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