Tillage of soils often decreases soil organic matter content and increases CO2 emission. Enhanced CO2 emission induced by tillage may provide an early indication of the likely consequences of soil management on soil organic C. This study was conducted to investigate a link between the mechanical properties of soil and the short-term (24 h) dynamics in CO2 emission induced by soil deformations in controlled laboratory experiments. The degree to which soil is deformed depends on the normal and shear stresses acting upon the sample and on the stress history experienced by the soil. Respirometers were designed in which a range of combined shear and compressive forces could be applied to a soil sample while simultaneously measuring CO2 emission. Mechanical characteristics of repacked sieved aggregates of soil were first established. Following the critical state concept, we identified changes in bulk density that resulted from different combinations of normal and shear stresses, and for different stress histories of the soil. We selected treatments that resulted in permanent deformation (compression) and expansion or consolidation upon shear. The CO2 emission increased by 18%, even for samples that were severely compressed with a normal load of 430 kPa. Shearing of soil samples also resulted in greater CO2 emission, in particular when combined with a normal load. The increased CO2 emission following compression was maintained for a period longer than 7 d. As the increase did not correlate with the physical changes in pore volume, we concluded that an increase in availability of organic matter was the main mechanism responsible for enhanced CO2 emission. We discuss these findings from experiments under controlled conditions in the context of the mechanical deformations that can be expected during tillage and assess the implications for biological activity in the field.