Carbon exchange and species composition of grazed pastures and exclosures in the shortgrass steppe of Colorado

Grasslands comprise approximately 40% of the world's terrestrial surface. Consequently, grassland ecosystems are a significant component of the global carbon cycle. In order to better understand how grazing affects the carbon cycle of grasslands, this study measured CO2 exchange rate (CER) and soil respiration rate (SRR) on 130 ha pastures with a 56-year history of heavy (60% removal) and light (20% removal) grazing, and their accompanying 0.8 ha exclosures, on the shortgrass steppe of northeastern Colorado, USA. A CER chamber that covered 1 m2 of native grassland was used on five plots in each of the four areas. Mid-day CER and SRR were measured during the growing seasons of 1995-1997, along with green vegetation index (GVI, similar to leaf area index) and plant species composition. When averaged over each growing season, there was no significant difference in CER of grazed pastures versus exclosures. However, there were seasonal differences in CER, which varied over the 3 years. Differences in CER between grazed pastures and exclosures were not related to GVI, which rarely differed between treatments. Grazing treatment differences in CER were driven by climate variability and species composition differences resulting from long-term grazing and exclusion from grazing. Exclosures had more cool-season (C3) grasses and forbs than grazed plots, which contained more warm-season (C4) grasses (primarily Bouteloua gracilis (H.B.K.) Lag. Ex Steud.). The somewhat unique, cool spring of 1995 was favorable to cool-season plant metabolism and resulted in higher CER in exclosures compared with grazed pastures. Warm, dry conditions in spring of 1996 favored warm-season species, resulting in higher CER in the heavily-grazed pasture. In 1997, there was little difference in CER between grazed pastures and exclosures. There were very few sampling dates when SRR was different in grazed pastures and exclosures. This study suggests that these intensities of cattle grazing do not alter the photosynthetic and soil respiration components of the carbon cycle of the US shortgrass prairie. It appears that cattle grazing can be a sustainable component of managing this ecosystem for maximum global carbon sequestration.