Agricultural soils represent a very large, and growing, global
source of nitrous oxide. Current estimates for annual emissions
from this source range from 2 to about 4 million tonnes of nitrous
oxide-N globally. With a rapid increase in population growth,
and the consequent need for more food production, both the area
of agricultural soils and the intensity of their use is likely
to continue to rise rapidly in coming decades.
major direct source of nitrous oxide from agricultural soils is
that of synthetic fertilizer use. Widespread increase in the
use of such nitrogen based fertilizers has been driven by the
need for greater crop yields, and by more intensive farming practices.
Where large applications of fertilizer are combined
with soil conditions favorable to denitrification, large amounts
of nitrous oxide can be produced and emitted to the atmosphere.
Similarly, the widespread and often poorly controlled
use of animal waste as fertilizer can lead to substantial emissions
of nitrous oxide from agricultural soils. Some additional nitrous
oxide is thought to arise in agricultural soils through the process
of nitrogen fixation, though the true importance of this source
remains poorly defined.
Man's need for more food, as a result of an expanding global population,
has inevitably led to an increase in the use of both synthetic
fertilizer and the wider application of animal waste on agricultural
soils. However, the application of such nitrogen based fertilizers
in many areas has been excessive, with large proportions
of the added fertilizer providing no benefit to crop yield, but
inducing elevated nitrous oxide emissions.
Potential for control
The better targeting of fertilizer applications, both
in space and time, can significantly reduce nitrous oxide emissions
from agricultural soils. Land-management strategies which accurately
take account of the optimum amounts of fertilizer addition necessary
for maximum crop yield and minimum waste are crucial both environmentally
and economically. Similarly, the exact form of nitrogen based
fertilizer and the best time of year at which to use them is key
information on which to base fertilization campaigns
agricultural sources of nitrous oxide remain poorly defined in
most cases. There are several ways in which such indirect emissions
occur. The most important of these is nitrous oxide emission arising
from nitrogen leaching and run-off from agricultural soils.
After fertilizer application or heavy rain, large
amounts of nitrogen may leach from the soil into drainage ditches,
streams, rivers and eventually estuaries. Some of the nitrous
oxide produced in agricultural soils is lost in exactly this way,
being emitted to the atmosphere as soon as the the drainage water
is exposed to the air.
Still more nitrous oxide is produced from such drainage
waters when the leached nitrogen fertilizer they contain undergoes
the processes of nitrification or denitrification in aquatic and
estuarine sediments. Other important indirect nitrous oxide sources
from agricultural soils include the volatilization and subsequent
deposition of ammonia from fertilizer
application, and the consumption of crops followed by sewage treatment.
As with direct nitrous oxide emission from agricultural soils,
man takes full responsibility for indirect emission. Not only
do large quantities of leached nitrogen based fertilizer have
a significant impact on indirect nitrous oxide emissions, they
have also led to dangerously high nitrate concentrations in drinking
water and to eutrophication in rivers and estuaries around the
world. Increased food consumption and consequent increases in
municipal sewage treatment have also inevitably led to increased
indirect nitrous oxide emissions from this source.
Potential for control
Again, it is through properly informed land-management practice
and fertilization campaigns that nitrous oxide emissions can primarily
be reduced. Much of the impetus for control of nitrogen based
fertilizers has come from concern over high nitrate levels in
drinking water supplies and the threat of eutrophication in estuaries
and coastal waters. Individual governments have enacted changes
in policy to bring about reductions in such nitrogen leaching,
with the creation of 'Nitrate Sensitive Zones' (NSZs) requiring
particular attention in the UK.