Nitrogen-based fertilizers have hugely increased agricultural output, but rain washes them into rivers and oceans with dire consequences. Zhejiang University researchers are searching for a middle ground.
Every summer, vast quantities of algae and zooplankton flourish in coastal waters of the Gulf of Mexico. These algal blooms gobble up oxygen, leaving a ‘dead zone’ in the ocean that covers around 10,000 square kilometres, and is largely devoid of life. The oxygen-zapping algae and zooplankton are fed by synthetic nitrogen-based fertilizers applied to crops far inland in the United States, and washed hundreds of kilometres along the Mississippi River into the Gulf.
Hundreds of other such ocean dead zones exist around the world. Synthetic fertilizers also contribute to air pollution and greenhouse gas production, according to research led by Professor GU Baojing, an environmental economist at Zhejiang University.
But in a surprise positive finding, the Gu team has also discovered that elevated CO2 in the atmosphere could increase the ability of crops to use natural sources of nitrogen, reducing dependency on artificial fertilisers by up to 14%1.
Developing a complete picture of the influences on how nitrogen moves through plants, animals, water and the atmosphere is important, says Gu, because synthetic nitrogen-based fertilizers are critical for feeding humanity. Addressing the problem of nitrogen pollution requires a more considered response than simply banning the use of fertilizers, he says.
With that aim, his team has also been identifying opportunities to solve the nitrogen pollution problem and bring us closer to achieving United Nations Sustainable Development Goals (SDGs), such as eliminating hunger, protecting life underwater, slashing pollution and mitigating climate change.
“If we can accelerate these improvements to nitrogen use and management now, we will be reaping the rewards for a thousand years to come,” says Gu.
Double whammy
Nitrogen makes up 78% of the gas in the atmosphere. But because it’s inert, plants struggle to harvest it from the air and convert it into a chemically reactive form that they can use. “Natural biological systems all lack nitrogen,” says Gu.
Nitrogen was so precious in the 1800s, that sailors traveled from Europe and the United States far into the Pacific and Atlantic oceans to remote islands to mine the limited supplies of guano, or bird droppings, that contained it. In the early 20th century, the development of the Haber-Bosch process for making ammonia from nitrogen at an industrial scale provided a secure source of nitrogen, boosting agricultural production to feed billions.
But the overuse of nitrogen fertilizers in the past century has also had grave consequences.
Nitrogen pollution from farming doesn’t only run into rivers and oceans when it rains, it also escapes as ammonia and oxides of nitrogen into the atmosphere. There, these gases react to form PM2.5, tiny airborne particles smaller than 2.5 microns.
In a paper in published in Science in 2021, Gu and his collaborators reported that ammonia is an unexpectedly significant contributor to deadly PM2.5 air pollution2. These have been typically blamed on sulfur dioxide emissions from dirty coal combustion or nitrogen oxides produced in vehicle engines.
As a result, Gu says, most countries have overlooked ammonia gas as a pollutant coming from agriculture: “It is almost completely unregulated”.
Overall, the researchers estimated that nitrogen-containing gases caused some 39% of global PM2.5 pollution in 2013. Other than harming human health, nitrogen pollutants are also greenhouse gases — in particular, nitrous oxide is about 300 times more potent than carbon dioxide, Gu says.
Double win
As carbon dioxide becomes more abundant in the atmosphere, however, there could be a surprising way to provide crops with the nitrogen they need to grow and reduce nitrogen pollution1.
“We were stunned to see that we can achieve a double win, and that the hidden potential for generating benefits was massive,” Gu says.
He and his collaborators found that increases to atmospheric carbon dioxide levels, between 2000 and 2050, could then lead plants that fix nitrogen in special nodules on their roots to increase productivity by 28–85%. Nitrogen fixation is a trick whereby specific kinds of plants work with symbiotic bacteria to turn inert nitrogen gas into forms of nitrogen that can be used biologically.
Over the same period, with higher carbon dioxide availability, plants are predicted to suck up nitrogen from fertilizer about 19% more efficiently, resulting in a reduction of 45% in nitrogen runoff and about a 30% reduction in ammonia and nitrogen oxides leaking into the air.
The prediction that plants in a more carbon-dioxide-rich world can use nitrogen more efficiently also means that fertilizer use could be cut by 9-14% — depending on what actions are taken to optimize how nitrogen is used, says Gu. Although, he cautions, as other climate changes such as temperature and rainfall will also affect nitrogen uptake by plants, more studies are needed to work out the overall impact of climate change.
Feasible, practical, executable
But that won’t be enough to solve the problem of excessive fertilizer use, says Gu. Looking to the future three things will have to change, Gu says in Nature3: People in wealthy countries have to eat less, and eat less meat. Fertilizers must be used more efficiently, ensuring that the nitrogen winds up in our food, instead of the oceans or the atmosphere. Finally, reactive nitrogen compounds should be recycled, by using materials such as manure and straw as fertilizers.
Meanwhile, the Gu team continues to fill in the picture of how nitrogen moves through the environment, currently modeling extreme weather events to understand, for instance, how nitrogen runoff changed during periods of persistent heavy rain.
The team is also turning to advances in modelling and satellite imagery to monitor and predict changes in nitrogen systems.
“These are techniques that we didn’t even dare imagine just years ago,” Gu says. “So, I am hopeful that we can develop feasible, practical solutions that can quickly become reality.”
References:
1. Cui, J., et al.Nat Sustain (2023). DOI: 10.1038/s41893-023-01154-0
2. Gu, B., et al.Science 374, 758-762 (2021). DOI: 10.1126/science.abf8623
3. Gu, B., et al. Nature 613, 77–84 (2023). DOI: 10.1038/s41893-023-01154-0