| Description |
Managing nitrogen in the environment is one of the 14 Grand Challenges identified by the National Academy of Engineers. There is a lack of knowledge about the nitrogen transformation processes occurring in bioretention ponds aside from their contribution to nitrous oxide (N2O) production. In this study, sediment samples collected from three existing bioretention systems located near the University of Utah Campus (1) to investigate the role of nitrifiers and denitrifiers, in nitrogen transformation through nitrification and denitrification rates serum bottle tests, and molecular analysis, and (2) to estimate the N2O fluxes from bioretention pond. The nitrification rates in winter were estimated to be 50, 17.2, and 239% times higher than summer for non-vegetative, upland, and wetland bioretention ponds, respectively, demonstrating the effect of temperature on nitrification efficiency of ponds. Also, the average denitrification rates in summer were relatively lower than those recorded in winter months. The denitrification rates in winter were 96.5, 13.3, and 242% higher than summer for wetland, upland, and non-vegetative bioretention ponds, respectively. Overall, both nitrification and denitrification performances were found to be higher in winter than summer. Furthermore, a quantitative polymerase chain reaction represented high abundance of amoA, nirS, nirK, and atypical nosZ genes in all winter samples. Surprisingly, the atypical nosZ gene was present in all samples in both seasons in high numbers, but relatively less gene abundance was observed in almost all soils collected during summer. Also, some samples did not show iv any detectable gene abundance in summer samples. This may be potentially due to low precipitation events during summer, less availability of nutrients, and loss of other environmental factors that support the favorable growth of microbial communities. The high abundances of amoA and nir functional genes also support the rationale that nitrification and denitrification are occuring in bioretention ponds, which is further supported by serum bottle nitrification and denitrification rates. These results also support the notion that atypical nosZ contributing to N2O sequestration are present in bioretention ponds. Also, N2O emissions estimated from the bioretention ponds, using in-situ flux chambers, ranged between 0.05-0.197g N2O_N /m2/yr., thus suggesting that bioretention can also become a potential contributor of N2O to the atmosphere under certain environmental stress conditions. |
