The world’s farmers nourish their fields with more than 120 million metric tons of nitrogen-based fertilizer each year. Nitrogen is a key component of chlorophyll, and it’s necessary for photosynthesis, but it’s not harmless, and when it’s used in fertilizer, it doesn’t just stay in the soil.

A small percentage of the nitrogen in fertilizers is converted into nitrous oxide (N 2 O), a gas that accounts for about 5.6% of total greenhouse gas emissions in the United States. Nitrous oxide traps heat at about 300 times the rate of carbon dioxide and can comprise as much as half of a farm’s warming effects.

In North America’s Great Plains, tens of thousands of farm ponds store water for livestock and irrigation, and they’re squarely in the path of fertilizer runoff from large-scale agriculture throughout the region.

Researchers found that farm ponds “were undersaturated and were actually acting as N 2 O sinks, which was a big surprise.” Kerri Finlay, an assistant professor of biology at the University of Regina in Canada, and Jackie Webb, a postdoctoral fellow in biology at the University of Regina, hypothesized that these ponds would be heavy greenhouse gas emitters. Most inland bodies of water are sources of greenhouse gases, even without regular nitrogen deposits.

“We assumed that with all of the nitrogen fertilizer on the landscape there would be quite a lot of N 2 O emitted,” says Finlay.

“But we found they were undersaturated and were actually acting as N 2 O sinks, which was a big surprise.”

The researchers sampled emissions from 101 farm ponds in the Canadian province of Saskatchewan and found that just over two thirds of them were acting as N 2 O sinks. Their results were published in the Proceedings of the National Academy of Sciences of the United States of America.

Engineering Ponds to be N 2 O Sinks

Finlay and Webb are working on guidelines that could help farmers turn existing ponds into sinks and build new ones to absorb gases from the outset. To help with this, they have identified key characteristics associated with the ponds that were not N 2 O emitters.

Most of the N 2 O sinks were more than 3 meters deep and were surrounded by small hills that shielded them from the wind. Deep, still water encourages the growth of algae, which consume nitrogen before it becomes a gas. Eventually, the algae die and fall to the bottom of the pond, never allowing the nitrogen to escape into the atmosphere.

The researchers aren’t focused only on N 2 O in farm ponds. High soil pH levels in the study area mean that carbon dioxide in the ponds is converted to other forms of dissolved carbon, meaning the ponds act as carbon sinks as well as nitrogen sinks. Like pond depth and wind shelter, the pH level of ponds can be controlled.

“We are looking for consistency in the characteristics that keep methane, carbon dioxide, and N 2 O low.” Finlay and Webb also found that some ponds had low methane emissions, but they aren’t yet sure why.

“We are still trying to work that out,” Finlay says. “But we are looking for consistency in the characteristics that keep methane, carbon dioxide, and N 2 O low. We think we can find a subset of sites like that, and may be able to manage other sites to shift to that.”

Building guidelines for agricultural ponds could help transform a necessary farm installation into a carbon offset, but the policy instruments to encourage this don’t yet exist.

“We don’t have monitoring, reporting, and verification in place,” says Margot Hurlbert, a coordinating lead author of the Intergovernmental Panel on Climate Change’s Special Report on Climate Change and Land. Hurlbert was not involved in the farm pond study.

“But some countries are beginning to think about how to bring agricultural practices into the system, so that farmers can benefit from them. That would be really exciting, but currently it’s a gap. If we were to get serious about monitoring, verification, or reporting greenhouse gas emissions, then there could be a linkage.”

—Ty Burke, Science Writer