Carbon exchange rates in grazed and ungrazed pastures of Wyoming

The influence of cattle grazing on carbon cycling in the mixed grass prairi e 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 Hi gh 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 eve ry 3 weeks from April to October to measure midday CO2 exchange rate. Green vegetation index (similar to leaf area index), soil respiration rate, spec ies composition, soil mater content, soil temperature, and air temperature were also measured to relate to CO2 exchange rates of the 3 grazing treatme nts. Treatment differences varied among gears, but overall early season (mi d 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 v egetation index increased in all pastures, but more so in the ungrazed excl osure, resulting in occasionally higher (up to 2 X) CO2 exchange rate compa red with grazed pastures late in the season. Seasonal treatment differences mere not associated with soil temperature, soil respiration rate, or air t emperature, nor was there a substantial change in species composition due t o grazing. We hypothesize that early spring green-up and higher early seaso n CO2 exchange rate in grazed pastures may be due to better light penetrati on and a warmer microclimate near the soil surface because of less litter a nd standing dead compared to the ungrazed pastures. When all the measuremen ts mere averaged over the entire season, there mas no difference in CO2 exc hange rate between heavily grazed, lightly prated and ungrazed pastures in this ecosystem.