Mineral Fertilizer Use and the Environment International Fertilizer Industry Association United Nations Environment Programme

21
8. Air
Ammonia can also react in the atmosphere with
sulphur oxides to form ammonium sulphate,
which precipitates into the soil with rainfall and
causes acidification.
Although most emissions of ammonia are
from manure or natural sources, experiments
demonstrate that nitrogen losses to the
atmosphere in the form of ammonia following the
application of urea can amount to 20% or more,
under temperate conditions. Losses occur when
the urea is not incorporated into the soil
immediately after spreading and they are
particularly high on calcareous soils. The
expanding practice of reduced tillage cultivation
is increasingly the surface application of urea.
Losses are even higher, up to 40% or more,
under tropical conditions, on flooded rice and on
perennial crops to which the urea is applied on
the surface, such as bananas, sugar cane, oil palm
and rubber.
8.2. Greenhouse gases
Carbon dioxide (CO
2
), methane (CH
4
) and nitrous
oxide (N
2
O) are the three most important
greenhouse gases. They absorb solar radiation
rather than allowing the heat to be radiated away
from the earth. Their impact as greenhouse gases,
or global warming potential (GWP), is a function
of two factors, their “radiative forcing” and on
their lifetime in the air. Taking the GWP of CO
2
as 1, that of CH
4
is 21 and that of N
2
0 is 310.
8.2.1. Carbon dioxide
Fixation of carbon dioxide by photosynthesis is
the source of organic carbon in crops and
eventually in soils. Crop production practices that
enhance photosynthetic activity improve the
retention of carbon. Decomposition of organic
Nitrogen can be lost from agricultural systems in
three forms which may cause pollution; nitrate
loss by leaching, ammonia volatilization and
nitrous oxide loss during denitrification or
nitrification. Ammonia loss to the atmosphere and
its subsequent deposition contributes to the
eutrophication of natural habitats and marine
waters and to the acidification of soils and lakes
as the NH
4
is converted to NO
3
. Losses by
denitrification are harmless if the end product is
nitrogen gas but if the resulting gas is nitrous
oxide there is a contribution to the greenhouse
effect and to depletion of ozone in the upper
atmosphere.