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Atmospheric DOUBLE-WHAMMY: fertilizers deplete ozone, add to warming

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A new study poses a problem for biofuel proponents and agriculture alike: excessive and careless use of fertilizers has been fingered as the driver of growing atmospheric nitrous oxide concentrations.

The study, published in Nature (abstract here), drew on the 40 years of atmospheric samples collected by CSIRO (the Commonwealth Scientific and Industrial Research Organisation) at its Cape Grim pollution station in Tasmania.

As lead author Dr David Etheridge explained to The Register, recent advances in mass spectrometery have made it possible to analyse not just the concentration of N2O in the atmosphere, but also its isotopic makeup. This signature provides the key to understanding where the rising concentration of the gas is coming from.

While N2O exists in vastly lower concentrations than CO2, its molecules are much more potent as a greenhouse forcer, Dr Etheridge said, and as the long-banned CFCs finally wash out of the stratosphere, N2O will also become a more important ozone-depleting gas. Hence, even though the change in atmospheric concentrations of the gas has only been from a pre-industrial 270 parts per billion (about a thousand times lower than the concentration of CO2) up to about 320 parts per billion now, scientists are still keen to trace what’s going on with the gas.

To identify the sources, the researchers had to take into account variations in the long term as well as annual and seasonal fluctuations, as well as identifying the isotopic signatures of natural-versus-man-made N2O.

Spot the isotope

The baseline source of N2O is the ocean (with the stratosphere as its sink), and the observation of seasonal variations allows the researchers to account for the background levels while searching for the man-made contribution.

“When fertilizer is added to soil, microbes are activated to produce N2O”, Dr Etheridge said. “The N-15 to N-14 ratio in the nitrous oxide is the key indicator of agricultural processes.”

That, along with the isotope’s position, he explained: the N2O can contain two N-14s, or one each of N-14 and N-15 in either possible position in the molecule.

This, combined with the low concentration of N2O, makes measurement more subtle than for CO2, he said.

To extend the dataset back to 1940 – well beyond the sampling at Cape Grim – the team also analysed the N2O samples in Antarctic ice samples. These surface ice samples provide “liters” of air, sufficient for measurement with current techniques, and Dr Etheridge predicted that as the technology improves, measurements of much smaller samples in ice cores will be possible.

Dr Etheridge hopes the research can also contribute to knowledge of how fertilizer use – both in agriculture and, if it’s to become part of the global strategy to deal with climate change, biofuel production – can be managed so as to minimize N2O release.

“You can apply fertilizer more efficiently,” he said. “Soil management can be addressed to reduce the amount of fertilizer used – which reduces both the N2O and the cost to the farmer.”

Better agricultural practices, he said, “end up with a win both economically and environmentally … this is the sort of information that’s needed in global management now.”

Dr Etheridge also paid tribute to Cape Grim’s Dr Paul Fraser, who decided back in the 1970s to retain the samples he began collecting on the basis that future technologies would enable better analysis of the samples. ®

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