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% increase in atmospheric nitrous oxide since the Industrial Revolution. |
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.
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%—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.
Archived air from Cape Grim
Boering
and her colleagues, including former UC Berkeley graduate students
Sunyoung Park and Phillip Croteau, obtained air samples from Antarctic
ice, called firn air, dating from 1940 to 2005, and from an atmospheric
monitoring station at Cape Grim, Tasmania, which has archived air back
to 1978.
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
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.”
Law Dome, Antarctica. Bubbles inside ice cores from this region provide historical air samples going back to 1940. |
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% worldwide, while
the relative output of bacteria that produce N2O by
denitrification—typically in the absence of oxygen—dropped from 87 to
77%. 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.
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.
