Effect on N2O emissions

Assessing the influence of tillage systems on N2O emissions is not straightforward. Farming systems include a complex mix of tillage tools, timings and frequencies, combined with variations in fertilizer and residue management and crop type, all interacting with local climate, topography and soil type. Soil conditions in NT systems differ from those in tilled systems in several ways: SOC and microbial biomass tend to be concentrated near the surface because residues are not buried; bulk density and aggregation are often higher, affecting oxygen diffusion; and surface moisture may be higher, suppressing oxygen because higher microbial activity consumes oxygen and air-filled porosity is lower. Linn and Doran (1984) reported that counts of denitrifiers in the surface of NT soils were several-fold those in CT soils. These factors may favour gaseous nitrogen loss via denitrification.

The influence on N2O emissions of the higher soil water under NT likely depends on the range of WFPS typical for the location. If other factors are not limiting, N2O emissions from denitrification only begin to increase sharply at WFPS of 60% or higher. If, as is likely in the subhumid and semiarid regions, NT does not increase soil water content to this level, the impact on N2O emissions from denitrification may be negligible. Indeed nitrification may be the dominant source of N2O in these drier regions.

Tillage tends to favour decomposition and nitrification by disrupting soil aggregates and exposing physically protected SOC to decomposers. In addition, residues left on the surface in NT systems may be desiccated for prolonged periods in dry climates, slowing decomposition. In cool semiarid and subhumid regions, therefore, soil CO2 emissions are often lower in NT than in tilled systems (Cochran et al., 1997; Lupwayi et al., 1999, 2004; Curtin et al., 2000), indicating slower decomposition and, presumably, nitrification. Lower nitrification rates may partly explain the similar or lower N2O emissions reported on NT compared to tilled soils in cool subhumid and semiarid regions (Cochran et al., 1997; Lemke et al., 1999; Malhi et al., 2006). Conversely, where rainfall is higher, N2O emissions in NT often exceed those in tilled systems (Linn and Doran, 1984; MacKenzie et al., 1998; Ball et al., 1999; Grageda-Cabrera et al., 2004), most likely due to increased denitrification. However, this relationship is not always consistent; in high rainfall areas N2O emissions in NT can also be similar to, or lower than, those in tilled soils (Kaharabata et al., 2003; Helgason et al., 2005). Venterea et al. (2005) observed an interaction between fertilizer N placement and tillage. N2O emissions were lower on NT than on CT when nitrogen was injected as anhydrous ammonia, higher on NT when urea was surface-broadcast and similar between tillage treatments when liquid urea ammonium nitrate was surface-applied.

According to selected examples from the literature, N2O emissions from NT systems can range from 20% lower (ratio of NT/CT = 0.8) to 600% higher than their tilled counterparts (Table 5.1). The influence on N2O can significantly offset the benefits of NT on soil carbon storage, or appreciably augment these benefits, depending on location.

Table 5.1. The influence of tillage on N2O emissions in various studies, selected to illustrate diverse environmental conditions. Also shown is the annual change in soil carbon storage required to offset the increased N2O emissions from adoption of no-till (NT). In some cases, where NT reduces N2O emissions, the effects on carbon sequestration and on N2O emissions are additive, rather than offsetting.

Table 5.1. The influence of tillage on N2O emissions in various studies, selected to illustrate diverse environmental conditions. Also shown is the annual change in soil carbon storage required to offset the increased N2O emissions from adoption of no-till (NT). In some cases, where NT reduces N2O emissions, the effects on carbon sequestration and on N2O emissions are additive, rather than offsetting.

Annual/

Nitrogen

seasonal

A SOC required

applied

MAT

MAP

Ratio

N2O loss

to offset A N2O

Location

(kg N/ha)

(┬░C)

(mm)

NT/Till

(kg N/ha)

(kg C/ha/year)

Delta Junction,

Alaska, USAa

90

-1.9

303

~1.0

na

na

Ellerslie, Alberta,

Canadab

56

3.5

450

0.7

1.4-2.1

(50-80)j

Sidney, Nebraska,

USAc

0

8.2

41 1

0.8

na

na

Piketon, Ohio,

USAd

180

4.4

400

0.8

0.9-3.7

(20-90)

Ormstown, Quebec,

Canadae

0 and 180

6.4

949

1.2

3.4-4.2

80-90

Celaya, Guanajuato,

Mexico'

1 80

18

650

2.5-4.0

0.7-19.8

50-1880

Turitea, New Zealand®

68

13

1305

1.3i

9.2-12.0

2 70-3 60

Oxford, Englandh

70 and 140

10.4

642

2.1-6.0

0.5-8.6

330-91 0

aCochran et al, 1997;

bLemke et al., 1999; cKessavalou et al., 1 998; dJacinthe and Dick, 1997; eMacKenzie et al., 1998; fGrageda-Cabrera et al., 2004; gChoudhary et al., 2002; hBurford et al., 1981.

Difference between tillage systems not significant. 'Numbers in brackets indicate negative values.

aCochran et al, 1997;

bLemke et al., 1999; cKessavalou et al., 1 998; dJacinthe and Dick, 1997; eMacKenzie et al., 1998; fGrageda-Cabrera et al., 2004; gChoudhary et al., 2002; hBurford et al., 1981.

Difference between tillage systems not significant. 'Numbers in brackets indicate negative values.

The radiative forcing from a kilogram of N2O, over a 100-year period, is about 296 times that of a kilogram of CO2 (IPCC, 2001). Adjusting for this greater 'global warming potential' (GWP) and the molar proportions of nitrogen and carbon in N2O and CO2, the change in N2O emissions from adopting NT can be expressed as equivalent kg C/ha/year (Table 5.1). This provides an indication of how much soil carbon would need to be sequestered to offset an increase in N2O emissions. The values calculated in Table 5.1 frequently match, or even greatly surpass, the rate of soil carbon gain typically reported after adoption of NT practices. For example, at Turitea, New Zealand (Table 5.1), a carbon sequestration rate of ~0.3 t C/ha would be required to offset the increased N2O emissions from adoption of NT. In some locations, however, the effect on N2O amplifies the benefits on carbon sequestration (e.g. Ellerslie, Canada).

The influence of tillage on N2O may also depend on other concurrent practices. On the semiarid North American Great Plains, for example, summer fallow is often used to avert risk of drought. During the fallow year, weeds are controlled but no crop is grown, allowing a 21-month period for soil water recharge before the next crop is planted. Fallow may be included in rotations once every 2-4 years. Adopting NT, which improves moisture conservation, has allowed farmers to reduce fallow frequency, or to eliminate fallow entirely. How this affects N2O emissions is unclear. Fertilizer N requirements are usually higher under reduced fallow frequency, resulting in higher N2O emissions, but eliminating plant growth during the fallow phase increases soil moisture, favouring denitrification, both from the higher water content itself and by increasing soil NO- from enhanced

Arid

Humid

Climate

Fig. 5.3. A conceptual graph, illustrating a possible relationship between climate and the ratio of N2O emissions under no-till (NT) to that under conventional tillage (CT) (N2Ont/N2Oct). This relationship is likely oversimplified, but may point to a useful working hypothesis.

mineralization. In western Canada, for example, annual N2O emissions from fallow plots were as high or higher than those from fertilized cereal plots (Lemke et al., 1999).

How will N2O emissions from soil change if NT practices are adopted? It may depend on region. In cool subhumid to semiarid regions, similar to those found in western Canada, direct soil-emitted N2O emissions in NT are likely to be similar to, or less than, those in CT systems (Fig. 5.3). In more humid regions, conversely, emissions under NT may be similar to, or higher than, those under CT. Reversals have, however, been reported (e.g. Helgason et al., 2005), and these conclusions remain tentative. Results to date emphasize that N2O merits concerted attention in assessing the potential of NT practices for reducing GHG emissions. While the benefits of reduced tillage for soil carbon sequestration are important, they may not always override effects of tillage on N2O emissions.

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