Urbanization and associated increases in impervious cover and nutrient use can impact surrounding watersheds. Urbanization can result in increases to runoff volume, discharge rates, and nutrient pollution into receiving water bodies. Urban stormwater best management practices (BMPs) are engineered structures designed to mitigate the impacts of urbanization. Green infrastructure (GI) or low impact development (LID) uses these BMPs in a way that attempts to maintain the pre-development hydrologic regime. Nitrogen is one of the nutrient pollutants found in increasing levels in watersheds with urbanization. While certain urban stormwater BMPs are not designed to provide microbially-facilitated denitrification per se, they may potentially provide this benefit to the ecosystem. Selected BMPs, namely, bioretention facilities, dry ponds, and surface sand filters were sampled across various treatment trains to identify the overall bacterial community. Both DNA and RNA were extracted from soil and sequenced to establish the composition and viability of the community. In addition, we identified the presence of certain functional denitrification genes (nirK, nirS, nosZ) using specific primers. Soil variables were assessed to indicate the environmental conditions driving the underlying microbial community taxonomy in sampled BMPs.
Results/Conclusions
PCoA analysis showed bacterial community clustering by BMP type, with a high total diversity of 1093 genera. The DNA and RNA bacterial communities were different, with tight clustering by BMP type in the DNA. Communities were more widely distributed in the RNA, with some clustering by BMP type. Bioretention facilities and dry ponds had higher levels of nitrogen and carbon than surface sand filters which could impact the denitrification process. Certain soil variables were linked to the bacterial community taxonomy of specific BMP types. For example, CCA analysis indicated a strong influence of soil texture (percent fines) on dry pond community composition, while nitrogen and carbon content drove bioretention facility communities. Bulk density and percent sand were the primary soil variables influencing surface sand filter communities. In addition, known denitrifier taxa that have the nosZ gene and are able to conduct the entire denitrification cycle, were found in several samples. These results show bacterial taxonomic variation in the sampled BMPs. Successful RNA extraction indicates the presence of a viable, functioning, bacterial community. The presence of nirK, nirS and nosZ genes in the samples indicates that the denitrification cycle is possible in these BMPs under favorable conditions.