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Recent field studies have reported anomalous CO2 uptake using eddy-covariance techniques in arid and semiarid ecosystems. The rates of CO2 uptake are incompatible with changes in situ of organic carbon pools. Here, I examine several potential mechanisms of abiotic CO2 uptake in arid and semiarid soils: atmospheric pressure pumping, carbonate dissolution, and percolation of soil water through the vadose zone. Each mechanism is deemed inadequate to explain the observations of the eddy-covariance systems, which must now be questioned for their accuracy in desert ecosystems.