Emission of Nitrous Oxide

During cultivation of crops, i.e. during tillage, planting, fertilising and growth as well as during harvest and storage, various climate-effective gases emit from soil and plants. These so-called greenhouse gases, particularly the Kyoto gases CO2, CH4 SF6, PFC, HFC and N2O, impair the ecological benefit of energy crops. In addition to carbon dioxide (CO2), the most harmful and most investigated gas of crop production is nitrous oxide (N2O), also called laughing gas. Though it is only responsible for about 20% of the total GHG emissions from worldwide agriculture (EPA 2006), it may amount to 70% of the GHG emissions of energy crop production (Neubarth and Kaltschmitt 2000; Heinze 2001). On the field alone it emits nearly 50% of the CO2 equivalents of all GHGs (Hartmann and Kaltschmitt 2002).

Nitrous oxide, a by-product of fixed nitrogen fertilisation, has a 100-year average global warming potential (GWP) about 300 times higher than an equal mass of CO2. As a source of NOx , i.e. NO and NO2, N2O also plays a major role in stratospheric ozone chemistry (Crutzen et al. 2007). In soil, N2O is produced predominantly by two microbial processes, the oxidation of ammonium (NH4+) to nitrate (NO3-) and the reduction of NO3- to gaseous forms NO, N2O and N2 (Firestone 1982). The rate of N2O production depends on the availability of mineral N in the soil and the conversion factor (CF) depends on soil type and climate (e.g. Bouwman 1990, 1996; Granli and B0ckman 1994; Bouwman et al. 2002; Novoa and Tejeda 2006; Stehfest and Bouwman 2006).

Agronomic practices such as tillage and fertiliser applications can significantly affect the production and consumption of N2O because of alterations in soil physical, chemical, and biochemical activities. Following N-fertiliser applications, an increase in N2O flux rates has been observed in field and laboratory experiments (e.g. Mulvaney et al. 1997; Kaiser et al. 1998; Jackson et al. 2003). N2O emission from croplands at site scales occurs essentially with great spatial and temporal variability (Veldkamp and Keller 1997; Dobbie and Smith 2003; Hellebrand et al. 2003, 2005). The annual pattern of temporal variation of N2O emissions is determined in the temperate regions by the seasons and weather conditions, since soil N2O emissions are regulated by temperature and soil moisture and so are likely to respond to climate changes (Frolking et al. 1998; Ruser et al. 2006).

Because of these influencing factors, the measuring results vary over a wide range. Moreover, there are several high emission periods with emission rates of more than 1,000 mg N2O m-2 h-1. These longer-lasting high N2O emissions, called "hot spots" or "hotspots" (e.g. Christensen et al. 1990; Rover et al. 1990; Hellebrand et al. 2005; Wanga et al. 2006), were detected at fertilised blocks only. Thus, they can cause a local measured emission factor of more than 10%. The reason for these N2O hot spot emissions is not clear up to now. High emissions after harvesting were observed several times and might be connected with soil distortions.

Reliable long-term measurements have been performed by Hellebrand et al. (2008). They measured the N2O emissions on different fertilised energy crop plots on a sandy soil over a period of 9 years and found differences not only between the various fertilisation levels, but also between the crop species. In spite of the yearly spread it can be summarised that SRCs cause less N2O than cereals and grass. So the N2O emissions rate on non-fertilised poplar and willow fields is only 17-26% of the rate on conventionally fertilised cereal fields (Fig. 5.5).

In literature the absolute emission rates are scarcely discussed, but instead mainly the N2O nitrogen CF. It is defined as N2O-N emission caused by fertilisation in relation to the nitrogen fertiliser applied. The emission period considered is 1 year and the fertiliser-caused emissions are obtained by taking the difference between fertilised and non-fertilised fields (Bouwman 1996). The Intergovernmental Panel on Climate Change (IPCC) recommends an average factor of 1.25% (De Klein et al. 2006) and Hellebrand et al. (2005) measured an average value of 0.8% ± 0.1%. On the other hand, Crutzen et al. (2007) calculate a factor of 3-5% N2O-N on the base of global N2O emissions and Feehan and Petersen (2004) calculate even 10% N caused by further conversion processes of the N fertiliser. If they are right, the latter values had a considerable impact on the GHG balance of energy crops. However, these values are not verified and are widely discussed in the scientific community. There is thus a strong need for further research in this field.

Nitrous oxide emissions are generally induced by fertiliser application. Emission strength varies with soil type, temperature and moisture and is substantially crop-specific. There is a considerable difference between woody species and cereals. While the IPCC general N2O emission value is set to 1.25% of the nitrogen applied, an average of 0.8-1.0% could be found from sandy soils.

Fig. 5.5 Crop-specific N2O-N emissions with and without hotspots for different N fertilising rates according to Hellebrand et al. (2008), continuously measured for up to 9 years on sandy soil in Germany. The additional N2O emissions caused by the "hotspots" are not insignificant. However, the reason for these irregular increases is not yet clear

Fig. 5.5 Crop-specific N2O-N emissions with and without hotspots for different N fertilising rates according to Hellebrand et al. (2008), continuously measured for up to 9 years on sandy soil in Germany. The additional N2O emissions caused by the "hotspots" are not insignificant. However, the reason for these irregular increases is not yet clear

5.8 Energy Yield

One of the most important environmental criteria for the production of energy crops is the energy yield per hectare. The energy yield is mainly dependent on the biomass yield and on the conversion technology, and hence on the fraction of crop used as energy source (Table 5.11).

With approximately 170-230 GJ ha-1 year-1, the highest energy yields in total are achieved by solid fuels (dry bales, chips, briquettes, pellets) produced from whole crop cereals such as wheat, rye and triticale. Only M. sinensis and maize have higher figures. However, the former bases on an uncertain biomass yield and the latter can be only wetly harvested and stored as silage (in central Europe). The energy yield of grass, poplar and willow is a little lower compared with cereals, which in the case of SRC is partly caused by the high moisture content of >50%.

Liquid fuels have the lowest figures. With less than 40 GJ ha-1 year-1, the rape seed oil, which is similar to rapeseed methyl ester (RME, FAME), has lower figures than ethanol from several grain species. The highest energy yield of all liquid biofuels produced in Germany is obtained from sugar beets at more than 130 GJ ha-1 year-1, which is even better than BtL fuels produced from whole crop cereals. However, the input of energy for the production (conversion) of the ethanol is also very high and ranges between 15 and 25 MJ L-1, respectively 75-140 GJ ha-1 year-1 (Schmitz 2003; Quirin et al. 2004).

The energy yield of biogas produced from grains, beets or whole haulm-type crops lies mostly between the corresponding solid and liquid fuels, which predestines it for stationary (heat and) power generation (CHP).

Table 5.11 Average energy yields of different fuel types produced from energy crops

Biomass

Solid fuelsb

Liquid fuels0

Gaseous fuelsd

yield3

Energy

Energy

Energy

Crop species and used part

(ton ha-1 y-

') (GJ ha-1 year1)

cp aw1)

(GJ ha-1 year1)

CF (m3 tDM_1)

(GJ ha-1 year1)

Oil use Rape

Grain

3.0

-

380

39

-

-

Sunflower

Grain

2.2

-

440

33

-

-

Sugar/starch use Wheat

Grain

6.4

106

450

61

720

91

Winter rye

Grain

4.6

77

480

48

720

65

Winter triticale

Grain

4.9

81

460

47

720

69

Maize

Grain

7.6

126

450

73

-

-

Potato

Tuber

8.9

-

370

70

780

137

Sugar beet

Tuber

13.5

-

470

134

760

202

Ligno-cellulose use Wheat

Whole crop

14.0

233

2208

1028

600

165

Winter rye

Whole crop

9.8

169

2208

728

700

135

Winter triticale

Whole crop

10.5

174

2208

778

700

141

Maize

Whole crop

17.5

-

2208

1288

620

214

Perennial rye

Whole crop

8.5

146

2208

628

700

117

Grass

Whole crop

9.0

144

2208

668

640

113

Miscanthus

Whole crop

15.0®

258«

2208

1118

5008

1478

Poplar

Whole crop

10.0g

155®

2208

748

-

-

Willow

Whole crop

7.08

111«

2208

528

-

-

aMean yields in Germany according to Table 5.3

bBales, chips, pellets, or grain with moisture contents and heating values as in Table 5.3 (CF = 1.0) cStraight oils (37.2 MJ kg"1. 0.91 kg l"1), ethanol (26.8 MJ kg-1. 0.79 kg H) or BtL (43.9 MJ kg-1. 0.76 kg H) dBiogas (55% CH4, 19.7 MJ nr3, 1.28 g H)

eCF without losses according to Reinhardt (1993). Meierhofer (2006). Schmitz (2003) and Keppeler et al. (2004) 'Biogas yields according to recommended mean values of KTBL (2005b) 8 No practically verified figures

However, the area-related energy yield is not the sole criteria for evaluating the energy efficiency of a crop species and/or a fuel type. The energy inputs (CED) of cultivation and conversion processes (see Table 5.7), the DM losses, the energetic use of by-products and the further ways of utilisation must also be considered.

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