Grazing lands represent one of the largest land resources in the world, yet their role as net sinks or sources of greenhouse gases is essentially unknown. Previous research has emphasized the role of grazing management on the sequestration of atmospheric carbon dioxide as soil organic carbon. However, there is a lack of information regarding how grazing management impacts the flux of two potent green house gases, nitrous oxide and methane. The grazing management systems represented by two native vegetation pastures under medium and high grazing pasture management, and a heavily grazed seeded crested wheatgrass pasture receiving supplemental nitrogen indicated that grazing lands are strong sinks of soil organic carbon and minor sinks of methane, but small to moderate sources of nitrous oxide. Net global warming potential for the native grasslands was negative, implying an overall removal of greenhouse gases from the atmosphere. This finding underscores the value of grazed, mixed-grass prairie as a viable agro ecosystem to serve as a net greenhouse gas sink in the Plains. Conversely, the seeded forage nitrous oxide emissions were nearly three times that of the native grasses, which contributed a net positive net global warming potential, implying net greenhouse gas emission to the atmosphere.