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It is very difficult to
measure how much a chemical fertilizer emits GHG to the atmosphere once
we use it in the field. But when we look at the life cycle of the
production and use of fertilizer we can take a decision as to which
fertilizer is eco-friendly and emits less GHG in the atmosphere.
The life cycle of fertilizer contributes significantly to the overall
impact of conventional agriculture. The production of fertilizers is
energy intensive, and emits about 1.2% of the world's total GHGs (Wood
and Cowie, 2004). Generally, fertilizers containing N compounds consume
up to 10 times more energy and consequently result in more GHG
emissions. In comparison, fresh manure is a very low carbon emitting
alternative when it is available to provide land with nutrients (Lal,
2004c). However, the actual energy consumed during the production can
vary widely as very modern plants have the potential to efficiently use
the heat produced during the reaction process and hence may even have a
negative energy balance, for production of nitrate will also generate
nitrous oxide as a by-product. Considering that nitrous oxide has a
global warming potential of ~296 compared to carbon dioxide, this is
the main GHG in the nitrate production (Brentrup et al., 2004) (Wood
and Cowie, 2004). As a result, nitrous oxide contributes 26% of the
global total fertilizer production GHG emissions (Kongshaug, 1998).
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Additional greenhouse gas
emissions will arise from the transport of these fertilizers as mines
are not evenly distributed around the world (www.fertilizer.org).
Generally, transport and storage will add more to the total GHG
emission of fertilizer use.
The final use of the fertilizer on the farm will have again a variety
of impacts. The machinery used to apply the fertilizer will require
fuel, adding to the GHG emissions. Fertilizers are not used fully by
the crop, which on average globally only recover about 50% of
fertilizer N supplied (Eickhout et al., 2006).
Consequently, a great proportion accumulates in soils and is either
lost directly as nitrous oxide, or leaches into water courses,
enhancing downstream, indirect nitrous oxide emissions. The amount lost
will greatly depend on many other factors such as climate, soil and
management practices (Brentrup et al., 2004) (Eickhout et al., 2006).
Cool
farming report, Greenpeace, January 2008 [R.E31d.40]
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