Methane is a potent greenhouse gas, and undrained peatlands are among the most important natural sources of it. In these waterlogged ecosystems, oxygen is scarce below the surface, so microbes decomposing old plant material produce methane in the saturated peat. Once formed, the gas can reach the atmosphere along three main routes: it can diffuse through the water and soil, it can travel through the hollow internal tissues of vascular plants such as sedges, or it can build up as free-phase gas bubbles until buoyancy forces them upward in sudden releases. While the first two pathways are relatively well studied, ebullition has remained stubbornly difficult to characterize, because it is episodic, spatially patchy, and easily missed by conventional flux chambers that sample at fixed intervals.

To capture this elusive pathway, the team deployed gas traps across areas with different plant cover types in the fen, allowing bubbles rising from the peat to be collected and quantified directly. They then partitioned the ebullition contribution from chamber-based measurements of total methane flux, relating the results to soil properties, water table position, pore water methane concentrations, and plant biomass. This dual approach, pairing in situ bubble collection with flux chamber data, is one of the few ways to reliably separate the episodic bubble flux from the steadier diffusive and plant-mediated components of the total emission.

The field results revealed a clear seasonal and spatial pattern. Ebullition peaked in the warmest months of July and August and was greatest in sedge-dominated areas where the water table sat closest to the surface. That combination makes physical sense: warmer peat accelerates microbial methane production, while a shallow water table shortens the distance a bubble must travel before it can escape, reducing the chance that it dissolves or is oxidized along the way. When the researchers built statistical models incorporating soil temperature, precipitation, water table depth, and pore water methane concentrations, these variables explained 42 percent of the spatiotemporal variation in measured ebullition volume and 61 percent of the variation in ebullition methane flux. In other words, a substantial share of the variability can be predicted from a handful of routinely measured environmental parameters, which is encouraging news for anyone trying to model wetland emissions at larger scales.

Yet a considerable fraction of the variation remained unexplained, underscoring just how heterogeneous ebullition is. Previous work at other peatlands has shown that bubbles can accumulate in discrete pockets of trapped gas within the peat profile and release in bursts triggered by falling atmospheric pressure, changes in water table, or simply the gradual growth of bubbles until they exceed a critical size. The Ontario fen study adds to this picture by showing that even within a single wetland, the timing and magnitude of bubble release differ sharply among plant communities, meaning that point measurements can badly misrepresent the true flux if they do not sample across vegetation types.

To isolate the role of plants more rigorously, the researchers brought intact water-saturated monoliths from the fen into a greenhouse, where they could control conditions and observe ebullition under different vegetation cover. The greenhouse experiment produced a surprising twist. Dense sedge cover was associated with higher belowground biomass but lower concentrations of methane in the pore water and less frequent ebullition. This counterintuitive result likely reflects the complex interplay between sedges and methane dynamics: sedge roots transport oxygen into the rhizosphere, where it can fuel methane-oxidizing microbes and suppress production, while root exudates can simultaneously provide fresh carbon substrates for methanogens. The net effect observed here was that thick sedge stands actually dampened the bubble pathway, even though sedges are often associated with high overall methane emissions through their internal transport tissues.

The numbers from the greenhouse experiment highlighted how much the ebullition contribution can swing with vegetation. Across the different cover types, ebullition accounted for between 1.9 and 46.9 percent of total methane emissions, a more than twentyfold range. In the field, the researchers estimated that ebullition contributed approximately 16.4 percent of total methane emissions from the fen. That figure is too large to ignore. Many ecosystem-scale methane budgets rely on chamber measurements or eddy covariance techniques that implicitly average over episodic events, and if one in six units of methane is leaving the peat as bubbles, campaigns that fail to capture ebullition risk systematically underestimating emissions.

The findings carry particular weight for the calcareous fens that dot parts of southern Ontario and similar landscapes elsewhere. These minerotrophic wetlands, fed by groundwater rich in calcium and other ions, occupy a distinctive position in the peatland spectrum, and their hydrology is strongly shaped by the surrounding hydrogeological setting. Because fens often support lush sedge vegetation, they have frequently been treated in models as systems where plant-mediated transport dominates and bubbles matter less. The new study complicates that assumption, showing that even in sedge-rich areas, ebullition remains a meaningful emission pathway, and that dense sedge cover can actually reduce it locally while adjacent communities continue to vent methane through bubbles.

For climate modeling, the implications are twofold. First, the demonstrated predictive power of simple environmental variables suggests that ebullition could be incorporated into process-based methane models with greater confidence, using soil temperature, water table depth, precipitation, and pore water methane concentration as driving inputs. Second, the strong vegetation signal means that maps of plant community composition, which are increasingly available from satellite imagery and drone surveys, could help identify hotspots and cold spots of bubble release within a wetland. As warming accelerates methane production in northern and temperate peatlands, and as shifting precipitation regimes alter water tables, the episodic bubble pathway may become an even larger share of the total, making its accurate representation a matter of real consequence for emission projections.

The study also serves as a methodological reminder to the wetland science community. Gas traps are inexpensive and simple, yet they capture a flux component that standard chambers routinely miss or smear across sampling intervals. The researchers argue that quantifying ebullition directly in the field is essential for accurate estimation of peatland methane emissions, including from sedge-dominated areas where the bubble pathway was once thought to be negligible. As nations refine their wetland greenhouse gas inventories and restoration projects multiply across rewetted peatlands, distinguishing between methane that seeps, methane that streams through plants, and methane that erupts in bubbles will be key to knowing exactly how much warming power these carbon-rich landscapes are releasing into the atmosphere.

Subject of Research: Controls on methane ebullition as a methane emission pathway in a calcareous fen peatland

Article Title: Evaluating soil and plant controls on the contribution of ebullition to methane emission in a calcareous fen

Article References: Kangro, R. J., & Strack, M. (2026). Evaluating soil and plant controls on the contribution of ebullition to methane emission in a calcareous fen. Plant and Soil. https://doi.org/10.1007/s11104-026-09084-4

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09084-4

Keywords: methane, ebullition, peatland, fen, wetland emissions, sedge, soil temperature, water table, pore water methane, belowground biomass, greenhouse gas, Plant and Soil