are such significant natural sources of nitrous oxide that they
justify being divided up into tropical and temperate soils sources.
Temperate soils are estimated to add about 2 million tonnes of
nitrous oxide-N via forest soils, with the other half being emitted
from the soil of temperate grasslands. Nitrous oxide arises from
soils primarily via the two biological pathways of nitrification
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
Our increased use of nitrogen based fertilizers in recent decades
has given rise to much increased nitrous oxide emissions from
temperate soils. 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 natural temperate soil.
As with tropical soils, rates of nitrous oxide from natural temperate
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 required if nitrous oxide emissions from
temperate 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.
Changes in land-use can have large effects on the balance of
nitrification and denitrification in non-agricultural temperate
soils, a fully informed land-use change strategy can therefore
help to reduce emissions. Strict control of man-made atmospheric
nitrogen emissions could also help to reduce future nitrous oxide
emissions from this source.