No human brain can access all the data necessary to determine the net carbon output resulting from the use of ethanol from various processes. It takes a good computer algorithm to do that. However, one can get a general idea by just philosophizing about it.
For instance, accept that biomass carbon is being recycled no matter whether you use the energy to run automobiles or bacteria are using it to fill their own energy needs. When bacteria break glucose apart, a significant product is carbon dioxide. That causes the carbon dioxide produced by burning ethanol in an internal combustion engine a wash, or a near wash, with biodecomposition of biomass.
Carbon sequestered in gasoline from petroleum was done millions of years ago and, when released into the current atmosphere, causes a current increase in atmospheric levels of the carbon which cannot all be resequestered by plants at a rate fast enough to keep a balance. One need not have tests and statistics to realize these points.
A net zero in carbon dioxide change makes ethanol from biomass neutral as compared with emissions from coal, natural gas, or petroleum.
Current technology in conversion of biomass rich in hemicellulose (with a high xylose content), and cellulose (with a high glucose content), gives a truly economic road to ethanol for motor fuel with a net carbon neutral end use result. Though fermentation of glucose yields one molecule of carbon dioxide for one molecule of ethanol produced, and fermentation of xylose yields one molecule of carbon dioxide average in the three pathways use; two tons of biomass can yield one ton of motor fuel and the same amount of carbon dioxide as the same two tons of biomass burned in the field, or allowed to decay and compost.