Washing Away Nitrogen Through Roadside Ditches

August 12, 2021

By Brock Parker

Grass ditch on the side of the road.Ditches are a common feature along roadways. The main purpose of these human-made conduits is to provide a path for stormwater to escape, but a University of Alabama researcher’s curiosity discovered a relatively unknown benefit: the removal of nitrogen.

Dr. Corianne Tatariw’s daily commute from Mobile, Alabama, to Dauphin Island was filled with miles and miles of ditches filled with vegetation. They reminded her of natural ecosystems that help lessen the load of nitrogen into coastal waters.

“I thought maybe these can act like wetlands or rivers, which have a reasonably high capacity for nitrogen removal,” said Tatariw, a postdoctoral researcher in UA’s Department of Biological Sciences.

Too much nitrogen can act as a pollutant that can create or sustain harmful algal blooms and dead zones in coastal waters. Those can reduce oxygen levels which kill marine life and cause irritation in human respiratory systems.

The impact of nitrogen can be mitigated when microbes separate it from water, turning it into a gas that dissipates into the atmosphere.

Tatariw’s study, recently published in American Geophysical Union’s Journal of Geophysical Research: Biogeosciences, compared the nitrogen removal ability by microbes in ditches from three distinct landscapes surrounding Mobile.

“You have this shift as you move around the bay from less developed forested land to a highly urbanized area around Mobile itself,” Tatariw said. “On the east side of the bay, you have a lot of agricultural land use.”

Tatariw and her team spent four weeks gathering samples from 96 different ditches along paved two-lane roads in each of those three areas. What they discovered was these ditches were as effective at removing nitrogen as natural ecosystems, such as wetlands and streams.

“It seems that when the ditches are constructed, they’re pretty homogenized, and because they’re managed to a certain extent, it seems like any effects of land use might actually be kind of muted,” Tatariw said. “These drainage networks might actually be a little more resilient to land use change or land use effects.”

To identify and determine the effectiveness of microbial communities in ditches near the forested, urban and agricultural areas, Tatariw’s team looked at plant biomass, inorganic nitrogen content in water and soil characteristics. They identified the different microbes in each sample by using 16S rRNA gene sequencing.

The amount of nitrate removal from each sample was determined by creating a slurry from the soil samples and water. A stable isotope of nitrogen, 15-nitrate, was then added to the slurries to see by how much the microbes reduced it. The result was a potential removal of 89% on average.

“It was really important to connect the microbes and the landscapes,” said Dr. Behzad Mortazavi, professor and chair of UA’s Department of Biological Sciences. “We wanted to learn about their function across the landscapes and the magnitude of their impact.”

Now knowing that these ditches can serve as bonus filters from excess nutrients, Tatariw said information like this can help in future development choices.

“We can possibly design our roadways in a manner that helps reduce nitrogen runoff,” she said. “One choice that could be made is building vegetated ditches as opposed to putting in concrete culverts.”

Tatariw said their data points to plant biomass being an important driver of both nitrogen removal and shaping the microbial communities. Even though there have been some land use effects in rivers and wetlands close to the ditches, it didn’t seem to matter in ditches given their effectiveness at removing nitrogen in all land use types.

“People really haven’t done a lot of work on how roadside ditches might reduce nutrient loads or how much will they remove nitrogen,” she said. “By doing this study, we’ve now provided a baseline and opened a Pandora’s box of questions to ask.”

Information from an article in AGU Eos was used in this report.