2016 Salt Lake County Watershed Symposium

Revealing Intrinsics of Nitrogen Transformation in Green Infrastructure
Unmitigated rainfall runoff from the impervious surfaces common to urban infrastructure can lead to many environmental problems. Water quality of the surface runoff is degraded because of deposition of various types of pollutants. This load-magnified stormwater discharge does not meet the required discharge quality standards because high levels of nutrients may result in eutrophication or algal blooms. Eutrophication will lead to excessive plant growth thus lowering water-column oxygen levels. Excess levels of nutrients or build-up of toxins may stimulate growth of algal blooms in receiving waterbodies and thereby killing aquatic biota.

Bioretention basins, as stormwater control measures, take advantage of natural environmental processes through incorporation of vegetation and different types of soil media resembling a natural ecosystem. The different layers of soils present in bioretention basins provide an environment favorable for the growth of various microorganisms which help in degrading filtered pollutants like nitrogen and phosphorus. Unfortunately, research has shown that nitrous oxide (N2O) gas which is a potent greenhouse gas is a byproduct of nitrogen removal in soil in both nitrification and denitrification processes. However, very limited research has been done in the direction of understanding the role and abundance of microbial communities involved in the process of nitrogen transformations，and contributes to the release of N2O into the atmosphere from bioretention basins.

During the study, samples were collected from existing bioretention basins located near the University of Utah Campus. Core samples of soil, and composite effluent and influent stormwater samples were collected from the basins and then transported on ice to the laboratory and subsequently analyzed. The results suggest that the quantitative abundance amoA, atypical NosZ, NirK, and NirS gene in all soil samples collected from the bioretention basins during winter are ranging from 1.7×10^6 to 1.7×10^7copies/gm-soil, 1.2×10^6 to 3.1×10^7copies/g-soil, 2.7×10^6 to 3.9×10^8copies/gm-soil, and 3.4×10^7 to 7.4×10^8copies/gm-soil, respectively. Furthermore, results showed lower amoA, NirS and NosZ gene abundance and no amplification of NirK gene in control basin soil samples.
Methods for analyzing phosphorus removal efficiency are being developed. Potential links between emissions of greenhouses gas like N2O and seasonal distribution of microbial communities in bioretention basins with different plant communities are also being observed. Results indicate the presence of AOB strains, Nitrosospira sp. 40KI and Nitrosospira sp. NpAV which have the ability to produce N2O during a nitrifier denitrification process. Moreover, analysis of both winter and summer stormwater samples showed higher concentrations of nitrate in effluent than influent. Further analysis on summer samples and N2Oemissions from each basin are in progress.

The purpose of this research is to comprehend the mechanisms involved in biological nitrogen removal, to explore the contribution of various pathways in N2Oemissions, and understanding microbial ecology for developing a better design of bioretention basin in future which will result in reduced emissions of greenhouse gases into the atmosphere.