Chemists from the University of California, Berkeley have discovered evidence of a link between increased fertilizer use and a rise in atmospheric nitrous oxide, a major greenhouse gas.
Climate scientists have long assumed the cause of rising nitrous oxide levels in the atmosphere was due in part to nitrogen-based fertilizers. Such fertilizers are used to stimulate microbes in the ground to convert nitrogen to nitrous oxide at a faster rate.
Published in the April issue of the journal Nature Geoscience, the new study uses nitrogen isotope data to pinpoint direct blame on these nitrogen rich fertilizers.
“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, according to a Berkeley press release.
“We are not vilifying fertilizer. We can´t just stop using fertilizer,” she added. “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.”
Nitrous oxide is the most potent of the greenhouse gasses, trapping heat and contributing to global warming. Nitrous oxide has also been found to destroy the stratospheric zone, which protects the Earth from ultraviolet rays.
These levels have been dramatically increased in the past 50 years, according to the study. Part of this increase coincided with the new “green” movement of the 1960s. As global population increased, cheap and synthetic fertilizers became widely available, boosting food production all over the world.
Boering and colleagues, former UC Berkeley graduate students Sunyoung Park and Phillip Croteau, gathered their samples from Antarctic ice, or “firn air,” dating from 1940-2005 as well as at an atmospheric monitoring station at Cape Grim in Tasmania.
Analysis of this data revealed a previously discovered seasonal cycle. What surprised these researchers, however, were the isotopic measurements found by a sensitive isotope ratio mass spectrometer. The measurements also discovered a seasonal cycle, something the spectrometer had not done before. At Cape Grim, the seasonal cycle is due in part to the circulation of air returning from the stratosphere, where nitrous oxide is destroyed after an average of 120 years, and seasonal changes in the ocean.
“The fact that the isotopic composition of N2O shows a coherent signal in space and time is exciting, because now you have a way to differentiate agricultural N2O from natural ocean N2O from Amazon forest emissions from N2O returning from the stratosphere,” Boering said. “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.”
Using these results, the UC Berkeley students are suggesting a limiting of nitrous oxide emissions, like those found in fertilizer, could be a first step in reducing greenhouse gasses and the effects of global warming.
According to a Berkeley press release about these findings, Boering said, “Limiting N2O emissions can buy us a little more time in figuring out how to reduce CO2 emissions.”
Image 1: Law Dome, Antarctica. Air trapped in the consolidated snow from this region provides historical air samples going back to 1940.
Image 2: 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 of CSIRO.