Christian Knoblauch

Arctic warming causes permafrost thaw and accelerates microbial decomposition of soil organic matter (SOM) to carbon dioxide (CO 2 ) and methane (CH 4 ). The determining factors for the ratio between CO 2 and CH 4 formation are still not well understood due to scarce in situ measurements, particularly in remote Arctic regions. We quantified the CO 2 :CH 4 ratios of SOM decomposition in wet and dry tundra soils by using CO 2 fluxes from clipped plots and in situ CH 4 fluxes from vegetated plots. At the water‐saturated site, CO 2 :CH 4 ratios decreased sharply from 95 at beginning of July to about 10 in August and September with a median of 12.2 (7.70–17.1; 25%–75% quartiles) over the whole vegetation period. When considering CH 4 oxidation, estimated to reduce in situ CH 4 fluxes by 10%–31%, even lower CO 2 :CH 4 ratios were calculated (median 10.9–8.41). Active layer depth and soil temperature were the main factors controlling these ratios. Methane production was associated with subsoil (40 cm) temperature, while heterotrophic respiration was related to topsoil (5 cm) temperatures. As expected, CO 2 :CH 4 ratios were substantially higher at the dry site (median 373, 292–500, 25%–75% quartiles). Both tundra types lost carbon preferentially in form of CO 2 , and CH 4 ‐C represented only 0.27% of the dry tundra total carbon loss and 6.91% of the wet tundra total carbon loss. The current study demonstrates the dynamic of in situ CO 2 :CH 4 ratios from SOM decomposition and will help improve simulations of future CO 2 and CH 4 fluxes from thawing tundra soils.