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.

Human Impact

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.