Soils

Soil carbon is vulnerable to both pervasive warming and moisture disturbances, as well as to land-use decisions, all of which can strongly affect soil carbon contents. In northern latitudes, which are particularly vulnerable to soil carbon loss, some of the fastest warming trends (Cohen et al., 2014) and largest carbon stocks (Ping et al., 2008) occur. A significant portion of northern soil carbon is stored as organic peat horizons, which play a pivotal role in insulating permafrost from temperature changes but are particularly sensitive to changes in soil moisture (Johnson et al., 2013). Thus, the feedbacks among warming, moisture, and wildfire have important consequences to the carbon cycle at a global scale (Olefeldt et al., 2016). Meanwhile, localized “hotspots” for soil carbon storage, while also vulnerable to warming and soil moisture, can be sensitive to management practices as well and, therefore, can offer potential mitigation opportunities to avoid carbon emissions. For example, maintaining high water tables in carbon-rich peatlands potentially avoids carbon emissions that otherwise would accompany drainage.

Management options for actively sequestering carbon into soil are important opportunities for climate mitigation, but several issues arise before there is confidence in the outcome for a given soil under a given management setting. Topographical and mineralogical characteristics and disturbance histories (e.g., fire-return interval and land-use change history) likely influence the net balance between input and loss and yet are highly variable across North America. Strategic experimental designs with consistent oversight and methodologies could constrain the uncertainties and understanding of the processes that control carbon storage. Building spatially and temporally explicit databases could improve process-based models to provide better estimates for soil carbon trajectories and thereby empower land managers to chart the trajectory of soil carbon.

Increasingly, the development of policies to 1) promote improved soil health (Kibblewhite et al., 2008; Vrebos et al., 2017), 2) encourage soil carbon sequestration for GHG mitigation (Chambers et al., 2016; Follett et al., 2011), and 3) satisfy consumer demands for more sustainable products (Lavallee and Plouffe 2004) will demand strong scientific support for improved understanding of SOC dynamics, new technologies to increase SOC stocks, and decision-support tools to effectively assess options and monitor progress. Along with new research on more conventional practices to build soil carbon (e.g., improved rotations, reduced tillage, and cover crops), scientists are investigating newer practices and technologies to increase SOC stocks, including 1) applying biochar (Woolf et al., 2010) and compost (Ryals et al., 2015), 2) using deep tillage to increase the total depth and storage of SOC-rich soil (Alcantara et al., 2016), 3) deploying new crop varieties with increased allocation of carbon below ground and deeper into the soil profile (Paustian et al., 2016), and 4) planting perennial plants in place of annual crops (Cox et al., 2006). New research and best practices in forestry such as selective harvesting and residue management (Peckham and Gower 2011), tailored for particular soils (Hazlett et al., 2014), also have the potential to increase carbon retention in forest soils. As new knowledge is generated about the applicability of various practices in different environments, incorporating this new information into improved decision-support tools (see Ch. 18: Carbon Cycle Science in Support of Decision Making) will guide land managers, industry, policymakers, and other stakeholders in building heathier soils that are rich in organic matter.