The influence of cattle grazing on carbon cycling in the mixed grass prairie was investigated by measuring the CO2 exchange rate in pastures with a 13 year history of heavy or light grazing and an ungrazed exclosure at the High Plains Grasslands Research Station near Cheyenne, Wyo. In 1995, 1996 and 1997 a closed system chamber, which covered 1 m(2) of ground, was used every 3 weeks from April to October to measure midday CO2 exchange rate. Green vegetation index (similar to leaf area index), soil respiration rate, species composition, soil mater content, soil temperature, and air temperature were also measured to relate to CO2 exchange rates of the 3 grazing treatments. Treatment differences varied among gears, but overall early season (mid April to mid June) CO2 exchange rates in the grazed pastures were higher (up to 2.5 X) than in the exclosure. Higher early season CO2 exchange rates were associated with earlier spring green-up in grazed pastures, measured as higher green vegetation index. As the growing season progressed, green vegetation index increased in all pastures, but more so in the ungrazed exclosure, resulting in occasionally higher (up to 2 X) CO2 exchange rate compared with grazed pastures late in the season. Seasonal treatment differences mere not associated with soil temperature, soil respiration rate, or air temperature, nor was there a substantial change in species composition due to grazing. We hypothesize that early spring green-up and higher early season CO2 exchange rate in grazed pastures may be due to better light penetration and a warmer microclimate near the soil surface because of less litter and standing dead compared to the ungrazed pastures. When all the measurements mere averaged over the entire season, there mas no difference in CO2 exchange rate between heavily grazed, lightly prated and ungrazed pastures in this ecosystem.
