Is pedogenic carbonate an important atmospheric CO2 sink?

Nearly 18 years after the proposal of the weathering-related carbon sink concept (Berner R A. Weathering, plants and the long-term carbon cycle. Geochim Cosmochim Acta, 1992, 56: 3225-3231), it is an appropriate timing to re-evaluate its geological context with the updated dataset. Ryskov et al. (Ryskov Ya G, Demkin V A, Oleynik S A, et al. Dynamics of pedogenic carbonate for the last 5000 years and its role as a buffer reservoir for atmospheric carbon dioxide in soils of Russia. Glob Planet Change, 2008, 61: 63-69) lately claimed that in the course of soil formation for the last 5000 years the soils of Russia fixed atmospheric carbon dioxide as pedogenic carbonate during the arid periods at a rate of 2.2 kg C/(m(2) a) in chernozem, 1.13 kg C/(m(2) a) in dark-chestnut soil, 0.86 kg C/(m(2) a) in light-chestnut soil, on the basis of carbon isotopic data; however, their interpretations of the data do not appear straightforward nor persuading, and thus their claim is likely misleading. Their interpretations are also contrary to the conclusions drawn by Dart et al. (Dart R C, Barovich K M, Chittleborough D J, et al. Calcium in regolith carbonates of central and southern Australia: Its source and implications for the global carbon cycle. Palaeogeogr Palaeoclimatol Palaeoecol, 2007, 249: 322-334) who found that Australian regolith carbonates did not capture any additional CO2; instead the carbonate was simply being remobilized from one pool to another. Here we raise comments to these explanations on the following two issues: (1) origin of pedogenic carbonate: silicate weathering vs. carbonate weathering, and (2) problems in using carbon isotopic technique to distinguish carbonates formed by silicate weathering and carbonate weathering. It is concluded that pedogenic carbonate may not be an important atmospheric CO2 sink at all, i.e. carbonate weathering-related pedogenic carbonate does not capture any additional CO2, while the CO2 capture in silicate weathering-related pedogenic carbonate is small in short-term time scales due to the slow kinetics of silicate weathering.