This dissertation research will compare soil microbial denitrification rates among plots with native vegetation, invasive vegetation, and plots without vegetation in multiple constructed wetlands in Australia in order to determine the optimal conditions for N removal. Human activities have resulted in nitrogen (N) pollution, and this N is affecting water bodies across the globe. Excess N can contaminate drinking water, produce toxic algae, and decrease freshwater biodiversity. In addition, excess N can stimulate algal blooms; subsequently, the death and decomposition of these algae can rob the water column of oxygen. One approach to mitigating these problems is to enhance the biological removal of N via the construction of wetlands. In general, wetland soils are hotspots for denitrification, which is the microbial conversion of dissolved nitrate (NO3-) to N gases. Denitrification, therefore, removes N from wetlands and downstream aquatic systems. Although wetland soils are generally ideal for denitrification, it is unknown how wetland plant community structure influences denitrification rates, and whether denitrification rates can be optimized via management of wetland plant communities. Constructed wetlands often have more invasive species than natural wetlands, and it is also unclear whether removing the invasive species influences N removal via denitrification.
Plant community structure can influence a wetland's value for wildlife and biodiversity, and may influence its efficacy for denitrification, as well. The proposed research will directly inform the planning and implementation of constructed wetlands and ultimately can be used to help land managers to create and restore wetlands that better meet N mitigation goals.
Nitrogen pollution is a global problem that is linked to fisheries declines, drinking water contamination, and loss of biodiversity. Excess nitrogen primarily enters waterways through fertilizer run-off in agricultural and residential areas, and through inadequate sewer systems in urban areas. One strategy for mitigating nitrogen pollution is to enhance biological removal of nitrogen via the construction of wetlands. Constructed wetlands have a high nitrogen removal capacity, but they also often harbor invasive plants, which can negatively influence wildlife and biodiversity. It is not known whether invasive plants also influence nitrogen removal, and therefore, it is unclear if invasive species management influences nitrogen removal capacity. We measured nitrogen removal beneath a common invasive species, the giant reed, beneath a non-invasive cattail, and beneath un-vegetated wetland sediments. We found that plots containing the two plant species had higher nitrogen removal rates than the un-vegetated sediments, but they were not different from one another. This information will help with the management of constructed wetlands, because it suggests that invasive species removal will not influence the nitrogen removal efficacy of constructed wetlands, as long as the wetland remains vegetated. These results also indicate that wetlands can be managed to simultaneously meet several goals: promote biodiversity, create wildlife habitat, and mitigate nitrogen pollution.
