Persistent and increasing forest floor methane consumption in a boreal mineral-soil pine forest over seasons and years

Mineral soil forest floors often act as net methane (CH4) sinks. The contribution of the different components of the forest floor, such as soil, shrubs, mosses and their roots and the roots of trees, to the sink and the factors affecting their contribution are not well known. Predicting the CH4 flux dynamics of forests requires understanding the component fluxes and drivers, such as microbial population, soil moisture and temperature, and composition and coverage of the forest floor vegetation.The CH4 exchange of the forest floor at the SMEAR II experimental forest in central Finland (Hari & Kulmala, 2005) was monitored using manual and automated chambers for a total of more than 10 years (2006-2016 manually, 2021-mid 2023 and mid 2025 onwards automatically). In addition, a manipulation experiment was conducted using manual chambers where either tree, shrub or mycorrhizal roots were excluded by trenching. Also, the effect of above ground vegetation (shrubs, mosses) on CH4 flux dynamics was studied. The humus and soil layers next to the automated chambers were inspected for presence of methanotrophs.We found that the forest floor is a persistent sink for CH4 through the year, CH4 being consumed even during winter on all measurement plots. The general trend in the long-term measurements was towards a larger sink during growing season. Increasing soil temperature increased the sink during the growing season, while soil moisture decreased it. During growing season, a diurnal pattern was observed where higher CH4 consumption occurred during night time.In the trenching experiment the exclusion of tree, shrub or mycorrhizal roots did not affect soil CH4 uptake, however, the cutting all above ground vegetation increase CH4 uptake compared to presence of normal vegetation (shrubs and mosses). Based on the probe-targeted metagenomic sequencing, methanotrophic bacteria within the organic and mineral soil layers consisted mainly of alphaproteobacterial high-affinity oxidizers, including taxa potentially adapted to oxygen-limited conditions.Hari, P., & Kulmala, M. (2005). Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II). Boreal Environment Research, 10(5), 315-322.