8.4.1.2 Grazing land management and pasture improvement

Grazing lands occupy much larger areas than croplands (FAOSTAT, 2006) and are usually managed less intensively. The following are examples of practices to reduce GHG emissions and to enhance removals:

a. Grazing intensity: The intensity and timing of grazing can influence the removal, growth, carbon allocation, and flora of grasslands, thereby affecting the amount of carbon accrual in soils (Conant et al., 2001; 2005; Freibauer et al., 2004; Conant and Paustian, 2002; Reeder et al., 2004). Carbon accrual on optimally grazed lands is often greater than on ungrazed or overgrazed lands (Liebig et al., 2005; Rice and Owensby, 2001). The effects are inconsistent, however, owing to the many types of grazing practices employed and the diversity of plant species, soils, and climates involved (Schuman et al., 2001; Derner et al., 2006). The influence of grazing intensity on emission of non-CO₂ gases is not well-established, apart from the direct effects on emissions from adjustments in livestock numbers.

b. Increased productivity: (including fertilization): As for croplands, carbon storage in grazing lands can be improved by a variety of measures that promote productivity. For instance, alleviating nutrient deficiencies by fertilizer or organic amendments increases plant litter returns and, hence, soil carbon storage (Schnabel et al., 2001; Conant et al., 2001). Adding nitrogen, however, often stimulates N₂O emissions (Conant et al., 2005) thereby offsetting some of the benefits. Irrigating grasslands, similarly, can promote soil carbon gains (Conant et al., 2001). The net effect of this practice, however, depends also on emissions from energy use and other activities on the irrigated land (Schlesinger, 1999).

c. Nutrient management: Practices that tailor nutrient additions to plant uptake, such as those described for croplands, can reduce N₂O emissions (Dalal et al., 2003; Follett et al., 2001). Management of nutrients on grazing lands, however, may be complicated by deposition of faeces and urine from livestock, which are not as easily controlled nor as uniformly applied as nutritive amendments in croplands (Oenema et al., 2005).

d. Fire management: On-site biomass burning (not to be confused with bio-energy, where biomass is combusted off-site for energy) contributes to climate change in several ways. Firstly, it releases GHGs, notably CH₄ and, and to a lesser extent, N₂O (the CO₂ released is of recent origin, is absorbed by vegetative regrowth, and is usually not included in GHG inventories). Secondly, it generates hydrocarbon and reactive nitrogen emissions, which react to form tropospheric ozone, a powerful GHG. Thirdly, fires produce a range of smoke aerosols which can have either warming or cooling effects on the atmosphere; the net effect is thought to be positive radiative forcing (Andreae et al., 2005; Jones et al., 2003; Venkataraman et al., 2005; Andreae, 2001; Andreae and Merlet, 2001; Anderson et al., 2003; Menon et al., 2002). Fourth, fire reduces the albedo of the land surface for several weeks, causing warming (Beringer et al., 2003). Finally, burning can affect the proportion of woody versus grass cover, notably in savannahs, which occupy about an eighth of the global land surface. Reducing the frequency or intensity of fires typically leads to increased tree and shrub cover, resulting in a CO₂ sink in soil and biomass (Scholes and van der Merwe, 1996). This woody-plant encroachment mechanism saturates over 20-50 years, whereas avoided CH₄ and N₂O emissions continue as long as fires are suppressed. Mitigation actions involve reducing the frequency or extent of fires through more effective fire suppression; reducing the fuel load by vegetation management; and burning at a time of year when less CH₄ and N₂O are emitted (Korontzi et al., 2003). Although most agricultural-zone fires are ignited by humans, there is evidence that the area burned is ultimately under climatic control (Van Wilgen et al., 2004). In the absence of human ignition, the fire-prone ecosystems would still burn as a result of climatic factors.

e. Species introduction: Introducing grass species with higher productivity, or carbon allocation to deeper roots, has been shown to increase soil carbon. For example, establishing deep-rooted grasses in savannahs has been reported to yield very high rates of carbon accrual (Fisher et al., 1994), although the applicability of these results has not been widely confirmed (Conant et al., 2001; Davidson et al., 1995). In the Brazilian Savannah (Cerrado Biome), integrated crop-livestock systems using Brachiaria grasses and zero tillage are being adopted (Machado and Freitas, 2004). Introducing legumes into grazing lands can promote soil carbon storage (Soussana et al., 2004), through enhanced productivity from the associated N inputs, and perhaps also reduced emissions from fertilizer manufacture if biological N₂ fixation displaces applied N fertilizer N (Sisti et al., 2004; Diekow et al., 2005). Ecological impacts of species introduction need to be considered.

Grazing lands also emit GHGs from livestock, notably CH₄ from ruminants and their manures. Practices for reducing these emissions are considered under Section 8.4.1.5: Livestock management.