Grazing management alters the amount and consumption of biomass by domestic animals and wildlife to achieve production and other goals. This technique requires management of intensity, frequency, and seasonality of grazing and animal distribution.
Use and Potential
Grazing influences carbon and nutrient cycling, as well as many other properties of grassland ecosystems (e.g., species composition, light interception, soil compaction). Grazing results in some of the plant carbon being routed through the digestive tracts of animals-where some is converted to weight gain, some is emitted as CO2 and CH4, and 25-50 percent is returned in wastes to the grassland. The response of soil carbon stocks to grazing intensity varies for different grasslands. In general, where grazing is managed to maintain or increase plant productivity, soil carbon stocks can be maintained or increased (Conant et al., 2000). Overgrazing-the definition of which is determined by social and economic values, as well as ecosystem function (Abel, 1997)-to the extent of significantly decreasing primary productivity and stimulating erosion decreases soil carbon. For example, Li et al. (1997) reported that overgrazing during a 40-year period in Inner Mongolia resulted in a 12.4 percent loss of soil carbon.
In northern Australia, 40 percent of grazing lands have been degraded, resulting in increased incidence of annual grasses and reduced cover of perennial grasses (Tothill and Gillies, 1992). Adoption of reduced stocking rates to increase perennial grasses could sequester 315 Mt C in the top 10 cm of soil over 30 years (Ash et al., 1996); this management change often does not significantly reduce farm income (Stafford Smith et al., 1999). The 0-10 cm layer contains about 16 percent of the profile soil carbon (Dalal and Carter, 1999); thus, total storage may be much greater. The rate of establishment of perennial grasses appears to determine the rate of recovery of soil carbon (Burke et al., 1995). Soil carbon can be lost from these systems-through a combination of inappropriate management and drought-much more quickly than it can be replaced (Bridge et al., 1983; Northup and Brown, 1999). Thus, a continuity of management purpose is required to maintain or increase soil carbon reliably.
Developing countries face additional problems in that the change from nomadic/semi-nomadic grazing management to permanent settlements may result in overgrazing and increased soil carbon losses (Togtohyn et al., 1996). In tropical pastures established after deforestation, overgrazing and nutrient deficiencies have resulted in soil erosion and soil carbon losses, weed invasion, and land degradation (Feller, 1993; Woomer et al., 1997). In contrast, appropriate pasture management can lead to increased soil carbon levels compared with the native forest (Lugo et al., 1986; Cerri et al., 1991; Lugo and Brown, 1996); Neill et al. (1997) estimate a potential soil carbon increase of 12-18 t C ha-1 in the top 30 cm of soil.
Current Knowledge and Scientific Uncertainties
Data exist for many locations, predominately in temperate countries but increasingly in tropical regions. Most studies use paired-site measurements, which have higher uncertainties than time series-based measurements. The mechanisms by which grazing influences ecosystem carbon storage are relatively well understood, but the magnitude of carbon change as a function of grazing intensity is uncertain for many grasslands. Rates of increase of soil carbon are likely to be higher in more mesic environments than in drier ones (Table 4-4); woodlands and native grasslands show the greatest increases (Conant et al., 2000). Environmental changes such as climate variability, climate change, and atmospheric CO2 increases are likely to impact carbon stocks.
Where direct sampling is used, soil ideally should be measured throughout the profile (1 m or more) because of deep carbon storage with some species. In at least some environments, surface soil carbon is correlated with pasture condition (Ash et al., 1996), which could be assessed using field measurements and/or remote sensing (Bastin et al., 1998)-providing the potential for regular, large-scale, geographically referenced assessments. Rapid, direct assessments of aboveground pasture biomass at site (Tothill et al., 1992; Waite, 1994) and regional scales (Hassett et al., 1999) are well established. Allometric relationships allow estimation of root biomass.
Carbon accumulation from improved grazing could extend for 25-50 years or more depending on the rate of plant productivity response. Low rates of change in soil carbon (e.g., because of slow plant recovery from overgrazing) will be associated with long recovery times (Burke et al., 1995); conversely, rapid increases in soil carbon will tend to diminish more rapidly over time. Soil carbon content and bulk density can be measured at intervals of 5 years or more to depths of 1 m or more, depending on species and soil conditions (see Chapter 2).
Monitoring, Verifiability, and Transparency
Rates of change could be verified through repeated field measurements (soil and vegetation) over time, for representative grassland types and grazing regimes. Existing grassland carbon models that incorporate grazing, appropriately parameterized, could be used in verification. Conventional vegetation mapping and/or remote sensing can be used in conjunction with country statistics to verify the areal extent of grazing lands of different types. Statistics or surveys of animal stocking rates could be used for rough estimates of past and current grazing intensity.
Biomass is removed by livestock and other herbivores, by fire, and through detachment and decomposition. Soil carbon is removed mainly by decomposition and erosion. Both processes are strongly influenced by management.
If continuity of management purpose is maintained, storage of carbon in soils and root biomass can provide pools that persist for years to centuries.
Improved land conditions where overgrazing is addressed reduces erosion; reduces methane emissions by reducing animal numbers and improving intake quality; and is likely to reduce nitrous oxide emissions as a result of lower levels of excreted nitrogen (Howden et al., 1994) and probably greater uptake by vegetation.
Relationship to IPCC Guidelines
Procedures for estimating grazing effects on carbon stocks are not explicitly defined in the Guidelines. For soil carbon, however, grazing effects will be a function largely of changes in productivity and carbon additions to soils, which can be addressed through the selection of input factors (Reference Manual). In addition, default values are provided for improved, unimproved, and degraded pasture soils (Workbook).
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