Nitrogen cycling in constructed and natural coastal wetlands: Assessment using nitrogen stable isotopes

 

Situation: As coastal development increased, there has also been an increase in excess nitrogen (N) pollution in salt marsh estuaries. This excess N has been linked to eutrophication, toxic algal blooms, fish kills, and deaths of other species such as oysters and crabs. Understanding the impact of anthropogenic (i.e., human-derived) N pollution on the nutrient cycling processes of salt marsh estuaries is key to abating the pollution problem. 

 

Actions: In this study, researchers traced the flow of the natural abundance of 15N (stable nitrogen isotope) through two constructed wetlands and two natural coastal wetlands in South Carolina to examine N processes. Both constructed wetlands were located in golf course retention ponds draining into salt marsh estuaries. Both constructed sites received fertilizer and one also received wastewater inputs. The two natural sites were located within the salt marsh estuary. 

 

Take-home message: 

  • The natural systems had lower nitrate-N inputs than the constructed systems, and no nitrate-N was detected in surface water and pore water samples.
  • Surface waters at the site that had wastewater irrigation had higher nitrate-N concentrations and more enriched ammonium (15NH4+)stable isotope signatures compared to the site receiving only golf course fertilizers indicating greater anthropogenic impacts to water quality.
  • Enhanced microbial activity was not able to compensate for excess anthropogenic N in the constructed wetlands, suggesting additional best management practices are needed.
  • Stable isotope data and concentrations can be helpful in identifying the different N sources and potential N-cycling processes occurring in constructed and natural sites if concentrations are sufficient, and were particularly useful identifying ammonium cycling processes.

 

For more information:

  • Aelion, C.M., M.R. Engle, H. Ma. 2009. Use of 15N natural abundance and N species concentrations to assess N-cycling in constructed and natural coastal wetlands. Applied and environmental soil science Vol. 2010. doi:10.1155/2010/371259
  • Email C. Marjorie Aelion at maelion@schoolph.umass.edu