Grazing Effects on Carbon Dynamics in the Northern Mixed-Grass Prairie

The role of rangelands in the regulation of atmospheric CO2 concentrations is a critical issue in global climate change research. Rangelands are complex ecosystems that occupy about 50% of the land area in the world and USA. We studied the effects of seasonal grazing on CO2 flux on small plots located on a silty range site in the northern mixed-grass prairie with an Eapa fine loam soil. Treatments were no grazing or short-duration intensive grazing during mid-May or midJuly in 1996, 1997, and 1998. Data were collected from midApril to mid-October at about 30-day intervals to estimate standing crop, leaf area, soil organic C, root mass to a 30-cm soil depth, and diurnal variation of CO2 flux and soil respiration (at 08:00, 12:00, 16:00, and 24:00 hr) in closed chambers. Uptake of CO2 was greatest during spring and early summer, peak periods of precipitation and green biomass. Grazing removed an average of about 70% of the green standing crop with a subsequent reduction in CO2 uptake of 175% in May and 109% in July. Grazing in May reduced CO2 uptake for 30 days in two of the three years, whereas, grazing in July reduced CO2 flux only in 1998. Residual effects of grazing, however, declined in late summer and autumn with the onset of plant maturation. The potential C sink in the mixed-grass prairie of the Northern Great Plains appears to be small and will vary through time with intensity and timing of grazing as it interacts with climatic conditions. Rangelands (including grasslands, shrublands, deserts, and tundra) occupy about 50% of the world’s land area and contain more than 33% of aboveground and belowground terrestrial C reserves (Allen-Diaz 1996). Follett and others (2001) suggest that, given the size of the C pool in US rangeland, a better understanding is needed of the current and potential effects of management practices on C storage. Since 1990 researchers have been actively studying the potential role of grasslands for C sequestration with research in seeded pastures (Franzluebbers and others 2000); tall grass prairie (Verma and others 1989, 1992, Kim and others 1992, Ham and Knapp 1998, Mielnick and Dugas 2000, Dugas and others 1999, Rice and Owensby 2001, Suyker and Verma 2001); mixed-grass prairie (Dormaar and others 1995, Frank and others 1995, 2001, Manley and others 1995, Schuman and others 1999, 2001, 2002, LeCain and others 2000, Meyers 2001, Frank and Dugas 2001, Sims and Bradford 2001, Frank 2002, Reeder and Schuman 2002; shortgrass steppe (Reeder and Schuman 2002; Reeder and others this issue); and sagebrush steppe (Angell and others 2001). Inherent in all rangeland ecosystems are both diurnal and seasonal variation in CO2 flux, with the direction of flux controlled by the balance between photosynthesis and respiration during the growing and dormant seasons (Norman and others 1992, Bremer and others 1998, Dugas and others 1999, Mielnick and Dugas 2000, Frank and Dugas 2001, Frank 2002). Rangelands are also resource limited, particularly for nitrogen and water (Willms and Jefferson 1993). Severity, duration, and timing of grazing and drought in relation to plant phenology are important in determining the status of prairie vegetation as a C source or sink. Inconsistent responses of soil organic C to grazing have been reported on rangelands. Many of these differences appear to be the result of variations in climate, soil properties, landscape position, plant community composition, grazing management practices, soil organic matter, depth of soil profile, and depth of the soil profile sampled (Johnston and others 1971, Smoliak and others 1972, Dormaar and others 1977, Bauer and others 1987, Milchunas and Lauenroth 1993, Frank and others 1995, Manley and others 1995, Biondini and Manske 1996, Derner and others 1997, Povirk and others 2001, Schuman and others 1999, Wienhold and others 2001, Reeder and Schuman 2002). Some have suggested that grazing leads to an increase in soil microbial biomass that is related to changes in root exu