Say the words "greenhouse gas" and most people think of carbon dioxide, but a new study released January 4 points a finger of growing concern at nitrous oxide, a lesser-known but more powerful agent of warming whose presence in the atmosphere is on the rise.
Much of the nitrous oxide comes from the degradation of synthetic nitrogen fertilizers on which modern industrial agriculture relies – in 2005 the world produced 220 billion pounds of it. As it washes into rivers and streams, the fertilizer run-off undergoes chemical change and some of it eventually ends up in the air as nitrous oxide (N2O), a gas most commonly known as the laughing gas that dentists use for anesthesia.
But there is nothing funny about the colorless substance. Each molecule of N2O has 300 times the warming potential of a CO2 molecule, and the new study from Proceedings of the National Academy of Sciences places nitrous oxide levels in the world's waterways at three times the amount estimated by the Intergovernmental Panel on Climate Change.
Atmospheric nitrous oxide increased 20% during the last century, and it now accounts for 6% of anthropogenic climate change. It's also the leading culprit in destruction of the ozone layer.
Overall, the study concluded that streams and rivers are responsible for at least 10% of anthropogenic N2O output, or 0.6% of total greenhouse gas emissions.
"That makes it seem (like) a lot less when you put it that way," study co-author Stephen Hamilton, a professor at Michigan State University, told SolveClimate News. "But understanding global warming involves studying all the little wedges that contribute. It's the cumulative effect of all these little wedges that (matters), and it's important to understand (them all) if we're to address the problem."
Most plants need nitrogen to survive. Although the atmosphere is 78% nitrogen, it is nonreactive and useless to plants, which absorb nitrogen in the form of salts, such as nitrates in fertilizers.
"The world needs nitrogen to grow food," said James Galloway, a professor at the University of Virginia who was not involved in the study. "And there's not enough reactive nitrogen on the planet made available by natural processes."
In the past, farmers used nitrate-rich animal manure and other natural sources, but modern agriculture relies on factory-produced nitrogen fertilizers. The manufacturing process requires large inputs of fossil fuels to transform atmospheric nitrogen into reactive substances that plants can use.
Most of the applied fertilizer never makes it into the plant: corn, for example, may absorb only 30% of what's sprayed on the field. The remaining fertilizer might get picked up by soil bacteria, get washed away by rain, or evaporate into the atmosphere.
Any fertilizer that leaches into groundwater will join underground aquifers and eventually flow into connecting rivers and streams. Microbes in the water then convert nitrate back into atmospheric nitrogen, but a small amount of nitrous oxide is also created as a byproduct.
It can take decades for nitrogen to move from the soil into waterways. "(Even) if we stopped all fertilizers tomorrow, nitrogen would still increase in our rivers for some time," Hamilton said.
In addition to fertilizers, crops like soybeans, alfalfa and clover have bacteria on their roots that convert atmospheric nitrogen into a useable form. So when humans plant vast fields of these crops, said Hamilton, they're creating yet another source of nitrous oxide.
Hamilton and his colleagues collected data from 72 streams in the United States. All the experiments were conducted under the LINX (Lotic Intersite Nitrogen Experiment) program, a grassroots collaboration of scientists studying nitrogen cycling in streams.
The team used an isotope of nitrogen (15N) to trace how nitrogen behaves and interacts in water. "(LINX) has been pioneering whole system nitrogen addition to streams," said Jonathan Cole, a senior scientist at Cary Institute of Ecosystem Studies who was not involved in the experiments. "Instead of doing this in a jar, they're doing this to the real world."
From the stream data, the scientists then built a model to extrapolate nitrous oxide output from all the waterways of the world. The model took into account how nitrate concentrations change as tributaries mix and flow into rivers.
The final results showed that waterways account for 10% of global nitrous oxide output, three times as much as the Intergovernmental Panel on Climate Change estimated. IPCC figures were based on older studies with data from two or three streams, said Hamilton, whereas the LINX study surveyed over 70 streams using experimental methods that hadn't been applied before.
Hamilton hopes to see the LINX figures in future IPCC reports—unless, of course, a newer, better study comes along. Nitrous oxide budgets are a tricky business, said Hamilton; unlike carbon dioxide, it's much harder to track how N2O behaves in the environment.
Cole called the study "a sophisticated analysis and a big step forward."
Given more time and resources, Hamilton would like to conduct field experiments on larger rivers and wetlands, especially in developing countries with intensifying agriculture such as China and Brazil.
Nitrogen Used to Feed the World
The best way to reduce nitrous oxide in rivers is to decrease fertilizer use. Yet by all estimates, said Hamilton, fertilizer production will only increase as the world struggles to feed a growing population.
And developing countries will seek to improve their standard of living, said Galloway. "That almost always means increased meat consumption."
More meat leads to additional land planted with animal feed (often corn-based feed), which means more fertilizers.
Fertilizers also cause human health effects. Infants who drink groundwater with high levels of nitrate can develop methemoglobinemia, colloquially know as "blue baby syndrome." The nitrate binds to their blood in place of oxygen, and in extreme cases leads to asphyxiation.
There are ways to increase the efficiency of nitrogen uptake by plants, said Galloway. Farmers can adjust the timing and frequency of fertilizer application, or change the kind of fertilizer being used.
"If all the farmers in the world used the current knowledge, we could have significant savings (of 33 billion pounds per year)," said Galloway. "I believe we have the technology...but there's not the political will to do it in all countries."