Ion release in the C horizon of a forest soil triggered by Carbonic Acid

Abstract

AbstractIon release in the C horizon of a Skeletic Umbrisol on gneiss bedrock was investigated by percolation experiments at a water status near field capacity and with adjusted CO2 partial pressures in soil air. CO2 partial pressures of 0.001, 0.01, and 0.1 bar with pH‐values of 5.7, 4.9, and 4.5 yielded cation release rates of 3.0, 8.5, and 12.3 kmolc yr–1 ha–1 at a 1 m horizon depth and 800 mm seepage water. Within the pH range of 5.7 to 4.5, the activity of carbonic acid triggers ion release. For the treatment with a CO2 partial pressure of 0.001, the isolation of a weathering rate (0.5 kmolc yr–1 ha–1) was possible because parallel running processes such as the dissolution of solid compounds could be identified by dissolved anions, and exchange processes only modified internal ratios of mobilized basic cations. The weathering rate at a site‐typical CO2 partial pressure of 0.01 bar was about 5–10 times higher than usually assumed in the literature. There are three reasons that may account for this: (1) the consideration of actual carbonic acid activities and (2) specific site features such as the richness of basic minerals and/or the presence of a skeletal fraction with micro voids and fissures providing large internal surfaces. Furthermore, (3) it can not be completely excluded that parallel running exchange processes contribute to ion release.