Greenhouse Gases in Intensive Agriculture: Contributions of Individual Gases to the Radiative Forcing of the Atmosphere, G. Philip Robertson, Eldor A. Paul, Richard R. Harwood, 15 September 2000 Vol 289. (This is a pay service.) This paper measures the production of greenhouse gas emissions in a variety of cropping systems and concludes that the vast majority of cases, nitrous oxide production was the single greatest source of GWP (global warming potential).