Influence of sampling depth and stand species on the potential methane uptake of forest soil samples.

Methane (CH4) is one the most important greenhouse gas and is responsible for approximatively 20% of the global warming (IPCC, 2022). Soils and mainly upland forest soils where aerobic environment prevails, are one of the main global sink of methane (Dutaur et Verchot 2013). At the soil-atmosphere interface, the net methane efflux consists in a net balance between the production of CH4 by methanogenic bacteria mainly in deep anaerobic soil layers and the consumption by methanotrophic bacteria in the aerobic soil horizons of the methane produced in the soil or diffusing from the atmosphere into the soil.

Presence of trees could influence soil edaphic features (mainly carbon content, pH, nitrogen, soil structure and texture, water content, …) which can have an impact on the abundance of methanotrophic and methanogenic communities in the soil profile and thus on methane uptake. In the upper part of the well aerated mineral soil, the abundance of methanotrophs is highest, but the depth of this level depends on the thickness of the organic layer and then on the rate of litter mineralisation. Depending on the season and the tree species in the plot, the intensity of methane uptake and the pattern of methane consumption may change.

The objective of this project was to study the influence of the temporal dynamics of methane consumption in a soil profile of different forest stand types. For this purpose, we developed a method to sample intact soil cores. We took 5 soil cores of 3 different thicknesses (5, 10 and 15 cm) in a forest of spruce, beech, oak and pine at different dates in spring 2022. Methane and CO2 fluxes were measured in the week after sampling on the soil cores incubated at 20°C.

Regardless of season, methane consumption increased with sample thickness. In the upper 5 cm, methane consumption was highest of the beech forest compared to the other stand types. However, when considering the 15 cm of soil, methane consumption no longer differed between stands. This trend seems to be related to the sharp decrease in organic carbon content and the much lower water content in spruce and pine forests. It is also possible that methane consumption at depth in the beech forest is limited by the low availability of methane at depth, which has been consumed at the top of the profile.