Changes in Methane Flux along a Permafrost Thaw Sequence on the

Dec 25, 2017 - (47) Permafrost thaw and surface subsidence have been documented at the site since the early 1990s and have led to the initiation and e...
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Changes in methane flux along a permafrost thaw sequence on the Tibetan Plateau Guibiao Yang, Yunfeng Peng, David Olefeldt, Yongliang Chen, Guanqin Wang, Fei Li, Dianye Zhang, Jun Wang, Jianchun Yu, Li Liu, Shuqi Qin, Tianyang Sun, and Yuanhe Yang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04979 • Publication Date (Web): 25 Dec 2017 Downloaded from http://pubs.acs.org on December 25, 2017

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Changes in methane flux along a permafrost thaw sequence on the Tibetan

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Plateau

3 4

Guibiao Yang1,2, Yunfeng Peng1, David Olefeldt3, Yongliang Chen1, Guanqin Wang1,2,

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Fei Li1,2, Dianye Zhang1,2, Jun Wang1,2, Jianchun Yu1,2, Li Liu1,2, Shuqi Qin1,2,

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Tianyang Sun1,2, and Yuanhe Yang1,2*

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1.

State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China

9 10

2.

University of Chinese Academy of Sciences, Beijing 100049, China

11

3.

Department of Renewable Resources, University of Alberta, Edmonton, Alberta, Canada, T6G 2H1

12 13 14

*

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10-6283 6632, E-mail: [email protected]

Corresponding author: Dr. Yuanhe Yang, tel.: + 86 10-6283 6638, fax: + 86

16

1

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ABSTRACT: Permafrost thaw alters the physical and environmental conditions of

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soil and may thus cause a positive feedback to climate warming through increased

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methane emissions. However, the current knowledge of methane emissions following

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thermokarst development is primarily based on expanding lakes and wetlands, with

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upland thermokarst being studied less often. In this study, we monitored the methane

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emissions during the peak growing seasons of two consecutive years along a thaw

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sequence within a thermo-erosion gully in a Tibetan swamp meadow. Both years had

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consistent results, with the early and mid-thaw stages (3 to 12 years since thaw)

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exhibiting low methane emissions that were similar to those in the undisturbed

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meadow, while the emissions from the late thaw stage (20 years since thaw) were 3.5

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times higher. Our results also showed that the soil water-filled pore space, rather than

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the soil moisture per se, in combination with the sand content, were the main factors

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that caused increased methane emissions. These findings differ from the traditional

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view that upland thermokarst could reduce methane emissions owing to the

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improvement of drainage conditions, suggesting that upland thermokarst development

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does not always result in a decrease in methane emissions.

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TOC Art

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Keywords: carbon cycle, climate feedback, methane, methanogens, methanotrophs,

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permafrost, thermokarst.

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1. INTRODUCTION

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Permafrost ground in high altitude and high latitude regions contains more than half

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the global soil organic carbon (C)1,

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warming3-5. A warmer climate will cause widespread permafrost thaw, which can lead

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to land surface collapse and erosion, i.e., thermokarst, in certain landscape settings6-8.

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Thermokarst often causes abrupt changes in soil environmental conditions and thus

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strongly influences the production rates of microbial greenhouse gases, including both

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carbon dioxide (CO2) and methane (CH4)9-14. Understanding the impacts of

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permafrost thaw on CH4 emissions is particularly critical, considering the 25 to

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30-fold greater warming potential of CH4 compared to CO2 over a 100-year horizon15.

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However, thermokarst is considered one of the key uncertainties in our understanding

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of future CH4 emissions and the overall permafrost carbon feedback to climate

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change6, 10, 16.

2

and is considered vulnerable to climate

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Thermokarst occurs due to the melting of excess ground ice7 and can cause the

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development of a large number of distinct landforms depending on the landscape

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characteristics and position. Broadly, these landforms have been grouped into wetland,

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lake, and upland thermokarst landforms17. Upland thermokarst often occurs in upland

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settings on moderate slopes or along watercourses and includes active layer

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detachment slides, retrogressive thaw slumps, and thermo-erosion gullies18-20. While

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several studies have reported increased CH4 emissions from recently formed wetland

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and lake thermokarst landforms21-24, less is known about the impact on CH4 emissions 4

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from upland thermokarst landforms. It has been reported that upland ecosystems with

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stable permafrost are most often minor CH4 sources or CH4 sinks

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improved water drainage26, 27 following the development of upland thermokarst would

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lead to further reduced CH4 emissions or an increased sink function16, 28, 29. However,

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a survey of 26 upland thermokarst landforms on the North Slope of Alaska found

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elevated CH4 concentrations in soil profiles compared to the nearby locations

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unaffected by thermokarst10. This finding implies that upland thermokarst may also

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cause increased CH4 emissions, but direct observations of increasing CH4 emissions

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following upland thermokarst are still lacking.

13, 21, 25

, and

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Upland thermokarst can lead to both land surface collapse and erosion, which together

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abruptly alters the physiochemical characteristics of the soil, and thus potentially

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affect the processes controlling CH4 production, transport, and oxidation4,

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Concurrent changes to several soil characteristics may have counteracting influences

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that need to be considered to understand the resulting net CH4 emissions. For example,

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while thermokarst-induced lower volumetric water content (VWC) in the soil may

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reduce CH4 flux, other factors could enhance CH4 emissions. For instance, soil

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collapse often increases soil bulk density by reducing soil porosity, which may

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increase soil water-filled porespace (WFPS) in the soil30,

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though soil compaction reduces the VWC, which has been linked to reduced CH4

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emissions11-13, the concurrent increase in the WFPS may cause increasing anaerobic

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conditions

that

are

beneficial

to

methanogens

and

10

.

31

. Consequently, even

adverse

for

aerobic 5

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methanotrophs32-34, increase CH4 production and inhibit aerobic CH4 oxidation35. At

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the same time, increases in both bulk density and WFPS increases the thermal

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conductivity of the soil36, which would lead to higher soil temperatures and thus

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stimulate microbial CH4 production. In addition, thermokarst may increase the sand

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content as a result of the water scouring action37, 38, which may retain less CH4 and

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increase the potential for the transport of CH4 from soils to the atmosphere39, 40.

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Increased sand content may also influence the net CH4 emissions following shifts in

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pH, since both methanogen and methanotroph activity increase as conditions go from

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acidic to neutral41, 42. However, it remains poorly understood about the factors that

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dominate the response of net CH4 emissions following thermokarst development,

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which is a prerequisite for predicting future CH4 emissions from permafrost regions.

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The Tibetan permafrost region represents approximately three quarters of the total

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alpine permafrost area in the Northern Hemisphere43, yet its vulnerability to climate

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change has attracted limited attention compared to the boreal and tundra permafrost

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regions1, 44-46. In this study, we conducted a two-year survey (2015 and 2016) of CH4

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emissions during the peak growing season in a thermo-erosion gully underlaid by

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discontinuous permafrost on the Tibetan Plateau. We also measured the biotic and

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abiotic parameters relevant to the processes of CH4 production and consumption.

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Structural equation modeling (SEM) was used to evaluate the relative importance of

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various pathways that regulate CH4 emissions. Overall, our current study aimed to 1)

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examine how the CH4 emissions during peak growing season change along a thaw 6

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sequence; and 2) disentangle the biotic and abiotic regulating pathways of net CH4

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emission pattern along the thaw sequence.

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2. MATERIALS AND METHODS

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2.1. Site description

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This study was conducted within the permafrost region of the northeastern Tibetan

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Plateau, China (N 37°28′, E 100°17′, altitude ~3900 m above sea level; Figure 1a).

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The study site is located on a hillslope with a gentle south-facing 9° incline. The mean

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annual temperature is -3.3 °C and the average annual precipitation is 460 mm. The

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vegetation type is swamp meadow dominated by Kobresia tibetica, K. royleana,

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Carex atrofuscoides, Saussurea pulchra Lipsch, Potentilla saundersiana Royle, etc.

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The soil has a silty loam texture with a high organic carbon content (19.4 ± 0.5%) and

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high-water content at a depth of 10 cm (seasonal mean VWC of 62.2 ± 4.4%

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measured in 2015). Average active layer thickness (ALT) was estimated to be 0.86 m

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based on the combination of GSSI georadar (SIR-20, Laurel, Santa Clara, USA) and

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thaw-probe measurements. The excess ground ice is classified as intrusive ice47.

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Permafrost thaw and surface subsidence have been documented at the site since the

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early 1990s and have led to the initiation and expansion of the thermo-erosional gully.

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The gully had been experiencing thermo-erosion processes and retreating forward and

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laterally along the slope. The gully is present ~240 m long, with a maximum depth of

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approximately 2 m. During the thawing season, a stream flows along the gully from

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the head to the bottom where it exits into a larger watercourse. 7

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Four 15 × 20 m2 plots were established in 2015. One plot, referred to as the control,

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was located away from the thermo-erosional gully and showed no evidence of surface

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subsidence due to permafrost thaw. Another three plots were located within the gully,

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and reflected different stages of thaw, with the specific age since thaw initiation

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estimated by two steps. We first estimated the rate of gully retreat (~8 m yr-1) by

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comparing to satellite images from 2007 to 2013 and ground measurements between

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2014 and 2016. We then determined the time since collapse for each thaw stage by

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dividing the distance between the gully head and each site by the rate of gully retreat,

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respectively. The early, mid- and late stage plots were thus estimated to have

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undergone surface collapse due to thaw 3, 12, and 20 years prior to the study,

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respectively (Figure 1c-f).

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2.2. CH4 flux measurements

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We measured the CH4 fluxes using the static chamber methodology48. Ten sampling

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locations were randomly selected within each of the four plots. A collar (diameter 26

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cm and height 12 cm) with an anchor ring was installed at each location to a depth of

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10 cm. The static chambers (25 cm tall) were made of PVC and were covered with

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insulation materials. A fan was installed on the top wall of each chamber to provide

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headspace mixing. Gas samples (50 ml) were taken from the headspace 0, 10, 20, 30

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and 40 min after chamber closure. The chamber was vented before it was moved to

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the next collar. During gas sampling, the soil temperature and VWC in the top 10 cm, 8

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as well as the ALT, were measured adjacent to each collar. To ensure that the

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measurements were not biased by differences in the microclimate over time, or by

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artificial and system errors, four people collected gas samples simultaneously in the

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four quadrats, and the people were rotated among the quadrats during gas collection.

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Gas samples were collected between 9 am and 12 noon in July and August of 2015

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(three times per month with 10-day intervals) and 2016 (once in early July, three

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times in a row at the end of July, and once at the end of August). Note that we

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conducted the gas measurements during three consecutive days near the end of July

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rather than at equal intervals in 2016, since we aimed to match the gas and soil

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samplings to better reveal the biotic and abiotic mechanisms regulating CH4 fluxes.

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All gas samples were analyzed for CH4 concentration on a gas chromatograph

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(Agilent 7890A, Agilent Technologies Inc., Santa Clara, California, USA). The CH4

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flux was then calculated according to Eq.148.

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V P T 0 dC F = ρ× × × × t A P0 T dt

(1)

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where F is the CH4 flux (mg m-2 h-1); ρ is the density of CH4 under standard

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conditions (mg m-3); V and A are the volume of the chamber (m3) and the base area

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(m2), respectively; P is the air pressure (hPa) and T is the air temperature (K). Po and

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To are the standard pressure (1013 hPa) and the standard temperature (273 K),

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respectively. dCt/dt is the growth rate of the CH4 concentration (10-6 h-1).

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2.3. Soil sampling and analyses

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Soil samples were collected from the upper 15 cm using a standard soil probe 9

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(inside-diameter 2.5 cm) at the end of July 2016 near each collar. The soil samples

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were sieved (2 mm), homogenized, and divided into two subsamples. One set of

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subsamples was immediately flash-frozen in liquid nitrogen and stored at -80° C for

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DNA extraction. The other set of subsamples was air-dried and used to determine soil

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pH and sand content. The soil pH was determined in a soil water suspension (1:5 dry

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soil to deionized water ratio) using a pH electrode (PB-10, Sartorius, Germany). The

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soil texture was measured using a particle size analyzer (Malvern Masterizer 2000,

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Malvern, Worcestershire, UK) after organic matter and carbonates were removed

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using 30% hydrogen peroxide and 30% hydrochloric acid, respectively49. The bulk

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density of each sample was obtained using a standard container with a fixed volume

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size of 100 cm3 and was then measured based on the dry soil weight after it was

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oven-dried at 105 °C for 24 h. The WFPS was determined according to Eq.2.

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W FPS = SW C × BD /(1-BD/PD )

(2)

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In the equation, WFPS and BD represent the soil water-filled pore space (%) and soil

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bulk density (g cm-3), respectively, while the soil water content (SWC) is calculated

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based on the oven-dry method (g g-1), and PD represents the particle density of the

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soil (2.65 g cm-3).

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2.4. DNA extraction and quantitative PCR analysis

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We quantified the abundance of CH4-related functional genes (mcrA and pmoA) to

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examine the effects of specific microorganisms on the CH4 fluxes. The abundance of

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the mcrA gene has been widely used to quantify methanogens34, 35, as it codes for a 10

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subunit of Methyl-coenzymeM reductase, which is a crucial enzyme in CH4

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production. Conversely, the abundance of the pmoA gene is used as an indicator of the

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abundance of methanotrophs32,

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monooxygenase, which is a key enzyme for CH4 oxidation. During the quantification

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process, the DNA was extracted from 0.4 g of soil preserved at -80 °C using a Power

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Soil® DNA Isolation Kit (MoBio Laboratories, Carlsbad, CA, USA) according to the

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manufacturer protocol. The quality of the DNA was assessed based on 260/280 nm

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and 260/230 nm absorbance values using a NanoDrop-2000 (Thermo Fisher Scientific,

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Madison, WI, USA). The abundances of the pmoA and mcrA genes were determined

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on a StepOne Plus real-time PCR system (Applied Biosystems, Inc., CA, USA). The

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20 µl reaction mixture contained 10 µl SYBR Premix Ex Taq (Tli RNaseH Plus), 0.4

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µl forward and reverse primers, 0.4 µl ROX Reference Dye (50×), 6.8 µl sterile water

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and 2 µl five-fold diluted template DNA. The thermal-cycling conditions, primer pairs,

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number of cycles and references used to quantify the abundances of the pmoA and

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mcrA genes were shown in SI Table S1. The standard curves were generated using

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ten-fold serial dilutions of purified plasmids containing the respective genes with r2 >

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0.9. The copy numbers in the samples were calculated based on the comparison to the

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threshold cycle values of the standard curve, and were given in per gram soil (dry

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weight).

35

, as it codes for a subunit of methane

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2.5. Statistical analyses

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The data were analyzed following three steps. First, a repeated-measures analysis of 11

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variance (ANOVA) with interactions was carried out with the thaw stage as the main

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(between-subject) factor and sampling date as the within-subject factor to assess the

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effect of the main factors on the CH4 fluxes during the peak growing seasons of 2015

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and 2016. One-way ANOVAs were used to determine the differences in the biotic

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factors (the mcrA and pmoA abundances) and abiotic factors (soil temperature, VWC,

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WFPS, ALT, pH, bulk density, and sand content) among the different thaw stages.

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Multi-comparison of Tukey's HSD was conducted to test whether each parameter

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exhibited significant differences among the thaw stages at the significance level of P

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< 0.05.

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Second, linear regressions were performed to explore the relationships of the CH4

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fluxes with the biotic and abiotic factors, in which we adopted the mean values of

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CH4 fluxes, soil temperature, VWC and ALT measured three times in a row at the end

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of July, and the abundances of the mcrA and pmoA genes, WFPS, pH, bulk density,

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and sand content in the soils collected during this time. The mean CH4 fluxes were

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log-transformed to achieve a normal distribution of the data to meet the assumptions

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of the analysis. All abovementioned statistics were performed using SPSS 20.0 (IBM

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SPSS, Chicago, IL, USA).

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Third, structural equation modeling (SEM) was used to determine the major

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controlling pathways regulating the CH4 fluxes along the thaw sequence. SEM is an

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extension of traditional regression and pathway analyses that is used to model 12

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multivariate relations based on a collection of simultaneous procedures that determine

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the hypothetical pathways of direct and indirect influence among many variables

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using the covariance among those variables50, 51. A base model was established on the

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basis of the current understanding of the key CH4 emission controls as described in

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the literature (SI Figure 1). In the base model, we assumed that: 1) methanogen

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microbial abundance, as expressed by the mcrA gene, has a direct effect on the CH4

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flux. By contrast, the CH4 oxidation process was not included in the model because no

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significant correlation was observed between the CH4 fluxes and the abundance of the

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pmoA gene (Figure 4g). 2) Soil pH affects microbial growth and enzyme activities and

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thus controls the CH4 emissions through both direct and indirect pathways via the

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mcrA gene abundance. 3) Sand content, VWC and WFPS are linked to CH4 emissions

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directly and indirectly through microbial abundance and pH. The model χ2 test and

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root mean squared error approximation (RMSEA) value were used to evaluate the fit

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of the final model. The model was considered to have a good fit when the χ2 test was

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not significant (P>0.05) and the RMSEA was between 0 and 0.150. The coefficient of

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each pathway was presented as a standardized coefficient in the final model, and was

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determined using the analysis of the correlation matrices. The pathway coefficients

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reflect how many standard deviations the effect variable would be changed when the

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causal variable was changed by one standard deviation. SEM analyses were

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conducted using AMOS 21.0 (Amos Development Corporation, Chicago, IL, USA).

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3. RESULTS 13

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3.1. Variability in CH4 emissions along the thaw sequence

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Our field observations showed that permafrost collapse had significant effects on CH4

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emissions (SI Table S2), with consistent trends along the permafrost thaw sequence

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during the 2015 and 2016 peak growing season. In both years, significantly increased

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CH4 emissions compared to the control were only observed at the late thaw stage,

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where surface collapse due to permafrost thaw had occurred 20 years prior (Figure 2).

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3.2 Effects of upland thermokarst on biophysical variables

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Thermokarst significantly altered the soil properties, including soil temperature, VWC,

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WFPS, bulk density, sand content and pH (Table 1). Compared with the control, the

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soil temperatures in the top 10 cm at the middle and late thaw stages were 0.4 and

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0.6 °C higher, respectively. Both the VWC and WFPS were lower at the early thaw

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stage compared to the control, but progressively increased at the mid- and late thaw

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stages. Interestingly, at the late stage, the WFPS was significantly higher, but the

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VWC was still lower than the control. Both bulk density and sand content

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progressively increased along the permafrost thaw sequence, ranging from 0.27 to

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0.32 g cm-3 and from 44.3% to 58.7%, respectively. The soil pH was slightly acidic,

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ranging from 5.5 to 5.8, with a slight decrease at the early thaw stage compared to the

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control, but progressively higher at the mid- and late thaw stages. In contrast,

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permafrost collapse did not cause any significant change in the ALT among the thaw

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stages, possibly because a large amount of material in the active layer eroded during

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thermokarst development. 14

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Accompanying the changes in the soil properties, the abundances of the mcrA gene

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were substantially altered following thermokarst formation (Figure 3). Specifically,

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while the abundances of the mcrA were not significantly different among the control,

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early and mid-thaw stages (Figure 3a), but significantly higher at the late stage of

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permafrost collapse (Figure 3a). In contrast, thermokarst development did not exert a

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significant effect on the abundance of the pmoA gene (Figure 3b).

286 287

3.3 Linking CH4 fluxes to biotic and abiotic variables

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The regression analyses were performed between the average CH4 fluxes during three

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consecutive days and the biophysical variables measured in late July 2016. The CH4

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fluxes exhibited positive correlations with the WFPS (r2 = 0.43, P < 0.001; Figure 4c),

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pH (r2 = 0.53, P < 0.001; Figure 4d) and sand content (r2 = 0.64, P < 0.001; Figure 4e),

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but did not show any significant associations with either soil temperature (P = 0.054;

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Figure 4a) or VWC (P = 0.094; Figure 4b). Moreover, the CH4 fluxes significantly

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increased with higher mcrA gene abundances (r2 = 0.58, P < 0.001; Figure 4f).

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However, there was not a significant relationship between CH4 fluxes and pmoA gene

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abundance (P = 0.31; Figure 4g).

297 298

The SEM analysis indicated that the fit of this model was good (χ2 = 5.5, df = 5, P =

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0.35; RMSEA = 0.05; Figure 5). The combination of biotic (the abundance of the

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mcrA gene) and abiotic factors (sand content, VWC, WFPS and pH) explained 79% of 15

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the total variance in CH4 emission along the permafrost thaw sequence. Among the

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explanatory variables, sand content, WFPS, pH and the abundance of the mcrA gene

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had direct positive effects on CH4 emissions, with standardized coefficients ranging

304

from 22% to 48% (SI Figure 2a), whereas VWC, WFPS and sand content had indirect

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positive effects, ranging from 18% to 32% (SI Figure 2b). Specifically, sand content

306

and VWC affected CH4 emissions by modifying the WFPS, soil pH and mcrA gene

307

copies. The WFPS regulated CH4 emissions by altering the soil pH and the abundance

308

of the mcrA gene. Of these biotic and abiotic variables, sand content was the most

309

dominant factor responsible for the variations in the CH4 fluxes along the permafrost

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thaw sequence (SI Figure 2c).

311 312

4. DISCUSSION

313

We found that permafrost thaw and the development of a thermo-erosion gully in a

314

Tibetan swamp meadow led to a 3.5-fold increase in methane emissions, but elevated

315

CH4 emissions emerged only 20 years following the thaw. The CH4 emissions from

316

the undisturbed meadow during peak growing season were relatively low and similar

317

to the emissions from the early and mid-thaw stages within the thermo-erosion gully

318

(0.20 mg m-2 h-1 in 2015; 0.22 mg m-2 h-1 in 2016), and also similar to those observed

319

in an arctic upland permafrost region13, 52 (Figure 2). The methane emissions from the

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late thaw stage were moderate (0.74 mg m-2 h-1 in 2015; 1.33 mg m-2 h-1 in 2016) and

321

more similar in magnitude to the emissions from boreal bogs (Figure 2). Our finding

322

is supported by Abbott et al. (2015) who also demonstrated an increase in soil CH4 16

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concentrations after thermokarst formation in upland regions. However, our finding is

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conflicted by a recent study that reported a decrease in soil CH4 fluxes in another

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thermo-erosion gully on the Tibetan Plateau29. This difference may be related to

326

different hydrological conditions of the soils in the two study sites. Specifically, the

327

volumetric water content (VWC) observed in this study was higher than that reported

328

by Mu et al. (2017) (~67% vs. ~40%), and may have caused the greater sand content

329

by increasing the potential for the erosion of finer material37, 38. The higher sand

330

content at the late thaw stage has been indicated to enhance the CH4 emissions at our

331

study site39, 40. Such a difference between the two study sites could also result from

332

the different thermokarst-induced changes in the soil bulk density. Specifically, soil

333

bulk density has been reported to be relatively stable after permafrost collapse by Mu

334

et al. (2017). However, the soil bulk density increased significantly at our study site

335

(Table 1), which resulted in decreased soil porosity and increased WFPS. The increase

336

in the WFPS likely enhanced CH4 production at our study site53. Lastly, the

337

thermokarst-induced changes in methanogen abundance could differ between the two

338

study sites, which may also be responsible for the different CH4 flux responses to

339

thermokarst formation. Nevertheless, this possibility should be tested in future studies.

340 341

Our results showed that sand content was the dominant driver of CH4 emissions along

342

the thaw sequence. The close association between CH4 fluxes and sand content

343

observed in this study could be attributed to the following three aspects. First, sand

344

content could directly regulate CH4 emissions by controlling the transport from soils 17

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345

to the atmosphere40, 41. It is widely accepted that coarse soil textures are usually

346

favorable for CH4 diffusion, and thus increase CH4 emissions39, 40. Although increased

347

sand content could also stimulate methanotrophs by favoring the transportation for

348

oxygen import54, it could inhibit methanotrophs in other ways. For instance, the

349

increase in sand content was accompanied by the decrease in soil nitrogen content

350

along the thaw sequence (SI Figure 4), which could then increase the competition for

351

nitrogen resources between methanotrophs and lead to a lower gene abundance of

352

methanotrophs55 (SI Figure 5). Nevertheless, considering the non-significant

353

relationship between CH4 fluxes and methanotrophs (P = 0.31), the increased sand

354

content led to greater CH4 emissions along the thaw sequence mainly by enhancing its

355

transportation from soils to the atmosphere. Second, sand content could exert indirect

356

effects on CH4 release by adsorbing less H+, increasing the soil pH, and thus

357

promoting the CH4 fluxes via regulating CH4 production in the acidic soil39, 42. Third,

358

sand content could further regulate CH4 emissions by indirectly increasing the

359

WFPS56, which has been demonstrated to be another key factor that determines CH4

360

emissions along the thaw sequence (Figure 5). Taken together, our results highlight

361

that changes in soil structure cannot be overlooked when examining CH4 dynamics

362

under upland thermokarst.

363 364

Our results also revealed that upland thermokarst formation increased CH4 emission

365

despite the decreased VWC. The soil water status regulates CH4 emission primarily

366

via controlling CH4 production and the oxidation process30, 31. Actually, the WFPS, 18

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367

rather than the VWC itself, may better reflect these processes because the WFPS is

368

not only related to the VWC but also significantly influenced by soil bulk density.

369

Given that soil bulk density was elevated at the late stage of permafrost collapse

370

(Table 1), the soil environment can still become more anaerobic despite the decrease

371

in the VWC, thus increasing CH4 production and reducing oxidation of CH4 when it

372

passes through the soil pores. In support of this deduction, our results revealed that the

373

CH4 flux had a non-significant correlation with the VWC (Figure 4b), and the VWC

374

was only indirectly related to CH4 emissions via the WFPS (Figure 5). Thereby,

375

thermokarst-induced increases in the WFPS could be responsible for the higher CH4

376

emission rates occurred at the late stage of permafrost collapse.

377 378

Our results further illustrated that CH4 flux was significantly related to the

379

methanogen abundance, while it had no relationship with the abundance of

380

methanotroph genes (Figure 4f-g), demonstrating that the CH4 production process

381

dominates the CH4 flux pattern along this permafrost thaw sequence. The

382

methanogens are an important group of archaea that convert acetate, CO2 and H2 into

383

CH4 under anaerobic conditions57,

384

abundance exhibited a pattern similar to that of the CH4 emissions along the

385

permafrost thaw sequence (Figure 3a). At the early stage of permafrost collapse, the

386

abrupt changes in the soil environments caused the decline in the WFPS, which was

387

not beneficial for the growth of methanogens, thus decreasing the abundance of the

388

mcrA genes. After a certain period of disturbance, the abundance of microorganisms

58

. We found that the changes in methanogen

19

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389

would gradually recover (the mid-stage), and at the late stage of permafrost thaw, the

390

more anaerobic conditions may lead to an increased abundance of methanogens32.

391

Thus, the thermokarst-induced changes in methanogen abundance affected the CH4

392

production potential (SI Figure 3) and the in situ CH4 emission rates (Figure 4f;

393

Figure 5). Nevertheless, the microbial gene abundances at the DNA level cannot

394

represent the activity of microbes although they were widely linked to greenhouse gas

395

emissions in previous studies35, 59, 60. Further experiments at the RNA or protein levels

396

should be conducted in future studies to better reflect the activity of microbes.

397 398

It is generally assumed that soil temperature is one of the major factors that control

399

CH4 emissions across permafrost zones13. However, we did not find any significant

400

correlation between CH4 fluxes and soil temperature along the thaw sequence. This is

401

likely due to the narrow range of soil temperature measured during the end of July,

402

which was only approximately 1.5 °C among the various thaw stages. Actually, the

403

soil temperature may be found to regulate CH4 emissions if the data from other times

404

of day or year are included. To test this point, we compared the seasonal dynamics of

405

soil temperature and CH4 fluxes and found that they displayed consistent trends (SI

406

Figure 6a). Moreover, a significant linear relationship was observed between CH4

407

fluxes and soil temperature (r2 = 0.12, P < 0.05; SI Figure 6b). Therefore, soil

408

temperature dynamics are still important to CH4 fluxes across upland permafrost

409

regions.

410 20

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411

In conclusion, the results from our two-year field survey indicated that upland

412

thermokarst significantly increased CH4 emissions during the peak growing season

413

from a typical thermo-erosion gully on the Tibetan Plateau, particularly at the later

414

thaw stage (Figure 2). Our results also revealed that the WFPS, rather than the VWC

415

per se, regulated the CH4 emission patterns along the thaw sequence, and sand content

416

was the most dominant factor that influenced CH4 flux by modifying the transport

417

processes (Figure 5). These findings have two important implications for

418

understanding the carbon-climate feedback in permafrost regions. First, upland

419

thermokarst enhanced the CH4 emissions in our study, which differ from the

420

traditional view that upland thermokarst could result in a decrease in CH4 flux. These

421

findings suggest that thermokarst-associated changes in CH4 emissions vary across

422

upland permafrost regions, and projecting CH4 emissions after the development of

423

upland thermokarst is more challenging than previously thought. Second, the changes

424

to the soil structure following upland thermokarst cannot be ignored as a major driver

425

of the response of methane emissions. Despite the fact that previous studies

426

emphasized that the VWC was an important factor in regulating CH4 emissions in

427

permafrost thaw regions12, 13, 61, our study highlights that the changes in soil structure

428

(e.g., sand content and bulk density) may become predominant following the

429

development of upland thermokarst despite the condition of decreased VWC. Hence,

430

the effects of thermokarst on soil structure should be considered in Earth System

431

Models used to project the permafrost carbon-climate feedback.

432 21

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433

ACKNOWLEDGMENTS

434

This work was supported by the National Basic Research Program of China on Global

435

Change (2014CB954001), National Natural Science Foundation of China (31670482),

436

Key Research Program of Frontier Sciences, Chinese Academy of Sciences

437

(QYZDB-SSW-SMC049), Chinese Academy of Sciences-Peking University Pioneer

438

Cooperation Team, and Thousand Young Talents Program.

439 440

Supporting Information

441

CH4 production and oxidation potential measurements; q-PCR conditions for pmoA

442

and mcrA genes; results of repeated-measures ANOVA for CH4 fluxes; base model for

443

CH4 fluxes; standardized effects of biotic and abiotic variables on CH4 fluxes;

444

changes in CH4 production and oxidation potentials along the thaw sequence;

445

relationship between TN and sand content; relationship between pmoA gene

446

abundance and TN content; seasonal dynamics of soil temperature and its effects on

447

CH4 fluxes.

448 449

This information is available free of charge via the Internet at http://pubs.acs.org.

450 451

REFERENCES

452

(1) Ding, J.; Li, F.; Yang, G.; Chen, L.; Zhang, B.; Liu, L.; Fang, K.; Qin, S.; Chen,

453

Y.; Peng, Y. The permafrost carbon inventory on the Tibetan Plateau: a new evaluation

454

using deep sediment cores. Global Change Biol 2016, 22 (8), 2688-2701.

455

(2) Hugelius, G.; Strauss, J.; Zubrzycki, S.; Harden, J. W.; Schuur, E. A. G.; Ping, C. 22

ACS Paragon Plus Environment

Page 23 of 39

Environmental Science & Technology

456

L.; Schirrmeister, L.; Grosse, G.; Michaelson, G. J.; Koven, C. D.; O'Donnell, J. A.

457

Estimated stocks of circumpolar permafrost carbon with quantified uncertainty ranges

458

and identified data gaps. Biogeosciences 2014, 11 (23), 6573-6593.

459

(3) Helbig, M.; Chasmer, L. E.; Kljun, N.; Quinton, W. L.; Treat, C. C.; Sonnentag,

460

O. The positive net radiative greenhouse gas forcing of increasing methane emissions

461

from a thawing boreal forest-wetland landscape. Global Change Biol 2017, 23 (6),

462

2413-2417.

463

(4) Schuur, E. A. G.; Bockheim, J.; Canadell, J. G.; Euskirchen, E.; Field, C. B.;

464

Goryachkin, S. V.; Hagemann, S.; Kuhry, P.; Lafleur, P. M.; Lee, H. Vulnerability of

465

Permafrost Carbon to Climate Change: Implications for the Global Carbon Cycle.

466

BioScience 2008, 58 (8), 701-714.

467

(5) Pautler, B. G.; Simpson, A. J.; Mcnally, D. J.; Lamoureux, S. F.; Simpson, M. J.,

468

Arctic permafrost active layer detachments stimulate microbial activity and

469

degradation of soil organic matter. Environ. Sci. Technol 2010, 44 (11), 4076-4082.

470

(6) Olefeldt, D.; Goswami, S.; Grosse, G.; Hayes, D.; Hugelius, G.; Kuhry, P.;

471

McGuire, A. D.; Romanovsky, V. E.; Sannel, A. B.; Schuur, E. A. Circumpolar

472

distribution and carbon storage of thermokarst landscapes. Nat Commun 2016, 7

473

(13043), 13043.

474 475

(7) Kokelj, S. V.; Jorgenson, M. T. Advances in Thermokarst Research. Permafrost

Periglac 2013, 24 (2), 108-119.

476

(8) Prater, J. L.; Chanton, J. P.; Whiting, G. J. Variation in methane production

477

pathways associated with permafrost decomposition in collapse scar bogs of Alberta, 23

ACS Paragon Plus Environment

Environmental Science & Technology

478

Page 24 of 39

Canada. Global Biogeochem Cy 2007, 21 (4), 312-314.

479

(9) Andresen, C. G.; Lara, M. J.; Tweedie, C. E.; Lougheed, V. L. Rising

480

plant-mediated methane emissions from arctic wetlands. Global Change Biol 2017, 23

481

(3), 1128-1139.

482

(10) Abbott, B. W.; Jones, J. B. Permafrost collapse alters soil carbon stocks,

483

respiration, CH4 , and N2O in upland tundra. Global Change Biol 2015, 21 (12),

484

4570-4587.

485

(11) Klapstein, S. J.; Turetsky, M. R.; David, M. G. A.; Harden, J. W.; Czimczik, C.

486

I.; Xu, X.; Chanton, J. P.; Waddington, J. M. Controls on methane released through

487

ebullition in peatlands affected by permafrost degradation. J. Geophys. Res.:

488

Biogeosci 2014, 119 (119), 418–431.

489

(12) Natali, S. M.; Schuur, E. A. G.; Mauritz, M.; Schade, J. D.; Celis, G.; Crummer,

490

K. G.; Johnston, C.; Krapek, J.; Pegoraro, E.; Salmon, V. G.; Webb, E. E. Permafrost

491

thaw and soil moisture driving CO2 and CH4 release from upland tundra. J. Geophys.

492

Res.: Biogeosci 2015, 120 (3), 525-537.

493

(13) Olefeldt, D.; Turetsky, M. R.; Crill, P. M.; McGuire, A. D. Environmental and

494

physical controls on northern terrestrial methane emissions across permafrost zones.

495

Global Change Biol 2013, 19 (2), 589-603.

496

(14) Abbott, B. W.; Jones, J. B.; Godsey, S. E.; Larouche, J. R.; Bowden, W. B.

497

Patterns and persistence of hydrologic carbon and nutrient export from collapsing

498

upland permafrost. Biogeosciences 2015, 12 (12), 3725-3740.

499

(15) IPCC Climate Change 2013: The Physical Science Basis. Contribution of 24

ACS Paragon Plus Environment

Page 25 of 39

Environmental Science & Technology

500

Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on

501

Climate Change. Cambridge University Press: Cambridge. 2013.

502

(16) Koven, C. D.; Ringeval, B.; Friedlingstein, P.; Ciais, P.; Cadule, P.;

503

Khvorostyanov, D.; Krinner, G.; Tarnocai, C. Permafrost carbon-climate feedbacks

504

accelerate global warming. Proc. Natl. Acad. Sci. U. S. A 2011, 108 (36),

505

14769-14774.

506 507

(17) Kokelj, S. V.; Jorgenson, M. T., Advances in Thermokarst Research. Permafrost

Periglac 2013, 24, (2), 108-119.

508

(18) Kokelj, S. V.; Lantz, T. C.; Tunnicliffe, J.; Segal, R.; Lacelle, D. Climate-driven

509

thaw of permafrost preserved glacial landscapes, northwestern Canada. Geology 2017,

510

45 (4), 371-374.

511

(19) Kokelj, S. V.; Tunnicliffe, J. F.; Lacelle, D. The Peel Plateau of northwestern

512

Canada: an ice-rich hummocky moraine landscape in transition. In Landscapes and

513

Landforms of Western Canada; O Slaymaker Eds.; Springer International Publishing:

514

Switzerland 2017; pp 109-122.

515

(20) Bowden, W. B.; Gooseff, M. N.; Balser, A.; Green, A.; Peterson, B. J.; Bradford,

516

J. Sediment and nutrient delivery from thermokarst features in the foothills of the

517

North Slope, Alaska: Potential impacts on headwater stream ecosystems. J. Geophys.

518

Res.: Biogeosci 2015, 113 (G2), 187-193.

519

(21) Hodgkins, S. B.; Tfaily, M. M.; McCalley, C. K.; Logan, T. A.; Crill, P. M.;

520

Saleska, S. R.; Rich, V. I.; Chanton, J. P. Changes in peat chemistry associated with

521

permafrost thaw increase greenhouse gas production. Proc. Natl. Acad. Sci. U. S. A 25

ACS Paragon Plus Environment

Environmental Science & Technology

522

Page 26 of 39

2014, 111 (16), 5819-5824.

523

(22) McCalley, C. K.; Woodcroft, B. J.; Hodgkins, S. B.; Wehr, R. A.; Kim, E. H.;

524

Mondav, R.; Crill, P. M.; Chanton, J. P.; Rich, V. I.; Tyson, G. W. Methane dynamics

525

regulated by microbial community response to permafrost thaw. Nature 2014, 514

526

(7523), 478-481.

527

(23) Nauta, A. L.; Heijmans, M. M. P. D.; Blok, D.; Limpens, J.; Elberling, B.;

528

Gallagher, A.; Li, B.; Petrov, R. E.; Maximov, T. C.; van Huissteden, J.; Berendse, F.

529

Permafrost collapse after shrub removal shifts tundra ecosystem to a methane source.

530

Nat. Clim. Change 2014, 5 (1), 67-70.

531

(24) Wik, M.; Varner, R. K.; Anthony, K. W.; Macintyre, S.; Bastviken, D.

532

Climate-sensitive northern lakes and ponds are critical components of methane release.

533

Nat Geosci 2016, 9 (2), 99-105.

534

(25) Juncher Jørgensen, C.; Lund Johansen, K. M.; Westergaard-Nielsen, A.;

535

Elberling, B. Net regional methane sink in High Arctic soils of northeast Greenland.

536

Nat Geosci 2014, 8 (1), 20-23.

537 538

(26) Harms, T. K.; Abbott, B. W.; Jones, J. B. Thermo-erosion gullies increase nitrogen available for hydrologic export. Biogeochemistry 2013, 117 (2-3), 299-311.

539

(27) Perreault, N.; Lévesque, E.; Fortier, D.; Lamarque, L. J. Thermo-erosion gullies

540

boost the transition from wet to mesic tundra vegetation. Biogeosciences 2016, 13 (4),

541

1237-1253.

542

(28) Kwon, M. J.; Beulig, F.; Ilie, I.; Wildner, M.; Kusel, K.; Merbold, L.; Mahecha,

543

M. D.; Zimov, N.; Zimov, S. A.; Heimann, M.; Schuur, E. A. Plants, microorganisms, 26

ACS Paragon Plus Environment

Page 27 of 39

Environmental Science & Technology

544

and soil temperatures contribute to a decrease in methane fluxes on a drained Arctic

545

floodplain. Global Change Biol 2017, 23 (6), 2396-2412.

546

(29) Mu, C.; Abbott, B. W.; Zhao, Q.; Su, H.; Wang, S. F.; Wu, Q.; Zhang, T.; Wu, X.,

547

Permafrost collapse shifts alpine tundra to a carbon source but reduces N2O and CH4

548

release on the northern Qinghai-Tibetan Plateau. Geophys Res Lett 2017, 44, (17),

549

8945-8952.

550

(30) Zhang, T.; Zhu, W.; Mo, J.; Liu, L.; Dong, S. Increased phosphorus availability

551

mitigates the inhibition of nitrogen deposition on CH4 uptake in an old-growth

552

tropical forest, southern China. Biogeosciences 2011, 8 (9), 2805-2813.

553 554

(31) Franzluebbers, A. J. Microbial activity in response to water-filled pore space of variably eroded southern Piedmont soils. Appl Soil Ecol 1999, 11 (1), 91-101.

555

(32) Aronson, E. L.; Dubinsky, E. A.; Helliker, B. R. Effects of nitrogen addition on

556

soil microbial diversity and methane cycling capacity depend on drainage conditions

557

in a pine forest soil. Soil Biol and Biochem 2013, 62 (62), 119-128.

558 559

(33) Hanson, R. S.; Hanson, T. E., Methanotrophic bacteria. Oxford Univ Press: New York. 1996.

560

(34) Peltoniemi, K.; Laiho, R.; Juottonen, H.; Bodrossy, L.; Kell, D. K.; Minkkinen,

561

K.; Mäkiranta, P.; Mehtätalo, L.; Penttilä, T.; Siljanen, H. M. P., Responses of

562

methanogenic and methanotrophic communities to warming in varying moisture

563

regimes of two boreal fens. Soil Biol and Biochem 2016, 97, 144-156.

564

(35) Freitag, T. E.; Toet, S.; Ineson, P.; Prosser, J. I., Links between methane flux and

565

transcriptional activities of methanogens and methane oxidizers in a blanket peat bog. 27

ACS Paragon Plus Environment

Environmental Science & Technology

566

Page 28 of 39

FEMS Microbiol Ecol 2010, 73, (1), 157-166

567

(36) Abu-Hamdeh, N. H.; Khdair, A. I.; Reeder, R. C., A comparison of two methods

568

used to evaluate thermal conductivity for some soils. Int J Heat Mass Tran 2001, 44,

569

(5), 1073-1078.

570

(37) Foster, G. R. Transport of soil particles by shallow flow. Amer Soc Agr Eng Trans

571

Asae 1972, 51 (5), 99-102.

572

(38) Wang, J.; Li, Z.; Cai, C.; Ma, R. Particle size and shape variation of Ultisol

573

aggregates affected by abrasion under different transport distances in overland flow.

574

Catena 2014, 123, 153-162.

575

(39) Wang, Z.; LindauR, C. W.; Delaune, D.; Patrick Jr, W. H. Methane emissions

576

and entrapment in flooded rice soils as affected by soil properties. Biological

577

Fertilization Soils, 1993 16, 163-168.

578

(40) Minami, K.; Mosier, A.; Sass, R. CH4 and N2O : global emissions and controls

579

from rice fields and other agricultural and industrial sources. National Institute of

580

Agro-Environmental Sciences Press: Tsukuba. 1994.

581

(41) Dunfield, P.; Knowles, R.; Dumont, R.; Moore, T. R. Methane production and

582

consumption in temperate and subarctic peat soils: response to temperature and pH.

583

Soil Biol and Biochem 1993, 25 (3), 321-326.

584 585 586 587

(42) Moore, I. D.; Gessler, P. E.; Nielsen, G. A.; Peterson, G. A. Soil attribute prediction using terrain analysis. Soil Sci Soc Am J 1993, 57 (2), 443-452. (43) Wang, B.; French, H. M. Permafrost on the Tibet Plateau, China. Quaternary

Sci Rev 1995, 14 (3), 255-274. 28

ACS Paragon Plus Environment

Page 29 of 39

Environmental Science & Technology

588

(44) Mu, C.; Zhang, T.; Zhang, X.; Li, L.; Guo, H.; Zhao, Q.; Cao, L.; Wu, Q.;

589

Cheng, G. Carbon loss and chemical changes from permafrost collapse in the northern

590

Tibetan Plateau. J. Geophys. Res.: Biogeosci 2016, 121 (7), 1781-1791.

591

(45) Niu, F.; Lin, Z.; Lu, J.; Luo, J.; Wang, H. Assessment of terrain susceptibility to

592

thermokarst lake development along the Qinghai–Tibet engineering corridor, China.

593

Environ Earth Sci 2014, 73 (9), 5631-5642.

594

(46) Li, F.; Peng, Y.; Natali, S. M.; Chen, K.; Han, T.; Yang, G.; Ding, J.; Zhang, D.;

595

Wang, G.; Wang, J.; Yu, J.; Liu, F.,; Yang, Y. Warming effects on permafrost

596

ecosystem carbon fluxes associated with plant nutrients. Ecology 2017, 98 (11),

597

2851-2859.

598 599

(47) Zhou Y. Geocryology in China. Chinese Academy of Sciences Press: Lan Zhou., 2000.

600

(48) Kaufman, M. Soil-atmosphere exchange in nitrous oxide, nitric oxide, and

601

methane under secondary succession of pasture to forest in the Atlantic lowlands of

602

Costa Rica. Global Biogeochem Cy 1994, 8 (4), 399-410.

603

(49) Fierer, N.; Colman, B. P.; Schimel, J. P.; Jackson, R. B. Predicting the

604

temperature dependence of microbial respiration in soil: A continental-scale analysis.

605

Global Biogeochem Cy 2006, 20 (3), 273-274.

606 607 608 609

(50) Grace, J. B. Structural Equation Modeling and Natural Systems. Cambridge Univ. Press: Cambridge, 2006. (51) Shipley, B. Cause and Correlation in Biology. Cambridge University Press: Cambridge, 2007. 29

ACS Paragon Plus Environment

Environmental Science & Technology

Page 30 of 39

610

(52) Zona, D.; Gioli, B.; Commane, R.; Lindaas, J.; Wofsy, S. C.; Miller, C. E.;

611

Dinardo, S. J.; Dengel, S.; Sweeney, C.; Karion, A., Cold season emissions dominate

612

the Arctic tundra methane budget. Proc. Natl. Acad. Sci. U. S. A 2016, 113, (1), 40.

613

(53) Louro, A.; Sawamoto, T.; Chadwick, D.; Pezzolla, D.; Bol, R.; Báez, D.;

614

Cardenas, L., Effect of slurry and ammonium nitrate application on greenhouse gas

615

fluxes of a grassland soil under atypical South West England weather conditions. Agr

616

Ecosyst Environ 2013, 181, 1-11.

617

(54) Elberling, B.; Askaer, L.; Jørgensen, C. J.; Joensen, H. P.; Kühl, M.; Glud, R. N.;

618

Lauritsen, F. R., Linking soil O2, CO2, and CH4 concentrations in a Wetland soil:

619

implications for CO2 and CH4 fluxes. Environ Sci Technol 2011, 45, (8), 3393.

620

(55) Kou, Y.; Li, J.; Wang, Y.; Li, C.; Tu, B.; Yao, M.; Li, X., Scale-dependent key

621

drivers controlling methane oxidation potential in Chinese grassland soils. Soil Biol

622

Biochem 2017, 111, 104-114.

623

(56) Davidson, E. A.; Verchot, L. V.; Cattânio, J. H. Effects of soil water content on

624

soil respiration in forests and cattle pastures of eastern Amazonia. Biogeochemistry

625

2000, 48 (1), 53-69.

626

(57) Sakai, S.; Imachi, H.; Hanada, S.; Ohashi, A.; Harada, H.; Kamagata, Y.

627

Methanocella paludicola gen. nov., sp. nov., a methane-producing archaeon, the first

628

isolate of the lineage 'Rice Cluster I', and proposal of the new archaeal order

629

Methanocellales ord. nov. Int J Syst Evol Micr 2008, 58 (Pt 4), 929-936.

630

(58) Woese, C. R.; Kandler, O.; Wheelis, M. L. Towards a natural system of

631

organisms: proposal for the domains Archaea, Bacteria, and Eucarya. Proc. Natl. Acad. 30

ACS Paragon Plus Environment

Page 31 of 39

Environmental Science & Technology

632 633 634

Sci. U. S. A. 1990, 87 (12), 4576-4579. (59) Morales, S. E.; Cosart, T.; Holben, W. E., Bacterial gene abundances as indicators of greenhouse gas emission in soils. ISME J 2010, 4, (6), 799-808.

635

(60) Lammel, D. R.; Feigl, B. J.; Cerri, C. C.; Nüsslein, K., Specific microbial gene

636

abundances and soil parameters contribute to C, N, and greenhouse gas process rates

637

after land use change in Southern Amazonian Soils. Front Microbiol 2015, 6, (1057),

638

1057.

639

(61) Schädel, C.; Bader, M. K. F.; Schuur, E. A. G.; Biasi, C.; Bracho, R.; Čapek, P.;

640

Baets, S. D.; Diáková, K.; Ernakovich, J.; Estoparagones, C. Potential carbon

641

emissions dominated by carbon dioxide from thawed permafrost soils. Nat Clim

642

Chang 2016, 6 (10), 950-953.

31

ACS Paragon Plus Environment

Environmental Science & Technology

643

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Table 1 Changes in soil physical parameters in different thaw stages. Parameter

Control

3 years

12 years

20 years

pH

5.6±0.03ab

5.5±0.03b

5.7±0.02a

5.8±0.06a

Soil Temperature (°C)

8.6±0.13c

8.8±0.10bc 9.0±0.12ab

9.2±0.13a

Volume Water Content (%)

67.6±0.6a

48.8±2.1c

62.8±1.1b

63.1±1.8b

Soil Water-filled Pore Space (%)

77.2±1.8b

62.7±2.8c

79.1±3.8ab

84.1±1.5a

Active Layer Thickness (cm)

68.5±5.1a

76.2±17.5a

73.7±7.5a

68.7±2.2a

Soil Bulk Density (g cm-3) Sand Content (%)

0.27±0.008b 0.32±0.014a 0.28±0.012b 0.32±0.021a 44.3±0.4b

46.5±0.9b

49.7±1.8b

58.7±4.2a

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Data are expressed as the means of ten replicate plots (±1 standard error). Different

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letters indicate significant differences among the different stages since permafrost

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collapse (one-way ANOVA, P < 0.05).

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Figure legends

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Figure 1. Location map of the study area (a), image of the thermo-erosion gully (b),

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and photographs of the different thaw stages (c-f). The red dot indicates our study site,

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and the image of the thermo-erosion gully was obtained by a high-resolution

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topographic model with LiDAR (VZ-400, Riegl, Horn, Austria, analyzed with Riscan

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pro 2.0 software).

653 654

Figure 2. Changes in CH4 fluxes along the thaw sequence in 2015 (a) and 2016 (b).

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The error bars represent the standard error determined among replicates (n = 10). The

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colors correspond to the different collapse times. The gray line indicates the mean

657

value of the CH4 flux in an arctic dry tundra, and the red line indicates the mean value

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of the CH4 flux in an arctic bog (data from Olefeldt et al.13). Significant differences

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were denoted by different letters among the different stages since permafrost collapse

660

(repeated-measures ANOVA, P < 0.05). The insert panels show the site-averaged CH4

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fluxes for each sampling time during the entire sampling period, with the error bars

662

representing the standard error determined among replicates.

663 664

Figure 3. Changes in the abundance of the mcrA (a) and pmoA (b) genes along the

665

thaw sequence. The error bars represent the standard error determined among

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replicates (n = 10). The different letters indicate significant differences among the

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different thaw stages (one-way ANOVA, P < 0.05).

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Figure 4. Relationships between the logarithm transformed CH4 fluxes and soil

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temperature (ST), volume water content (VWC), soil water-filled porespace (WFPS),

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soil pH, sand content, and the gene copies of mcrA and pmoA. The black lines and

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shades represent the regression lines with 95% confidence intervals. Statistics (r2 and

673

P values) for the linear regression are shown (***P < 0.001).

674 675

Figure 5. Final results of structural equation model (SEM) analysis examining the

676

effects of edaphic and microbial properties on CH4 flux. In the model, square boxes

677

indicate variables, and the abbreviations of the variables are explained in Table 1. The

678

arrows connecting the boxes indicate the direction of causation. The red arrows denote

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positive relationships. The arrow widths are proportional to the standardized path

680

coefficients, which reflect the importance of the factors in the model. The proportion

681

of explained variance (r2) is below each response variable in the model. The final

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model fit was evaluated by a χ2 test and RMSEA value. * P < 0.05, ** P < 0.01, ***

683

P < 0.001.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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