Decalcification of soils subject to periodic waterlogging

Intense decalcification of fine-grained organic-rich soils subject to perio dic oxidation and reduction takes place in the Biesbosch, a freshwater, tid ally influenced wetland area in the Rhine-Meuse delta in The Netherlands. S oil chemical (sulphide concentration and pore-water characteristics) and hy drological variables (drainage) were measured in three representative Fluvi sols differing in hydrology to identify processes inducing calcium carbonat e dissolution. Both oxidation of previously formed iron sulphides during pe riods of low ground water and infrequent inundation, and increased carbon d ioxide pressure in the soil during periods of waterlogging combined with dr ainage of pore-water solutes, contribute significantly to decalcification o f the hydric soils. The effects of these individual processes on decalcific ation are in the same order of magnitude in the studied soils. Depending on site-specific hydrological conditions, approximately 0.1-0.3% calcium carb onate may be dissolved per year by a combination of these two processes, wh ich is comparable to actual decalcification rates at these sites. Estimates of long-term decalcification rates, based on knowledge of the hydrogeochem istry, may be used to assess the risks accompanying the conversion of agric ultural soils into wetlands.