Ratio of In Situ CO2 to CH4 Production and Its Environmental Controls in Polygonal Tundra Soils of Samoylov Island, Northeastern Siberia

• Published in: Journal of geophysical research. Biogeosciences Vol. 128; no. 4
• Main Authors: Galera, Leonardo de A., Eckhardt, Tim, Beer, Christian, Pfeiffer, Eva‐Maria, Knoblauch, Christian
• Format: Journal Article
• Language: English
• Published: 01.04.2023
• Subjects: active layer thickening; chamber; greenhouse gases; heterotrophic respiration; methanogenesis; thaw depth
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1029/2022JG006956

Arctic warming causes permafrost thaw and accelerates microbial decomposition of soil organic matter (SOM) to carbon dioxide (CO2) and methane (CH4). The determining factors for the ratio between CO2 and CH4 formation are still not well understood due to scarce in situ measurements, particularly in remote Arctic regions. We quantified the CO2:CH4 ratios of SOM decomposition in wet and dry tundra soils by using CO2 fluxes from clipped plots and in situ CH4 fluxes from vegetated plots. At the water‐saturated site, CO2:CH4 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 CH4 oxidation, estimated to reduce in situ CH4 fluxes by 10%–31%, even lower CO2:CH4 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, CO2:CH4 ratios were substantially higher at the dry site (median 373, 292–500, 25%–75% quartiles). Both tundra types lost carbon preferentially in form of CO2, and CH4‐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 CO2:CH4 ratios from SOM decomposition and will help improve simulations of future CO2 and CH4 fluxes from thawing tundra soils. Plain Language Summary Global warming causes the thaw of the permanently frozen soil in Arctic regions, exposing soil organic matter (SOM) previously frozen to decomposition, increasing the emission of carbon dioxide (CO2) and methane (CH4), which are greenhouse gases. It is crucial to quantify the ratio of CO2 and CH4 produced because CH4 has a stronger global warming potential than CO2. We partitioned SOM decomposition into CO2 and CH4 formation (CO2:CH4 ratios) in wet and dry tundra soils on Samoylov Island, Northeastern Siberia, and we related these ratios to environmental variables. Deeper active layer, which is the topsoil layer that freezes and thaws annually, and higher subsoil (40 cm) temperature at the interface between the active layer and the permafrost, foster CH4 production and decrease CO2:CH4 ratios. Carbon was preferentially lost in form of CO2 by the soils, but CH4 had a larger contribution to the carbon loss in the wet tundra. Our study indicates that warming and deepening of the active layer can result in rising CH4 production. Further understanding of in situ CO2:CH4 ratios from SOM decomposition will help improve simulations on future CO2 and CH4 fluxes from thawing tundra soils. Key Points Topsoil (5 cm) warming increases the CO2:CH4 production ratio, while warming of subsoil (40 cm) leads to lower CO2:CH4 production ratios The CO2:CH4 production ratio is associated with active‐layer depth (ALD) due to a direct effect of ALD on CH4 production Carbon was preferentially lost in form of CO2 at wet and dry sites, but CH4 had a higher contribution at the wet tundra site