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Reducing tillage intensity and diversifying crop rotations may improve the sustainability of irrigated cropping systems in semi-arid regions. The objective of this study was to compare the greenhouse gas (GHG) emissions, soil organic matter, and net global warming potential (net GWP) of a sugar beet (Beta vulgaris L.)-corn (Zea mays L,) rotation under conventional (CT) and reduced-tillage (RT) and a corn-dry bean (Phaseolus vulgaris L.) rotation under organic (OR) management during the third and fourth years of 4-year crop rotations. The gas and soil samples were collected during April 2011-March 2013, and were analyzed for carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) emissions, water-filled pore space (WFPS), soil nitrate (NO3 (-)-N) and ammonium (NH4 (+)-N) concentrations, soil organic carbon (SOC) and total nitrogen (TN), and net global warming potential (net GWP). Soils under RT had 26% lower CO2 emissions compared to 10.2 kg C ha(-1) day(-1) and 43% lower N2O emissions compared to 17.5 g N ha(-1) day(-1) in CT during cropping season 2011, and no difference in CO2 and N2O emissions during cropping season 2012. The OR emitted 31% less N2O, but 74% more CO2 than CT during crop season 2011. The RT had 34% higher SOC content than CT (17.9 Mg ha(-1)) while OR was comparable with CT. Net GWP was negative for RT and OR and positive for CT. The RT and OR can increase SOC sequestration, mitigate GWP and thereby support in the development of sustainable cropping systems in semiarid agroecosystems.