per unit land area, for example, large inputs of organic amendments or conversion to grassland or woodland. However, there are other impacts of different options which also need to be considered, such as biodiversity and soil erosion benefits. The options presented in Table 5.1 are not all mutually exclusive - options for the management of both productive and surplus cropland can be combined to maximise the C mitigation potential (Smith et al. 2000).

Robertson (2004) emphasises the needs for a systems approach for assessing GHG mitigation potential in agriculture. For example, increasing soil carbon stocks in the soil through reduced tillage can lead to anaerobic zones in some soils and thereby increase N2O emissions (MacKenzie et al. 1998; Smith et al. 2001; Li et al. 2005; Six et al. 2004). Similarly, management to reduce CH4 emissions in paddy rice fields might increase N2O emissions. Trade-offs between the GHGs are complex (Robertson et al. 2000), but to be considered. Wherever possible, we have indicated any likely trace gas trade-offs in Table 5.1, although the magnitude and direction of trace gas trade-offs are often uncertain. For instance, altering inorganic fertiliser applications could either increase or reduce N2O emissions depending on the timing and amounts applied. The different GHGs also have different impacts -for instance, since CO2has direct physiological effects on plant transpiration via its influence on stomatal conductance, it has the potential to alter run-off in addition to altering climate directly (Betts et al. 2007a).

5.2.3 Climate Mitigation Opportunities in Global Croplands

Cropland soils can be a large source of carbon dioxide (Janssens et al. 2003). Hence, there is a significant potential to reduce the efflux of carbon from agricultural soils, and to sequester carbon in them. Estimates of the potential for additional soil carbon sequestration vary widely. The most recent global estimate is that of Lal (2004a) 0.9 ± 0.3PgCyr-1, or approximately 11% of the 1990's annual anthropogenic CO2 flux (fossil fuel combustion, cement production and land use change). Over 50 years, this level of C sequestration would restore a large part of the carbon lost from soils historically. However, soil carbon sequestration rates have a limited duration and cannot be maintained indefinitely - in cool temperate climates, soil carbon changes often reach a new equilibrium 50-100 years, following a land use change (Poulton et al. 2003), after which no additional sequestration may be achieved. Geographically, the greatest potential for GHG mitigation on a unit land area basis is in the Cool Moist and Warm Moist climate zones, while in the Cool and Warm dry zones, the mitigation potential is generally much lower (Smith et al. 2007b).

Estimating mitigation potential is often confounded by the choice of constraints. Some authors quote biological potentials (Metting et al. 1999), while others quote potentials as limited by available land or resources (Smith et al. 2000), and many others consider economic and social constraints (Cannell 2003; Freibauer et al. 2004). Smith (2004a) provided a figure showing how these mitigation potential estimates differ and how the potential is reduced by a number of constraints

Fig. 5.1 How different constraints reduce the GHG mitigation potential from its theoretical biological maximum to realistically achievable potentials that are much lower (adapted from Smith 2004a)

Maximum value

Minimum value

GHG mitigation potential

Biological potential |

Biologically / physically constrained potential (e.g. land suitability)

Economically constrained potential

Socially / politically constrained potential - estimated realistically achievable potential (~10% of biological potential)

(Fig. 5.1). An analysis of the estimates presented by Freibauer et al. (2004) and the assumptions used by Cannell (2003) suggest that the realistic sustainable (or conservative) achievable potential of GHG mitigation (taking into account limitations in land use, resources, economics, and social and political factors) may be only about 10-20% of the biological potential. Although this value is derived predominantly from expert judgment, it may be useful in assessing how different estimates of GHG mitigation potential can be compared and how they might realistically contribute to GHG stabilization. This value did not, however, consider how future changes in climate might impact GHG mitigation potential. More recent sectoral and multi-sectoral economic assessments allow the mitigation potential for each activity to be quantified more precisely (Smith et al. 2007a, b). These studies show that the implementation of different measures depends upon the carbon price and measures are implemented to different extents at differing carbon price (Smith et al. 2007a, b).

Smith (2004b) calculated how future carbon emissions and CO2 stabilization targets might influence the relevance of soil carbon sequestration as a GHG mitigation measure. The IPCC standard reference emission scenarios (SRES) provide estimates of possible emissions under a range of different possible futures (IPCC 2000), which depend upon the degree to which greenhouse gas mitigation policies become global and upon whether environmental or economic concerns take precedence over the next century. In all of these scenarios, the global population will grow, the population will become wealthier and per-capita energy demand will increase over the next century (IPCC 2000), but the extent of these changes differs between scenarios. For each of the scenarios, carbon emission trajectories have been determined (IPCC 2000). Annual carbon emissions (PgCyr-1) by 2100 would be A1FI-30, A1B-17, A1T-7, A2-28, B1-6, B2-18.

Emissions trajectories can also be calculated for a range of atmospheric CO2 stabilization targets (e.g. 450, 550, 650, 750 ppm). For each stabilization target, the allowed carbon emission trajectories, which cannot be exceeded if the target is to be reached, can be calculated. The difference between the allowed emission trajectory for stabilisation at a given target concentration, and the emissions associated with the estimated global energy demand are the carbon emission gaps. For a stabilization target of 550ppm, carbon emission gaps for each scenario by 2100 (PgCyr-1) are

A1FI = 25, A1B = 12, A1T = 2, A2 = 22, B1 = 1, B2 = 13 (IPCC 2001); however, climate-carbon cycle feedbacks could increase these gaps substantially (Jones et al. 2006).

Carbon emission gaps by 2100 could be as high as 25PgCyr-1, almost four times the current annual emission of CO2-carbon to the atmosphere (6.3±1.3PgCyr-1). Since the maximum annual global C sequestration potential is 0.9 ± 0.3 PgCyr-1, this implies that even if these rates could be maintained until 2100, soil carbon sequestration would contribute a maximum of 2-5% towards reducing the carbon emission gap under the highest emission scenarios in the long-term. The limited duration of carbon sequestration options in removing carbon from the atmosphere, also implies that carbon sequestration could play only a minor role in closing the emission gap by 2100. However, if atmospheric CO2 levels are to be stabilised at concentrations below 450-650 ppm by 2100, drastic reductions in emissions are required over the next 20-30 years (IPCC 2000). During this critical period, all measures to reduce net carbon emissions to the atmosphere would play an important role - there will be no single solution (IPCC 2000) and options, such as reduced emissions from deforestation (Gullison et al. 2007) could also contribute significantly. Since carbon sequestration is likely to be most effective in its first 20 years of implementation, it could form a central role in any portfolio of measures to reduce atmospheric CO2 concentrations over the next 20-30 years, whilst new energy technologies are developed and implemented (Smith 2004b).

5.2.4 Climate Change Impacts on Greenhouse Gas Fluxes from Cropland Soils

As mentioned in the introduction, climate change alone could alter the future levels of carbon storage and trace gas fluxes from cropland soils, since changes in temperature, precipitation and atmospheric CO2 concentration will affect net primary production (NPP), carbon/nitrogen inputs to soil, soil carbon decomposition rates and soil nitrogen cycling. We will next examine predicted changes in each of these drivers, and how these could affect GHG fluxes from cropland soils.

5.2.5 Predicted Changes in the Climate Drivers of Cropland Greenhouse Gas Fluxes

Most General Circulation Models (GCMs) agree that much of the globe is likely to experience considerable warming over the next century, with best estimates of the global mean temperature increase by 2100 ranging from 1.4 to 4.0°C (likely range 1.1-6.4°C), dependent on GCM and emissions scenario (IPCC 2007a). Warming is predicted to be greatest over land and high northern latitudes (Fig. 5.2a). However, there is much less agreement regarding which regions will experience increases or decreases in precipitation and soil moisture. Increases in the total annual amount

Fig. 5.2 Change in (a) temperature (0C), (b) top-level (0-10 cm) soil moisture (kg m-2) and litter C inputs (kg C m-2), from 2000 to 2100 - HadCM3LC model (IS92a emissions scenario) (Cox et al. 2000)

Fig. 5.2 Change in (a) temperature (0C), (b) top-level (0-10 cm) soil moisture (kg m-2) and litter C inputs (kg C m-2), from 2000 to 2100 - HadCM3LC model (IS92a emissions scenario) (Cox et al. 2000)

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