| 29 Jan 2026
Plant belowground traits indicate increased plant-mediated methane transport along a peatland permafrost thaw gradient
Abstract. Permafrost thaw in subarctic peatlands alters ecosystem methane (CH4) fluxes. Collapsing permafrost palsas change hydrology, interstitial oxygen availability, and vegetation composition, and each of these factors contribute to net CH4 flux by influencing CH4 production, consumption and transport. However, changes in plant-mediated CH4 fluxes have mostly been estimated using aboveground characteristics, such as biomass and leaf area, leaving belowground parts (roots and rhizomes) understudied despite their direct contact to depth-dependent CH4 flux processes. Here, we explored the potential of using root and rhizome traits as proxies for plant-mediated CH4 cycling along a peatland permafrost thaw gradient in subarctic Sweden. We investigated changes in root and rhizome biomass, surface area (SA), diameter, tissue density (TD), and specific root length (SRL) along the permafrost thaw gradient, and how these traits relate to early-, middle-, peak- and season median CH4 fluxes. We utilized chamber CH4 flux and pore water CH4 concentration and isotopic measurements during the productive season. Shrub SRL, diameter and isotopic data suggested increased plant-mediated carbon substrates available for acetoclastic methanogenesis across the thaw gradient. Root TD, proxy for root porosity, decreased with thaw and had negative correlations with CH4 fluxes throughout the season. Simultaneously, herbaceous rhizome SA-CH4 flux correlations were positive and pore water CH4 concentrations were lowest in the fully thawed stage. These results indicated increasing herbaceous plant-mediated transport of acetoclastically-produced CH4 with thaw. Our study demonstrates that integrating plant belowground traits with environmental and biogeochemical data can help improve CH4 flux predictions in thawing landscapes and revealed key mechanistic insights regarding the interplay between substrate availability for methanogenesis and gas transport.
Controls (or drivers) of the exchange of methane between the atmosphere and the surface of various wetlands have always been complicated, because of the nature of the processes and pathways and the large spatial variability across short distances, combined with temporal variability. Initially, simple variables such as temperature, water table and plant functional type were useful in ‘explaining’ patterns of methane exchange across landscapes. Since then, attention has been given to a variety of properties and processes which have led to a better ‘explanation’ and perhaps ‘prediction’ of the methane exchange. For belowground activities, important processes are provision of carbon for methanogenesis and the ability of plants to transport methane directly (avoiding methanotrophy in the profile) as well allowing oxygen to create a potential zone of methanotrophy in the rhizosphere. Thus, linking belowground root/rhizome traits and methane emission can be ‘complicated’ but a necessary advance.
This manuscript is an example of the latter, focusing on the role of root and rhizome traits in the exchange (emission) of methane, based along a gradient created by the thawing of permafrost palsa in northern Sweden. Three ‘thaw stages’ have been recognized at this site, with changes in thermal regime, hydrology and vegetation characteristics creating a very large variation in methane emission rates. The research focuses on determining root and rhizome traits across triplicates of each thaw phase, though this is not done in the automatic flux chambers, but from sites located nearby with similar properties. As noted by the authors, there is not complete species similarity in mean coverage between the triplicate chamber and core data but when combined into herbaceous and shrub categories there is general agreement (Table 1). Daily median flux of methane was calculated, to avoid the usual ‘burp’ of methane occurring occasionally which would be unlikely to be derived from root processes, and chamber carbon dioxide flux was used to define seasonal patterns. Root and rhizome data were collected carefully from depth increments and biogeochemical data were used to provide evidence for some of the processes leading to methane production, and how they might be related to root activity.
The manuscript is well structured and written and provides valuable evidence for the role of belowground plant activities in controlling methane emissions. It is ‘digestible’ in that the information in the manuscript is restricted to two Tables and five Figures, with voluminous supporting data in Appendices. The strong Introduction ends with several expectations/hypotheses based on previous studies, but in this case applied to the specific Stordalen site, in which palsa are thawing and thus perhaps similar to features elsewhere in the Arctic. Based on methane fluxes from the nine chambers and root characteristics from the ‘similar’ sites, the study presents strong evidence for correlations between methane emission rate and the herbaceous and shrub root and rhizome properties, and whether patterns changed with the season, based on carbon dioxide exchange. In general, it appears that correlations were stronger for root properties compared to rhizomes, though there are mixed relationships shown in Figures 3 and 4. The reasons for positive, negative or no correlations were examined in the Discussion and the relevance of these relationships may vary with the specific features of Stordalen, which are somewhat unusual. The relationships between the traits and methane emission were examined in terms of processes related to changes from shrubs to herbaceous plants and the root trait structure (e.g. density, specific length and diameter), and drew upon results presented elsewhere which could be applicable at Stordalen. The overall conclusion was that belowground root/rhizome activities were an important component of methane emission, that change from shrubs to herbaceous would lead to increased emissions, but there is a bit of ‘ying-yang’: shrubs may supply more carbon substrates, whereas herbaceous plants may be important conduits of methane to the atmosphere. Such is life in the real world of Nature and ‘teams’ like these authors are needed to see at least part of the whole.
The study is ‘well referenced’, with 145 in the References, perhaps too many?
Specific comments:
In Table 1, it seems that mosses formed a significant plant cover in the chambers (e.g. Sphagnum balticum, capillifolium and riparium in the three thaw stages). Would methane emission rates be affected by this coverage and some differences between chambers and cores and different wetness? This is addressed in 4.2 line 486 onwards …..
A tremendous amount of work went into this study, particularly in the measurements of root and rhizome properties collected from the cores. If I understand correctly, a 12 cm diameter peat core was dived into 10 cm depth increments and these were then divided into four vertical quarters, which was used for trait measurements, resulting in subsampling of the core sections (and subsampling of that in some cases). This are measurements based on an initial volume of 1130 cm3, and it must take considerable time and patience to extract roots and rhizomes, weighing in some cases to 0.00001 g. pH in 0.01M Cacl2 is much easier and quicker …….
While I accept that the belowground contribution is important to processes such as methane emission, are there any ways of making it ‘easier’ to define and measure these properties over a wider range of sites? If so the properties could be included and tested within models which are grappling with the high spatial and temporal variability of methane emission rates.
Technical issues:
I found few. I thought that the first two sentences of 2.6 (lines 214-215) could be combined into one: they are repetitive. Minor typos in places.
Fig. B13. I was confused the right hand diagram. I think the dashed lines have circles whereas the solid lines have triangles …… not what is written in the legend?
I did find that the total root length of shrubs in the intact sites was about 180 km per square meter (to 30 cm depth). Jeez, as it is 10 km from Stordalen to Abisko, it means that one square meter of intact permafrost has enough shrub root to go back and forth 18 times or perhaps all the way to Norway or the Norwegian Sea? Might get the attention of readers that permafrost is not ‘dead’ but rich in roots and though root length decreases in thawed sites, the impact of roots and rhizomes and change from shrub to herbaceous plants allows for larger methane emission rates.