CH4 and CO2 fluxes at sites with different hydrological patterns in the polygonal tundra of Samoylov Island, Northeastern Siberia
Authors: Galera Leonardo de Aro , Knoblauch Christian , Eckhardt Tim , Beer Christian , Pfeiffer Eva-Maria 
Published: March 2021

&lt;p&gt;In the last two decades, there were registered record high permafrost temperatures promoting permafrost thawing and leading to additional CO&lt;sub&gt;2&lt;/sub&gt; and CH&lt;sub&gt;4&lt;/sub&gt; emissions. It is crucial to assess the amount of C that is mineralized to CH&lt;sub&gt;4&lt;/sub&gt;, due to its higher global warming potential (GWP) compared to CO&lt;sub&gt;2&lt;/sub&gt;. The role of CH&lt;sub&gt;4&lt;/sub&gt; in the total C emissions is mainly governed by the hydrological patterns of ecosystems. CH&lt;sub&gt;4&lt;/sub&gt; oxidation is another critical process and is largely controlled by vegetation. The soil CO&lt;sub&gt;2&lt;/sub&gt;:CH&lt;sub&gt;4&lt;/sub&gt; production ratio shows the contribution of CH&lt;sub&gt;4&lt;/sub&gt; to the C emission budget of a determined area. Few studies evaluated &lt;em&gt;in situ&lt;/em&gt; CO&lt;sub&gt;2&lt;/sub&gt;:CH&lt;sub&gt;4&lt;/sub&gt; production ratios. Our objective was to assess CH&lt;sub&gt;4&lt;/sub&gt; emissions and the heterotrophic CO&lt;sub&gt;2&lt;/sub&gt;:CH&lt;sub&gt;4&lt;/sub&gt; production ratios in the Siberian tundra during the growing season. To accomplish these goals, we measured CH&lt;sub&gt;4&lt;/sub&gt; and CO&lt;sub&gt;2&lt;/sub&gt; fluxes using the chamber technique in the polygonal tundra of Samoylov Island in the Lena River Delta, Northeastern Siberia. The plant-mediated CH&lt;sub&gt;4&lt;/sub&gt; transport and the heterotrophic respiration (R&lt;sub&gt;h&lt;/sub&gt;) 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 &lt;em&gt;in situ&lt;/em&gt; CO&lt;sub&gt;2&lt;/sub&gt;:CH&lt;sub&gt;4&lt;/sub&gt; ratios from the R&lt;sub&gt;h&lt;/sub&gt; of the soil. The CH&lt;sub&gt;4&lt;/sub&gt; emissions at the polygon center were much higher than the rim and showed evident seasonality. The polygon center median CH&lt;sub&gt;4&lt;/sub&gt; flux of 26 mg.m&lt;sup&gt;-2&lt;/sup&gt;.d&lt;sup&gt;-1&lt;/sup&gt; decreased by 80% when the vegetation was removed, indicating the relevance of plant-mediated CH&lt;sub&gt;4&lt;/sub&gt; transport in these emissions. This was not detected at the polygon rim that had much lower emissions (1.8 mg.m&lt;sup&gt;-2&lt;/sup&gt;.d&lt;sup&gt;-1&lt;/sup&gt;). The heterotrophic CO&lt;sub&gt;2&lt;/sub&gt;:CH&lt;sub&gt;4 &lt;/sub&gt;ratios varied from 1 to 100 at the polygon center, and from 100 to 1000 at the polygon rim, showing the greater importance of CH&lt;sub&gt;4&lt;/sub&gt; 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 CO&lt;sub&gt;2&lt;/sub&gt;-C sequestration rate (-23 kg CO&lt;sub&gt;2&lt;/sub&gt;-C.ha&lt;sup&gt;-1&lt;/sup&gt;.d&lt;sup&gt;-1&lt;/sup&gt;) more than 3 times higher than the dry tundra (-7 kg CO&lt;sub&gt;2&lt;/sub&gt;-C.ha&lt;sup&gt;-1&lt;/sup&gt;.d&lt;sup&gt;-1&lt;/sup&gt;). Overall, the CH&lt;sub&gt;4&lt;/sub&gt; emissions represent a decrease of just 5% in the total CO&lt;sub&gt;2&lt;/sub&gt;-e offset of the tundra in Samoylov during the growing season. The CH&lt;sub&gt;4&lt;/sub&gt; 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 CO&lt;sub&gt;2&lt;/sub&gt; and CH&lt;sub&gt;4&lt;/sub&gt;, specially related to hydrology, topography, and vegetation.&lt;/p&gt;