No laughing matter - Laughing Gas is a potent agent in global climate change

Figure 1: atmospheric Nitrous Oxide has increased by 5% in the last 20 years and is still increasing by 0.25% per year

The most infamous greenhouse gases in the public eye are currently carbon dioxide and methane and there is much international focus on reducing emissions of these by countries signed up to the Kyoto Protocol. However, nitrous oxide (N2O) now ranks alongside these as a cause for great concern. At present nitrous oxide is perhaps best known to the general public as laughing gas. However throughout the 20th century (Fig. 1), and continuing into the 21st century, nitrous oxide in the environment has increased by 50 parts per billion and this atmospheric loading is increasing further by 0.25% each year. Although it only accounts for around 9% of total greenhouse gas emissions it has a 300-fold greater global warming potential than carbon dioxide over the next 100 years and an atmospheric lifetime of 150 years. This is most definitely not a laughing matter and it is recognised in the Kyoto protocol that is important to begin to mitigate these releases.

Where is the nitrous oxide coming from?

N2O is produced naturally in soils through the microbial processes of denitrification. These natural emissions of N2O can be increased by a variety of agricultural practices and activities including:

Direct addition of nitrogen to soils:

  • use of synthetic and organic fertilisers
  • production of nitrogen-fixing crops
  • cultivation of high organic content soils
  • application of livestock manure to croplands and pasture


Indirect addition of nitrogen to soils:

  • surface run-off and leaching of applied nitrogen into ground water and surface waters
  • denitrification of the organic nitrogen in livestock manure and urine

The Nitrogen Cycle

Image illustrating the Nitrogen Cycle

When faced with a shortage of oxygen many bacterial species, are able switch from using oxygen to using nitrates to support respiration in a process known as denitrification, during which the water-soluble nitrates are converted to gaseous products, including nitrous oxide, that are emitted into the atmosphere (Fig. 2).


 

Denitrification is central to the cycling of nitrogen in agriculture and has industrial applications in water purification and wastewater treatment. Consequently more than 80% of nitrous oxide emissions globally are associated with the agricultural and waste-treatment industries (Fig. 4).

Figure 4 - illustration of the fine balance in nutrient cycling

Figure 4: Illustration of the fine balance in nutrient cycling. Depending upon microbial status, the effects of nitrogen-containing fertiliser input may vary and greatly impact upon overall greenhouse gas emissions. Modelling this phenomenon could lead to the ability to predict when and why this might happen.


How can we mitigate nitrous oxide release?

Much of the increase in production of nitrous oxide correlates to the increased application of nitrogenous fertilisers onto soils that began in the early 1900s. Since the UK signed up to the Kyoto Protocol, many non-biological emissions have been reduced, but emissions from agriculture are essentially unchanged (Fig.3).

Nitrous oxide Emissions by source: 1990-2004. Source:Defra

Figure 3: Nitrous Oxide emissions by source:1990-2004
Source: http://www.defra.gov.uk/news/2006/060123b.htm
It is important that this does not remain the case. Efforts to improve the management of nitrous oxide emissions will benefit from the better understanding of the factors that influence the net production of nitrous oxide by bacteria. Thus a study such as this to understand the regulation of denitrification will be instrumental for the development of appropriate management techniques for a range of soil systems to mitigate emissions and to improve existing agricultural and waste treatment practices.