You’ve heard of managing your carbon footprint. But how about your nitrogen footprint? Emissions of reactive nitrogen into the environment have increased more than 10-fold over the past 150 years, contribute to deaths from air pollution and water pollution, and have countless other impacts including acid rain and degradation of ecosystems such as the Great Barrier Reef.
Photo by AgriLife Today
Now, for the first time, researchers have calculated the average nitrogen footprint of people from 188 countries, as well as where exactly they cause that pollution, helping pave the way to policy that could help the world reduce its emissions of reactive nitrogen.
Almost 80 percent of the atmosphere is made of nitrogen in the form of N2. But in that form it hardly interacts with other chemicals — so it is not useful for humans or plants, and it is not harmful either. And for most of Earth’s history, pretty much the only way N2 could be turned into a reactive form like ammonia or nitrous oxide (also known as laughing gas) was either by bacteria, lightning and legumes.
“It takes a lot of energy to turn N2 into reactive nitrogen,” says Arunima Malik from the University in Sydney in Australia.
But since the industrial revolution, humans have been spewing reactive nitrogen into the atmosphere as byproducts from burning fossil fuels. And since the start of the 20th century it has been poured into the ground as fertilizer.
Regulations can be effective at reducing emissions from fossil fuel use, so long as they are not subverted the way they were by Volkswagen. But limiting emissions from nitrogen fertilizers has proved more difficult.
The manufacture of nitrogen-based fertilizer through the Haber process is responsible for feeding about 40 percent of the world’s people, according to Cameron Gourley, an agricultural scientist and secretary of the International Nitrogen Initiative Conference 2016, who was not involved in the study.
“We need to realize this is one of the world’s major breakthroughs,” he said. But we had been making too much, he said. Anything not captured by plants or animals ends up as pollution. “There is no doubt in my mind that nitrogen surplus and the environmental problems that produces is going to be a major problem for us ... It’s very much overlooked.”
To reduce nitrogen emissions effectively, it was not enough just to know where it was produced, said Malik. Instead, we also needed to know where it was going and where it was being consumed.
So Malik and colleagues analyzed a database with 5 billion supply chains of 15,000 thousand commodities in 188 countries. They combined that with a global nitrogen emissions database and a model of how reactive nitrogen moves through various systems.
“We wanted to know who’s producing the products that are eventually being put on the shelves of other countries and who’s being affected by it,” Malik said.
Overall, they found just four countries were responsible for almost half the world’s nitrogen footprint: China, India, the US and Brazil.
But the results, published in Nature Geoscience, demonstrate a remarkable disparity between developed and developing countries on a per-capita basis. On average, each person in Liberia and Papua New Guinea were responsible for less than 7kg of reactive nitrogen pollution each year. Meanwhile, people in Hong Kong and Luxembourg were responsible for more than 100kg each year.
Developed countries usually imported their nitrogen-releasing products from elsewhere. As such, countries like Japan, Germany, the UK and the US had per-capita nitrogen footprints twice as large as the amount they produced locally. The one exception was Australia, which exported a lot of livestock, meaning it exported more of those products than it consumed locally.
And because the researchers used databases showing exactly where the products were produced, and a model that showed exactly where the pollution moved through the environment, they were able to drill down to show precisely how one group’s actions impacted another.
For example, Tulare County in California produces a lot of dairy and cattle, much of which is exported to Japan. As a result of that industry, 15 percent of residents, many of whom are from a Latino background, were exposed to drinking water with fertilizer- and manure-derived nitrogen of more than half the maximum contaminant level, which could cause health problems, especially to newborn babies.
The results showed that to properly limit global nitrogen emissions, international treaties were needed, Malik said. “We need international conventions to control the trade of nitrogen that flows across borders,” she said.
Malik suggested that labeling on products showing its nitrogen footprint could allow consumers to make informed choices too.
Gourley said the problem was the low cost of nitrogen fertilizer. “When the fertilizer input is a smaller part of the cost, there’s no economic incentive to drive down its use,” he said.
Something like a nitrogen tax, which would put a cost on reactive nitrogen use, was needed, he said. “We need to look at economic signals, and how best to manage these externalities.”
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