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the proposed practices will need to be evaluated for individual
agricultural systems according to the specific climatic, edaphic,
social settings, and historical land use and management (Smith
et al., 2007a). The effectiveness of mitigation strategies also
changes with time. Some practices such as those which elicit
soil carbon gain have diminishing effectiveness after several
decades while others can reduce energy use restricting emis-
sions indefinitely. For instance, there is a strong time depen-
dency of emissions from no-till agriculture, in part because
of changing influence of tillage on N
2
O emissions (Six et al.,
2004). Many of the climate change effects have high levels of
uncertainty but demonstrate that the practices chosen to reduce
GHG emissions may not have the same effectiveness in coming
decades. Consequently, programmes to reduce emissions in the
agricultural sector will need to be designed with flexibility for
adaptation in response to climate change (Smith et al., 2007a).
1.7
Co-benefits and trade-offs of
mitigation options
The merits of an agricultural GHG emission mitigation practice
cannot be judged solely on the effectiveness of GHG mitigation.
Agro-ecosystems are inherently complex and very few prac-
tices yield purely beneficial outcomes, but instead involve some
trade-offs above certain levels or intensities of implementation
(DeFries et al., 2004; Viner et al., 2006). The co-benefits and
trade-offs of a practice may vary from place to place because
of differences in climate, soil or the way the practice is adopted
(Smith et al., 2007a). Land use changes and agricultural man-
agement can either be beneficial or harmful to biodiversity; for
instance, loss of biodiversity due to intensification of agriculture
or large-scale production of biomass energy crops while peren-
nial crops often used for energy production can favour biodiver-
sity if they displace annual crops or degraded areas (Berndes and
Börjesson, 2002; Anonymous, 2006e; Feng et al., 2006; Xiang
et al., 2006). Agricultural mitigation practices may influence
non-agricultural ecosystems. Increasing the productivity on
existing croplands may 'spare' some forest or grasslands (West
and Marland, 2003; Balmford et al., 2005; Mooney et al., 2005);
however, the net effect of such trade-offs has not yet been fully
quantified (Huston and Marland, 2003; Green et al., 2005).
Implementation of agricultural GHG mitigation measures
may allow expanded use of fossil fuels and may have some neg-
ative effects through emissions of sulphur, mercury and other
