Correlations between substrate availability, dissolved CH₄, and CH₄ emissions in an arctic wetland subject to warming and plant removal

Abstract

The Arctic is warming which may potentially affect substrate availability, organic matter decomposition, plant growth, and plant species composition. This may lead to changes in the exchange of methane (CH4) and carbon dioxide (CO2) between the soil system and the atmosphere. Yet the correlations among substrate availability, CH4 production, and net emissions of CH4 have been scarcely studied in arctic wetlands. Presently, the impact of increasing temperatures on CH4 exchange is uncertain as the two existing reports on field warming in arctic wetlands present opposite results. We here report results on how summer warming and shrub removal affect soil water substrate (acetate, formate, oxalate, and dissolved organic carbon) concentrations as well as dissolved CH4 and CH4 emissions in a fen at Disko Island (West Greenland). The peak in dissolved CH4 followed the peak in acetate concentration, and appeared after the peak in CH4 emissions, which indicates a lack of correlation between CH4 production and emissions. The peak in CH4 emissions coincided with maximum gross ecosystem production suggesting that CH4 emissions are closely linked to photosynthesis. This was supported by an experiment with removal of the sedge Carex aquatilis ssp. stans which contributed with up to 77% of the CH4 emitted from the ecosystem. By contrast, shrub removal and summer warming did not significantly affect CH4 emissions, possibly due to the treatments impacting CH4 production more than emissions. This implies that such wetlands may be less sensitive to moderate warming and changes in shrub cover than previously assumed.

Responses of tundra plant community carbon flux to experimental warming, dominant species removal and elevation

“…In turn, such reductions would result in no net shift in NEE if heterotrophic respiration is similarly reduced. In partial support of this, Nielsen et al (2017) found that NEE was unresponsive to shrub removal and its interaction with experimental summer warming in a Greenland fen. Pairing plant removal (Díaz, Symstad, Chapin, Wardle, & Huenneke, 2003) and global change experiments can thus provide important information on how global 4.…”

“…carbon, ecosystem respiration, global warming, gross primary productivity, leaf area index, normalized difference vegetation index, plant-plant interactions change factors may mediate plant-plant interactions to influence high latitude terrestrial ecosystem processes (Aerts, 2010;Nielsen et al, 2017).…”

High gas-phase mixing ratios of formic and acetic acid in the High Arctic

“…The water-air partitioning of FA and AA can be estimated from Eq. (3) using measured soil temperature and pH, effective Henry's law constants, and estimated soil formate and acetate concentrations (formate and acetate were chosen as 2 mg L −1 , probably an upper limit for this region; Nielsen et al, 2017;Ström et al, 2012). Further uncertainty is introduced by using literature values for soil formate and acetate concentrations because it requires making the assumption that all the formate and acetate extracted from the soils in those studies resided in the soil pore water, which is unlikely to be the case.…”

“…A very acidic soil would also promote the emission of FA and AA, but the soil samples were taken from three different microenvironments (albeit all within ∼ 5 km of the station), and all samples had near-neutral pH. Additionally, other work examining Arctic soils has generally found near-neutral pH (Brummell et al, 2012;Nielsen et al, 2017). While a combination of lower pH and higher soil reservoirs of formate and acetate could perhaps lead to somewhat larger emissions than those estimated here, we conclude that equilibrium soil-air partitioning is unlikely to account for the high mixing ratios of FA and AA observed in this study.…”

“…To our knowledge, no information on the direct emissions of FA and AA from Arctic flora exists in the literature. OVOCs, such as FA and AA, are produced as by-products of plant metabolism (Niinemets et al, 2014), and can either be emitted to the atmosphere or consumed by further metabolic processes. The direct emission of FA and AA from plants has been documented (Kesselmeier et al, 1998;Kuhn et al, 2002;Guenther et al, 2000), but is not generally considered to be an important source of these compounds to the atmosphere globally (although emission estimates vary widely; Paulot et al, 2011).…”