Agricultural ecosystems hold large carbon reserves (IPCC, 2001a), mostly in soil organic matter.Historically, these systems have lost more than 50 Pg Carbon, but some of this carbon lost can be recovered through improved management, thereby withdrawing atmospheric CO2 (Paustian et al., 1998; Lal, 1999, 2004a).
Land management options for mitigation fall in the following four categories: a) cropland management; b) grazing land management/pasture improvement; c) management of agricultural lands and d) restoration of degraded lands. This description focuses on the management of grazing lands and pastures.
Grazing lands occupy much larger areas than croplands (FAOSTAT, 2006) and are usually managed less intensively. Several management techniques can be identified that will support climate change mitigation efforts:
- Grazing intensity management
- Increased productivity
- Nutrient management
- Fire management
- Species introduction
The total mitigation potential of land management techniques is substantial, as is illustrated in Figure 1.
Figure 1: Total technical mitigation potentials (all practices, all GHGs) for each region by 2030 (click image to enlarge) Source: IPCC, 2007; Smith et al., 2007
N2O is typically the dominant greenhouse gas source from agricultural systems and is produced naturally in soils through the processes of nitrification and denitrification. These are soil microbial processes whereby ammonium (NH3) is reduced to nitrate (NO3) under aerobic or oxygen-rich conditions (nitrification), and nitrate is reduced to molecular nitrogen (N2) under anaerobic or oxygen-poor conditions (denitrification).
A number of activities add nitrogen to soils, thereby increasing the amount available for nitrification and denitrification, and ultimately the amount of N2O emitted to the atmosphere. Activities may add nitrogen to soils either directly or indirectly.
Direct additions occur through nitrogen fertilizer use, application of managed livestock manure and sewage sludge, production of nitrogen-fixing crops and forages, retention of crop residues, and cultivation of histosols (i.e., soils with high organic-matter content, also known as organic soils). Indirect emissions occur through volatilization and subsequent atmospheric deposition of applied nitrogen, as well as through surface runoff and leaching of applied nitrogen into groundwater and surface water.
Other soil management activities, such as irrigation, drainage, tillage practices, and fallowing of land, can also affect fluxes of N2O, as well as soil carbon and fossil fuel CO2 emissions. Fossil fuel CO2 emissions can be generated on-farm by agricultural equipment and off-farm or upstream through the energy-intensive production of fertilizers.
Enhancing soil carbon sequestration represents a significant greenhouse gas mitigation option, potentially more viable than N2O reductions (see USEPA [2005a]). Croplands often emit CO2 as a result of conventional tillage practices and other soil disturbances. This occurs when soils containing organic matter that would otherwise be protected by vegetative cover are exposed to the air through tillage disturbances and become susceptible to decomposition.
Conservation tillage—defined in the United States as any tillage system that maintains at least 30 percent of ground covered by crop residue after planting (Conservation Technology Information Center [CTIC], 1994)—eliminates one or several practices associated with conventional tillage, such as turning soils over with a moldboard plow and mixing soils with a disc plow. Conservation tillage, including no-till, allows crop residues to remain on the soil surface as protection against erosion.
Feasibility of technology and operational necessities
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.
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).
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).
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 woodyplant 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.
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). However, it is very important to consider the ecological impacts of species introduction need to be considered.
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).