Carbon dioxide fluxes reflect plant zonation and belowground biomass in a coastal marsh

Coastal wetlands are major global carbon sinks; however, they are heterogeneous and dynamic ecosystems. To characterize spatial and temporal variability in a New England salt marsh, greenhouse gas (GHG) fluxes were compared among major plant‐defined zones during growing seasons. Carbon dioxide (CO₂) and methane (CH₄) fluxes were compared in two mensurative experiments during summer months (2012–2014) that included low marsh (Spartina alterniflora), high marsh (Distichlis spicata and Juncus gerardii‐dominated), invasive Phragmites australis zones, and unvegetated ponds. Day‐ and nighttime fluxes were also contrasted in the native marsh zones. N₂O fluxes were measured in parallel with CO₂ and CH₄ fluxes, but were not found to be significant. To test the relationships of CO₂ and CH₄ fluxes with several native plant metrics, a multivariate nonlinear model was used. Invasive P. australis zones (−7 to −15 μmol CO₂·m⁻²·s⁻¹) and S. alterniflora low marsh zones (up to −14 μmol CO₂·m⁻²·s⁻¹) displayed highest average CO₂ uptake rates, while those in the native high marsh zone (less than −2 μmol CO₂·m⁻²·s⁻¹) were much lower. Unvegetated ponds were typically small sources of CO₂ to the atmosphere (