The effect of floating vegetation on denitrification and greenhouse gas production in wetlands

Background/Question/Methods

Anthropogenic intensification of nitrogen (N) loading to aquatic ecosystems is widespread and can lead to the degradation of these systems. Wetlands are important sites for N removal via denitrification, the microbially mediated reduction of reactive nitrate to inert N₂ gas, but they can also produce high levels of greenhouse gases. Floating plants play an important role in encouraging denitrification, since they create low oxygen conditions that may favor denitrification. We investigated whether a wetland with floating plant cover had higher denitrification and greenhouse gas production rates than a wetland without floating plants. A wetland on the WSU-Vancouver campus covered in azolla, a native floating fern, was divided in half and azolla was removed from half of the wetland. Additionally, mesocosms were constructed in a growth chamber to replicated this system. Denitrification and greenhouse gas production rates were calculated by measuring excess N₂gas, methane, and nitrous oxide concentrations in the water column and measuring the gas exchange rates between the water column and the atmosphere. Gas exchange rates were measured using the relative loss rate of a volatile tracer added to the wetland inlet and measured at the wetland outlet.

Results/Conclusions

N₂ concentrations were statistically greater on the azolla-covered side of the wetland compared to the uncovered side of the wetland. Additionally, N₂ concentrations were higher in the morning compared to the evening indicating a diel pattern in N₂ production. Greenhouse gas concentrations were not different between the two sides. Over the course of our short-term experiment, our observations were consistent with greater N₂ gas production under floating vegetation, but further seasonal sampling is necessary to determine how the system responds to different inflows and temperatures.