Abstract. Carbon dioxide and methane fluxes were measured at a tundra site near Chokurdakh, in the lowlands of the Indigirka river in north-east Siberia. This site is one of the few stations on Russian tundra and it is different from most other tundra flux stations in its continentality. A suite of methods was applied to determine the fluxes of NEE, GPP, R eco and methane, including eddy covariance, chambers and leaf cuvettes. Net carbon dioxide fluxes were high compared with other tundra sites, with NEE=−92 g C m −2 yr −1 , which is composed of an R eco =+141 g C m −2 yr −1 and GPP=−232 g C m −2 yr −1 . This large carbon dioxide sink may be explained by the continental climate, that is reflected in low winter soil temperatures (−14 • C), reducing the respiration rates, and short, relatively warm summers, stimulating high photosynthesis rates. Interannual variability in GPP was dominated by the frequency of light limitation (R g <200 W m −2 ), whereas R eco depends most directly on soil temperature and time in the growing season, which serves as a proxy of the combined effects of active layer depth, leaf area index, soil moisture and substrate availability. The methane flux, in units of global warming potential, was +28 g C-CO 2 e m −2 yr −1 , so that the greenhouse gas balance was −64 g C-CO 2 e m −2 yr −1 . Methane fluxes depended only slightly on soil temperature and were highly sensitive to hydrological conditions and vegetation composition.

The behavior of tundra ecosystems is critical in the global carbon cycle due to their wet soils and large stores of carbon. Recently, cooperation was observed between methanotrophic bacteria and submerged Sphagnum, which reduces methane emissions in this type of vegetation and supplies CO₂ for photosynthesis to the plant. Although proven in the lab, the differences that exist in methane emissions from inundated vegetation types with or without Sphagnum have not been linked to these bacteria before.

To further investigate the importance of these bacteria, chamber flux measurements, microbial analysis and flux modeling were used to show that methane emissions in a submerged Sphagnum/sedge vegetation type were 50% lower compared to an inundated sedge vegetation without Sphagnum. From examining the results of the measurements, incubation experiments and flux modeling, it was found that it is likely that this difference is due to, for a large part, oxidation of methane below the water table by these endophytic bacteria.

This result is important when upscaled spatially since oxidation by these bacteria plays a large role in 15% of the net methane emissions, while at the same time they promote photosynthesis of Sphagnum, and thus carbon storage. Future changes in the spread of submerged Sphagnum, in combination with the response of these bacteria to a warmer climate, could be an important factor in predicting future greenhouse gas exchange from tundra

Abstract. Most plot-scale methane emission models – of which many have been developed in the recent past – are validated using data collected with the closed-chamber technique. This method, however, suffers from a low spatial representativeness and a poor temporal resolution. Also, during a chamber-flux measurement the air within a chamber is separated from the ambient atmosphere, which negates the influence of wind on emissions. Additionally, some methane models are validated by upscaling fluxes based on the area-weighted averages of closed-chamber measurements, and by comparing those to the eddy covariance (EC) flux. This technique is rather inaccurate, as the area of upscaling might be different from the EC tower footprint, therefore introducing significant mismatch. In this study, we present an approach to validate plot-scale methane models with EC observations using the footprint-weighted average method. Our results show that the fluxes obtained by the footprint-weighted average method are of the same magnitude as the EC flux. More importantly, the temporal dynamics of the EC flux on a daily time scale are also captured (r2 = 0.7). In contrast, using the area-weighted average method yielded a low (r2 = 0.14) correlation with the EC measurements and an underestimation of methane emissions by 27.4%. This shows that the footprint-weighted average method is preferable when validating methane emission models with EC fluxes for areas with a heterogeneous and irregular vegetation pattern.

&lt;p&gt;&lt;strong&gt;Abstract.&lt;/strong&gt; Following the recent Global Carbon project (GCP) synthesis of the decadal methane (CH&lt;sub&gt;4&lt;/sub&gt;) budget over 2000&amp;#8211;2012 (Saunois et al., 2016), we analyse here the same dataset with a focus on quasi-decadal and inter-annual variability in CH&lt;sub&gt;4&lt;/sub&gt; emissions. The GCP dataset integrates results from top-down studies (exploiting atmospheric observations within an atmospheric inverse-modelling frameworks) and bottom-up models, inventories, and data-driven approaches (including process-based models for estimating land surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations). &lt;br&gt;&lt;br&gt; The annual global methane emissions from top-down studies, which by construction match the observed methane growth rate within their uncertainties, all show an increase in total methane emissions over the period 2000&amp;#8211;2012, but this increase is not linear over the 13 years. Despite differences between individual studies, the mean emission anomaly of the top-down ensemble shows no significant trend in total methane emissions over the period 2000&amp;#8211;2006, during the plateau of atmospheric methane mole fractions, and also over the period 2008&amp;#8211;2012, during the renewed atmospheric methane increase. However, the top-down ensemble mean produces an emission shift between 2006 and 2008, leading to 22 [16&amp;#8211;32] Tg CH&lt;sub&gt;4&lt;/sub&gt; yr&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; higher methane emissions over the period 2008&amp;#8211;2012 compared to 2002&amp;#8211;2006. This emission increase mostly originated from the tropics with a smaller contribution from mid-latitudes and no significant change from boreal regions. &lt;br&gt;&lt;br&gt; The regional contributions remain uncertain in top-down studies. Tropical South America and South and East Asia seems to contribute the most to the emission increase in the tropics. However, these two regions have only limited atmospheric measurements and remain therefore poorly constrained. &lt;br&gt;&lt;br&gt; The sectorial partitioning of this emission increase between the periods 2002&amp;#8211;2006 and 2008&amp;#8211;2012 differs from one atmospheric inversion study to another. However, all top-down studies suggest smaller changes in fossil fuel emissions (from oil, gas, and coal industries) compared to the mean of the bottom-up inventories included in this study. This difference is partly driven by a smaller emission change in China from the top-down studies compared to the estimate in the EDGARv4.2 inventory, which should be revised to smaller values in a near future. Though the sectorial partitioning of six individual top-down studies out of eight are not consistent with the observed change in atmospheric &lt;sup&gt;13&lt;/sup&gt;CH&lt;sub&gt;4&lt;/sub&gt;, the partitioning derived from the ensemble mean is consistent with this isotopic constraint. At the global scale, the top-down ensemble mean suggests that, the dominant contribution to the resumed atmospheric CH&lt;sub&gt;4&lt;/sub&gt; growth after 2006 comes from microbial sources (more from agriculture and waste sectors than from natural wetlands), with an uncertain but smaller contribution from fossil CH&lt;sub&gt;4&lt;/sub&gt; emissions. Besides, a decrease in biomass burning emissions (in agreement with the biomass burning emission databases) makes the balance of sources consistent with atmospheric &lt;sup&gt;13&lt;/sup&gt;CH&lt;sub&gt;4&lt;/sub&gt; observations. &lt;br&gt;&lt;br&gt; The methane loss (in particular through OH oxidation) has not been investigated in detail in this study, although it may play a significant role in the recent atmospheric methane changes.&lt;/p&gt;

), a few typographical errors have been brought to our attention which have unfortunately slipped through, and we apologize for any inconvenience they may have caused.[2] In equation (1), the conditions of L > 0 and L < 0 were reversed, and in the second part of the equation, a minus was given where a plus should have appeared. However, these mistakes are purely typographical. The same errors were not made in the code used, and all calculations presented in the paper were made with the correct equation, shown here in equation (1). Therefore, both the results and the conclusion of the paper remain unchanged. The correct equation is:[3] Furthermore, in the last sentence of paragraph 21, the word "unstable" was given twice, while the second occurrence should have read "stable." The correct sentence is: "As a consequence, unstable conditions are expressed in the range 0 < F < 1, neutral conditions occur at F = 1, and stable conditions are expressed for values of F > 1."[4] We'd like to thank Bernard Heinesch and students for bringing these typographical errors to our attention.