Carbon dioxide is the “face” of the greenhouse gases, but nitrous oxide (N2O) merits its own spotlight. The same “laughing gas” once used by dentists as an anaesthetic and used today by people looking for a quick, giggly high, turns out to be pretty bad for the environment.
Nitrous oxide (a molecule made of two nitrogen atoms and an oxygen atom) is over 300 times more potent as a greenhouse gas than CO2 and accounts for 6.3% of all UK greenhouse gas emissions. If nations are to meet their climate change targets, they need to pay attention to N2O.
While the gas is best known for its recreational uses, most of it is actually generated through farming, where microbes in the soil combine oxygen (from the air) and nitrogen (added to farmland) to create new compounds. This results in the leaking of N2O gas from the soil. As more nitrogen is added to the soil more N2O is emitted, so the best way to manage emissions is to control the nitrogen added via synthetic fertilisers, manures and slurries.
This century, the world faces a challenge to supply enough nitrogen to maximise crop yields while reducing the release of excess nitrogen into the surrounding environment as pollution. It’s an issue I looked at in a recent report for the Parliamentary Office of Science & Technology.
Nitrogen is an essential element for life, but it is mostly present as an unreactive gas, dinitrogen (N2), which only a few organisms can use directly. Agriculture was revolutionised in the early 20th century when the large-scale industrial synthesis of nitrogen fertiliser became possible. Food production increased and population growth followed; but huge amounts of nitrogen have subsequently been added to soils and the increase of N2O emissions is the inevitable result.
There are some scientific developments which could help to reduce nitrogen emissions while maintaining crop yields and so global food production levels. A few are listed here.
Instead of applying fertilisers equally across a field, precision farming allows farmers to fine-tune the location and amount of fertiliser spread by machines. This is based on soil and plant condition measurements and associated software-generated maps – optimising the yield and reducing fertiliser waste (pollution) and cost. In 2012, 20% of English farms used soil mapping to optimise fertiliser applications.
Plants could be bred to enable a reduction in nitrogen fertiliser. Most commercial plant breeding focuses on maximising crop yields under optimal plant growth conditions, which include a requirement for high levels of nitrogen (usually delivered via fertilisers). Some researchers have argued for programmes which focus on breeding plants that perform better under lower nitrogen conditions.
The final option is further away from realisation: the crops’ genetics can be altered to reduce the need for nitrogen fertilisers. Some plants such as legumes (e.g. clover and beans) work with bacteria to convert unreactive N2 from the air into a form that is available to the plant. Scientists at the John Innes Centre in Norwich have recently begun research that aims to transfer this capability into cereal crops.
These research efforts are part of an international focus to sustainably intensify agricultural production: increasing yields without adversely affecting the environment or cultivating more land. Nitrous oxide is critical to the debate on climate change, which means that farming is too.