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Are desert ecosystems a stable sink for atmospheric CO 2 ? Although the uptake of atmospheric CO 2 by soil during the nighttime is detected in deserts and some semi-deserts, the underlying mechanisms remain elusive. In order to determine the factors affecting the CO 2 fluxes into soil and to reveal the relationship between CO 2 fluxes and carbonate formation and recrystallization in saline and alkaline soils, four soils with contrasting salinity from two sites, Aksu and Yingbazar, along the Tarim River were analysed. Soils were incubated in 14 CO 2 labelled atmosphere for 90 days at three CO 2 concentrations: 0.04%, 0.4% and 4%. The 14 C activity was measured in soil water and air, as well as in carbonates after 2, 14 and 90 days. The 14 C incorporation in CaCO 3 increased with corresponding 14 C decrease remaining in the CO 2 . The highest 14 C incorporation into CaCO 3 (54%) was observed in the Yingbazar saline soil. The carbonate recrystallization rates increased logarithmically (r 2 > 0.97) with the CO 2 concentration. The average carbonate recrystallization rate from three sampling times was highest in the Yingbazar saline soil under 4% CO 2 (8.45 × 10 − 6 day − 1 ) and lowest in the Aksu alkaline soil under 0.04% CO 2 (0.03 × 10 − 6 day − 1 ). The average carbonate recrystallization rate increased with electric conductivity (corresponding to soil salinity) but decreased with pH ( R 2 = 0.99 for 4% CO 2 ). The alkaline soils incorporated less 14 C into CaCO 3 than the saline soils. At the highest CO 2 concentration of 4%, the full recrystallization period of the remaining primary carbonates was 10- to 100-fold shorter than at the lower CO 2 concentrations. Therefore, besides soil chemical parameters (e.g. pH, CaCO 3 content), CO 2 concentration (respiration of microorganisms and roots) is an important factor for CaCO 3 recrystallization and formation of pedogenic carbonates in desert soils. It is the most important factor for CaCO 3 recrystallization and so, for the formation of pedogenic carbonates. [ABSTRACT FROM AUTHOR]