Agriculture Reference
In-Depth Information
contribute to the greenhouse effect and are largely, directly or indirectly, as a result of the
burning of non-renewable resources. On a global scale, agriculture is responsible for roughly
15% of the trace gas emissions with climatic impact (Burdick 1994, Cole
et
al
. 1997, Stolze
et
al
.
2000). However, agriculture also provides a sink for CO
2
because of the fixation of carbon by
crops and pasture.
An important side-effect of global warming could be that cultivation zones will shift pole-
wards and that agricultural yields will be affected. Reilly
et
al
. (1996) and others expect that
extreme climatic events will occur more frequently and that this will jeopardise plant
production.
Carbon dioxide
Carbon dioxide emissions from the agricultural sector in OECD countries are estimated at less
than 1% of overall CO
2
emissions (IPCC 2001). Net emissions of CO
2
from agriculture depend
upon the direct and indirect use of fossil fuels, and on the amount of carbon sequestration in
soil organic matter and crop growth (Shepherd
et
al
. 2003).
In order to compare farming systems CO
2
emissions need to be differentiated between the
emission due to the burning of fuel (direct energy) and the fuel used for the production and
transport of fertilisers, machinery and synthetic pesticides (indirect energy). Haas
et
al
. (1995)
found that 70% of CO
2
in organic farming resulted from fuel consumption and the production
of machinery, whereas 75% of the CO
2
emissions in conventional systems were due to N ferti-
lisers, feedstuff and fuels.
On a per hectare scale, most studies found lower (up to 40-60%) CO
2
emissions in organic
systems (Burdick 1994, Haas and Köpke 1994, Stolze
et
al
. 2000). The main reasons for these
positive effects are the renouncement of the use of mineral N fertilisers with high energy con-
sumption, lower use of high energy consuming feedstuffs and mineral fertilisers as well as the
elimination of pesticides. But, on a per unit output scale, which mainly depends on the yield
that is achieved, CO
2
emissions tend to be higher in organic farming.
Carbon sequestration
According to Tilman (1998), soil carbon levels have decreased under agricultural land use.
Therefore, sustainable agricultural strategies including recycling of organic matter, tightening
nutrient cycles, and low-tillage or no-tillage practices may rebuild organic matter and reduce
losses from the system. Haas and Köpke (1994) calculated that, despite generally lower crop
yields, plant productivity in organic farming accounts for almost the same organic matter
return as in conventional systems. Drinkwater
et
al
. (1998) found over a 15 year period that
mixed farming with manure application and the combination with other organic farming
techniques lead to significantly higher organic matter levels in soil as compared to conven-
tional farming.
There is a huge potential for CO
2
sequestration, which differs for tropical and temperate
countries. Developing countries, mostly in the tropical belt, have a 30-60% higher potential
than industrial countries, mainly located in temperate climatic regions. Agroforestry holds the
highest potential of agricultural carbon sequestration in tropical countries and is seen as a viable
alternative to slash-and-burn agriculture in the humid tropics (Kotschi and Müller-Sämann
2004). Simple agroforestry systems with one species (e.g. oil palm, rubber) can regain 35% of the
original carbon stock of the forest, which is three times more than cropland and pastures (Palm
et
al
. 2000). Extensive research on the situation and on the potential impact of agroforestry has
been undertaken by ICRAF for East and Southern Africa. Carbon stocks can be tripled over 25
years, similar results can be assumed for subhumid West Africa and subhumid South America,
but there is little researched evidence (FAO 2001). Sound technologies have been developed to
