Introduction

Nitrous oxide (N₂O) is a powerful greenhouse gas produced both naturally and by human activities. Its concentration in the Earth's atmosphere has risen by around 15% since the Industrial Revolution. Atmospheric mixing ratios for nitrous oxide now stand at around 315 parts per billion (ppb) compared to a pre-industrial high of 275ppb. Though its concentration the atmosphere is much smaller than that of carbon dioxide, N₂O is a much more effective greenhouse gas having a Global Warming Potential of 296 over a 100-year time span. This means that 1kg of N₂O released into the atmosphere has a global warming effect equivalent to 296 kg of carbon dioxide over a 100 year period.

Sources of Nitrous oxide

Natural

Natural sources of nitrous oxide are dominated by temperate and tropical soils, with the world's oceans also being an important contributor. Total annual emissions of N₂O from natural sources are etimated to be about 10 million tonnes of N₂O-N.

Human-made

The human-made (anthropogenic) emission of N₂O is of a similar magnitude to natural sources. Here, agricultural soils are by far the biggest source. When nitrogen-rich fertilizers are added to fields, some of the nitrogen can end up being emitted as N₂O through the processes of nitrification and denitrification. Both these processes are usually a result of microorganisms (nitrifiers and denitrifiers). Nitrification is common where the soil is well aerated, while denitrification tends to predominate where the soils are more water-logged and the supply of oxygen is limited. Globally, between 2 and 4 million tonnes of N₂O-N are thought to be released into the atmosphere each year via this pathway.

Industry, in particular the production of nitrogen fertilizers and nylon, is an important source of global N₂O emissions, as is fossil fuel burning in power generation. Other important human-made sources of N₂O include biomass burning, cattle and cattle feed production.

Further Reading

• Climate Change 2001: The Scientific Basis. IPCC 2001. Full text.
• Greenhouse Gas Sinks. Reay et al. (eds). CABI Publishing (in press).