Agriculture Reference
In-Depth Information
sequester additional carbon in cropland management and in agroforestry and they are systemati-
cally applied and improved in organic farming. The evidence of their performance is still frag-
mentary, however, and does not allow regular comparison and quantification for the various
agroclimatic regimes and socioeconomic patterns (Kotschi and Müller-Sämann 2004).
Burdick (1994) concluded that organic farming enables ecosystems to better adjust to the
effects of climate change and it also offers a major potential to reduce the emissions of agricul-
tural greenhouse gases. Moreover, mixed farming and the diversity of organic crop rotations
are protecting the fragile soil surface and may even counteract climate change by restoring the
organic matter content (Haas and Köpke 1994). The carbon sink idea of the Kyoto protocol
may therefore be accomplished efficiently by farming organically (Alföldi
et
al
. 2002).
Nitrous oxide
Nitrous oxide contributes severely to global warming and the depletion of ozone in the strato-
sphere (Crutzen 1981, Bouwman 1996). Almost 90% of the global atmospheric N
2
O is formed
during the microbial transformation of nitrate (NO
3
-
) and ammonia (NH
4
+
) in soils and
water. In OECD countries the agricultural contribution to N
2
O emissions is estimated at 58%
(IPCC 2001). Soils fertilised with inorganic fertilisers and manure stores are seen as the largest
sources (Chadwick
et
al
. 1999, Brown
et
al
. 2002).
Nitrous oxide emissions are very difficult to measure and, therefore, have been related to
the total N input in the form of fertilisers, manures and crop residues (Flessa
et
al
. 2002). Con-
sequently, it has been largely assumed that, because organic farming operate at a much lower
intensity, with lower N inputs and less available mineral N in both manures (Shepherd
et
al
.
1999) and soils, N
2
O losses will also be lower (Köpke and Haas 1994, Stolze
et
al
. 2000, Alföldi
et
al
. 2002). However, until recently there have been no quantitative comparisons between
organic and conventional systems. Within conventional agriculture, the main risks arise from
manures and from the waterlogging of soils by heavy rainfall following fertiliser application.
Within organic farming, the main risks come from manures and the incorporation of residues
from legumes. In the absence of direct measurement, one may assume that the amount of N
2
O
lost per unit of yield is unlikely to differ to that from conventional systems, but losses per unit
area may differ, depending on the cropping system and input of organic manures (Stolze
et
al
.
2000, Shepherd
et
al
. 2003).
Methane
Schönwiese (1995) calculates the CH
4
share in the greenhouse effect of about 2.5% and agri-
culture is believed to account for roughly two-thirds of the total human-generated CH
4
(Watson
et
al
. 1996). While paddy rice fields, cattle feedlots and the burning of biomass con-
tribute to methane emissions, about 75% of methane on farms is emitted directly from
ruminant animals, from digestive processes and excretion (Stolze
et
al
. 2000, Alföldi
et
al
.
2002, Shepherd
et
al
. 2003).
In order to assess the overall methane emissions from farming systems, several factors like
animal numbers and type, diet and manure management system need to be considered as well
as their interactions. Diets, that are high in roughage, for example, will release higher rates of
methane than diets high in starch. This may result in higher emissions from organic systems,
where diets tend to be high in roughage and low in concentrates (Shepherd
et
al
. 2003). The
higher proportion and lower productivity of ruminants in organic farming may lead to slightly
higher CH
4
emissions. But according to Alföldi
et
al
. (2002), standards and breeding programs
in organic systems aim at longevity in order to prolong the productive period in relation to the
'unproductive' life of young cattle. Correspondingly, the 'unproductive' CH
4
emission of calves
and heifers may be reduced (Sundrum and Geier 1996).
