Stoichiometry and temperature sensitivity of methanogenesis and CO 2 production from saturated polygonal tundra in Barrow, Alaska

Abstract

Arctic permafrost ecosystems store ~50% of global belowground carbon (C) that is vulnerable to increased microbial degradation with warmer active layer temperatures and thawing of the near surface permafrost. We used anoxic laboratory incubations to estimate anaerobic CO 2 production and methanogenesis in active layer (organic and mineral soil horizons) and permafrost samples from center, ridge and trough positions of water‐saturated low‐centered polygon in Barrow Environmental Observatory, Barrow AK , USA . Methane ( CH 4 ) and CO 2 production rates and concentrations were determined at −2, +4, or +8 °C for 60 day incubation period. Temporal dynamics of CO 2 production and methanogenesis at −2 °C showed evidence of fundamentally different mechanisms of substrate limitation and inhibited microbial growth at soil water freezing points compared to warmer temperatures. Nonlinear regression better modeled the initial rates and estimates of Q 10 values for CO 2 that showed higher sensitivity in the organic‐rich soils of polygon center and trough than the relatively drier ridge soils. Methanogenesis generally exhibited a lag phase in the mineral soils that was significantly longer at −2 °C in all horizons. Such discontinuity in CH 4 production between −2 °C and the elevated temperatures (+4 and +8 °C) indicated the insufficient representation of methanogenesis on the basis of Q 10 values estimated from both linear and nonlinear models. Production rates for both CH 4 and CO 2 were substantially higher in organic horizons (20% to 40% wt. C) at all temperatures relative to mineral horizons ( CO 2 than the active layer and negligible CH 4 . High concentrations of initial exchangeable Fe( II ) and increasing accumulation rates signified the role of iron as terminal electron acceptors for anaerobic C degradation in the mineral horizons.

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