Methane in Two Stream Networks: Similar Contributions From Groundwater and Local Sediments While Oxidation Was a Large Sink Controlling Atmospheric Emissions

Streams are important sources of methane (CH₄) to the atmosphere but magnitudes and regulation of stream CH₄ fluxes remain uncertain. Stream CH₄ can come from groundwater and/or produced in anoxic sediments. A fraction can be microbially oxidized to carbon dioxide (CO₂) when passing redox gradients in soil, sediment, or water, while the fraction escaping oxidation is emitted to the atmosphere. The relative importance of the CH₄ sources (groundwater inputs vs. sediment production) and the fraction oxidized is typically unknown, yet key for the regulation and magnitude of stream emissions. In this study, we followed the transport of CH₄ from below-stream soils to the stream water surface and to the atmosphere using a combination of CH₄ concentration and stable carbon isotope gradient measurements, high resolution stream flux and discharge assessments, and inverse mass-balance modeling. Sampling was done in multiple locations in the stream network of two independent catchments in Sweden to consider spatial variability. We show that the surface water, sub-surface, and groundwater CH₄ concentration, CH₄ oxidation, and emission were highly variable in space. Our results indicate that the variability could be related to stream morphology and soil characteristics. Of the total CH₄ input into the streams, roughly half of it was estimated to come from groundwater CH₄ in both catchments (39% and 57%; the rest from sediment production), and most of the CH₄ was oxidized (97%–99%) before emission to the atmosphere. Our results indicate that CH₄ oxidation is a major sink for CH₄ in the studied streams.