Taking the heat: soil warming optimum of CH4 uptake in subarctic mineral soils

Atmospheric methane (CH₄) uptake in subarctic and Arctic mineral soils is significant for the CH₄ budget of high-latitude regions, but its response to warming is not well understood. The effect of soil warming on net CH₄ uptake was studied in situ across a natural warming gradient (ambient to +  57.5 °C) in a geothermal area in Southwest Iceland. The study site represented a subarctic grassland on mineral soil with field measurements conducted in summer and fall 2021. Combined automatic and manual dynamic chamber CH₄ flux measurements across the warming transect showed that net CH₄ uptake increased with 0.26 nmol CH₄ m⁻² s⁻¹ per 1 °C of soil warming from ambient soil temperature up to about + 4 °C of soil warming. Soil warming above + 4 °C resulted in a gradual decrease of net CH₄ uptake corresponding to 0.1 nmol CH₄ m⁻² s⁻¹ per 1 °C of soil warming up to + 13 °C. With further soil warming, in situ net CH₄ fluxes were probably affected by geogenic emissions during the effective study period. These trends of enhanced in situ net CH₄ uptake with mild soil warming followed by a decreasing uptake rate with further warming were confirmed in a laboratory incubation experiment showing that the in situ response to temperature <  + 13 °C was biogenic rather than geogenic. It is still not known whether the observed trends are due to adaptation of the community structure to temperature, differential regulation of activity or abundance. Our findings point to a window of future soil warming up to about + 4 °C where net CH₄ uptake in subarctic grassland mineral soils is likely to increase, while further soil warming may result in a decrease of this important CH₄ sink below ambient level. To expand the representativeness of these findings, we encourage future studies to include similar incubation experiments of the warming response for soils across the Arctic.