Agricultural production generates sizeable methane and nitrous oxide emissions. Nitrous oxide is not only a potent greenhouse gas, but also contributes to the depletion of the stratospheric ozone layer.
Essentially, greenhouse gas emissions are calculated by multiplying an activity data (e.g. the excreted manure nitrogen of a particular animal category) by an emission factor that defines the amount of greenhouse gas released into the atmosphere per amount of activity.
In order to compare the climate effect of different greenhouse gases, further factors besides the quantities emitted must be taken into consideration. Different greenhouse gases absorb radiation in different ranges of the wavelength spectrum. That is why, for example, a tonne of methane in the atmosphere has a greater thermal effect than a tonne of carbon dioxide. In addition, the lifetime of the greenhouse gas in the atmosphere must be taken into account. The calculated methane and nitrous oxide emissions are thus ultimately converted into so-called CO2-equivalents to facilitate comparison of the different greenhouse gases. Here, a time horizon for the climatic effects of 100 years is chosen as standard.
Greenhouse gas emissions are calculated according to the framework methodologies and system boundaries of the IPCC (IPCC 2006). The respective system boundaries comprise primarily all emissions originating (directly or indirectly) from agricultural activities on the farms, excluding energetic emissions caused e.g. by tractor diesel consumption or the heating of greenhouses. The IPCC assigns these later emissions to the energy sector that accounts for energetic emissions arising from the manufacturing of agricultural inputs such as synthetic fertilisers. Animals which are absent (e.g. on alpine pastures) cause no emissions at farm level. Changes in carbon stocks in agricultural soils are not taken into account, owing to lack of data.
Below, we will briefly represent the gases ammonia (NH3), methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2).
While not a greenhouse gas itself, ammonia can be converted into nitrous oxide. A slightly modified version of the Agrammon model is used as a basis for calculating nitrogen and farmyard-manure flow; for more information please consult the ‘Agri-Environmental Indicator Ammonia Emissions’.
Methane is emitted both during the animals’ digestive process and during the storage of farmyard manures (slurry and dung). Emissions depend primarily on the quantity of feed consumed and on the manner in which the manures are stored. The more concentrate a dairy cow eats, the higher its milk yield, but also its methane emissions. For dairy cows, the calculation of methane emissions therefore depends on the milk yield. Given the absence of reliable details on farm-specific productivity and feeding strategies for the remaining livestock categories, a standard feed consumption is assumed in each case.
The amount of fermentable substrate excreted by the animals can also be derived from the amount of feed ingested. Methane is mainly produced when these substrates are stored in liquid form (e.g. slurry).
In addition to producing ammonia and methane, farmyard-manure storage also gives rise to nitrous oxide. Furthermore, nitrous oxide is formed during the cultivation of agricultural soils, due to the spreading of nitrogen fertilisers and soil nitrogen turnover. Particularly in drained peatland soils (over 30% humus content), mineralisation of soil organic matter releases nitrogen and emits nitrous oxide. Grazing livestock also contribute to nitrous oxide emissions via their excrements. Generally, cattle, pig and poultry emissions are higher than those of sheep and other animals.
In addition to direct nitrous oxide emissions there are also indirect emissions. These are generated when nitrogen compounds are translocated by air or via leaching and then lead to increased nitrous oxide emissions in soils or in surface waters. The amount lost to the atmosphere in the form of ammonia is calculated using the Agrammon model. In addition to ammonia, the emission and deposition of nitric oxides from farmyard-manure storage and from soil nitrogen turnover are estimated. With regard to leaching, it is assumed that around 22% of the total nitrogen input to agricultural soils is lost into water. 0.75% of the respective nitrogen is released into the atmosphere in the form of nitrous oxide.
In addition to methane and nitrous oxide emissions, carbon dioxide emissions from the application of urea and lime fertilisers (fertiliser lime, dolomite lime) are calculated.
