Livestock grazing on the Colorado Plateau: Impacts on above and belowground carbon stocks

Background/Question/Methods

Soils store more carbon than plants, animals, and the atmosphere combined; thus, subtle changes to terrestrial carbon cycling, such as those associated with livestock grazing, could significantly affect atmospheric CO₂ concentrations. Drylands comprise 40% of Earth’s terrestrial surface and grazing in these ecosystems can substantially impact carbon dynamics, both via direct effects on plant and soil microbial carbon cycling and via indirect effects on soil resources and physical characteristics. Native grasslands historically covered large areas throughout the arid and semi-arid ecosystems of the Southwest, but have been impacted by land-use, such as livestock grazing. Heavy grazing can result in declines in biological soil crust cover, decreased native perennial grass and forb cover, increased invasive plant cover, as well as increased woody encroachment. In turn, these changes alter the quantity and quality of litter inputs into soil pools and can affect both soil carbon stocks and soil CO₂ flux to the atmosphere. Here, our goal was to assess how removal of livestock grazing affects above- and belowground carbon stocks. Using a combination of ecological site descriptions and the information provided by the Utah Exclosure Inventory Final Report, we selected longterm grazer exclosures across the Colorado Plateau in Utah that varied in range type, historic land use, and applied treatments, yielding 23 ecosites that are representative of common vegetation cover types and land use practices. Aboveground plant biomass was allometrically estimated using published relationships, based on field measurements made inside and outside of livestock exclosures. Soil cores were collected in a spatially discrete manner in order to characterize spatial heterogeneity of soil carbon and nitrogen at smaller scales. 

Results/Conclusions

Preliminary results indicated no effect of livestock exclusion on percent cover of grasses and shrubs across ecosites. Furthermore, there were no differences in Bromus tectorum cover. In turn, the effects of grazing on soil carbon and nitrogen were site-dependent. The results from this work will provide model parameters to aid in the prediction and mitigation of future ecosystem change and be valuable to land management agencies and the general public.