SMART THINKING

Researchers are building bioreactors to prevent this vital nutrient from polluting our waterways.

Scientists are using reactors to protect our waterways from excess nitrogen.

The nutrient nitrogen is essential for plant and animal growth, but excess nitrogen in water is a serious environmental problem. The nutrient contributes to harmful algal blooms that starve fish and other organisms of oxygen. Agriculture is the main source of excess nitrogen, where nutrients used in fertilisers and animal manure can end up in waterways, causing the algal blooms.

The burning of fossil fuels also increases the amount of nitrogen in the air, which can enter waterways when it rains. Closer to home, fertilisers, pet waste, soaps and detergents contain nitrogen which can also flow into waterways.

Researchers from the University of Illinois are developing a solution that can significantly reduce the amount of nitrogen in drainage water, regardless of the production system or location: denitrifying bioreactors. These come in many shapes and sizes, but in their simplest form, they’re trenches filled with wood chips. Water from fields flows through the trench, where bacteria living in wood chip crevices turn nitrate into a harmless gas that escapes into the air. “People are worried we’re just transferring nitrate in water for nitrous oxide, which is a greenhouse gas,” said lead researcher Professor Laura Christianson.

“We don’t know the full story on nitrous oxide with bioreactors yet, but we can say with good confidence they’re not creating a huge nitrous oxide problem.” This conservation practice has been studied for years, but most of what scientists know about nitrogen removal rates is based on laboratory replicas and smaller-scale experimental set-ups. The USDA’s National Resource Conservation Service published a set of standardised bioreactor guidelines in 2015, based in part on Christianson’s early field-scale work, and now more and more farmers around the world are adding bioreactors.

“After gathering all the data, the message is bioreactors work,” says Christianson. “We’ve shown a 20-40 per cent reduction in nitrate from bioreactors in the Midwest, and now we can say bioreactors around the world are pretty consistent with that.”

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