Implications for Other Practices

This brief look at NT farming has implications also for broader questions of GHG

mitigation from agroecosystems. Perhaps the most obvious is the urgency of accounting for all GHGs in assessing how well a proposed practice might reduce emissions (Robertson and Grace, 2004; Mosier et al., 2005). Although much of the early focus, justifiably, was on soil carbon sequestration, agriculture is a major contributor of N2O and CH4, both potent GHGs. Because of the high GWPs of these gases, small shifts in their emissions can substantially augment or offset the benefits from any soil carbon gain. So we cannot consider only the soil carbon accrual from reduced tillage; we have to estimate the effects on N2O emission. We cannot limit our attention only to the SOC gains from planting grasses; we have to think about the CH4 emitted when those grasses are fed to livestock. We cannot examine only the SOC benefits of practices that favour higher yields; we have to quan-tiffy the N2O emitted from higher fertilizer rates needed to support those yields and the energy consumed in making that fertilizer. Thus, GHG mitigation in agriculture depends increasingly on looking at farms as ecosystems, considering the entire web of intertwined flows of carbon, nitrogen and energy in that system. Perhaps the best way of doing this, quantitatively, is by building models of varying sophistication. Some progress has already been made in that direction (e.g. Flessa et al., 2002; Soussana et al., 2004; Janzen et al., 2006; Schils et al., 2005), but the complexity of such analyses suggests that immediate success in such ventures is unlikely. A particular challenge, and an aim worth pursuing, is to link the nutrient and energy flows between livestock and cropping facets of farming systems, facets that heretofore have often been examined separately.

A second finding is that mitigation practices such as NT farming cannot be advocated blindly without taking into account local conditions. NT practices offer a powerful opportunity to reduce GHG emissions in some settings; elsewhere they have minimal net benefits, and may even enhance GHG emissions. While NT and other practices have been widely recommended as mitigation practices - with good reason - the next step may be to identify those conditions and complementary farming practices where they are most beneficial (or at least not counterproductive).

Further, the preceding analysis implies that the choice of mitigation practices may involve trade-offs. Although NT farming is often promoted as a win-win opportunity, it (like other proposed mitigation practices) may not be without possible costs. For example, while NT has many benefits for soil conservation, what does the farmer do if there is animal manure to apply? Does the farmer maintain strict NT or compromise aversion to tillage by incorporating the manure, thereby minimizing environmental effects of that manure? What if NT farming reduces net GHG emissions but poses higher risk to water quality via leaching through macropores - who decides the relative costs and merits of each? The value judgements needed to resolve such trade-offs may need to consider not only scientific data but also social factors (Lubchenco, 1998; Ludwig et al., 2001).

Finally - and perhaps most daunting - is the challenge to include time into the analysis of GHG mitigation. As observed for NT, the net benefit of a practice depends on how much time has elapsed since the practice was adopted. Eventually the SOC gains must cease as soil carbon approaches a new equilibrium, and in the long run - over many decades - the net benefit of NT may depend less on the SOC gained than on the continued emissions of N2O and energy-derived CO2 needed to continue the practice (Fig. 5.4). Further, the effectiveness of a proposed practice will depend on history - the net accrual of carbon under NT, for example, depends not only on the practice now in place on a farm, but also on the practices imposed on that land years (even generations) before. Lastly, time needs to be included to acknowledge future global changes. Conditions a halfcentury from now may be different than those observed in our current careful studies - the climate may be different, atmospheric CO2 certainly will be higher, energy availability may be altered. What will happen, in that changed world, to the carbon that we have so carefully extracted from the atmosphere and stored away in our soils? Do we risk losing the soil carbon

T C o t 0

/ \N2O 1 \

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I (energy use) CO2

(soil sink)

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Time (years since adoption of NT)

Fig. 5.4. Conceptual illustration of how the rates of greenhouse gas (GHG) emissions, individually and together, might change with time after the adoption of no-till (NT) farming. Arbitrary rates are expressed as the increase (or decrease) over those in the preceding tilled system. A value less than zero denotes reduced emission (or enhanced removal) compared to the tilled system. The actual rates, as well as the pattern over time, will vary among sites.

so carefully sequestered, perhaps accentuating CO2 emissions in future decades (e.g. Knorr et al., 2005)?

These examples of challenges, and doubtless many others, demonstrate the need to develop new ways of studying and understanding our ecosystems. We will need to study them as systems, with complex interactions within their borders, and many ties to those outside. Such efforts, likely aided by building better simulation models, will help us not only reduce GHG emissions from farms, but also preserve and augment other ecosystem functions that are no less important.

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