Denitrification at the microscale in treatment wetlands

Abstract

Treatment wetlands (TWs) have been in use for over three decades for wastewater treatment, agricultural water treatment, and some industrial wastes. Thousands of TWs exist for treating wastewater globally, but the microbial processes and controls in situ primarily responsible for water treatment are poorly understood. In this study, 16 separate model TW columns consisting of three plant groups and one non-planted group were fed synthetic post-secondary wastewater with half receiving no added carbon and half receiving 0.391 g L -1 as sucrose. Core samples were taken from each of the TW columns and separated into three distinct habitats (roots, gravel, particulates). Each habitat was assayed for its ability to produce N 2O, consume N 2O, and emit N 2O, as well as for denitrification gene abundances (nirS, nirK, and nosZ) and bacterial gene abundance (16S rDNA). The addition of organic carbon to the wetland was found to increase denitrification activity and gene copy abundance in non-root fractions, but organic carbon addition did not affect the root fraction. Plant presence within the TW was found to increase gas assay and gene abundance values in non-root habitats. Differences between three plant species were minor compared to differences attributed to carbon addition and plant presence. Of all habitats, gravel was found to have the highest denitrification activity and denitrification gene copy abundance relative to the number of 16S rDNA copies, as well as the highest ratio of N 2O produced to N 2O emitted. Implications for this study suggest the gravel and root fractions should be studied in further detail for their ability to accommodate denitrifying microbes.