Micrometeorological assessments of seasonal variation in CO₂ fluxes and the partitioning of fluxes into photosynthetic and respiratory components provide important mechanistic information about how environment and management affect the ability of crop and grazing lands to sequester carbon. Depending on weather conditions and management practices, agricultural systems in a given year can vary from being strong sources to strong sinks for CO₂. Year to year variation in weather and management appears to be the primary determinant of whether a site will be a source or sink. Increased atmospheric CO₂ concentration and temperature, and altered rainfall patterns associated with future climate change will affect both photosynthetic and respiratory rates in ways that are difficult to predict. In 2001, agriculture was a net greenhouse gas source of about 460 Tg CO₂ with methane and nitrous oxide the primary greenhouse gases emitted. It has been estimated that crop and grazing lands could sequester more than 750 Tg CO₂ yr^-1 and results suggest that western rangelands are currently sequestering about 180 Tg CO₂ yr^-1. Adaptation of management strategies to increase carbon sequestration, however, will have to take into account concurrent effects on nitrous oxide and methane fluxes to avoid increasing total greenhouse gas emissions.