Soils and vegetation of North American crop and grasslands have been identified as CO2 sinks with the potential to offset greenhouse gas emissions associated with global warming. The sink is a collection of ecosystems, which act independently alternating as both sinks and sources within and among years. Agroecosystems differ from natural ecosystems in that inputs and outputs are managed. Natural ecosystems are more likely to be at equilibrium with regard to CO2 flux. Like natural ecosystems the status of agroecosystems as CO2 sinks or sources is affected by micro and macroclimate, soil characteristics and history, but agroecosystems are affected by soil, crop and animal management and economic conditions. Beneficial practices which affect field scale CO2 sequestration have been identified using long-term soil sampling techniques and soil organic matter assays. Micrometeorological techniques which measure intra annual and annual net CO2 flux have emerged to evaluate and develop management practices for CO2 sequestration at the ecosystem level. Year-round measurement of daily net CO2 flux separates seasons into active and dormant periods for crop growth. Spatial comparisons between ecosystems compare management effects on net CO2 flux within a farm or landscape and over larger geographical regions. Observations of CO2 flux provide information about soil, root and canopy respiratory and photosynthetic processes, which may assist in management design. Collectively, micrometeorological studies involving measurement of CO2 flux may be used to optimize CO2 sequestration within and among intensive and extensive agroecosystems to enhance the agricultural CO2 sink.