Environmental correlates of peatland carbon fluxes in a thawing landscape: do transitional thaw stages matter?

Abstract

Abstract. Peatlands in discontinuous permafrost regions occur as a mosaic of wetland types, each with variable sensitivity to climate change. Permafrost thaw further increases the spatial heterogeneity in ecosystem structure and function in peatlands. Carbon (C) fluxes are well characterized in endmember thaw stages such as fully intact or fully thawed permafrost but remain unconstrained for transitional stages that cover a significant area of thawing peatlands. Furthermore, changes in the environmental correlates of C fluxes, due to thaw, are not well described -a requirement for modeling future changes to C storage of permafrost peatlands. We investigated C fluxes and their correlates in end-member and a number of transitional thaw stages in a sub-arctic peatland. Across peatland-lumped CH 4 and CO 2 flux data had significant correlations with expected correlates such as water table depth, thaw depth, temperature, photosynthetically active radiation and vascular green area. Within individual thaw states, bivariate correlations as well as multiple regressions between C flux and environmental factors changed variably with increasing thaw. The variability in directions and magnitudes of correlates reflects the range of structural conditions that could be present along a thaw gradient. These structural changes correspond to changes in C flux controls, such as temperature and moisture, and their interactions. Temperature sensitivity of CH 4 increased with increasing thaw in bivariate analyses, but lack of this trend in multiple regression analyses suggested cofounding effects of substrate or water limitation on the apparent temperature sensitivity. Our results emphasize the importance of incorporating transitional stages of thaw in landscape level C budgets and highlight that end-member or adjacent thaw stages do not adequately describe the variability in structure-function relationships present along a thaw gradient.

Methane Production Pathway Regulated Proximally by Substrate Availability and Distally by Temperature in a High‐Latitude Mire Complex

“…In addition to model structural uncertainty, the differences between the modeled and measured CH 4 emissions could result from the lack of micrometeorological forcing and quantitative vegetation description at each automated chamber. Future model improvement requires better constraints from additional biometeorological measurements to address the high spatial heterogeneity presented in high‐latitude peatlands (Malhotra & Roulet, ; Olefeldt et al, ). Some episodic CH 4 emission pulses (Mastepanov et al, ) were modeled during shoulder seasons in the bog and fen, although the magnitudes were relatively lower than the modeled summertime emissions.…”

Thaw Transitions and Redox Conditions Drive Methane Oxidation in a Permafrost Peatland

“…Permafrost thaw in permafrost peatlands can cause palsa mounds with intact permafrost to subside into submerged fens (Malmer et al, ), increasing methane (CH 4 ) emissions as the water table rises above the peat surface and active layer depth and graminoid vegetation cover increase (Johnston et al, ; Malhotra & Roulet, ; Turetsky et al, ). These changes are associated with shifts in the abundance and activity of CH 4 ‐cycling microbes, including aerobic methane oxidizing bacteria (MOB; Singleton et al, ; Woodcroft et al, ).…”

“…Hydrology and vegetation vary across transitional thaw stages (Malhotra et al, ; Malhotra & Roulet, ) and influence redox potential (Eh) and rates of CH 4 transformation and emission (Street et al, ; Svensson & Rosswall, ). For instance, in intermediately thawed bogs, rates of CH 4 oxidation are predicted to be highest at the oxic‐anoxic interface near the water table, where both CH 4 and O 2 are available for MOB (Kettunen et al, ; Moore & Dalva, ; Nedwell & Watson, ); however, isotopic evidence and the relative abundance of MOB lineages suggest substantial CH 4 oxidation in peat that is frequently below the water table (Singleton et al, ).…”

“…Previous work that compares intermediately thawed bogs and fully thawed, saturated sites with aerenchymous vegetation indicates higher MOB activity in fully thawed sites (Singleton et al, ; Whalen & Reeburgh, ). However, comparing only the intermediately thawed and fully thawed stages disregards the variability of vegetation, hydrology, and redox conditions found in transitional thaw stages that are abundant and expanding areas of permafrost peatlands (Johansson et al, ; Malmer et al, ) contributing to landscape‐level CH 4 flux (Malhotra & Roulet, ). Furthermore, while aerobic CH 4 oxidation is primarily controlled by O 2 and CH 4 availability (Hornibrook et al, ; Lofton et al, ), the availability of alternative terminal electron acceptors (TEAs) may impact MOB by providing substrates for obligate methanotrophs in oxygen‐limited environments (Kits et al, ; Skennerton et al, ), inhibiting methanogenesis (Gao et al, ; Knorr & Blodau, ; Nedwell & Watson, ), and buffering redox potential as other microbes oxidize and reduce TEAs (Fiedler et al, ).…”

Ecosystem carbon response of an Arctic peatland to simulated permafrost thaw

“…Methane oxidation frequently occurs in ecosystems of the northern high latitudes (dry upland tundra: Bartlett & Harriss, , Jørgensen, Johansen, Westergaard‐Nielsen, & Elberling, , Lau et al, , Christiansen et al, , D'imperio, Nielsen, Westergaard‐Nielsen, Michelsen, & Elberling, , Arctic peatlands: Flessa et al, , Malhotra & Roulet, , upland forests: Olefeldt, Turetsky, Crill, & Mcguire, ). Methane fluxes are the balance between CH 4 production by methanogens under anoxic conditions, and CH 4 consumption under oxic conditions (Lai, ).…”