Net Ecosystem Carbon Balance of Grazing Lands across the continental United States, 2013-2023

Grazing lands underpin U.S. beef production, store roughly one-third of global soil organic carbon, deliver multiple ecosystem services, and are closely tied to the prosperity and resilience of rural communities. In this study, we calculated net ecosystem carbon balance (NECB), the net status of grazing lands as a carbon sink or source, by integrating carbon uptake from photosynthesis, and carbon loss through ecosystem respiration, enteric fermentation and manure from livestock. Our objective was to synthesize multiple years of annual NECB of grazing lands measured by eddy covariance towers from 16 pastures across seven USDA Long-term Agroecosystem Research Network (LTAR) sites and enteric fermentation and manure emissions derived from the stocking rates. We evaluated annual NECB against mean annual precipitation (MAP), mean annual temperature (MAT), vegetation, soil, fire history, grazing pressure index (GPI) and fertilization history. We found: (1) grazing lands were a carbon sink or neutral in most sites, and NECB was not significantly different between grasslands and shrublands, mesic and xeric conditions, and fertilized and unfertilized sites; (2) NECB increased with precipitation and temperature, but decreased with a higher GPI; and (3) precipitation, temperature, and GPI interacted such that temperature had a positive effect when MAP was greater than 700 mm and GPI had a negative effect when MAP was less than 1000 mm. Thus, most grazing lands in our study function as a carbon sink unless coupled with water deficit, low temperature, or heavy grazing. NECB is most sensitive to precipitation when water was limited with high interannual variability. Future work to improve our understanding of NECB on grazing lands should directly measure enteric fermentation and ecosystem emissions in different systems and add measurements on carbon loss through wind erosion and leaching.