Gwp Analysis For Conventional And Notillage Maize Production In The United States

A comprehensive analysis of the effect of a change from CT to NT on the carbon dioxide flux for grain production in the United States has showed that emissions associated with machinery were reduced, but those associated with agricultural inputs increased (West and Marland, 2002). These changes produced a net savings of 31 kg C/ha/yr in addition to that achieved by carbon sequestration in soil (337 kg C/ha/year) for a total carbon benefit of 368 kg C/ha/year. However, the impact of the change from CT to NT on contributions to greenhouse gas build-up from N2O and CH4 were not considered. A more complete analysis is undertaken here for rainfed corn. Additional sources of GWP (Table 17.2) were associated with increased emissions of N2O from soil under NT and during manufacture of the additional 42.6 kg N/ha that is used in NT maize (West and Marland, 2002). Changes in GWP associated with emissions of N2O and CH4 from use of farm machinery and soil fluxes of CH4 were small and are not presented.

The N2O and CH4 emissions associated with increased fertilizer N input under NT were estimated for production,

Table 17.2 Effect of Tillage Practice on Relative GWP Sources from Rainfed Maize Production in United States kg C iv/Wy^

GWP Component Conventional Till No-Till

Soil C sequestration 0 -337a

Carbon dioxide emissions

* Agricultural inputs +156a +202a

* Machinery +72a +23a

Net C flux +228 -112

Relative C flux 0 -340

Relative N2O emission

Additional N input (42.6 kg/ha) a

* Manufacture 0 +11b

Switch to no-till 0 +219d

Total additional GWP flux 0 +294

Revised relative GWP flux 0 -46

a From West, T.O., and G. Marland. 2002. Agric. Ecosyst. Environ., 91:217-232; positive and negative signs indicate carbon emission and sink, respectively.

b Based on 22% of fertilizer as NH4NO3, and N2O release from Kramer, K.J., H.C. Moll, and S. Nonhebel. 1999. Agric. Ecosyst. Environ., 72:9-16.

c Using Intergovernmental Panel on Climate Change formula of 1.25% fertilizer N released as N2O from 90% of applied N.

d Average value from Smith, K.A., F. Conen, B.C. Ball, A. Leip, and S. Russo. 2002. Emissions of non-CO2 greenhouse gases from agricultural land, and the implications for carbon trading. In J. van-Ham, A.P.M. Baede, R. Guicherit, and J.G.F.M. Williams-Jacobese, Eds. Non-CO2 Greenhouse Gases: Scientific Understanding, Control Options and Policy Aspects. Proceedings of 3rd International Symposium, Maastricht, Netherlands, 21-23 January 2002. Millpress Science, Rotterdam, Netherlands.

Note: GWP = greenhouse warming potential.

transport, and use. Only N2O emission during N fertilizer production and use added significant amounts of GWP (Table 17.2). Nitrous oxide emission during fertilizer manufacture is associated with production of nitric acid that, in turn, is used to manufacture ammonium nitrate. Ammonium nitrate accounts for 22% of the fertilizer N consumption in

U.S. agriculture (Brady and Weil, 1999), and it was assumed that this proportion was used on maize. The value for N2O release during production of ammonium nitrate (4.68 g N2O/kg NH4NO3) was taken from Kramer et al. (1999). Use emission of N2O was based on the IPCC formula of 1.25% N2O release from 90% of added fertilizer N, which is derived from the work of Bouman (1996).

An increase in emissions of N2O from soils associated with the change from CT to NT was estimated by using the mean value of 1.65 kg N2O-N/ha/year from Smith et al. (2002), who summarized published information. This does not duplicate the N2O input from increased N use, as CT/NT comparisons were made at the same N input levels. All N2O emission values were converted to CO2 equivalents using a factor of 310 (IPCC, 1996) and then to CO2-C.

When all three greenhouse gases (CO2, N2O, and CH4) were included in the analysis, the net annual GWP benefit associated with the change from CT to NT was 46 kg CO2-C/ha, compared to 340 kg CO2-C/ha if only soil carbon sequestration is considered.

The revised estimate of the GWP benefit created by a change from CT to NT is largely driven by the estimate of increased N2O emission from soil when NT is adopted. However, this value is quite uncertain due to the limited number of comparisons and the inadequate sampling methodologies used in most studies. Fluxes of N2O from soils are highly variable in space and time (Mosier and Hutchinson, 1981; Duxbury et al., 1982), which presents a challenge to anyone generating annual flux data using chamber measurements. Only one of the reported comparisons of N2O emissions from CT and NT practices used an automatic chamber system (Ball et al., 1999), which addresses temporal variability very well, and there are no reports using micrometeorological methods, which can address both temporal and spatial variability.

Nitrous oxide emissions from soil are episodic, and both short-term comparisons between CT and NT practices and estimates of annual N2O fluxes would be improved by a focus on events that lead to bursts of N2O emission. Such events are rainfall, freeze-thaw cycles, plowing, fertilizer application, and residue, and manure additions (Skiba et al., 2002; Yam-ulki and Jarvis, 2002; Davies et al., 2001; Van Bochove, 2000; Lemke et al., 1999; Mackenzie et al., 1997; Chen et al., 1995; Duxbury et al., 1982).

Most investigators recognize the variability issue but do not adjust sampling strategies, especially in terms of frequency, to provide confidence that annual flux values are accurate. Nevertheless, the majority of studies (Six et al., 2002; Skiba et al., 2002; Mackenzie et al., 1997; Hilton et al., 1994; Hutsch, 1991; Aulakh et al., 1984, Burford et al., 1981), but not all (Choudhary et al., 2002; Lemke et al., 1999; Jacinthe and Dick, 1997) find higher emissions of N2O under NT. There is also evidence to support the contention that denitrification, the primary source of N2O, is greater under NT (Linn and Doran, 1984; Rice and Smith, 1982; Staley et al., 1990).

It is also important to recognize that soil physical and biological properties are dynamic for many years following a shift from CT to NT. Soil physical conditions improve considerably with time, and the reestablishment of macro fauna, especially earthworms, leads to the development of macropore channels and much improved drainage. Organic matter and biological activity are concentrated close to the soil surface where gas exchange with the atmosphere is most rapid, promoting aeration of soils but also emission of N2O when it is being generated. It is probable that N2O emissions change over time after adoption of NT. Initially they are greater than for CT, as soils have poor structure and are poorly drained, but later they may become similar to or even less than CT as soil structure and drainage improve. Unfortunately, there has not been any systematic study of temporal effects of changing from CT to NT on N2O emissions from soils.

Soil compaction, which increases N2O emission from soils (Ball et al., 1999; Yamulki and Jarvis, 2002), is another important parameter to consider, and inadequate attention is given to this in experiments and also in current NT agriculture in the United States. No tillage, and hence carbon sequestration, is unlikely to be agronomically sustainable on many soils without the use of controlled traffic patterns to minimize compaction, and a mulch to prevent surface sealing by raindrops.

These practices are aimed at maintaining a desirable physical condition at the soil's surface, which would also be expected to reduce N2O generation and emission. Researchers also need to be cognizant of potential differences in soil physical condition between researchers' plots and farmers' fields, and hence in their N2O emissions. Overall, it is clear that much additional work is needed to determine the impact of any change from CT to NT on N2O emission from soils. Such information is critical for determining whether a change from CT to NT is, on balance, an effective means of reducing GWP.

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