Effects of floating vegetation on denitrification,nitrogen retention,and greenhouse gas production in wetland microcosms

Wetlands are biogeochemical hotspots that have been identified as important sites for both nitrogen (N) removal from surface waters and greenhouse gas (GHG) production. Floating vegetation (FV) commonly occurs in natural and constructed wetlands, but the effects of such vegetation on denitrification, N retention, and GHG production are unknown. To address this knowledge gap, we used microcosm experiments to examine how FV affects N and GHG dynamics. Denitrification and N retention rates were significantly higher in microcosms with FV (302 μmol N m^?2 h^?1 and 203 μmol N m^?2 h^?1, respectively) than in those without (63 μmol N m^?2 h^?1 and 170 μmol N m^?2 h^?1, respectively). GHG production rates were not significantly different between the two treatments. Denitrification rates were likely elevated due to decreased dissolved oxygen (DO) in microcosms with FV. The balance of photosynthesis and respiration was more important in affecting DO concentrations than decreased surface gas exchange. The denitrification fraction (N₂-N production: N retention) was higher in microcosms with FV (100 %) than those without (33 %) under increased (tripled) N loading. A 5 °C temperature increase resulted in significantly lower denitrification rates in the absence of FV and significantly lowered N₂O production with FV, but did not significantly change CH₄ production or N retention in either treatment. These results suggest that intentional introduction of FV in constructed wetlands could enhance N removal while leaving GHG production unchanged, an insight that should be further tested via in situ experiments.