Effect of permafrost thaw on CO2 and CH4 exchange in a western Alaska peatland chronosequence

Carmel E Johnston,M Torre Jorgenson,Christopher C Fuller,Joshua C Koch,Jennifer W Harden,Kristen Manies,Stephanie A Ewing,Kimberly P Wickland,Ruth K Varner

doi:10.1088/1748-9326/9/8/085004

Carmel E Johnston, M Torre Jorgenson + Show 7 more

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https://doi.org/10.1088/1748-9326/9/8/085004

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Abstract

Permafrost soils store over half of global soil carbon (C), and northern frozen peatlands store about 10% of global permafrost C. With thaw, inundation of high latitude lowland peatlands typically increases the surface-atmosphere flux of methane (CH4), a potent greenhouse gas. To examine the effects of lowland permafrost thaw over millennial timescales, we measured carbon dioxide (CO2) and CH4 exchange along sites that constitute a ∼1000 yr thaw chronosequence of thermokarst collapse bogs and adjacent fen locations at Innoko Flats Wildlife Refuge in western Alaska. Peak CH4 exchange in July (123 ± 71 mg CH4–C m−2 d−1) was observed in features that have been thawed for 30 to 70 (<100) yr, where soils were warmer than at more recently thawed sites (14 to 21 yr; emitting 1.37 ± 0.67 mg CH4–C m−2 d−1 in July) and had shallower water tables than at older sites (200 to 1400 yr; emitting 6.55 ± 2.23 mg CH4–C m−2 d−1 in July). Carbon lost via CH4 efflux during the growing season at these intermediate age sites was 8% of uptake by net ecosystem exchange. Our results provide evidence that CH4 emissions following lowland permafrost thaw are enhanced over decadal time scales, but limited over millennia. Over larger spatial scales, adjacent fen systems may contribute sustained CH4 emission, CO2 uptake, and DOC export. We argue that over timescales of decades to centuries, thaw features in high-latitude lowland peatlands, particularly those developed on poorly drained mineral substrates, are a key locus of elevated CH4 emission to the atmosphere that must be considered for a complete understanding of high latitude CH4 dynamics.

Highlights

Over half of global belowground carbon (C) is stored in permafrost soils, largely at high latitudes where rates of climate warming are greatest (Yu 2012, Tarnocai et al 2009, Kuhry et al 2013)
The young bogs are small collapse scars (2.0 ± 0.6 m2 in area) within the permafrost plateaus that lack woody vegetation, exhibit thaw-induced lowering of the ground surface by 1.20 ± 0.11 m relative to plateaus, host shallow water tables with acidic pH (4.1–4.6), and are dominated by S. riparium, and S. lindbergii
Young and intermediate bogs maintained the shallowest water tables and supported a plant community dominated by S. riparium, and S. lindbergii, with a less consolidated surface peat mat compared to that of the old bogs, which were dominated by S. fuscum

Summary

Introduction

Over half of global belowground carbon (C) is stored in permafrost soils, largely at high latitudes where rates of climate warming are greatest (Yu 2012, Tarnocai et al 2009, Kuhry et al 2013). It is well established that as thawing peatlands are warmed and inundated, methane (CH4) release becomes an important pathway of C loss (Frolking et al 2011, Jorgenson et al 2013, Schuur et al 2008, Turetsky et al 2002, 2007 Wickland et al 2006, Christensen 2004, Bäckstrand et al 2010, Malmer et al 2005) This is of concern because estimates suggest that northern peatlands currently contribute ∼3–9% (15–50 Tg yr−1) of global CH4 emissions, and atmospheric CH4 is a strong contributor to global radiative forcing (Kirschke et al 2013, McGuire et al 2010, Wofsy et al 2007, Dlugokencky et al 2011). It is well known that methane emissions respond to soil temperature and soil moisture, which together with plant community composition explain >60% of the variance in CH4 flux in high-latitude wetlands (Olefeldt et al 2012)

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