Effects of seasonality, transport pathway, and spatial structure on greenhouse gas fluxes in a restored wetland

Abstract

Wetlands can influence global climate via greenhouse gas ( GHG ) exchange of carbon dioxide ( CO 2 ), methane ( CH 4 ), and nitrous oxide (N 2 O). Few studies have quantified the full GHG budget of wetlands due to the high spatial and temporal variability of fluxes. We report annual open‐water diffusion and ebullition fluxes of CO 2 , CH 4 , and N 2 O from a restored emergent marsh ecosystem. We combined these data with concurrent eddy‐covariance measurements of whole‐ecosystem CO 2 and CH 4 exchange to estimate GHG fluxes and associated radiative forcing effects for the whole wetland, and separately for open‐water and vegetated cover types. Annual open‐water CO 2 , CH 4 , and N 2 O emissions were 915 ± 95 g C‐ CO 2 m −2 yr −1 , 2.9 ± 0.5 g C‐ CH 4 m −2 yr −1 , and 62 ± 17 mg N‐N 2 O m −2 yr −1 , respectively. Diffusion dominated open‐water GHG transport, accounting for >99% of CO 2 and N 2 O emissions, and ~71% of CH 4 emissions. Seasonality was minor for CO 2 emissions, whereas CH 4 and N 2 O fluxes displayed strong and asynchronous seasonal dynamics. Notably, the overall radiative forcing of open‐water fluxes (3.5 ± 0.3 kg CO 2 ‐eq m −2 yr −1 ) exceeded that of vegetated zones (1.4 ± 0.4 kg CO 2 ‐eq m −2 yr −1 ) due to high ecosystem respiration. After scaling results to the entire wetland using object‐based cover classification of remote sensing imagery, net uptake of CO 2 (−1.4 ± 0.6 kt CO 2 ‐eq yr −1 ) did not offset CH 4 emission (3.7 ± 0.03 kt CO 2 ‐eq yr −1 ), producing an overall positive radiative forcing effect of 2.4 ± 0.3 kt CO 2 ‐eq yr −1 . These results demonstrate clear effects of seasonality, spatial structure, and transport pathway on the magnitude and composition of wetland GHG emissions, and the efficacy of multiscale flux measurement to overcome challenges of wetland heterogeneity.

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