SOIL RESPIRATION AND GEORESPIRATION DISTINGUISHED BY TRANSPORT ANALYSES OF VADOSE CO2, (CO2)-C-13, AND (CO2)-C-14

Georespiration and soil respiration operate on organic carbon pools of vastly different sizes and mean residence times (MRT). Both processes occur in the shallow subsurface at the Dalmeny site in southern Saska tchewan, Canada. Steady and transient, heterogeneous simulations of va dose CO2, (CO2)-C-13, and (CO2)-C-14 show that at least 98% of all sub surface respiration occurs in the solum where the MRT of labile soil c arbon is about 10 years. Root respiration dominates the total during t he growing season. Remaining CO2 generation occurs near the capillary surface at 6.5-7.5 m depth, where delta(14)C of respired CO2 indicates an MRT of about 22,000 years. This value is consistent with a respira tion substrate dominated by Cretaceous-age kerogen in the till. The si mulated oxidation/georespiration rate at this depth is also consistent with observed depletion of kerogen C from the vadose zone during the Holocene. Field relations in this setting indicate that georespiration is controlled hydrogeologically by the development of aerobic vadose zones; we speculate that this may be more generally true on a global b asis. Where soil parent materials contain ancient carbon, georespirati on should be considered as a possible factor complicating studies of s oil carbon turnover.