Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/3/2008
Publication Date: 4/8/2008
Citation: MacNeil, M.D., Haferkamp, M., Vermeire, L.T., Muscha, J.M. 2008. Prescribed fire and grazing effects on carbon dynamics in a northern mixed-grass prairie. Agriculture, Ecosystems and Environment 127:66-72. Interpretive Summary: It is frequently hypothesized that rangelands may sequester large amounts of carbon and thus significantly reduce emission of greenhouse gases to the atmosphere. On rangelands, precipitation or soil water content is frequently the most limiting resource affecting photosynthesis and respiratory processes which determine CO2 flux and ultimately net primary production. Our goal was to determine the effects of fire and grazing on CO2 fluxes and other measures of the biotic and abiotic state of a northern mixed prairie rangeland. Despite the relatively short (2 yr) duration of this study, temporal changes in the state of this northern mixed-grass prairie rangeland ecosystem observed between the two years and within years were substantially greater than the effects of either fire or grazing. We estimated that carbon released to the atmosphere in the present study would not be replenished on the rangeland until the second year following the burn, most likely due to the limited precipitation during the immediately subsequent growing season. In contrast to some other studies, CO2 flux was reduced following grazing bouts. Thus, it seems any statement about grazing effects on CO2 flux should be tempered by consideration of the grazing system. Soil water availability was the primary identifiable factor that either directly or indirectly controlled much of the CO2 fluxes for the studied northern mixed-grass prairie. Year-to year variation in precipitation patterns had greater effects on the northern mixed-grass prairie system than either grazing or burning as imposed here. These results suggest that semi-arid rangelands may contribute less to mitigation of greenhouse gas effects than has been previously suggested.
Technical Abstract: Rangelands are complex systems that occupy more than 50% of the land area in the world. Carbon cycling on rangelands is generally understood, but details concerning daily and seasonal CO*2 fluxes and the influences of fire and grazing are not well defined. Field experiments were conducted on northern mixed grass prairie in eastern Montana with undisturbed, burned, and grazed treatments established in 2003. Designated plots were burned during the dormant season (December) in 2003. Grazing occurred in 2004 and 2005, removing 45 to 50% of then currently available leaf area. Responses were evaluated monthly from April to October of 2004 and 2005. Abiotic measurements included: precipitation, soil water content, photosynthetically active solar radiation, and temperatures of soil and leaves. Biotic responses were indicated by: standing crop, leaf area, root mass to a soil depth of 30 cm, and CO*2 fluxes above the mixed grass prairie canopy and bare soil. Substantially more biomass was produced in 2005 than in 2004 with maximum green biomass of 1154±38 kg•ha**-1 and 404±38 kg•ha**-1, respectively. Temporal changes in green biomass were affected by treatment (P < 0.01). However, treatment effects on these seasonal dynamics were consistent across years (P > 0.20) and 98x smaller than the differences between years. Moisture available to the northern mixed grass prairie system at this location was the primary identifiable factor that either directly or indirectly controlled much of the CO*2 flux. Depending on conditions during the growing season, C removed from northern mixed grass prairie rangeland may not be re-sequestered during the growing season immediately post-treatment.