Book excerpt: Landscape position, grazing, and seasonal variation in precipitation and temperature create spatial and temporal variability in soil processes, and plant biomass and composition in grasslands. However, it is unclear how this variation in plant and soil properties affects carbon dioxide (CO2) fluxes. The aim of this study is to explore the effect of grazing, topographic position, and seasonal variation in soil moisture and temperature on plant assimilation, shoot and soil respiration, and net ecosystem CO2 exchange (NEE). Carbon dioxide fluxes, vegetation, and environmental variables were measured once a month inside and outside long-term ungulate exclosures in hilltop (dry) to slope bottom (mesic) grassland throughout the 2004 growing season in Yellowstone National Park. There was no difference in vegetation properties and CO2 fluxes between the grazed and the ungrazed sites. The spatial and temporal variability in CO2 fluxes were related to differences in aboveground biomass and total shoot nitrogen content, which were both related to variability in soil moisture. All sites were CO2 sinks (NEE> 0) for all our measurments taken throughout the growing season; but CO2 fluxes were four- to fivefold higher at sites supporting the most aboveground biomass located at slope bottoms, compared to the sites with low biomass located at hilltops or slopes. The dry sites assimilated more CO2 per gram aboveground biomass and stored proportionally more of the gross-assimilated CO2 in the soil, compared to wet sites. These results indicate large spatio-temporal variability of CO2 fluxes and suggest factors that control the variability in Yellowstone National Park.