Soil surface carbon dioxide fluxes induced by spring, summer, and fall moldboard plowing in a sandy loam

Soil tillage is believed to influence soil organic matter decomposition. We hypothesized that the short-term impacts of tillage on soil surface CO2 emissions would vary following spring, summer, and fall moldboard plowing of a sandy loam (loamy, mixed, frigid Umbric Dystrochrept). At all dates, plowing increased CO2 emissions by 7 to 20 mu mol m(-2) s(-1) during the first hours following tillage due to degassing of soil CO2. The effects of tillage following this initial burst varied with date of plowing. Fall incorporation of fresh maize residues increased CO2 losses by 173 kg C ha(-1) during a period of 4 d compared with a non-tilled control, until halted by a sudden drop of 6 degrees C in soil temperature with the onset of winter. In contrast, the cumulative CO2 fluxes following spring plowing of fall-applied residues were 186 kg C ha(-1) low er than on a non-tilled control. This effect of spring plowing on CO2 fluxes lasted for 2 wk, during which soil temperatures in the plow layer were below 10 degrees C. Summer plowing increased CO2 emissions try as much as 2 mu mol m(-2) s(-1) for a period of 65 d, resulting in cumulative soil C losses of 685 kg C ha(-1) compared with the undisturbed control. Variations in tillage induced CO2 emissions between dates were attributed to differences in soil microclimatic conditions following plowing events. The study of the impact of tillage on soil organic matter decomposition should consider the time of year when the soil is tilled.