Nitrogen, heat trapping gases and climate
4/25/12 12:33 AM
Changes in fertilizer use can reduce N2O emissions
Limiting nitrous oxide emissions could be part of a first step toward reducing all greenhouse gases and lessening global warming, Boering said, especially since immediately reducing global carbon dioxide emissions is proving difficult from a political standpoint. 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.”
By Robert Sanders, Media Relations | April 2, 2012 BERKELEY
University of California,
Berkeley, chemists have found a smoking gun proving that increased fertilizer use over the past 50 years is responsible for a dramatic rise in atmospheric nitrous oxide, which is a major greenhouse gas contributing to global climate change.
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.
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 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.”
Since the year 1750, nitrous oxide levels have risen 20 percent – from below 270 parts per billion (ppb) 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.
Not surprisingly, a steep ramp-up in atmospheric nitrous oxide coincided with the green revolution that increased dramatically in 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.
Analysis of N2O levels in the Cape Grim air samples revealed a seasonal cycle, which has been known before. But isotopic measurements by a very sensitive isotope ratio mass spectrometer also displayed a seasonal cycle, which had not been observed before. At Cape Grim, the isotopes show that the seasonal cycle is due both to the circulation of air returning from the stratosphere, where N2O is destroyed after an average lifetime of 120 years, and to seasonal changes in the ocean, most likely upwelling that releases more N2O at some times of year than at others.
“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. 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.”
Changes in fertilizer use can reduce N2O emissions
Finding the fingerprint of fertilized microbes
Boering was able to trace the source of N2O because bacteria in a nitrogen-rich environment, such as a freshly fertilized field, prefer to use nitrogen-14 (14N), the most common isotope, instead of nitrogen-15 (15N).
“Microbes on a spa weekend can afford to discriminate against nitrogen-15, so the fingerprint of N2O from a fertilized field is a greater proportion of nitrogen-14,” Boering said. “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.”
Just as telling is the isotope ratio of the central nitrogen atom in the N-N-O molecule. By measuring the nitrogen isotope ratio overall, the isotope ratio in the central nitrogen atom, and contrasting these with the oxygen-18/oxygen-16 isotope ratio, which has not changed over the past 65 years, they were able to paint a consistent picture pointing at fertilizer as the major source of increased atmospheric N2O .
The isotope ratios also revealed that fertilizer use has caused a shift in the way soil microbes produce N2O. The relative output of bacteria that produce N2O by nitrification grew from 13 to 23 percent worldwide, while the relative output of bacteria that produce N2O by denitrification – typically in the absence of oxygen – dropped from 87 to 77 percent. Although the numbers themselves are uncertain, these are the first numerical estimates of these global trends over time, made possible by the unique archived air dataset of this study.
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.
March 15, 2012 Contacts: Lee Tune, 301 405 4679 or firstname.lastname@example.org
One Downside of Early Spring: a Drop in Rocky Mt. Butterflies
A Mormon Fritillary butterfly feeding on an aspen fleabane daisy, a main nectar source. Credit: Carol Boggs
COLLEGE PARK, Md. - The early arrival of spring across the U.S. undoubtedly has warmed the hearts of many people, but for flowering plants and pollinating insects, the trend toward earlier springs brings complicated, and not always good, results. For example, a new study from the University of Maryland and Stanford University shows earlier snowmelts in the Colorado Rockies is causing a decline in the population of the Mormon Fritillary butterfly (Speyeria mormonia).
Seasonal impact of climate on vegetation and insects--
Inouye's work at the Rocky Mountain Biological Laboratory focuses on variation in the timing and abundance of wildflowers, and the consequences of climate change for the wildflowers and animals that interact with them. Previous work has shown that earlier spring thaws in the Rockies harm wildflowers rather than benefit them because flowers come out early, but are damaged by late frosts for which the timing has not changed. He has ongoing studies of the population biology of wildflowers and how that is changing with climate change, on ant-plant mutualisms, bumble bee ecology, and long-term studies of variation in insect populations.
Boggs' research addresses the effects of environmental variation on individuals, populations and interactions among species. Using butterflies as focal organisms, her current work includes tests of the effects of variation in climate and food availability on birth and death rates and population sizes.