Roadside ditches keep water clean

By Elise Cutts

Microbes in ditches remove harmful nitrogen pollutants just as well as those in natural wetlands

Life needs nitrogen, but this nutrient can easily become a pollutant. When too much ends up in the water, it can fuel dangerous algae blooms and create aquatic dead zones where there's too little oxygen for fish and other wildlife to survive.

Wetlands naturally recycle nitrogen, returning it to the atmosphere as harmless nitrogen gas. But more than half of the wetlands in the continental United States have already been lost, with many having been cleared for agriculture — itself a major source of nitrogen pollution.

Still, even as we lose natural wetlands, we may be inadvertently creating new ones. Roadside stormwater ditches might not inspire the way that wild wetlands do, but they're just as good at recycling nitrogen as their natural counterparts, researchers reported in May in the Journal of Geophysical Research: Biogeosciences.

The idea that roadside ditches might function a bit like natural wetlands came to University of Alabama ecologist Corriane Tatariw while she was driving Alabama’s backroads.

"It kind of just evolved through lots and lots of driving, and basically just looking at something that's there all the time that we take for granted," she said.

Tatariw had been splitting time between her home in Mobile and workplaces at the University of Alabama and Dauphin Island Sea Lab, and was putting in long hours each week driving the 250-miles of Alabama backroads connecting the Gulf of Mexico to Tuscaloosa. "I was watching the seasons change, and I could see how the plant growth was changing in the roadside ditches.” These ditches are behaving like wetlands, she realized.

The microbes that inhabit soggy wetland soils remove nitrogen pollutants from water. If ditches really did function a bit like wetlands, then the microbes living alongside roads might recycle nitrogen, too. Inspired by her observations on the road, Tatariw loaded up a bicycle with test tube racks and collected samples from ditches around Dauphin Island in southern Alabama to measure nitrogen-removal rates. With these preliminary results in hand, she set out to conduct a larger study

Tatariw and her colleagues measured nitrogen-removal rates and characterized the microbial communities present in hundreds of samples from roadside ditches around Mobile. They also made measurements of the soil chemistry and tracked whether the ditches were in regions that were forested, urban, or agricultural.

The results confirmed Tatariw’s suspicions: roadside ditches really did host microbial communities capable of recycling nitrogen. And even more remarkably, the nitrogen-removal rates she measured were comparable to those previously described for natural wetlands.

"Finding that ditches are valuable in terms of nitrogen cycling is really important, because ditches are everywhere," said Michael Piehler, a microbial ecologist at the University of North Carolina at Chapel Hill, not involved in the research. Having an accurate understanding of the nitrogen cycle is critical for maintaining clean water, and “impaired water quality has demonstrable economic impacts,” he said.

The microbial communities Tatariw and her colleagues found in the ditches didn't seem to be shaped strongly by the environment outside the ditch — samples from forested, urban, and agricultural areas all hosted similar organisms. While this lack of variation doesn't square with what's known about natural wetlands, Tatariw suspects that it might actually be good news for nitrogen removal.

"[Ditches] may be more resistant to anthropogenic disturbances,” said Tatariw. While natural ecosystems with lots of diversity can easily be disturbed by human meddling, Tatariw suspects that these less complicated, stress-adapted communities in ditches might be a bit more stable in the face of challenges like pollution. “Because of the way they're constructed and because of, essentially, the low level of stress that they are constantly under, it’s just not stress to them.”

While both Tatariw and Piehler cautioned that this result doesn't necessarily mean that building more ditches is a good plan, they ultimately saw the findings as hopeful.

“[This finding] is a little bit heartening in terms of our landscape's ability to assimilate nitrogen — even though a lot of the natural habitats are gone," reflected Piehler. He explained that, while ditches can’t replace natural wetlands, they do have an important advantage over restored and constructed wetlands in terms of nitrogen removal: their age.

Ecological functions like nitrogen removal can take a long time to recover after a wetland is restored — and constructed wetlands are starting from scratch, so it can take decades before they provide the same benefits as natural wetlands. "There have been some processes put in place to speed that up,” he said, “but trying to build something that is identical to its natural analog is really, really difficult." Many ditches have been in place for years and could have been removing nitrogen effectively for years.

"What we're seeing is that these very ubiquitous, often overlooked constructed ecosystems are actually potentially heavy hitters in terms of improving water quality,” said Tatariw.

Going forward, she hopes to build on this finding by determining how different ways of managing ditches could improve their already substantial nitrogen-removal capabilities. Tatariw suggested that even simple changes, such as adjusting how often ditches are mowed, have the potential to enhance nitrogen removal and protect the environment.

“The first thing that we want to look at is how vegetation management affects nitrogen removal potential in the ditches,” she said. “I think that's something that can have directly applicable results.”

Elise Cutts is a graduate student at MIT where she studies how sticky carbohydrate molecules in bacterial slime influence mineral growth. She is also a freelance science writer with words in Science News, Massive Science, and more with a focus on stories at the intersection of the life and Earth sciences, especially those that explore the microbial world. You can see her latest stories at www.elisecutts.com and get in touch either on Twitter @elisecutts or via email at elise.m.cutts@gmail.com.

Photo by Shess Khan Afridi on Unsplash


Elise Cutts

Katherine Kornei
Oct. 5, 2021

NYU School of Professional Studies