Policy challenges to enhance soil carbon sinks: the dirty part of making contributions to the Paris agreement by the United States.

The U.S. government is planning significant reductions in greenhouse gas emissions as part of their nationally determined contribution to the Paris Agreement. The plan includes a variety of activities, one of which is enhancing carbon sinks in soils through a climate-smart agriculture program. The nature of soil carbon along with market forces, cultural factors and other issues create challenges for a program in climate-smart agriculture. These challenges include quantification of soil carbon sequestration, targeting practice adoption that is additional to past adoption, and ensuring that emissions of other greenhouse gases do not increase with climate-smart practices. In addition, there are challenges associated with maintaining carbon storage in soils over a long-time horizon; and avoiding increases in greenhouse gas emissions on non-participating lands. We review and discuss options for addressing challenges with direct regulations, subsidies and tax incentives, carbon taxes, and carbon credit offsets. None of these policy interventions are likely to overcome all challenges, but there are ways to limit risks that challenges pose to each intervention. The ability of the U.S. government to limit or mitigate these risks through careful design of a climate-smart agriculture program will largely determine how much carbon is sequestered in soils, and associated contributions to their emissions reduction goal for the Paris Agreement. [ABSTRACT FROM AUTHOR]