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.