Dynamics and effects of soil CO2 on carbonate dissolution and transport in response to precipitation events

Dissolution of CaCO₃ in calcareous soils is mainly governed by CO₂ which forms a weak but ubiquitous acid in the aqueous phase. Soil CO₂ concentrations are generally higher than atmospheric concentrations due to the CO₂ production in the soil. It is generally assumed, that it is mainly the CO₂ concentration in the soil and the discharge that control the re-location of CaCO₃, and thus the further formation of soil and karst. In most cases soil and karst systems are considered to be static and that the CaCO₃ dissolution process is a steady state process. However, we know that soil CO₂ concentrations can be highly dynamic and are affected by soil temperature and soil moisture. Our objective was to investigate whether this steady state assumption regarding carbonate dissolution and transport can be applied or whether we have to consider the dynamics and interaction of soil CO₂ and dissolution of CaCO₃ in the aqueous phase.

We report on insights from a 3 year field study in a calcareous soil in which soil CO₂ concentrations and its response to soil moisture and precipitation were investigated. Low intensity precipitation resulted in slow increase in soil CO₂ concentration, since increased soil water content blocks formerly air-filled pores. Intense precipitation events were followed by fast infiltration and probably preferential flow. Intense precipitation also resulted in temporary drops in soil CO₂. These drops can be explained by a relative under-saturation of the soil solution at a certain depth. The soil solution is mixed with infiltrating rain water, which is still equilibrated with the lower atmospheric CO₂ concentrations and thus drawing CO₂ from the surrounding soil air. These mechanisms should results in a much stronger dissolution of local CaCO₃ and net transport of dissolved CaCO₃.

A following laboratory experiment on mesocosms of natural soil and restructured soil was used to test and reproduce the observed CO₂ patterns as well as dissolution and transport of carbonate due to precipitation events. These experiments also showed that higher intensity of precipitation results in stronger drops in soil CO₂ concentration and higher transport rates of dissolved CaCO₃. Hydrus1D was used to model soil CO₂ dynamics and dissolution of CaCO₃ in the aqueous phase for the measured scenarios. The observed general pattern of the “drops” of soil CO₂ could be easily reproduced confirming the assumption of CO₂ undersaturated soil water right after the precipitation events. The natural soil mesocosm showed comparable patterns in all precipitations experiments. The restructured soil mesocosm showed a high mobilization and drainage during the first precipitations experiments which then fast declined to the level of the natural soil mesocosm. We interpret this as fast dissolution and washing off of carbonates attached to the macropore surfaces in which preferential flow occurs.

We conclude that dynamics and interaction of soil CO₂ and dissolution of CaCO₃ in the aqueous phase are highly dynamic and affected by preferential flow. It seems that general patterns can be reproduced using Hydrus 1D, with the hydrological parametrization as a major challenge.

This research was financially supported by DFG (MA 5826/2-1).