Around 4.4 million ha of land in USDA Conservation Reserve Program (CRP) contracts will expire between 2013 and 2018 and some will likely return to crop production. No-till (NT) management offers the potential to reduce the global warming costs of CO2 , CH4 , and N2 O emissions during CRP conversion, but to date there have been no CRP conversion tillage comparisons. In 2009, we converted portions of three 9-21 ha CRP fields in Michigan to conventional tillage (CT) or NT soybean production and reserved a fourth field for reference. Both CO2 and N2 O fluxes increased following herbicide application in all converted fields, but in the CT treatment substantial and immediate N2 O and CO2 fluxes occurred after tillage. For the initial 201-day conversion period, average daily N2 O fluxes (g N2 O-N ha-1 d-1 ) were significantly different in the order: CT (47.5 ± 6.31, n = 6) ≫ NT (16.7 ± 2.45, n = 6) ≫ reference (2.51 ± 0.73, n = 4). Similarly, soil CO2 fluxes in CT were 1.2 times those in NT and 3.1 times those in the unconverted CRP reference field. All treatments were minor sinks for CH4 (-0.69 ± 0.42 to -1.86 ± 0.37 g CH4 -C ha-1 d-1 ) with no significant differences among treatments. The positive global warming impact (GWI) of converted soybean fields under both CT (11.5 Mg CO2 e ha-1 ) and NT (2.87 Mg CO2 e ha-1 ) was in contrast to the negative GWI of the unconverted reference field (-3.5 Mg CO2 e ha-1 ) with on-going greenhouse gas (GHG) mitigation. N2 O contributed 39.3% and 55.0% of the GWI under CT and NT systems with the remainder contributed by CO2 (60.7% and 45.0%, respectively). Including foregone mitigation, we conclude that NT management can reduce GHG costs by ~60% compared to CT during initial CRP conversion.