Prescribed fire and grazing effects on carbon dynamics in a northern mixed-grass prairie

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 agricultural management practices of prescribed 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 45–50% 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: standing crop, leaf area, root mass to a soil depth of 30 cm, and CO2 fluxes above the mixed-grass prairie canopy and bare soil. 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 CO2 flux. Treatment effects on seasonal changes in soil water content were consistent across years, with grazed and undisturbed plots being similar. Burned plots had less soil water content than grazed or undisturbed plots in late spring and early summer with soil water content becoming similar across all treatments in late summer and into autumn. Despite differences in soil water content, CO2 fluxes above either burned or undisturbed plots were dynamically similar. However, peak CO2 flux of grazed plots was shifted one month earlier in 2004 and was decreased in 2005, relative to the undisturbed and burned plots. Over the course of this experiment, cumulative CO2 flux on the grazed plots was 72% of that on the burned and undisturbed plots. 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-burning. We conclude that the studied ecosystem is unlikely to sequester considerable C, irrespective of imposed agricultural practices, due to the relatively low CO2 flux and the small magnitude of treatment observed.