Auburn researchers highlight fertilizer impact on climate change
A new study led by Auburn University researchers details streams and rivers’ contribution to climate change. The study’s co-authors are, front row from left: Hanqin Tian and Shufen Pan; and, back row from left, Naiqing Pan, Rongting Xu, Yuanzhi Yao and Hao Shi.Courtesy Auburn University
By Dennis Pillion | [email protected] Posted Jan 15, 2020
Researchers at Auburn University have detailed how overuse of fertilizers can contribute to global warming, in a new paper published in the journal Nature Climate Change.
Overuse of fertilizers can cause large amounts of nitrogen to run off from agricultural fields or lawns into small rivers and streams. This can cause local environmental problems, including algae blooms, but also releases nitrous oxide, a greenhouse gas 300 times more potent than carbon dioxide, into the air.
The research team, composed of scientists from Auburn and Australia, developed a new computer model to measure how much nitrous oxide is being released into the atmosphere from global rivers and streams, allowing scientists to get a better estimate of the climate impact of agriculture.
The team found there has been a fourfold increase in releases of nitrous oxide from global rivers and streams since 1900. That increase was largely tied to the widespread use of nitrogen-based fertilizers, according to the study’s lead author Hanqin Tian, director of the International Center for Climate and Global Change Research at Auburn’s School of Forestry and Wildlife Sciences.
“Nitrogen loads on headwater streams and groundwater from human activities, primarily agricultural nitrogen applications, play a significant role in the increase of global riverine nitrous oxide emissions,” Tian said in a news release.
Previously scientists’ estimates of nitrous oxide emissions were based on the amount of nitrogen detected in bodies of water multiplied by an “emissions factor,” estimating how quickly the substances broke down in a lab. But many factors can change the rate at which nitrous oxide is released into the atmosphere, including atmospheric carbon dioxide levels, weather, land use and farming practices.
For this study, the researchers built computer models to create more accurate representations of the actual conditions in 50 river basins across the globe to provide a more accurate estimate of emissions under varying conditions.
The model outputs were then compared to direct measurements from the river basins, where the researchers say the outputs “agreed well with observations both spatially and temporally.”
The increase in nitrous oxide emissions escalated quickly in the 1990s with the widespread increase in the use of nitrogen fertilizers, but decreased slightly after 2010, as total use of the fertilizers dipped and the impacts of over-fertilizing became more well-known.
"One of the most compelling new findings is the importance of surface and subsurface processes in nitrous oxide emissions from the world’s river networks,” said Shufen Pan, director of Auburn’s GIS and Remote Sensing Laboratory in the School of Forestry and Wildlife Sciences and co-author of the study. “Previous estimates ignored or underestimated large nitrous oxide emissions from headwater or small rivers.”
The researchers found that those small headwaters and streams accounted for up to 85 percent of total nitrous oxide emissions.
Australia-based climate scientist Josep Canadell, executive director of the Global Carbon Project, is a co-author on the paper and said that nitrous oxide is often described as the “unspoken greenhouse gas” because of its association with food production, which no one wants to compromise.
But knowing how much nitrous oxide is being released, and what conditions are most favorable to releasing it, can help scientists and policymakers decide how best to proceed in reducing those emissions without disrupting the food supply.
“We now have a more precise quantification of [nitrous oxide’s] impacts to help us target mitigation strategies which, in most cases, also makes farming more sustainable and increases farm-level economic gains,” Canadell said.
Tian said dealing with nitrous oxide emissions will be an important factor in climate mitigation strategies going forward, as well as general best practices in agriculture.
“All greenhouse gas emission pathways consistent with the goals of the Paris Climate Agreement require large and sustained reductions on nitrous oxide emissions, which in turn require improved quantification, process attribution and methodological transparency,” Tian said.
The full paper, titled “Increased global nitrous oxide emissions from streams and rivers in the Anthropocene,” was published on Dec. 23. Researchers on the study also included Auburn postdoctoral fellows Hao Shi and Rongting Xu, recent graduate Yuanzhi Yao and doctoral student Naiqing Pan.