The Cape Grim Baseline Air Pollution Station in Tasmania, where air samples have been collected since 1978. These samples show a long-term trend in isotopic composition that confirms that nitrogen-based fertilizer is largely responsible for the 20 percent increase in atmospheric nitrous oxide since the Industrial Revolution. Photo courtesy CSIRO.
Best management practices by farmers could help reduce levels without much additional cost
By Summit Voice
SUMMIT COUNTY — A detailed analysis of global nitrogen cycles shows without a doubt that a spike in atmospheric nitrous oxide can be traced to increased fertilizer use during the past 50 years.
Since 1970, nitrous oxide concentrations have increased by 20 percent, from below 270 parts per billion to more than 320 ppb. After carbon dioxide and methane, nitrous oxide (N2O) is the most potent greenhouse gas, trapping heat and contributing to global warming. It also destroys stratospheric ozone, which protects the planet from harmful ultraviolet rays.
Climate scientists have assumed that the cause of the increased nitrous oxide was nitrogen-based fertilizer, which stimulates microbes in the soil to convert nitrogen to nitrous oxide at a faster rate than normal.
The new study, reported in the April issue of the journal Nature Geoscience, uses nitrogen isotope data to identify the unmistakable fingerprint of fertilizer use in archived air samples from Antarctica and Tasmania.
“Our study is the first to show empirically from the data at hand alone that the nitrogen isotope ratio in the atmosphere and how it has changed over time is a fingerprint of fertilizer use,” said study leader Kristie Boering, a UC Berkeley professor of chemistry and of earth and planetary science.
“We are not vilifying fertilizer. We can’t just stop using fertilizer,” she said. “But we hope this study will contribute to changes in fertilizer use and agricultural practices that will help to mitigate the release of nitrous oxide into the atmosphere.”
The steep rise in atmospheric nitrous oxide coincided with the green revolution of the 1960s, when inexpensive, synthetic fertilizer and other developments boosted food production worldwide, feeding a burgeoning global population.
Tracking the origin of nitrous oxide in the atmosphere, however, is difficult because a molecule from a fertilized field looks identical to one from a natural forest or the ocean if you only measure total concentration. But a quirk of microbial metabolism affects the isotope ratio of the nitrogen the N2O microbes give off, producing a telltale fingerprint that can be detected with sensitive techniques.
Global warming impacts
Limiting nitrous oxide emissions could be part of a first step toward reducing all greenhouse gases, Boering said. In particular, reducing nitrous oxide emissions can initially offset more than its fair share of greenhouse gas emissions overall, since N2O traps heat at a different wavelength than CO2 and clogs a “window” that allows Earth to cool off independent of CO2 levels.
“On a pound for pound basis, it is really worthwhile to figure how to limit our emissions of N2O and methane,” she said. “Limiting N2O emissions can buy us a little more time in figuring out how to reduce CO2 emissions.”
One approach, for example, is to time fertilizer application to avoid rain, because wet and happy soil microbes can produce sudden bursts of nitrous oxide. Changes in the way fields are tilled, when they are fertilized and how much is used can reduce N2O production.
Boering’s studies, which involve analyzing the isotopic fingerprints of nitrous oxide from different sources, could help farmers determine which strategies are most effective. It could also help assess the potential negative impacts of growing crops for biofuels, since some feedstocks may require fertilizer that will generate N2O that offsets their carbon neutrality.
“This new evidence of the budget of nitrous oxide allows us to better predict its future changes– and therefore its impacts on climate and stratospheric ozone depletion – for different scenarios of fertilizer use in support of rising populations and increased production for bio-energy,” said coauthor David Etheridge of the Centre for Australian Weather and Climate Research in Aspendale, Victoria.
Archived air
The researchers reached their conclusions by carefully studying isotopes associated with nitrous oxide from air samples obtained in Antarctic ice and from a monitoring station at Cape Grim, Tasmania, which has archived air back to 1978.
The analysis revealed a seasonal cycle that enabled the scientists to differentiate nitrous oxide from different sources, whether from the Amazon rainforest or from the oceans.
“In addition, you also now have a way to check whether your international neighbors are abiding by agreements they’ve made to mitigate N2O emissions. It is a tool that, ultimately, we can use to verify whether N2O emissions by agriculture or biofuel production are in line with what they say they are,” Boering said.
The new data could also help assess the impacts of growing crops for biofuels, since some feedstocks may require fertilizer that will generate N2O that offsets their carbon neutrality.
The work was supported by UC Berkeley’s Atmospheric Sciences Center, NASA’s Upper Atmosphere Research Program, the Camille Dreyfus Teacher-Scholar Award, the Brain 21 Korea Program, a Korean government research grant through Seoul National University, and the Australian government’s Cooperative Research Centres Programme.