Nitrous Oxide Sources
Nitrous oxide Sources - Tropical Soils
Soils are such significant natural sources of nitrous oxide
that they justify being divided up into tropical and temperate
soils sources. Tropical soils are estimated to add about 4
million tonnes of nitrous oxide-N to our atmosphere each year.
Of this, around 3 million tonnes comes from wet forest soils,
with the remainder being emitted from the soil of dry savannas.
Tropical and temperate soils generally have different ratios
of nutrients, with tropical soils often being phosphorous
(P) limited, rather than nitrogen (N) limited like many temperate
soils. Because of this, extra N inputs to these P limited
tropical soils may cause nitrous oxide emissions hundreds
of times greater than that which would be seen in N-limited
temperate soils. Nitrous oxide arises from soils primarily
via the two biological pathways of nitrification and denitrification.
Nitrification in soils is carried out by aerobic, ammonia
oxidizing bacteria (AOB) which produce nitrate from ammonium
in the soil, but can also produce some nitrous oxide during
this process. Because the nitrification process relies on
a good availability of oxygen it is most important in well
drained and aerated soils. These AOB have also been shown
to oxidize certain amounts of the greenhouse gas methane as
part of the nitrification process, though whether they have
a significant impact on methane emissions from soil is still
open to debate.
In wetter or more compact soils, the anaerobic conditions
suitable for denitrification to occur become more prevalent.
Denitrification involves the reduction of nitrate in the soil
to gaseous nitrogen (N2) by anaerobic bacteria. Again, nitrous
oxide can be produced during this process and generally denitrification
produces more nitrous oxide than nitrification. During denitrification
the nitrous oxide produced can be further reduced to N2, but
usually a proportion escapes to the atmosphere. Soil conditions,
such as water content, temperature and the availability of
ammonium and nitrate are key determinants of how much nitrous
oxide a particular soil will produce.
Our rapidly increasing use of nitrogen based fertilizers on
tropical soils is giving rise to ballooning nitrous oxide
emissions from this source. Additionally, increased atmospheric
nitrogen deposition due to man-made nitrogen emissions, such
as intensive livestock rearing, can induce elevated rates
of nitrous oxide emission over large areas of otherwise untouched
tropical soil. Rates of nitrous oxide from natural tropical
soils are also likely to change in response to human-induced
variations in temperature and rainfall.
Potential for control
More efficient use of nitrogen based fertilizers and better
overall land-use practice are crucial if nitrous oxide emissions
from tropical soils are not to grow further. Ensuring that
nitrogen based fertilizer additions do not end up on natural
soils, whether directly or indirectly, makes sense both environmentally
and economically. Strict control of man-made atmospheric nitrogen
emissions could also help to reduce future nitrous oxide emissions
from this source.
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