The increasing amount of nitrous oxide in the atmosphere over the last 65 years is due to nitrogen-based fertilisers, according to a new study.
An international team of scientists, led by University of California-Berkeley researcher Dr Sunyoung Park, made the finding after studying air collected at the Cape Grim Station in Tasmania and sampled from the Antarctic ice sheet.
Previous studies have shown a 20 per cent increase in the level of N2O since 1750 - from below 270 parts per billion to more than 320 ppb.
Despite being relatively low in concentration, N2O is considered a significant contributor to global warming (about 6 per cent) and also destroys ozone in the stratosphere. It is produced naturally and by human activities such as agriculture.
According to this latest report, which appears this week in the online edition of Nature Geoscience, changes in the ratio of nitrogen-14 and nitrogen-15 point to the use of agricultural fertilisers as the main source of the increase.
"In this new paper we used isotopes - slightly different forms of nitrogen and oxygen atoms in the N2O molecule - as tracers of what the sources are behind that increase," says paper co-author Dr David Etherege, a principal research scientist at CSIRO in Marine and Atmospheric Research in Melbourne.
"We know that when fertiliser is added to soils it stimulates microbes to release N2O with the lighter form of nitrogen-14. Over the Industrial Period we see a decline in the ratio of [nitrogen-15]," he says. "So those two bits of matching pieces of information let us know that [the growth in N2O is] consistent with the use of agricultural fertilisers."
The researchers also measured the seasonal cycle in N2O and its isotopes, which provide a picture of its movement between the soils and oceans, atmosphere and stratosphere. They say this information could improve models used to predict future levels.
They also examined the subtle changes in the growth rate of N2O from year to year, which according to Etheridge highlight the effect of climate phenomena such as ENSO (El Nino-Southern Oscillation).
He says El Nino is typically associated with periods of dry weather, which could lead to increased biomass burning, while La Nina causes more mixing in the oceans. Both of these processes would release more N2O into the atmosphere.
Understanding the impact
The use of fertilisers in agriculture, both for food and biofuel production, is expected to increase as demand for both rises.
But Etheridge is quick to point out that the study isn't about demonising N2O and the use of fertilisers. Instead, he says this study will help feed into agricultural research aimed at improving fertiliser use and tilling practices, which could reduce emissions of N2O as well as the amount of fertiliser consumed.
"There's a potential economic win, as well as an environmental win, in not overly using fertilisers in crops," says Etheridge.
"The first thing to do is start with an understanding of 'Is there a problem?' and if so what is causing it, and then you can apply with a little more information how to mitigate the problem. I think that is what's happening here."