Seasonal analyses of in situ CO fluxes from forested and agricultural soils in Maine and Georgia, and more limited comparisons in Hawaii indicated that agricultural land use consistently enhanced CO consumption. Soils at an agricultural site in Maine consumed approximately 1.9 g CO m-2yr-1 while uptake in a nearby mixed forest was about 70% lower, 0.6 g CO m-2yr-1. A similar trend was observed for sites in Georgia, where annual uptake by agricultural sites was approximately 1.0 g CO m-2 while net emission (about -0.5 g CO m-2) was observed for neighboring pine stands. Net CO fluxes in Maine and Georgia were generally aseasonal. Accordingly, seasonal changes in temperature and water content played variable but often minimal roles as determinants of net fluxes and gross CO uptake and production. However, comparisons among sites suggested that soil organic matter contents were an important control of the magnitude of CO fluxes. In particular net CO consumption for a given soil type increased with decreasing organic matter content associated with forest to agriculture transitions in land use. Although interactions among soil organic matter and various microbiological, physical and chemical parameters in soils are complex, changes in organic matter at the sites described here appear to affect net CO fluxes primarily by reducing the relative rate of abiological CO production. ¿ 2000 American Geophysical Union