Optimizing fertilizer application with respect to nitrous oxide emission

Emissions of N2O from agricultural systems will not decrease to zero when no N fertilizer is applied, but will remain at some background level, as in any non-agricultural system. For example, when 383 non-manured, non-N-fertilized cropping systems were evaluated for N2O emissions, the average background emission was estimated at 0.55kg N2O ha-1 (Helgason et al, 2005). Natural N deposition and biological fixation, and N mineralization from soil organic

Table 5.2 Global estimates of manure-N excretion in animal houses and storage systems and excretion during grazing, spreading of stored manure in cropland and grassland, and N fertilizer use in cropland and grassland for the year 2000

Agricultural system

Tg yr 1

% of total

Animal manure management

Mixed and landless systems

72.6

65

Housing and storage

47.1

42

Grazing

25.5

23

Pastoral systems

28.8

26

Housing and storage

1.2

1

Grazing

27.6

25

Outside agricultural system

11.0

10

Total

112.4

100

Spreading of manure (excluding NH3 loss from animal houses and storage)

Cropland in mixed systems

32.4

84

Cropland in pastoral systems

0.9

2

Grassland in mixed systems

5.3

14

Grassland in pastoral systems

0.0

0

Total

38.7

100

N fertilizer use

Cropland

79.1

96

Grassland

3.4

4

Total

82.5

100

Biological N2 fixation

Cropland

21.9

Source: Data for year 2000 using the Integrated Model for the Assessment of Global Environment (IMAGE) model framework, data and settings (Bouwman et al, 2006a).

Source: Data for year 2000 using the Integrated Model for the Assessment of Global Environment (IMAGE) model framework, data and settings (Bouwman et al, 2006a).

matter, continue to drive the production of N2O via nitrification or denitrification. In natural ecosystems, N2O emissions are therefore commonly expressed on an area basis, not as a function of inputs.

Agricultural systems serve for the production of food, fibre, feed and biofuel crops. Production input factors, such as N fertilizers, are used to estimate N2O emissions which can then be converted into GWP (IPCC, 2006; Stehfest and Bouwman, 2006). The conclusion could be drawn that reducing fertilizer input would be an adequate means of reducing N2O emissions from agriculture. However, as agricultural activity will always remain essential for mankind to survive, a more revealing parameter is the N2O emission per output factor such as grain yield, total N production or protein production. Evaluating conventional and no-tillage systems, Mosier et al (2006) calculated net global warming potential and greenhouse gas intensities in irrigated maize

Figure 5.5 N2O emission from global arable land

Source: Computed with the model of Bouwman et al (2002b) for the year 2000

Table 5.3 Direct and indirect N2O emission from global arable systems for the standard 2000 situation, and for changing management in livestock and crop production systems

Case N2O-N emission (% change relative to standard)

Table 5.3 Direct and indirect N2O emission from global arable systems for the standard 2000 situation, and for changing management in livestock and crop production systems

Case N2O-N emission (% change relative to standard)

Direct

Indirect Gg yr-1

Total

Standard1

3582

442

4023

Reduce NH3 loss from stored manure2

3599 (0%)

452 (+2%)

4051

(+1%)

Reduce grazing3

3706 (+3%)

483 (+9%)

4189

(+4%)

Substitute fertilizer by manure4

3191 (-11%)

301 (-32%)

3492

(-13%)

Improved animal diets5

3426(-4%)

398 (-10%)

3825

(-5%)

Nitrification inhibitors6 -50%

Nitrification inhibitors6 -50%

Note:1 Standard is data for year 2000 using the model of Bouwman et al (2002b) and the Integrated Model for the Assessment of Global Environment (IMAGE) model framework, data and settings (Bouwman et al, 2006a).2 As standard, with reduction of NH3 loss from animal houses and storage by 20 per cent (from 20 to 16 per cent) leading to an increase of manure-N available for cropland of 33.3 to 34.9Tg yr1.3 As standard, with reduction by 20 per cent of the time spent by ruminants in the meadow. This leads to an increase in the amount of manure collected in animal houses and storage systems, and an increase of manure-N available for cropland from 33.3 to 40Tg yr1.4As standard, with substitution of fertilizer-N by manure assuming effectiveness of manure of 60 per cent resulting in a reduction of N fertilizer use by 33 per cent (from 82 to 56Tg yr1).5 As standard, with improved animal diets by 20 per cent (20 per cent less excretion), leading to a reduction of the total animal manure excretion from 112.4 to 89.9Tg yr1, and manure-N application to arable land from 33.3 to 26.2Tg yr1.6The impact of nitrification inhibitors is assumed to be a reduction of 50 per cent. However, there may also be an effect of nitrification inhibitors on NH3 emission, crop growth and the surface N balance through N uptake, and leaching. We therefore do not present an estimate for the N2O emission.

cropping systems and introduced the concept of greenhouse gas intensity (GHGI). The GHGI is obtained by dividing GWP by grain yield. A positive GHGI value indicates a net source of CO2 equivalents per kg grain yield; a negative GHGI value indicates a net sink of greenhouse gas. Once emissions per unit of yield are known, adjustments to management practices can be made to optimize the balance between economic viability and environmental conservation, through selection of appropriate levels of input such as fertilizer-N.

The concept of linking GHG emissions to productivity can be expanded and link the amount of N2O emitted to a unit of production. In this context, the following three relationships are determining factors controlling N2O emission per unit yield for any given agricultural production system:

1 the amount of N fertilizer applied versus the amount of N2O emitted;

2 the amount of N fertilizer applied versus N accumulation in the crop which affects fertilizer use efficiency; and

3 the amount of N in the crop versus yield.

Using a meta-analysis approach, Van Groenigen et al (2010) related the amount of N2O emitted to the amount of N accumulated in the crop. The meta-analysis included 22 field experiments encompassing 188 individual observations that reported N2O emissions and N accumulation in annual crops and grassland. To allow for a comparison between the different crops in the various studies, N2O emissions were related to total N accumulation in the crop rather than crop yield. As there is a strong linear relationship between total crop N and grain yield (Cassman et al, 2003), N2O emissions expressed per kg crop N would strongly correlate to N2O emitted per unit of grain yield. Van Groenigen et al (2010) estimate that on average 16.7g N2O kg-1 crop N is emitted when no or low rates of fertilizer-N were applied. The lowest relative N2O emissions occurred, i.e. 6.1g N2O kg-1 crop N, when on average 233kg N ha-1 of fertilizer-N was applied. Emissions increased again to 21.4g N2O kg-1 crop N when the rate of fertilizer-N input increased to >300kg N ha-1. The higher emissions of N2O at zero N fertilizer input is caused by the combination of background N2O emissions (i.e. without any N fertilizer input) and reduced yield/total crop N when a deficiency of available N is limiting crop growth. Higher N2O emissions per kg crop N at high rates of N fertilizer input are expected because increased N fertilizer input leads to a diminishing return in yield and crop N per unit of N input, i.e. to a low fertilizer use efficiency (see below). Furthermore, rates of N fertilizer input exceeding crop N demand lead to a non-linear increase in the rates of N2O emissions (Bouwman et al, 2002b).

The decrease in N2O emissions per kg crop N when fertilizer-N input increased from zero or low levels to median levels has agronomic significance. Management practices could be developed toward maximizing yield produced with the lowest amount of N2O emitted per unit of yield. Based on the result of this meta-analysis, both low and high fertilizer-N input would not lead to low N2O emissions per unit of crop N produced. Median and near-optimum levels of fertilizer-N input in association with management practices that optimize the yield potential and increase fertilizer-N use efficiency would lead to a reduction in N2O emissions as expressed per unit of crop N accumulated or grain yield.

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