Peatlands methane origin and fluxes to the atmosphere: towards an integrative conceptual model of a temperate French peatland

Peatlands cover only 3 % of emerged lands, but their carbon stock represents about 30 % of the global soil organic carbon. Climate change and local anthropogenic disturbances deeply affect the hydrological functioning of peatlands. This may trigger carbon fluxes to surface waters and the atmosphere, thus leading to a positive feedback for global warming. It is therefore crucial to better estimate carbon fluxes between peatlands and the atmosphere and to delineate their major controlling constraints. To achieve this goal, we studied the functioning of a temperate mid-mountain peatland located in the French Jura Mountains, named the Frasne peatland.

The methane (CH₄) dynamics of the Frasne peatland appear to be constrained by a range of hydrological, physical, biogeochemical, and biotic factors. From a hydrological point of view, the system is fed by local rainwater and injection of carbonated groundwater at the bottom of the peatland, which provides a major input of dissolved inorganic carbon (DIC) to the system. Values of the δ¹³C_DIC were high (even reaching positive values up to 8.1 ‰) compared to the expected values in a limestone and C3 plant-dominated area such as the Jura Mountains, supporting biotic CH₄ production within the peatland. Consistently, high-frequency eddy-covariance monitoring during 2.5 years allowed us to show that the site acted as a source of CH₄ to the atmosphere (23.9 ± 0.6 g C m^-2 year^-1) with interannual, seasonal, and diurnal time scale dynamics. In particular, we found an outstanding diurnal cycle for CH₄ with the highest fluxes at night and lower ones at mid-day. In addition, the mid-day fluxes were negative in spring, highlighting larger oxidative processes than CH₄ production attributed to photosynthesis activity (i.e., soil oxygen penetration and endosymbiotic methanotrophs of Sphagnum). The range of CH₄ emissions was also controlled by the interannual variation in precipitation amounts and by the seasonal temperature variation.

This conceptual production-emission model highlights that water-carbon interactions in the peatland depend on local biotic and abiotic factors but also on hydrological processes at the watershed scale. This also highlights the need to further constrain carbon transfers between the production and the emission zones (i.e., peatland-atmosphere interface and surface water exports). For this purpose, we will soon carry out a field campaign to measure the concentrations and isotopic values of dissolved gases in peat pore water along with an upstream downstream and a vertical gradient.