Changes in the quantity of carbon (C) in vegetation and soils play a major role in determining short- and long-term fluctuations in the concentration of carbon dioxide (CO2) in earth’s atmosphere. Human activities, such as the irrigation of agricultural soils in arid regions, can alter the accumulation of pedogenic carbonate in soils. For comparative purposes, biogeochemists calculate the mean residence time (MRT). Although the amount of pedogenic carbonate in world soils is quite large, this pool of C is relatively sluggish. The long MRT of the pedogenic carbonate pool ensures that it will not become a major sink or source of atmospheric CO2 over the next several centuries.