Carbon, fertilizer policies would also improve water quality

Dec. 21, 2023
A study links the reduction of nitrate groundwater contamination with climate policies that would raise the price of carbon- and fertilizer-intensive products.

A climate policy that raises the price of carbon-intensive products across the entire U.S. economy would yield a side benefit of reducing nitrate groundwater contamination throughout the Mississippi River Basin, according to a press release from Purdue University by Steve Koppes.

The Gulf of Mexico, an important U.S. fishery, would also see modest benefits from the nitrate reductions. These were among the conclusions of a recent study published in the Proceedings of the National Academy of Sciences (PNAS).

The study combined four scientific models to simulate how different aspects of climate policy, agricultural economics and the environment would interact. A major feature of the study traced how nitrogen that humans have converted into other forms, called reactive nitrogen, flows through the environment.

“Food production is generating a lot of the reactive nitrogen. Some comes from energy as well,” said study co-author Thomas Hertel, distinguished professor of agricultural economics at Purdue. “This is a big problem. The reactive nitrogen ends up in rural groundwater. Some of it ends up in the atmosphere. And of course, it ends up in the streams and eventually the Gulf of Mexico.”

Excessive fertilizer use creates a growing number of water-quality concerns, said Shan Zuidema, a research scientist at the University of New Hampshire’s Earth Systems Research Center.

“Our models showed that with this climate policy, U.S. carbon emissions could significantly decline, which would translate into about a 3% to 4% reduction of the Gulf of Mexico dead zone in an average year,” said Zuidema.

One of the study’s four linked models analyzed various issues related to climate change economics. Another model linked local policies to national and international agriculture prices, land use and the environment. A third produced an estimate of nitrate leaching in the entire Mississippi River Basin based on changes in fertilization rates and land cover. The fourth simulated vertical water exchange between the ground and atmosphere and horizontal transport through runoff and stream networks.

The study included an assessment of policy outcomes that assigned the social cost of carbon to estimates ranging from $51 to $152 per ton of carbon dioxide equivalents.

The highest carbon price in the models reduced U.S. carbon emission by almost 50%. Coupled with an increase in nitrogen fertilizer prices, this reduced fertilizer applications by about 15% for corn production across the Mississippi River Basin.

The team also considered a scenario that restored wetlands in the central U.S. Corn Belt without a climate policy. The farmers in the area grew less corn and applied less fertilizer. But that prompted increased fertilizer applications in the untreated regions, resulting in negative spillover.

The PNAS study looked only at carbon pricing and wetlands, but many other food system interventions lend themselves to similar analyzes, including how a change in our diets could affect the nitrogen cascade.

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