Metabolic Rift

Corn Belt N2O emissions outweigh soil carbon storage

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Nitrogen emissions can overwhelm the climate benefits of storing carbon in agricultural soil

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About 200 million tonnes of synthetic nitrogen fertilizer is applied to soils every year. About two-thirds of the nitrogen ends up in rivers and lakes and the atmosphere.

by Ian Angus

Two years ago, in a series of articles on Disrupting the nitrogen cycle, I described how fossil fuels and industrial agriculture have created a major rift in the Earth System’s metabolism, by releasing more than twice as much reactive nitrogen into the environment as nature alone has ever produced.

“In particular, close to 200 million metric tons of synthetic fertilizers are used every year — and most of the reactive nitrogen they contain escapes into the broader environment, polluting air and water and disrupting ecosystems. … It is painfully clear that any serious effort to prevent ecological catastrophes in this century must include reining in the overproduction of reactive nitrogen.”

An under-studied part of nitrogen pollution is the nitrous oxide gas that microorganisms in the soil give off as a byproduct of the nitrogen biochemical cycle. Nitrogen stimulates nitrous oxide production, so adding nitrogen fertilizers to soil increases emissions.

A study published this month in the Proceedings of the National Academy of Sciences (PNAS) quantifies the climate-changing impact of nitrous oxide (N2O) emissions from soil in the midwestern US Corn Belt, the largest and most productive agricultural region in North America. It finds that agricultural soils fertilized with the most widely-used chemicals emit high amounts of nitrous oxide across a wide range of environmental conditions, far exceeding the benefits of using the same soils as a means of sequestering carbon.

“Nitrous oxide (N2O) is a powerful greenhouse gas with 298 times the warming potential of carbon dioxide (CO2) over 100 years, and N2O is also the leading contributor to stratospheric ozone depletion. Agricultural soils are currently the primary anthropogenic source of N2O, as a consequence of increased application of synthetic nitrogen fertilizer and manure over the past century. Without efforts to reduce emissions, atmospheric N2O will continue to rise along with demand for agricultural products, threatening our ability to mitigate climate change and ozone depletion.”

Steven Hall of Iowa State University, the study’s senior author, says their study shows that “the climate warming effects of nitrous oxide emissions from local corn and soybean soils are two-fold greater than the climate cooling that might be achieved by increasing soil carbon storage with common agricultural practices.”

Storing carbon in agricultural soils can mitigate climate change, but the PNAS article shows that “the outsized impact of N2O emissions from drainage-impaired Corn Belt soils exceeds climate benefits of current efforts to increase soil carbon through agricultural management.”

“If we want to maximize our climate benefit, we want to be strategic about it,” Hall said. “We’re not simply going to flip the switch on climate just by putting more carbon in the soil. Nitrous oxide emissions need to be a priority as well.”

This article includes material provided by Iowa State University.