Chemistry Reference
In-Depth Information
Tange in Denmark and emission from two sites in Central Europe (Tange and
Langebrugge), where Tange is located about 400 km North of Langebrugge in
Germany, and the Tange station hence has a later peak in the emission from spring
application directly into growing crops. In general, then the environmental conditions
during application time have a large effect on both the temporal pattern and the total
amount of emission from the application. A good example is the effect of precipita-
tion: Precipitation before application limits soil infiltration and therefore enhances
emission, while precipitation after application can enhance infiltration and reduce the
emission [
52
]. The latter, however, also increases the risk run-off to nearby river
systems. This clearly shows how management taking into account environmental
variables (e.g. day-night temperatures) can modify emissions. Despite these known
effects on environmental variables, the mathematical description accounting for these
governing effects still remains to be developed for regional scale application [
53
,
54
].
2.3 Emissions from Grazing Animals
The emission from grazing animals depends on time spend in the field but also on
the N content in the grass. As a result, animals feeding from grass with high N
contents excrete large amounts of excessive N as TAN compared with animals on
grassland with less N-rich grass [
19
]. Furthermore, urine from grazing animals
more easily enters the soil compared with surface-applied slurry [
55
]. This reduces
the emission from grazing animals compared with stable-based systems. The main
reason for this difference is that the larger amount of dry matter content of slurries
limits infiltration into the soil and thus is allowing for enhanced evaporation of the
slurry compared with urine from grazing animals. In Southern Europe, animals are
in the field most of the year. Sheep may also stay in the field most of the time,
whereas dairy cattle in many countries are inside stables approximately half of the
year (see Fig. 2 in Skjøth et al. [
6
]). The number of grazing animals follows in
general the availability of grass, or in other words the season of growth. In
principle, the ambient air temperature should increase NH
3
emission related to
grazing animals and outdoor yards used by livestock. This was supported by studies
by Ryden et al. [
56
] who found increased NH
3
emissions from the urine fraction
during summer compared to autumn. However, clear ambient air temperature
responses have not always been seen in practice [
57
]. This suggests that some of
the main processes governing NH
3
emissions from grazing animals have been
identified, but several knowledge gabs still exist. The latter include developing a
full dynamic description of emission from gazing animals that go beyond the farm
scale. Such descriptions would be useful in policy and scenario studies on the
international level in relation to air quality. Due to this, there is still significant
uncertainty regarding the effect of environmental variables on the NH
3
emission
from grazing animals.
