Agriculture has a role in mitigating the increase of atmospheric carbon dioxide. Indigenous soil carbon, crops, and management practices are all critical for determining the level of mitigation. Grasslands occupy extensive acreages in the US and generally are not intensively managed, whereas croplands are most often intensively managed. The seasonality of carbon uptake by an annual crop coincides with the period from crop emergence to physiological maturity, which can vary from about 80 days per year for spring wheat to 135 days or more for corn. In contrast, perennial grasslands or forages sequester carbon for periods that often exceed 180 days. The magnitude of carbon losses from soil respiration during the dormant or non-growing period can be sufficiently large to determine if ecosystems are a net carbon source or sink. We measured annual soil CO2 efflux that averaged 1.8 g CO2-C m-2 d-1 for wheat and 2.8 g CO2-C m-2 d-1 for permanent grasslands. Differences in CO2 efflux are associated with soil organic carbon content and microbial activity. Precipitation can also be a major factor influencing annual variations in net carbon captured. Improved understanding of intra-seasonal dynamics of CO2 efflux is important in identifying agricultural management practices that are most effective in mitigating the increase of atmospheric CO2.