the dominant source of uncertainty in determining chemical budgets. Uncertainty in annual chemical budgets due to simulation of unsaturated soil water fluxes generally does not exceed 10%. A new method was developed for in situ measurement of unsaturated soil water fluxes at a similar accuracy as obtained by numerical simulation.Weathering of Al is the dominant process for proton buffering in acid soils. In dutch acid soils, the most reactive pool for Al is present in hydrated oxides, often in association with organic matter. On average the weathering rate of Al increases nearly proportional with (H <sup><font size="-1">+</font></SUP>), and decrease strongly with increasing depletion of reactive Al. Total exhaustion of the reactive pool of hydrated oxides may become a problem in the coming century.A new column percolation technique was developed to measure weathering rates at controlled pH in absence of mechanical disturbance. In all cases base cation weathering rates from laboratory experiments were considerably higher than estimated ' from field mass balance studies. A yet unconsidered effect for explanation of this discrepancy, is the increase of weathering rate with (by approximation the square root of) the percolation rate. Moreover, evidence was found that assumed patchy coatings of secondary Al may be protective against weathering of Ca from plagioclases.The ILWAS model was adapted for simulation of soil acidification. The model proved to be appropriate to simulate annual budgets and seasonal variation of concentrations for all major components. Concentration peaks near the end of summer appeared to be overestimated, but also scarce field observations may be biased. The model seemed somewhat too crude to predict N-dynamics. The model was used to analyze the response of the soil system to 50% reduction of deposition, fertilization, and removal of forest vegetation. Scenario analyses, demonstrated that pH is not a sensitive indicator for changes in soil chemistry. Simple models, using large time steps, may be adequate to evaluate average annual changes of soil solution characteristics over periods of several decades.