Chapter 5: SOCCR2: Current State of the Carbon Cycle in Agricultural Systems in North America

The purpose of the 2^nd State of the Carbon Cycle Report (SOCCR-2) is to assess the current state of the carbon cycle in North America and to present advances in our understanding of the various factors that influence the carbon cycle. Agriculture is a significant source (and sink) of carbon; emissions of agricultural greenhouse gases (GHG) totaled 567 teragrams (Tg) of carbon dioxide equivalent (CO₂e) in the United States and 73 Tg CO₂e in Canada, not including land-use change. Agricultural GHG emissions in Mexico were 85 Tg CO₂e in 2014. Most cropland carbon stocks are in the soil and cropland management practices can increase or decrease soil carbon stocks. The magnitude and longevity of management-related carbon stock changes have strong environmental and regional differences, and they are subject to subsequent changes in management practices. Various strategies are available to mitigate livestock enteric and manure CH₄ emissions, yielding potential reductions of 20% to 30% (for enteric fermentation) to as much as 50 to 80% (for manure management). Methane mitigation strategies have to be evaluated on a production-system scale to account for emission tradeoffs and co-benefits such as improved feed efficiency or productivity in livestock (high confidence, likely). Potential effects of climate change on agricultural soil carbon stocks are difficult to assess, because they will vary according to the nature of the change, onsite ecosystem characteristics, production system, and management type.