Sinks and Sources: The Dynamic Contributions of Riparian Wetlands to Catchment Carbon Budgets

Abstract:

Wetlands are important sites of catchment carbon cycling: they receive, store, and emit carbon. However, the contribution of wetlands to carbon emissions remains largely unquantified, especially for ephemeral wetlands. As inundation levels change across stream corridors, riparian wetlands can shrink or dry completely, which may change the magnitude of CO₂ and CH₄ they emit. To quantify the role of wetlands and changing water levels in stream corridor carbon emissions, we measured CO₂ and CH₄ fluxes in four riparian wetlands from June - August 2019 at Coweeta Hydrologic Laboratory, NC and in the Jefferson National Forest, VA. We measured CH₄ and CO₂ emissions at multiple sites within each ecosystem using a flux chamber attached to a portable greenhouse gas analyzer. Coweeta wetlands were sources of CO_2; their emission fluxes ranged from 22 to 1831 mmol m^-2 d^-1 (median: 211 mmol m^-2 d^-1) . CH₄ fluxes measured from riparian wetlands in Coweeta were heterogeneous, likely captured ebullitive fluxes, and ranged from -1.1 to 2713 mmol m^-2 d^-1 with a median of 0.55 mmol m^-2 d^-1. Riparian wetlands in the Jefferson Forest were drier and lower emitters; they were net sources of CO₂ (median: 193 mmol m^-2 d^-1; range -334 to 3370) and net sinks for CH₄ (median: -0.01 mmol m^-2 d^-1; range -0.32 to 0.03) when we combined data from wet and dry sampling periods. Though we predicted that temperature might increase CO₂ and CH₄ fluxes, there was no relationship between water temperature and emissions. We also deployed inundation sensors to monitor the presence and absence of water across wetland transects. Ongoing work is linking changes in wetland water levels with CH₄ and CO₂ fluxes. Gaining insight into how much carbon is emitted from riparian wetlands and how changing inundation controls carbon fluxes is a critical next step towards recognizing their role in catchment carbon budgets.