Spectral evidence for substrate availability rather than environmental control of methane emissions from a coastal forested wetland

Knowledge of the dynamics of methane (CH₄) fluxes across coastal freshwater forested wetlands, such as those found in the southeastern US remains limited. In the current study, we look at the spectral properties of ecosystem net CH₄ exchange (NEECH₄) time series, and its cospectral behavior with key environmental conditions (temperature (Tₛ₅), water table (WTD) and atmospheric pressure (Pₐ)) and physiological fluxes (photosynthesis (GPP), transpiration (LE), sap flux (Jₛ)) using data from a natural bottomland hardwood swamp in eastern North Carolina. NEECH₄ fluxes were measured over five years (2012 – 2016) that included both wet and dry years. During the growing season, strong cospectral peaks at diurnal scale were detected between CH₄ efflux and GPP, LE and Jₛ. This suggests that the well understood diurnal cycles in the latter processes may affect CH₄ production through substrate availability (GPP) and transport (sap flow and LE). The causality between different time series was established by the magnitude and consistency of phase shifts. The causal effect of Tₛ₅ and Pₐ were ruled out because despite cospectral peaks with CH₄, their phase relationships were inconsistent. The effect of fluctuations in WTD on CH₄ efflux at synoptic scale lacked clear indications of causality, possibly due to time lags and hysteresis. The stronger cospectral peak with ecosystem scale LE rather than Jₛ suggested that the evaporative component of LE contributed equally with plant transpiration. Hence, we conclude that while the emission of dissolved gases through plants likely takes place, it may not contribute to higher CH₄ emissions as has been proposed by aerenchymatous gas transport in sedge wetlands. These findings can inform future model development by (i) highlighting the coupling between vegetation processes and CH₄ emissions, and (ii) identifying specific and non-overlapping timescales for different driving factors.

Duke Scholars

Author Kevan Minick  

Author Jean Christophe Domec Environmental Natural Science