Plowing (tillage) greatly increases release of CO2 in soil. Here are some comparisons.
Takeaway: "The moldboard plow lost 13.8 times as much CO2 as the soil area not tilled while different conservation tillage tools lost only 4.3 times" (ordinary plow)
Abstract: "The influence of agricultural production systems on greenhouse gas generation and emission is of interest as it may affect potential global climate change. Agricultural ecosystems can play a significant role in production and consumption of greenhouse gases, specifically, carbon dioxide. Information is needed on the mechanism and magnitude of gas generation and emission from agricultural soils with specific emphasis on tillage mechanisms. This work evaluated four different tillage methods on the short-term CO2 and water vapor flux from a clay loam soil in the Northern Cornbelt of the USA. The four tillage methods were moldboard plow only, moldboard plow plus disk harrow twice, disk harrow and chisel plow using standard tillage equipment following a wheat (Triticum aestivum L.) crop compared with no tillage. The CO2 flux was measured with a large portable chamber commonly used to measure crop canopy gas exchange initiated within 5 minutes after tillage and continued intermittently for 19 days. The moldboard plow treatment buried nearly all of the residue and left the soil in a rough, loose, open condition and resulted in maximum CO2 loss. The carbon released as CO2 during the 19 days following the moldboard plow, moldboard plow plus disk harrow, disk harrow, chisel plow and not tilled treatments would account for 134%, 70%, 58%, 54% and 27% respectively of the carbon in the current year's crop residue. The short-term carbon dioxide losses 5 hours after four conservation tillage tools was only 31% of that of the moldboard plow. The moldboard plow lost 13.8 times as much CO2 as the soil area not tilled while different conservation tillage tools lost only 4.3 times. The smaller CO2 loss following conservation tillage tools is significant and suggests progress in developing conservation tillage tools that can enhance soil carbon management. Conservation tillage reduces the extent, frequency and magnitude of mechanical disturbance caused by the moldboard plow and reduces the air-filled macropores and slows the rate of carbon oxidation. Any effort to decrease tillage intensity and maximize residue return should result in carbon sequestration for enhanced environmental quality."
D.C. Reicosky, "Tillage-induced CO2 emission from soil," Nutrient Cycling in Agroecosystems 49 (1997), 273, accessed June 5, 2014, doi:10.1023/A:1009766510274.