Roadside ditches are effective at nitrogen removal

Roadside ditches are a catchall for water, from both sheets of rain that fall on roads and runoff from lawns or fields. Although ditches are ubiquitous in the landscape, they have the potential to be much more than a storm water conduit. In fact, ditches are human-made lowlands that often act as wetlands, complete with fluctuating water levels and a broad array of vegetation and microbes.

In these human-made landscapes, resident microbes and vegetation have the ability to strip nitrogen out of the entering waters, removing it from the system. In the process, nitrogen removal in ditches can reduce the downstream effects of excess nutrients, such as algal blooms and dead zones.

But how effective are ditches at removing nitrogen? Until now, it was poorly understood.

In a new study, Tatariw et al. compared how ditches—those next to forests, urban areas, and agriculture fields—remove nitrogen and what sorts of microbes live in each locale. They looked at three different watersheds near Mobile Bay in Alabama and sampled 96 different ditches that stretched along paved two-lane roads. Each watershed represented ditches along forested, developed, or agricultural lands.

To characterize the ditches, the team looked at plant biomass, inorganic nitrogen content in water, and soil characteristics. Because microbes are so small, they can't be identified even using a microscope, so the scientists used 16S rRNA genes to identify and analyze the different microbes in each sample.

Last, the researchers calculated the potential of nitrate removal for each sample by taking the soil samples, adding water, and making a slurry of ditch material. A stable isotope of nitrogen (15-nitrate) was added to the slurries to see how much nitrogen was reduced by the microbes in the sample.

They found that the microbes in ditches had the potential of removing nitrate (NO₃–) by upward of 89% on average. Although the soil characteristics between types of ditches were similar, the team notes that specific microbes—classified as Nitrososphaeraceae, Nitrosomonadaceae, Gaiellales, and Myxococcales—were more abundant in urban and agricultural ditches where human activity is prevalent.

Overall, ditches were found to have a nitrogen removal potential similar to many natural ecosystems such as wetlands and rivers. The new research shows that roadside ditches may be important areas for removing nitrogen from the environment.