Nitrous Oxide, a Greenhouse Gas, Three Times Higher in Rivers than IPCC Estimates

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."