Nitrous oxide fluxes in undisturbed riparian wetlands located in agricultural catchments: Emission, uptake and controlling factors

Riparian wetlands can mitigate nutrient pollution to the aquatic environment when they serve as biogeochemically active buffer zones between arable land and water bodies. Nevertheless, as a result of the extensive nutrient transformation, wetlands hold a potential of atmospheric emission of greenhouse gases such as nitrous oxide (N2O). To quantify this potential, fluxes of N2O were measured over a year at 48 sub-plots located in four Danish riparian wetlands with contrasting characteristics of soil parameters and groundwater dynamics. The wetlands were hydrologically and physically relatively undisturbed, but they were all located in catchments dominated by agriculture. Individual fluxes of N2O measured using the static chamber technique ranged from −44 to 122 μg N2O-N m−2 h−1 (n = 800) while cumulative fluxes ranged from −0.25 to 0.50 g N2O-N m−2 yr−1 (n = 48), i.e., showing both uptake and emission of N2O. Modeling of the fluxes using linear mixed models revealed that ammonium in the groundwater was the only tested variable having a significant effect on N2O fluxes. Tentative maximum estimates showed that only about 2.2% of the total Danish N2O emissions could be related to freshwater wetlands (representing about 1.3% of the land area). Further, the low and frequently negative N2O fluxes (n = 294) indicated that riparian wetlands, at least under some conditions, may actually reduce atmospheric N2O pollution, although the measured N2O uptake was weak. In conclusion, riparian ecosystems with only minor disturbances are not generally to be considered as hotspots of N2O emissions in the landscape.