Which gives off less Co2's a tractor plowing an acre of ground or a mule plowing an acre of ground?

I don�t have any data to calculate this problem but I would venture into saying it is the mule that produces the most CO2. The tractor, being a diesel engine, would be able to do the same job faster and more energy efficient. But on the other hand the mule just exhales CO2 that is produced during methabolism of carbohydrates that is biological in origins. So for every CO2 molecule exhaled and O2 molecule inhaled by the mule a plant absorbed a CO2 molecule while exhaling an oxygen molecule. So no net increase in atmospheric CO2 levels while the diesel in the tractor is fossil in origin.