Nitrous Oxide: An Inconspicuous Climate Warming Gas From Agricultural Land

Agricultural activities are known to be one of the major sources of greenhouse gas emissions (GHGs), which contributes to 16%-27% of the total anthropogenic-related emissions. Yet, the climate warming impact from agriculture does not solely lie on the production of carbon dioxide (C02), but also on nitrous oxide (N2O) due to the overuse of nitrogen fertiliser. How is nitrous oxide produced? Why should we be concerned about this laughing gas? And what measures can be taken to reduce its emission?

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Where Does Nitrous Oxide Come From?  

There are multiple sources of nitrous oxide both anthropogenically and naturally occurring – ranging from ocean, atmosphere, and soils. Agricultural activities including animal husbandry is considered the largest source of N2O, which accounts for more than one-third of the total N2O emission. 

One of the reasons why agriculture contributes to the significant amount of N2O is due to the over application of fertilisers.  With the development of the Haber-Bosch process allowing fast production of chemically synthesised fertilisers, commercial farmers can apply synthetic fertilisers in soil to boost crop yield. Another major source of N2O is the feed production and manure deposition from managed pastures, which accounts for 45% and 39% respectively of total greenhouse gas emissions by livestocks. Nitrogen that is not taken up by plants or remains in the manure is then consumed by the soil microbes, where nitrous oxide is produced through microbial processes.

There are two processes contributing to the production of N2O; namely nitrification and denitrification. Nitrification involves the transformation of ammonia into nitrate that can then be directly consumed by plants, while denitrification helps in the balance of nitrogen present in the soil by converting excess nitrate back into atmospheric nitrogen. N2O is produced as a by-product during these nitrogen conversions. Soil eutrophication due to excess use of nitrogen in soil offers a favourable condition for these microbes to proliferate, resulting in N2O emitted from soil.

Why Should We Be Concerned With N2O? 

The quantity of N2O is comparatively less than carbon dioxide; however, this gas is long-living, which on average, stays in the atmosphere for 114 years whilst containing much higher (i.e. 298 times more) global warming potential than carbon dioxide.

N2O also leads to other environmental problems such as ozone depletion. It has been recognised as a dominant anthropogenic ozone-depleting substance (ODPs) and it is anticipated to be the biggest anthropogenic emission source of ozone depleting compounds in the foreseeable future. 

Aside from the environmental issues from N2O itself, the release of nitrogen also raises other environmental problems such as leaching of nitrogen-containing nutrients into the water bodies causing eutrophication that damages the local environment. Production of synthetic fertilisers by the Haber-Bosch process is also energy demanding; contributing up to 1% of global energy consumption and 1.4 % of CO2 emissions, it further intensifies the impact on global warming.

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Solutions to Mitigate N2O Emissions

Scientists have proposed multiple approaches in tackling the issue of  N2O production. Given that the major issue of N2O emission from agricultural land is due to the excess nitrogen retained in the soil body, one of the solutions is to avoid the overuse of fertilisers by adopting remote sensing technology to suitably apply fertiliser to the crops only as and when (and where) needed. Other source control measures include applying nitrification inhibitors to suppress microbial activity for generating N2O,  allowing a longer retention time in soil for plants to uptake the nitrogen. 

New technologies are also developing to reduce reliance on synthetic fertilisers, which in turn could reduce the production of N2O. One such example is genetically modifying microbes to supply nitrogen to the plants, per the model of symbiotic relationships as nitrogen fixing bacteria have with legumes, where the bacteria provide nitrogen to the plants to grow, while the plants in turn offer shelter for survival of the bacteria. 

Another solution to reduce emissions is to promote better management practices by avoiding or reducing tillage to the soil. A meta-analysis study with over 200 papers was conducted to compare the soil N2O emission by conventional tillage (CT), and no-tillage and reduced tillage (NT/RT) practices. It concluded that a long period (i.e. 10 years) of implementing NT/RT practices can help reduce N2O emissions. Given that denitrification is an anoxic process, which is carried out in the absence of oxygen, it is suggested that microbes produce less N2O in undisturbed farmland with loose soil due to the presence of a higher oxygen content.

We are all now facing the increased frequency of extreme weather such as wildfires in Greece, flooding in both Australia and China, and a severe storm surge by Hurricane Ida in the United States. Cutting off greenhouse gas emissions is necessary to avoid further worsening of our climate. N2O is a significant source of greenhouse gas emissions and is at its highest level since human existence. Unfortunately, atmospheric N2O is still rising continuously due to the growing number of agricultural activities to meet our food demand. Implementing more sustainable agricultural practices may help lower the GHGs emissions, and hopefully alleviate the impacts that have already and still being done to the Earth.

Featured image by: Pxfuel