Nitrous oxide | A greenhouse gas, atmospheric levels of which have increased by 20% since 1750. The abstract below highlights how the rise in nitrous oxide is mainly due to conventional fertilizers.

Abstract, ‘Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940’, Nature Geoscience, Volume 5,261–265 (2012).

The atmospheric nitrous oxide mixing ratio has increased by 20% since 1750. Given that nitrous oxide is both a long-lived greenhouse gas and a stratospheric ozone-depleting substance, this increase is of global concern. However, the magnitude and geographic distribution of nitrous oxide sources, and how they have changed over time, is uncertain. A key unknown is the influence of the stratospheric circulation, which brings air depleted in nitrous oxide to the surface. Here, we report the oxygen and intramolecular nitrogen isotopic compositions of nitrous oxide in firn air samples from Antarctica and archived air samples from Cape Grim, Tasmania, spanning 1940–2005. We detect seasonal cycles in the isotopic composition of nitrous oxide at Cape Grim. The phases and amplitudes of these seasonal cycles allow us to distinguish between the influence of the stratospheric sink and the oceanic source at this site, demonstrating that isotope measurements can help in the attribution and quantification of surface sources in general. Large interannual variations and long-term decreasing trends in isotope composition are also apparent. These long-term trends allow us to distinguish between natural and anthropogenic sources of nitrous oxide, and confirm that the rise in atmospheric nitrous oxide levels is largely the result of an increased reliance on nitrogen-based fertilizers.

Other sources

An article in Living Earth, Spring 2006, p.8, published by the Soil Association said Nitrogen fertiliser ‘is made from fossil fuel – mainly natural gas. Hence N fertiliser is the largest source of carbon dioxide (C02) emissions in agriculture. Its manufacture also emits large amounts of nitrous oxide, a far more powerful greenhouse gas than CO2,’ (‘N fertiliser and climate change’,

The Economist (11th April 2009, p.3) reported ‘some controversial findings published in 2007 by Paul Crutzen of the Max Planck Institute for Chemistry in Mainz, Germany. Dr Crutzen concluded that most analyses had underestimated the importance to global warming of a gas called nitrous oxide N2O by a factor of between three and five…[this gas] is a more potent greenhouse gas than CO2, and it hangs around for longer. N2O is made by bacteria that live in soil and water and, these days, their raw material is often the nitrogen-rich fertiliser that modern farming requires. Since the 1960s the amount of fertiliser used by farmers has increased sixfold, and not all of that extra nitrogen ends up in their crops.’