Effect of CO2 concentration on the initial recrystallization rate of pedogenic carbonate — Revealed by 14C and 13C labeling

In calcareous parent material, pedogenic carbonate formation mostly involves dissolution and recrystallization of lithogenic carbonates with CO2 of soil air, leading to a complete exchange of lithogenic carbon with soil-derived carbon. Interest in pedogenic carbonates has increased in recent decades because they are a useful tool for reconstructing paleoclimatic conditions (δ 13C and δ 18O) and past atmospheric CO2 concentrations as well as for radiocarbon dating of soils. For such investigations, the recrystallization rate of primary CaCO3 by pedogenic carbonate formation and the dependence of the recrystallization rate on environmental factors are essential, but still unquantified factors. The recrystallization rate of primary CaCO3 of loess at three CO2 concentrations was estimated by isotopic exchange between primary CaCO3 and the 14C of artificially labeled CO2. Loess was used for the study as a parent substrate for soil formation to simulate initial rates of CaCO3 recrystallization. CO2 concentrations of 380ppm, 5000ppm and 50,000ppm lead to recrystallization rates of 4.1·10−7 day−1, 8.1·10−7 day−1 and 16.9·10−7 day−1, respectively. The relation between CO2 concentrations and recrystallization rates was described by a saturation curve. Under the tested experimental conditions, complete (95%) recrystallization of loess carbonate and formation of pedogenic carbonate would take 4.9–20.0·103 years, strongly depending on CO2 concentration. We expect faster recrystallization rates under field conditions because of permanent CO2 supply by root and rhizomicrobial respiration. This impedes the equilibrium between the inorganic C pools in solid, liquid and gaseous phases.