CH4 and CO2 fluxes at sites with different hydrological patterns in the polygonal tundra of Samoylov Island, Northeastern Siberia

In the last two decades, there were registered record high permafrost temperatures promoting permafrost thawing and leading to additional CO₂ and CH₄ emissions. It is crucial to assess the amount of C that is mineralized to CH₄, due to its higher global warming potential (GWP) compared to CO₂. The role of CH₄ in the total C emissions is mainly governed by the hydrological patterns of ecosystems. CH₄ oxidation is another critical process and is largely controlled by vegetation. The soil CO₂:CH₄ production ratio shows the contribution of CH₄ to the C emission budget of a determined area. Few studies evaluated in situ CO₂:CH₄ production ratios. Our objective was to assess CH₄ emissions and the heterotrophic CO₂:CH₄ production ratios in the Siberian tundra during the growing season. To accomplish these goals, we measured CH₄ and CO₂ fluxes using the chamber technique in the polygonal tundra of Samoylov Island in the Lena River Delta, Northeastern Siberia. The plant-mediated CH₄ transport and the heterotrophic respiration (R_h) were determined by comparing plots with and without vegetation through a trenching experiment. To account for the differences between wet and dry tundra, one representative polygon was selected, measurements were made at its water-saturated center and at its drained rim. We also estimated the C budget of the polygonal tundra of Samoylov Island during the measurement period. This is the first study measuring and calculating in situ CO₂:CH₄ ratios from the R_h of the soil. The CH₄ emissions at the polygon center were much higher than the rim and showed evident seasonality. The polygon center median CH₄ flux of 26 mg.m^-2.d^-1 decreased by 80% when the vegetation was removed, indicating the relevance of plant-mediated CH₄ transport in these emissions. This was not detected at the polygon rim that had much lower emissions (1.8 mg.m^-2.d^-1). The heterotrophic CO₂:CH₄ ratios varied from 1 to 100 at the polygon center, and from 100 to 1000 at the polygon rim, showing the greater importance of CH₄ production to the heterotrophic C release at the polygon center. The polygonal tundra on Samoylov Island was a C sink during the measurement period. The wet tundra had a CO₂-C sequestration rate (-23 kg CO₂-C.ha^-1.d^-1) more than 3 times higher than the dry tundra (-7 kg CO₂-C.ha^-1.d^-1). Overall, the CH₄ emissions represent a decrease of just 5% in the total CO₂-e offset of the tundra in Samoylov during the growing season. The CH₄ emissions measured in this study were low. However, it is important to point out that only the growing season is considered, and the off-season and winter C emissions might be significant. Our results stress the high microscale variability of emissions of CO₂ and CH₄, specially related to hydrology, topography, and vegetation.