Evaluation of batch and column techniques to measure weathering rates in soils

Abstract

Six batch and column techniques were evaluated with respect to their potential to measure realistic weathering rates in soil. Soil samples were taken from a C-horizon of a Dystrochrept. Rates observed in the laboratory were one to three orders of magnitude higher than those inferred from field studies. Batch techniques that involve stirring or shaking, generally gave the highest weathering rates, probably due to abrasion of mineral particles. Column techniques are to be preferred, but in a classical column leaching experiment chemical conditions in the pore water may vary considerably within the column and over the duration of the experiment. To reduce variation of chemical conditions in a column titration experiment, a new technique was used, in which the influx of protons was automatically adjusted proportionally to the overall buffering rate. Results of the column techniques show that dissolution rates for base cations and aluminum strongly decrease with the extent of reaction. Experimental evidence was found that an increase of the column percolation rate will increase the weathering rate. Although the underlying mechanism for a relationship between percolation rate and weathering rate is not yet clear, this relationship may explain an important part of the discrepancy between weathering rates of base cations from column studies and rates that are observed in the field.