The natural soil sources

Soils are significant natural sources of N2O on a global scale, with both tropical and temperate soils being important. Tropical soils are estimated to add ~4 Tg N2O/year to our atmosphere. Of this, ~3 Tg comes from wet forest soils, with the rest being emitted from the soils of dry savannahs.

Temperate soils are estimated to add ~2 Tg N2O to the atmosphere, with about half being emitted by forest soils, and the other half by the soils of temperate grasslands.

Tropical and temperate soils generally have different ratios of nutrients, with tropical soils often being phosphorus-limited, rather than nitrogen-limited (like many temperate soils). Because of this, extra nitrogen inputs to these phosphorus-limited tropical soils may cause N2O emissions hundreds of times greater than that seen in nitrogen-limited temperate soils. N2O 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 N2O during this process. As the nitrification process relies on 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 GHG CH4 as part of the nitrification process, though whether they have a significant impact on CH4 emissions from soil is still open to debate.

In the wetter or more compact soils common to many temperate soils, the anaerobic conditions suitable for denitrifi-cation become more prevalent. As for oceanic N2O emissions, denitrification involves the reduction of nitrate to gaseous nitrogen (N2) by anaerobic bacteria. Again, N2O can be produced during this process and, generally, denitrification produces more N2O than nitrification. During soil denitrification, as in the oceans, the N2O that is 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 nitrogen are key determinants of how much N2O a particular soil will produce.

Increased atmospheric nitrogen deposition following NOx and ammonia emissions can induce elevated rates of N2O emission over large areas of otherwise pristine soil. Rates of N2O from such soils at all latitudes are also likely to change in response to variations in temperature and rainfall resulting from global climate change. Ensuring that fertilizers do not end up on natural soils, whether directly or indirectly, makes sense both environmentally and economically.

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