Denitrification in a best management practice bioretention system

Abstract

This study investigates the occurrence of denitrifying soil bacteria in a bioretention system located in Singapore and containing a saturated anoxic zone intended to facilitate denitrification. Soil samples were collected from six depths within the rain garden, four of which were within the saturated anoxic zone. These samples were analyzed using endpoint PCR, targeting total bacterial 16S rRNA or a denitrification gene (nosZ) in order to determine presence or absence of denitrifying bacteria. Three dilutions were used to produce semiquantitative results for the abundance of denitrifying bacteria in a sample relative to samples from other depths. The highest numbers of nosZ amplicons per gram of soil were observed in the deeper levels of the saturated anoxic zone as well as within the root zone of the rain garden. Subsurface water samples from the saturated anoxic zone were also analyzed for oxidation-reduction potential, dissolved oxygen, and nitrogen and phosphorus species. Concentrations of nitrate and nitrite were below the detection limit for most samples, indicating consumption by denitrifying bacteria and high rates of removal for long detention times. Ammonia and phosphorus concentrations are of potential concern because they appear to increase within the saturated anoxic zone.