Nineteen months of continuous data from two sites within the atmospheric boundary layer experiments (ABLE) facility are used to compare surface energy fluxes, carbon dioxide fluxes and controlling parameters. One site, which has been monitored continuously since 1997, is rangeland, with a mixture of C3 and C4 grasses. The other site is active cropland that was planted in sorghum for the first growing season of the observation period and in winter wheat for the second. The uptake of carbon dioxide was well-defined within the respective growing seasons, with peak uptake rates for sorghum being greater by almost a factor of 2 than those for wheat and rangeland. The longer growing season for rangeland almost countered this effect. Net production of carbon dioxide occurred at both sites at the beginning and end of the growing seasons because of root growth respiration and enhanced decay of tilled soil.