As explained in the previous section, the ability of biofuels to reduce fossil energy use and GHG emissions rests on the additional absorption of solar energy and carbon dioxide from the atmosphere by growing plants. These benefits are offset to some extent by the use of fossil energy in producing and processing the crop, and by land use change emissions. Although not a major factor in energy terms in the whole pathway, farming is a major source of GHG emissions associated with biofuels.
CO2 emissions associated with farm equipment use and manufacture of fertilizers and chemicals are not the only GHG emissions to be considered. Significant quantities of another greenhouse gas, nitrous oxide (N2O), are produced from nitrogen fertilizer production and emissions of N2O from the field.
Although N2O emissions are not very large in absolute terms, the very high greenhouse effect of this gas (about 300 times as much as CO2 on a mass basis) makes them very significant. In particular, the huge uncertainty in estimates of GHG emissions from soils dominates the errors in the final GHG balances of biofuels pathways. Measured N2O emissions for individual fields vary by at least three orders of magnitude, depending on soil characteristics, climate, tillage, fertilizer rates and crop (in approximate descending order of importance). In Europe, emissions generally show much greater local variation than in America, due to the heterogeneity of soils and drainage. Therefore it is worthwhile putting a large effort into improving the accuracy of the soils-emissions estimates.
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3.4.2.2 Sources of N
2O emissions
Emissions of N2O resulting from anthropogenic nitrogen inputs to agricultural soils occur through both:
o a direct pathway
• i.e. directly from the soils to which the nitrogen is added/released, and:
o two indirect pathways9:
(i) following volatilisation of NH3 and NOx from managed soils and the subsequent re-deposition of these gases and their products NH4+ and NO3- to soils and waters; and (ii) after leaching and runoff of nitrogen, mainly as NO3-.
Where they have considered them at all, other biofuels studies have adopted two approaches to estimating nitrous oxide emissions from soils. One is to extrapolate from measurements on individual fields; the other is to use the “tier 1” estimates under the IPCC guidelines. These are designed to estimate national greenhouse gas emission inventories, not emissions for particular crops or fields.
The revised tier 1 method guidelines in [IPCC 2006 (1)] assume N2O emissions from managed fields are a constant fraction of the nitrogen applied (as synthetic fertilizer, manure, crop residues or from nitrogen-fixing crops). The fraction is called an “emission factor”. Separate emission factors are used for “direct” emissions from the soil and for “indirect” emissions from nitrogen leached off the field.
To account for variables other than N input, IPCC tier 1 specifies a wide error range with max/min ratio varying from 10 (for direct emissions) to 25 (for indirect emissions). The IPCC emission factors were designed to help countries report national greenhouse gas inventories, not to predict emissions from individual fields. Thus the error ranges can represent the uncertainty in national average emissions from a crop, but the uncertainty in emissions from smaller regions or individual fields is very much higher. The reason for that is that IPCC tier 1 approach assumes that the main local determinants of N2O emissions, predominantly soil/drainage properties, tend to average out on the national scale.
3.4.2.3 Methodology
Soil N2O field measurements are expensive and are not generally available for a particular crops and locations. In addition N2O emissions vary by orders of magnitude over quite short distances, and also vary significantly between years.
[IPCC 2006] suggests that where suitable input data is available, national emission inventories can be based not on tier 1 but on the more sophisticated ‘tier 2’ and ‘tier 3’ approaches.
o tier 2 is the same approach as tier 1 except that the equation linking N2O emissions to nitrogen inputs is replaced by a more sophisticated one where other, localized, parameters (soils, drainage, climate, crops) are also taken into account. Both equations are based on statistical fitting of measured N2O data to reported parameters.
o tier 3 foresees the use of experimentally-validated soils chemistry models, or direct measurements, to calculate N2O on the basis of local parameters.
IPCC tier 1 and tier 2 both start with an equation showing a statistical regression of all (at the time) known N2O measurements against the known controlling parameters of soils/drainage, climate, management, crop and nitrogen inputs from all sources. In order to estimate the tier 1 emission factor for N inputs, [Bouwman 2002] applied this equation to global GIS data in order to estimate N2O emissions from global crops. For tier 1 this fit was applied to estimate world N2O emissions from crops on a 0.5 degree global grid; then the total emissions are divided by the total applied nitrogen to give an estimate of the average emission factor for all crops and locations. A tier 2 approach is similar, except that more results would be averaged for locations in a particular country. In our calculation, the results are averaged for particular crops.
In version 3 of this study we applied a tier 3 approach, estimating N2O using a soils chemistry model applied to the extensive local data available for sites growing particular crops in the EU15 in the
9These “indirect” nitrous oxide emissions should not to be confused with nitrous oxide emissions resulting from food production displaced by biofuels.
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LUCAS land-cover survey. Unfortunately, the same approach was impossible for crops grown outside EU because there was not enough input data available; in these cases we were obliged to fall back on IPCC tier 1.
In this version we use an IPCC “tier 2” approach, because it requires less detailed input data than tier 3, and could thus be applied equally to crops grown both inside and outside EU. Furthermore, although a tier 3 soils model should give a more accurate prediction of emissions on a particular field if all the input data is known, we fear that inaccuracies in the soils chemistry model may lead to systematic errors in the average of emissions for a particular crop. This is avoided in our new approach.
The new tier 2 methodology used in this version was developed by the Climate Change Unit of JRC’s Institute for Environment and Sustainability (IES), and called 'Global crop and site specific Nitrous Oxide emission Calculator (GNOC)'. It is being developed as on-line tool which will enable estimates to be made of N2O emissions for any crop in any place in the world, using estimated or specified input parameters.
Example output from the model is shown in Figure 3.4.2-1. The mean line is close to the IPCC generic figure, but takes account of fertiliser input rates. The GNOC models the effect of soil organic carbon and pH, and the impact can be seen by the minimum and maximum cases. Even with this improved discrimination, there remains a large uncertainty in the estimates.
Figure 3.4.2-1: Nitrous oxide emissions from the GNOC calculator for temperate climates
0.00 0.01 0.02 0.03 0.04 0.05 0.06
1 51 101 151 201 251 301 351 401 451 501
N input kg ha-1 Fertilizer induced emissions (kg N2O-N Emissions / kg Fertilizer N input)
Agricultural Fields: Minimum case for Cereals in Temperate Oceanic Climate (SOC
<1% ; pH >7.3; medium soil texture)
Agricultural Fields: Mean case for Cereals in Temperate Oceanic Climate (SOC 1-3% ; pH 5.5-7.3; coarse soil texture)
Agricultural Fields: Maximum case for Cereals in Temperate Oceanic Climate (SOC
>3% ; pH <5.5; fine soil texture)
IPCC (2006) factor for direct N2O emissions from fertilizer input
Soils emit some N2O even if they are not farmed (so-called “background emissions”). These can be quite significant, especially for organic soils. In previous versions of this study we have subtracted N2O emissions for a reference case of unfertilised grassland to obtain the N2O emissions directly attributable to biofuel production. With the new GNOC modelling tool used in this version a reference case is no longer needed, since the emissions from an unfertilised control plot are subtracted internal to the model.
The significance of field N2O emissions in the overall WTT calculation is illustrated by these example figures for the main EU crops considered in the study and shown in Table 3.4.2-1. Note that the total WTT GHG emissions may be less than the cultivation emissions after by-product credits are taken into account.
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Table 3.4.2-1: N2O contribution to GHG emissions for the main biofuels crops