Effects of plant biomass on nitrogen transformation in subsurface-batch constructed wetlands: a stable isotope and mass balance assessment.

Nitrate is commonly found in the influent of subsurface-batch constructed wetlands (SSB CWs) used for tertiary wastewater treatments. To understand the effects of plants and the litter on nitrate removal, as well as on nitrogen transformation in SSB CWs, six laboratory-scale SSB CW microcosms were set up in duplicate and were operated as batch systems with hydraulic residence time (HRT) of 5d. The presence of Typha latifolia enhanced nitrate removal in SSB CWs, and the N removed by plant uptake was mainly stored in aboveground biomass. Typha litter addition greatly improved nitrate removal in SSB CWs through continuous input of labile organic carbon, and calculated enrichment factors (ε) were between -12.1‰--13.9‰ from the nitrogen stable isotope analysis, suggesting that denitrification plays a dominant role in the N removal. Most significantly, simultaneous sulfur-based autotrophic and heterotrophic denitrification was observed in CWs. Finally, mass balance showed that denitrification, sedimentation burial and plant uptake respectively contributed 54%-94%, 1%-46% and 7.5%-14.3% to the N removal in CWs.