Grasslands

Because grassland vegetation is predominantly herbaceous (i.e., nonwoody), biomass carbon stocks in grassland systems are a small, transient carbon pool with soil constituting the dominant carbon stock. The main processes governing the carbon balance of grassland soils are the same as for other ­ecosystems—the photosynthetic uptake and assimilation of CO₂ into organic compounds and the release of gaseous carbon, primarily CO₂ but also methane (CH₄), through respiration and fire (see Key Finding 4). In grasslands, carbon assimilation is directed toward production of forage by manipulating species composition and sometimes growing conditions (e.g., soil fertility and irrigation).

10.4.1 Grazing Management

For most grasslands in North America, grazing management is the primary feasible management practice that can be manipulated to alter soil carbon stocks. The capacity to increase grassland system carbon stocks is a function of 1) carbon stock changes that might be realized with a shift from suboptimal to best management practices and 2) the areal extent of grasslands that are not optimally managed (Conant and Paustian 2004). Estimates of the potential to sequester carbon in North American grasslands by improving grazing management practices seem likely to be on the order of tens of teragrams of carbon per year (Follett et al., 2001). Uncertainty across these and similar estimates stems from variation in soil carbon responses to management practices, which vary substantially from place to place. Some uncertainty also arises from limited information about past management and the extent to which those historical practices have depleted soil carbon stocks. Additionally, plot-level research indicates that a wide variety of practices could drive increases in soil carbon stocks (Chambers et al., 2016; Conant et al., 2001; Henderson et al., 2015). What is not clear is whether practices used in field experiments can be replicated reasonably under real-world conditions or the extent to which experiments are indicative of potentially observed real-world carbon stock rate changes (Conant et al., 2017).

Removal of some (30% to 50%) aboveground biomass through grazing can reduce the amount of carbon returned to the soil, potentially leading to reduced soil carbon stocks (Conant et al., 2017). Similarly, shifts in species composition in response to grazing could lead to reductions in carbon inputs and soil carbon stocks. Some of the carbon lost from grassland soils can be recovered with changes in management practices that increase carbon inputs, stabilize carbon within the system, or reduce carbon losses (Conant et al., 2017; Eagle and Olander 2012). Adaptive and intensive grazing practices can increase soil carbon stocks (Machmuller et al., 2015; Teague et al., 2011). However, the management practices that promote soil carbon sequestration would need to be maintained over decades to avoid subsequent losses of sequestered carbon.

10.4.2 Fire Suppression and Woody Encroachment

Grazing management, fire suppression, and climate interactively control grassland species composition and productivity, and these responses vary regionally. Woody plant cover is increasing in many grasslands because of management activities such as fire suppression and anthropogenic GHG emissions that increase atmospheric CO₂ concentrations (Kulmatiski and Beard 2013). The most recent syntheses suggest that carbon in aboveground pools decreases in regions with more-limited water (mean annual precipitation < 330 mm) but increases in regions with greater precipitation (Barger et al., 2011; Knapp et al., 2008b). For example, fire suppression in Kansas allowed the expansion of Juniperus virginia that was associated with rapid increases in carbon stocks in vegetation and soils (McKinley and Blair 2008). In the more arid Chihuahuan Desert, shrub encroachment related to historical over-grazing led to higher net carbon uptake rates (Petrie et al., 2015) but may lead to additional loss of grass vege­tation (Thomey et al., 2014). Soil carbon pools may increase with woody encroachment, depending on other disturbance factors, especially fire (Barger et al., 2011). If management policies continue to allow woody plants to expand into native grasslands, the central United States may become a significant regional carbon sink (McKinley and Blair 2008), given sufficient precipitation.

Regional responses to management and climate change are partly related to distinct evolutionary pressures. The combination of grazing and aridity in the Great Plains grasslands may have favored traits that impart resistance to both those disturbances (Milchunas et al., 1988; Moran et al., 2014; Quiroga et al., 2010). In contrast, desert grasslands evolved the ability to rapidly respond to and effectively use highly variable precipitation (McClaran 1997), though often requiring years to recover from disturbance (Peters et al. 2012) and thus allowing rapid expansion of woody species (McClaran et al., 2010). If the frequency of burning increases in mesic tallgrass prairie, decreased nitrogen may become a limiting factor, eventually diminishing aboveground production (Soong and Cotrufo 2015). Thus, fire regime management can influence carbon storage via its effects on above- and belowground production, as well as inputs of recalcitrant, pyrogenic organic matter to soil.

10.4.3 Land Conversion

Agricultural policies can have a large influence on land-use change. For example, in the U.S. Great Plains during 1973 to 2000, grassland and shrub­land area expanded by 2.2% while agricultural area decreased by 1.8%, in part related to farm policy programs such as the Conservation Reserve Program (CRP; landcovertrends.usgs.gov/gp/eco43Report.html). However, the area held in CRP peaked in 2007 at 37 million acres and has since declined (Ahlering et al., 2016). In the coming three decades, agricultural expansion is expected to continue to reduce the extent of grasslands by 2% to 9% by 2050 (see Section 10.3.2; Zhu et al., 2011), depending on annual crop prices (Stubbs 2014).

Grasslands generally take up and store more carbon than croplands; for example, in the Great Plains, the average uptake rates were about 45 g C per m² per year for grasslands and 31 g C per m² per year for croplands from 2000 to 2008 (Wylie et al., 2016). Soil carbon losses occur when native grasslands are initially tilled, with the amount determined by the tillage method and the soil’s initial carbon content. In a modeling study, this “carbon debt” was repaid after 2 to 25 years of no-till corn ethanol production, but that process was 50% longer in a full-tillage production scenario (Kim et al., 2009). Moreover, GHG emissions from croplands tend to be higher than those from grasslands, especially when CH₄ and N₂O are considered. Protection of grasslands from conversion to croplands in the northern mixed-grass prairie pothole region of the Dakotas would reduce emissions significantly, but carbon offsets alone cannot compete with high market prices for corn (Ahlering et al., 2016). For more details on the effects of agricultural management on carbon cycling, see Ch. 5: Agriculture.