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The flux of CO2 from soil determines the extent to which carbon (C) deposited as plant litter is retained in the soil. Retention of soil C is beneficial for soil physical, chemical and biological properties, and is essential if soils are to be used as a repository of C to mitigate atmospheric CO2 increases. Although tillage is assumed to have a major influence on soil C retention, the extent to which tillage enhances the transfer of soil C to the atmosphere is uncertain. We assessed the short-term (50 h) influence of tillage on CO2 fluxes from Chernozemic soils under a two-year wheat (Triticum aestivum L.)-summerfallow rotation in a semi-arid region of the Canadian Prairie, The tillage effect and its persistence were assessed by using a portable CO2 analyzer to record several temporal series of CO2 fluxes, along undisturbed and tilled transects, at successive time intervals (from -0.5 to 50 h) after a single pass with a heavy-duty cultivator. Immediately after tillage, CO2 fluxes along the tilled transects increased from 2 to 4-fold above pre-tillage fluxes, but the increases were short-lived and fluxes along undisturbed and tilled transects were again similar within 24 h of cultivation. Total amounts of CO2 released by a tillage operation were quantified by:

1. linear interpolations among successive fluxes along tilled and undisturbed transects, and

2. by fitting a model to successive differences between fluxes along the transects.

Both methods estimated the amounts of tillage-susceptible CO2 to be in the range of 3.6-7.2 kg C ha(-1). The tillage-induced flush of CO2 was attributed mainly to enhanced transport of CO2 already in the soil, but enhanced production of CO2 by heterotrophic soil organisms also may have contributed to the flush. Regardless of the sources of CO2 released by single tillage operations, amounts of tillage-susceptible soil C were minor; even 10 passes with a cultivator would account for less than 5% of annual soil CO2 emissions or crop residue production in these cropping systems. Our study suggested that the short-term influence of tillage on the transfer of soil C to atmospheric CO2 is small under semi-arid conditions like those in southern Alberta, Canada. Crown copyright (C) 1999 Published by Elsevier Science B.V, All rights reserved.