Are grassed roadside ditches biogeochemical hotspots?

Background/Question/Methods

Roadside ditches are an ubiquitous feature in developed landscapes. They are implemented to route water off of roads for safety reasons, which leads to pulses of pollutant –laden runoff moving towards streams and other downstream water bodies. Though there is a growing body of research on biogeochemical cycling in various stormwater control measures, there is minimal data available on road ditches. Our research focuses on nutrient cycling in grassed ditches in a suburban watershed in central New York. Nutrients like nitrogen can come from atmospheric deposition on impervious surfaces as well as from fertilizer on adjacent lawns. Specifically, we are measuring fluxes of methane (CH₄), nitrous oxide (N₂O), and carbon dioxide (CO₂) as well as denitrification potential. We are comparing measurements within twelve ditches to reference locations directly adjacent to the ditches to assess whether ditches are hotspots for any of these processes. To discern what factors are driving observed patterns, we are comparing this flux data to measured environmental variables such moisture, temperature, and soil carbon, as well as landscape characteristics such as ditch slope and application of fertilizer to adjacent lawns. 

Results/Conclusions

Potential denitrification, which transforms nitrate into nitrogen gases, in the ditches was significantly higher than in adjacent lawns (p<0.05; avg= 3.45 vs. 0.98 mg N kg^-1 hr^-1). These rates are comparable to those measured in riparian areas, landscape features traditionally viewed as biogeochemical hotspots. Preliminary results on trace gas fluxes indicate that ditches are not consistently sources of these gases. CH₄ fluxes were significantly higher in ditches compared to adjacent lawns. Chronically saturated ditches had CH₄ emissions as high as 45 mg C m^-2 hr^-1, which is comparable to rates observed in temperate wetlands. Across all data, soil moisture was significantly positively correlated to CH₄ fluxes. N₂O and CO₂ fluxes were both higher in adjacent lawns than ditches, though N₂O emissions (max ~80 ug N m^-2 hr^-1) were not as high as those reported in other lawn studies. Since these trace gases are potent greenhouse gases, it’s important to understand what factors drive their production in various parts of the landscape. As we continue this research, we hope to inform better design of these ditches to maximize ecosystem services like denitrification and minimize disservices like greenhouse gas emissions.