Nitrogen fertilisers and climate change

Over recent years, primarily since the ‘Green Revolution’ in the 1960s, nitrogen has been increasingly used in agriculture, now more nitrogen is produced artificially for fertilisers than is produced naturally by the earth.  

Nitrogen naturally forms in soils in tropical and temperate regions of the earth and in the oceans.  However recently it has been used as a fertiliser, is spread over fields (see picture below) as it increases crop yields as more nutrients, namely nitrogen is available in the soil encouraging greater plant growth.  This has consequently led to an increase in nitrous oxide concentrations in the atmosphere, as more nitrogen in the soil means a greater rate of microbe activity, which creates and releases nitrous oxide. This is important, because nitrous oxide has a warming affect on the planet, being a greenhouse gas, but it also according to Crutzen and Ehhalt (1977)  has a depleting affect on the ozone in the stratosphere, which means more UV rays can penetrate through warming the planet.  It is thought that nitrous oxide accounts for 6% of total anthropogenic radiative forcing (Davidson 2009).  Nitrous oxide production only occurs under specific conditions and results from the combination of aerobic and anaerobic processes.  Nitrification is the process of ammonium transforming to nitrate, an aerobic process and denitrification, the formation of nitrogen gas from nitrate reduction, an anaerobic process.  This nitrogen gas is then oxidised to form nitrous oxide (Monteny et al 2005)

Nitrogen fertilisers are used to increase crop yields so it is possible to produce more food from the same amount of space, making food production more efficient.  This consequently led to an increase in population, and is one of the reasons why we have seen the population expand so rapidly over recent years.  But in turn the increasing population needs food so results in higher crop yields being needed.  This means that fertilisers are used at an increased rate having negative effects on the planet.  Not only does it result in increased concentrations of nitrous oxide in the atmosphere but also the use of fertilisers causes loss of soil nutrients, soil acidification and erosion reducing soil quality in the long run.  

The work of Boering et al. is discussed in this article, who looked at samples of Antarctic ice dating between 1940 and 2005, to reconstruct nitrous oxide concentrations in the atmosphere between these times.  They found that it is possible to differentiate between natural and agricultural nitrous oxide, due to its isotopic composition.  This is useful as it allows us to determine when concentrations of nitrous oxide rose significantly as a result of agriculture and see how successful mitigation strategies are at reducing it.  

The use of nitrogen as a fertiliser is important due to the negative affect it has on climate change, as it is the predominant cause of increase in nitrous oxide concentrations. For more information on the effects of nitrous oxide on the atmosphere, I found this link useful. 
The diagram below shows how nitrogen fertilisers are incorporated into the nitrogen cycle.