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Impacts of Temperature and Soil Characteristics on Methane Production and Oxidation in Arctic Tundra

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Impacts of Temperature and Soil Characteristics on Methane Production and Oxidation in Arctic Tundra Biogeosciences, 15, 6621–6635, 2018 https://doi.org/10.5194/bg-15-6621-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Impacts of temperature and soil characteristics on methane production and oxidation in Arctic tundra Jianqiu Zheng1, Taniya RoyChowdhury1,a, Ziming Yang2,b, Baohua Gu2, Stan D. Wullschleger2,3, and David E. Graham1,3 1Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA 2Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA 3Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA anow at: Department of Environmental Science & Technology, University of Maryland, College Park, Maryland, USA bnow at: Department of Chemistry, Oakland University, Rochester, Michigan, USA Correspondence: David E. Graham ([email protected]) Received: 29 December 2017 – Discussion started: 29 January 2018 Revised: 29 September 2018 – Accepted: 25 October 2018 – Published: 8 November 2018 Abstract. Rapid warming of Arctic ecosystems accelerates Arctic polygonal tundra. Future changes in temperature and microbial decomposition of soil organic matter and leads to soil saturation conditions are likely to divert electron flow increased production of carbon dioxide (CO2) and methane to alternative electron acceptors and significantly alter CH4 (CH4). CH4 oxidation potentially mitigates CH4 emissions production, which should also be considered in CH4 models. from permafrost regions, but it is still highly uncertain whether soils in high-latitude ecosystems will function as a net source or sink for CH4 in response to rising temperature Copyright statement. This manuscript has been authored by UT- and associated hydrological changes. We investigated CH4 production and oxidation potential in permafrost-affected Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains soils from degraded ice-wedge polygons on the Barrow En- and the publisher, by accepting the article for publication, acknowl- ˙ vironmental Observatory, Utqiagvik (Barrow), Alaska, USA. edges that the United States Government retains a non-exclusive, Frozen soil cores from flat and high-centered polygons were paid-up, irrevocable, world-wide license to publish or reproduce sectioned into organic, transitional, and permafrost layers, the published form of this manuscript, or allow others to do so, ◦ and incubated at −2, C4 and C8 C to determine potential for United States Government purposes. The Department of En- CH4 production and oxidation rates. Significant CH4 pro- ergy will provide public access to these results of federally spon- duction was only observed from the suboxic transition layer sored research in accordance with the DOE Public Access Plan and permafrost of flat-centered polygon soil. These two soil (http://energy.gov/downloads/doe-public-access-plan). sections also exhibited highest CH4 oxidation potentials. Or- ganic soils from relatively dry surface layers had the low- est CH4 oxidation potential compared to saturated transition 1 Introduction layer and permafrost, contradicting our original assumptions. Low methanogenesis rates are due to low overall microbial Arctic ecosystems store vast amounts of organic carbon in activities measured as total anaerobic respiration and the active layer soils and permafrost (Hugelius et al., 2014; Shik- competing iron-reduction process. Our results suggest that lomanov et al., 2010). Rising temperatures, increased annual CH4 oxidation could offset CH4 production and limit surface thaw depth, and a prolonged thaw season accelerate micro- CH4 emissions, in response to elevated temperature, and thus bial degradation of this carbon reservoir (Shiklomanov et al., must be considered in model predictions of net CH4 fluxes in 2010; Schuur et al., 2015; Schuur et al., 2013). The poten- tial carbon loss due to these direct effects is estimated to be Published by Copernicus Publications on behalf of the European Geosciences Union. 6622 J. Zheng et al.: Impacts of temperature and soil characteristics 92 ± 17 Pg carbon over the coming century (Schuur et al., under oxygen-limiting conditions as well (Roslev and King, 2015). The extent of soil organic matter (SOM) decomposi- 1996). In the classical model of CH4 oxidation profiles, there tion and partitioning between CO2 and CH4 emissions highly is usually a vertical gradient of decreasing O2 concentration depend upon soil saturation conditions. Thawing of ground in the top centimeters of the soil column that is inversely cor- ice and ice-wedge degradation cause ground subsidence and related with an increasing gradient of CH4 through the sub- significant changes in soil water saturation (Liljedahl et al., oxic/anoxic active layer. Methanogens are relatively abun- 2016) generating heterogeneous surface CH4 fluxes (Schädel dant in the deeper layer where oxygen is limiting (Lee et al., et al., 2016), with a current estimate of net CH4 exchange 2015; Kim and Liesack, 2015). Methane oxidation potential from tundra to the atmosphere ranging widely from 8 to in wetlands and peat bogs is highest near the water table, −1 29 Tg C yr (McGuire et al., 2012). Understanding the fac- while most CH4 is produced below the water table (Whalen tors that control CH4 fluxes is key to reducing model uncer- and Reeburgh, 2000). In contrast, methanogenic and methan- tainties and predicting future climate feedbacks. otrophic communities can overlap in the rhizosphere (Lieb- The unique polygonal ground in Arctic coastal plain tun- ner et al., 2012; Knoblauch et al., 2015), where roots or dra creates natural gradients in hydrology, snow pack depth Sphagnum create an oxic/anoxic interface providing a sub- and density, and soil organic carbon storage (SOC) that con- stantial amount of oxygen for methanotrophs (Laanbroek, trol CH4 fluxes (Liljedahl et al., 2016; Lara et al., 2015). 2010; Parmentier et al., 2011) and organic acid substrates for Thermal contraction processes create cracks in the tundra methanogenesis. soil, which can fill with water that freezes to produce massive Soil CH4 fluxes result from the net effect of microbial ground ice (French, 2007). This ice forms wedges that cre- CH4 production and oxidation, coupled with transport pro- ate the borders of three dominant polygon types, defined by cesses. The rates of CH4 oxidation are mainly governed by their surface relief and subsurface hydrology: low-centered the abundance and composition of methanotrophic microbial polygons (LCPs), flat-centered polygons (FCPs), and high- communities and environmental factors including CH4 and centered polygons (HCPs) (MacKay, 2000). Poorly drained O2 availability, soil air-filled porosity, and soil-water content LCPs are characterized by wet centers bordered by raised, (Preuss et al., 2013). Previous studies of boreal lakes and relatively dry rims and wet troughs. FCPs lack the rims of wetlands showed that CH4 production is more sensitive to LCPs and are drier (Wainwright et al., 2015). When ice temperature changes than CH4 oxidation, as CH4 oxidation wedges erode and water drains from the polygons, troughs rates respond more strongly to CH4 availability than tem- subside and rims disappear to form drier HCPs. Methane perature increase (Liikanen et al., 2002; Segers, 1998). Soils emissions from wet and inundated LCP sites were 1–2 or- at the Barrow Environmental Observatory in Utqiagvik˙ (Bar- ders of magnitude larger than emissions from drier FCP and row), Alaska, experience a wide range of arctic temperatures, HCP sites (Vaughn et al., 2016; Sachs et al., 2010). Al- from −20 to C4 ◦C (Shiklomanov et al., 2010). Soil respira- though a number of factors – including vegetation height and tion and methanogenesis continue at low temperatures close plant composition (von Fischer et al., 2010), soil inundation to 0 ◦C, even after the soil surface freezes trapping gas un- (Sturtevant et al., 2012), thaw depth (Sturtevant and Oechel, der ice. Therefore, substantial annual CH4 and CO2 emis- 2013; Grant et al., 2017), and season (Chang et al., 2014) sions from the Alaskan Arctic occur during the spring thaw – were suggested as explanatory factors for CH4 flux varia- (Commane et al., 2017; Raz-Yaseef et al., 2017; Zona et al., tions, the 100-fold difference in CH4 flux between polygon 2016). However, it is unclear how accelerated warming in types could not be fully explained by variations in moisture Arctic soils affects the opposing processes of CH4 produc- or temperature (Sachs et al., 2010; Vaughn et al., 2016). Mea- tion and oxidation due to their nonlinear response to temper- surements of dissolved CH4 concentrations in soil pore wa- ature changes (Treat et al., 2015). ters suggested a huge disconnect between > 100 µM concen- In this study, we investigated the rates and tempera- trations of dissolved CH4 in the active layer below 20 cm and ture sensitivities of CH4 production and oxidation from negligible dissolved CH4 at 10 cm soil depth (∼ 1 µM). This permafrost-affected soils in Utqiagvik˙ (Barrow), Alaska. Al- 13 CH4 gradient is particularly steep in FCPs. δ C-CH4 data though various studies have identified significant and fre- suggested CH4 oxidation played an important role in miti- quently correlated factors affecting CH4 and CO2 produc- gating CH4 in soil pore water and limiting surface CH4 emis- tion in permafrost ecosystems upon thawing, the oxidation sions (Vaughn et al., 2016). However, CH4 oxidation poten- of CH4 is
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