Soils

Globally, soils contain more than three times as much carbon as the atmosphere and four and a half times more carbon than the world’s biota (Lal 2004); therefore, even small changes in soil carbon stocks could lead to large changes in the atmospheric concentration of carbon dioxide (CO₂). Despite their importance, however, stocks of soil organic carbon (SOC), which is the carbon component of soil organic matter (SOM), have been depleted through changes in land use and land cover and unsustainable land management practices associated with agriculture, grazing, and forest management. To better manage and sustain SOC stocks, a focused understanding of microbial and biogeochemical processes that interact in soils, regardless of land cover, to control soil carbon stabilization and destabilization is needed. Soil organic matter (the organic component of soil, consisting of organic residues at various stages of decomposition, soil organisms, and substances synthesized by soil organisms) also is considered a central indicator of soil health because it regulates multiple ecosystem services that humanity derives from soils, including moderation of climate. SOM stores nutrients, increases water-holding capacity to promote plant growth, limits leaching of nutrients, and adds structure that improves drainage and reduces erosion (Oldfield et al., 2015).

The current best estimates for global SOC stocks are 1,400 ± 150 petagrams of carbon (Pg C) to 1 m in depth and 2,060 ± 220 Pg C to 2 m in depth (Batjes 2016). These values are derived from the Harmonized World Soil Database with corrections for underrepresented regions, including the Northern Circumpolar Region, using measured soil profiles and geospatial modeling. The resulting values are consistent with other global SOC pool estimates (Govers et al., 2013; Köchy et al., 2015). An estimated 90 to 100 Pg C is released by soils to the atmosphere as soil respiration each year, an efflux that represents both heterotrophic (approximately 51 Pg C) and autotrophic (approximately 40 Pg C) respiration (Bond-Lamberty and Thomson 2010; Hashimoto et al., 2015), roughly balanced by carbon incorporated into SOC from plant residues. This flux value can be compared to estimates from the most recent Intergovernmental Panel on Climate Change (IPCC) report that estimated the gross efflux from surface ocean water to the atmosphere as 78.4 Pg C per year (with a net sink of 2.3 ± 0.7 Pg C per year), carbon emissions from fossil fuel combustion and cement production as 7.8 ± 0.6 Pg C per year, and outgassing from freshwater as 1.0 Pg C per year (Ciais et al., 2013). Soil carbon storage and flux at a given location are controlled by variations in 1) soil-forming factors (Jenny 1941; McBratney et al., 2003; Mishra et al., 2010), 2) anthropogenic activities (Lal 2004), and 3) climatic forcings (Heimann and Reichstein 2008; Richter and Houghton 2011). Future change in the frequency of climatic extremes (Seneviratne et al., 2012) and land use and land management (Nave et al., 2013; Ogle et al., 2010; Wills et al., 2014) may alter SOC stocks and fluxes that affect land feedbacks to climate change, changing the magnitude of, or even reversing (i.e., change from sink to source), the land carbon sink (Friedlingstein et al., 2014).

Soils of North America store 366 to 509 Pg of organic carbon to 1 m in depth based on continental-scale analyses (Batjes 2016; Liu et al., 2013). Breakdown of SOC stocks by country are discussed in more detail later in this chapter. At the continental scale, nearly 75% of SOC stocks down to 1 m are found in the top 30 cm (Liu et al., 2013), which also is the portion of the soil profile most vulnerable to changes induced by land-use and land-cover changes, disturbance and extreme events, management practices, and climate change. Several knowledge gaps exist in the current ability to measure SOC stocks and fluxes across North America. Researchers employ diverse analytical methods to measure carbon concentration and take measurements at different depths; furthermore, many measurements lack bulk density estimates that are needed to calculate stock estimates. Most SOC stock estimates lack systematic uncertainty (i.e., error propagation) estimates. Consequently, this chapter shows many values of stocks and fluxes without companion uncertainty values. Therefore, significant risks exist for biased conclusions due to inadequate and uneven distributions of SOC profile observations, especially in permafrost regions (Mishra et al., 2013), for depths >1 m and in bulk density estimates for organic soils (Köchy et al., 2015). Recent updates to soil databases have improved coverage, but distributions of available samples across geographic regions are uneven and thus not sufficient to fully characterize SOC dependence on climate, edaphic factors, and land-cover types (Hengl et al., 2014; Mishra and Riley 2012). However, recent efforts, notably the U.S. Department of Agriculture’s (USDA) Rapid Carbon Assessment (RaCA), will yield a much more consistent estimate of current soil carbon stocks (see Section 12.4.1). Similarly, RaCA recently initiated a field-based soil carbon inventory for Mexico, and comprehensive stock estimates for different regions and land uses are forthcoming (see Section 12.4.2).

Since cultivation of land began nearly 12,000 years ago, humans have been altering soil carbon stocks. Just since 1850, human degradation of soil worldwide may have resulted in a loss of 44 to 537 Pg SOC, largely through land-use change and conversion to agriculture (Lal 2001; Paustian et al., 1997). Globally, agricultural soils have lost 20% to 75%, or 30 to 40 megagrams of carbon (Mg C) per hectare (ha), of their antecedent SOC pool (Lal et al., 2015). In contrast, afforestation (the establishment of forest cover on land that previously did not have tree cover) and land restoration have the potential to recover depleted SOC stocks from the atmosphere (Lal 2004). For example, newly afforested lands cover 4 billion ha globally and have a carbon sequestration potential of 1.2 to 1.4 Mg C per year (Lal et al., 2015). Meta-analysis of afforestation effects on soil carbon storage in the United States and Canadian border provinces found that land conversion to forest from agriculture, industry, or wild grassland increased SOC by 21% + 9% (Nave et al., 2013). The researchers found that the largest increase was in lands previously used for industrial purposes such as mining (173%), for areas with woody encroachment into unmanaged grassland (31%; see Ch. 10: Grasslands), and for agricultural areas in the Northern Plains (32%; see Ch. 5: Agriculture). Such SOC increases via afforestation and reforestation contribute to the net carbon sequestration by U.S. forests, currently estimated at 313 ± 40 teragrams of carbon (Tg C) per year (Lu et al., 2015).