Agriculture Reference
In-Depth Information
Comparative empirical studies on CH
4
emissions in different farming systems are scarce.
Flessa
et
al
. (2002) compared a conventional and an organic farm rearing beef cattle in southern
Germany and calculated that CH
4
emissions were about 25% higher on the conventional farm.
On the basis of the literature reviewed and expert knowledge, only the following conclu-
sions (based mainly on studies for milk production) can be drawn: as a result of lower stocking
densities, organic farming might have a lower CH
4
emission potential on a per hectare scale,
whereas per unit output, the CH
4
emission potential tends to be higher than in conventional
farming (Stolze
et
al
. 2000, Shepherd
et
al
. 2003). However, in the absence of extensive solid
data, no significant differences between the two farming systems with respect to CH
4
emis-
sions can be identified.
Ammonia
Ammonia does not contribute to the greenhouse effect, but causes acidification and eutrophi-
cation when redeposited to soils and water and can damage sensitive habitats (Roelofs and
Houdijk 1991). In agriculture, livestock production, in particular, accounts for the main part
of NH
3
emissions. Ammonia is produced when urea in urine and dung comes into contact
with the enzyme urease, which can be found in both manure and soil. Therefore, animal
housing, manure stores and the spreading of manures to land are major sources of NH
3
(Shepherd
et
al
. 2003). There has been a large amount of research into NH
3
emissions from
conventional animal production but only a few studies specifically on organic farms (Stock-
dale
et
al
. 2001).
Differences in dietary N intake and N excretion, housing system and period, manure storage
and spreading, and livestock density will affect the amount volatilised (Stolze
et
al
. 2000). Since
organic systems operate at a lower level of intensity, NH
3
losses may be lower too.
Emissions from housed animals are considered to be greater than those from grazed, as
urine is quickly absorbed into soils. In organic systems, maximum grazing is recommended,
which tends to reduce housing periods. Therefore, the potential for ammonia loss is likely to be
less, although this has not been tested (Stolze
et
al
. 2000). Straw-based systems also tend to
have lower emission rates than systems based on slurry; the latter are more common in con-
ventional farming (Pain
et
al
. 1998). Furthermore, much evidence suggests that ammonia
losses are greater from composted manures compared to those which are just stockpiled
(Shepherd
et
al
. 1999, Gibbs
et
al
. 2000).
Studies in Europe reviewed by Stolze
et
al
. (2000) suggest that organic farming tends to
bear a lower potential for NH
3
emissions than conventional farming systems. In a scenario of
complete conversion to organic farming, Köpke (2002) estimated a potential reduction of acid-
ification by more than 30%, caused mainly by reduced ammonia emissions. In contrast,
Unwin
et
al
. (1995) provides a risk assessment upon which NH
3
emissions will not necessarily
be lower in organic farming than in conventional. In the absence of direct measurements, one
may assume that the amount of NH
3
lost per unit of yield is unlikely to differ to that from con-
ventional systems, but that losses per unit area are likely to be less, due to lower livestock densi-
ties (Shepherd
et
al
. 2003).
Energy
The OECD (1997) proposed to use energy intensity and efficiency as appropriate indicators to
measure and evaluate energy use. The corresponding parameters are:
• energy consumption (per hectare and per output); and
• energy efficiency (input/output ratio).