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SUPPLEMENTARY INFORMATION DOI: 10.1038/NCLIMATE2058 NATURE CLIMATE CHANGE | www.nature.com/natureclimatechange 1 by M. Lupascu, J. M. Welker, U. Seibt, K. Maseyk, X. Xu, C.I. Czimczik . 1 - Material and Methods 2 - Supplementary Figure and Legend 1; 3 - Supplementary Table 1-2 High Arctic wetting reduces permafrost carbon feedbacks to climate warming

High Arctic wetting reduces permafrost carbon feedbacks to climate warming

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Page 1: High Arctic wetting reduces permafrost carbon feedbacks to climate warming

SUPPLEMENTARY INFORMATIONDOI: 10.1038/NCLIMATE2058

NATURE CLIMATE CHANGE | www.nature.com/natureclimatechange 1

  1  

Supplemental Information of the manuscript entitled 1  

High Arctic wetting dampens permafrost carbon feedbacks to climate 2  

warming 3  

by M. Lupascu, J. M. Welker, U. Seibt, K. Maseyk, X. Xu, C.I. Czimczik. 4  

5  1 - Material and Methods 6   7  2 - Supplementary Figure and Legend 1; 8  

3 - Supplementary Table 1-2 9  

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High Arctic wetting reduces permafrost carbon feedbacks to climate warming

Page 2: High Arctic wetting reduces permafrost carbon feedbacks to climate warming

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Material and Methods 26  

Field site 27  

The field site is located at North Mountain near Thule U.S. Air Force Base in 28  

Northwest Greenland (76o32’N, 68o50’W, 200–350 m a.s.l.). The study area is drained 29  

into North Star Bay by the North River, which originates from the Store Landgletscher (a 30  

segment of the Greenland Ice Sheet) and has a land drainage area of about 100 km2 31  

(excluding ice sheet drainage)6. 32  

The soil at our site is a Typic Haploturbel with a maximum thaw depth of about 1 33  

m. Soils are developed from glacial till overlying alternating bands of Proterozoic shale 34  

and fine-grained sandstones interbedded with dolostone and calcareous sandstone6. Soils 35  

are subject to cryoturbation. In vegetated areas, the topsoil (0-12 cm) texture is 67-74% 36  

sand, 20–34% silt and 5-8% clay20, with a bulk density (0-60 cm) of approximately 1.54 37  

g soil cm-3 and an organic carbon (C) content between 0.26-0.05 kg C m-2 (Table S2). In 38  

bare areas, the topsoil (0-12 cm) texture is 54–64% sand, 33–38% silt and 3-7% clay, 39  

with a bulk density of approximately 1.75 g soil cm-3 and a C content (0-12 cm) between 40  

0.3 and 0.4 kg C m-2 (ref. 20). Vascular plant cover is approximately 50% and the 41  

patterned ground is a mixture of nonsorted nets, weakly formed stripes and frost boils. 42  

The vascular plant community, which maintains an open canopy less than 5 cm in height, 43  

is dominated by three species: Salix arctica PALL., Dryas integrifolia VAHL. and Carex 44  

rupestris ALL20. 45  

46  

47  

48  

Page 3: High Arctic wetting reduces permafrost carbon feedbacks to climate warming

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Experimental set-up 49  

A multifactorial climate change experiment was established at the site in 2003 to 50  

mimic conditions of today and 2050 (ref. 20, 23).  Two similar 70×16 m blocks of tundra 51  

were delineated within a 70×60 m2 area and control conditions and three treatments were 52  

assigned to 2.0×0.8 m2 plots in a randomized complete block design: (1) +4°C warming, 53  

(2) wetting and (3) +4°C soil warming × wetting. 54  

Infrared radiators (160×12 cm2, Kalglo Electronics Co. Inc., Bethlehem, PA, 55  

USA) were suspended 125 cm above the soil surface with rebar tripods installed at the 56  

edge of each plot. Precipitation records for the climate period 1971–2000 were used to 57  

design the irrigation experiment to increase the magnitude of summer precipitation by 58  

approximately 50% while maintaining seasonal patterns. Historically, record analysis 59  

exhibited that precipitation during July was approximately double of that in June and 60  

August. Plots were hence irrigated weekly with gardening cans with 2 mm of 61  

supplemental water in June and August and with 4 mm in July. 62  

Each treatment regime and control was replicated twice in each block, such that n 63  

= 4 at the site-level. Plots were oriented to cover the shift between vascular plants and 64  

bare soil to facilitate scaling from the plot- to ecosystem-level. 65  

CO2 measurements and collection 66  

Measurements of ecosystem respiration (Reco) and net ecosystem exchange (NEE) 67  

were conducted on vegetated and bare areas from 2010 to 2012 (2010-2011 for NEE) 68  

from the end of May to the end of August (average temperature and precipitation, see 69  

Supplementary Table 1). Ecosystem respiration and CO2 concentrations at depth were 70  

analyzed as a minimum of three times a week through the use of an infrared gas analyzer 71  

Page 4: High Arctic wetting reduces permafrost carbon feedbacks to climate warming

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(800, 820, 840, 1400, LI-COR, Lincoln, NE, USA). Ecosystem respiration was measured 72  

with opaque chambers placed on the top of collars previously inserted in the ground. Soil 73  

CO2 concentrations were monitored via gas wells inserted vertically to 20, 30, 60 or 90 74  

cm depth. Wells consisted of 1.27 cm o.d. stainless steel pipes, perforated at the end 75  

inserted into the ground, and with a 0.64 cm i.d. swagelock union with septa for sampling 76  

on the aerial end (Grace, Deerfield, USA). 77  

Net ecosystem exchange was measured using clear chambers (two designs: ABT-78  

Sorime, Paris, France, and LI-COR-8100-104C) coupled to a cavity ring-down CO2, CH4, 79  

H2O analyzer (G1301, Picarro Inc., Santa Clara, CA, USA). Chambers were closed for 15 80  

minutes each on an automated schedule, and kept open for the remaining time. During 81  

closure periods, the sampling line to the analyzer was switched every 2-3 minutes 82  

between chamber inlet and outlet lines. Net CO2 fluxes (Fc) were calculated by fitting an 83  

exponential function [1] to the changes in chamber CO2 mole fraction (Cch) vs. closure 84  

time (t): 85  

86  

!!! ! = !!" + !!/!   ∗ (1  −  e(!!∗!!))   [1], 87  

88  

where Cin is the CO2 mole fraction of incoming air, f is the flowrate, and V is the chamber 89  

volume. Data processing was implemented in IDL7. 90  

Gas samples (Reco, atmospheric CO2, root-respired CO2) for 14C analysis were 91  

collected on molecular sieve traps monthly. To sample Reco, chambers were left closed 92  

until the CO2 concentration accumulated to more than twice of that of ambient air. Roots 93  

were manually extracted from a 30×30×5 cm soil block, rinsed with water and incubated 94  

Page 5: High Arctic wetting reduces permafrost carbon feedbacks to climate warming

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in CO2-free air for 24 hours. Soil gas was collected in pre-evacuated, stainless steel 95  

canisters via flow-restricting capillaries. 96  

Understanding radiocarbon measurements 97  

Radiocarbon (14C) analysis of Reco is a valuable tool for understanding the sources 98  

of Reco, i.e. the respiration of plants that fix CO2 from the atmosphere vs. the respiration 99  

of microbes that decompose soil C to CO2 (ref. 25). Radiocarbon is produced in the upper 100  

atmosphere, oxidized to CO2 and enters ecosystems via photosynthesis, so that every 101  

living thing contains 14C. The 14C content of recent photosynthetic products, and 102  

therefore the 14C content of plant and rhizosphere respiration, is similar to the 14C content 103  

of current atmospheric CO2. Due to radioactive decay, century- to millennium-old soil C 104  

is depleted in 14C (t½=5730 years). 105  

In addition, 14C can be used to identify CO2 derived from the decomposition of 106  

soil C made from photosynthetic products years to decades ago25. This C (“modern C”) is 107  

enriched in 14C due to the production of 14C during above ground testing of nuclear 108  

weapons in the 1950s and 1960s. Since test cessation, the amount of bomb-14C in the 109  

atmosphere is declining as a consequence of mixing with terrestrial and ocean C pools 110  

and emissions of fossil (14C-free) CO2. 111  

Distinguishing Reco sources is crucial for assessing soil C feedbacks to rising 112  

atmospheric CO2 levels. The rapid cycling of modern C between plants and soil microbes 113  

has no net effect on atmospheric CO2 levels, but decomposition of older C pools, that 114  

were formerly disconnected from the C cycle, represents a net flux of C to the 115  

atmosphere. 116  

117  

Page 6: High Arctic wetting reduces permafrost carbon feedbacks to climate warming

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Isotope analyses of CO2 118  

In order to analyze its 14C content, CO2 was released from molecular sieve traps 119  

by baking at 650°C for 45 min or extracted from canisters on a vacuum line. Carbon 120  

dioxide was then purified cryogenically and reduced to graphite through sealed tube zinc 121  

reduction28. A split of the purified CO2 was analyzed for its δ13C signature (GasBench 122  

coupled with DeltaPlus IRMS, Thermo). 123  

Samples were analyzed at UC Irvine’s W. M. Keck Carbon Cycle Accelerator 124  

Mass Spectrometer Facility. The 14C content of Reco was mathematically corrected for 125  

CO2 from ambient air in the chamber headspace based on CO2 concentration and δ13C 126  

measurements. 127  

Collection and analysis of soil carbon 128  

Soil samples were collected from a pit, stored frozen, dried at 60°C, sieved (<2 129  

mm), ground to powder and acid-washed. Concentrations of organic C were measured by 130  

an elemental analyzer (Carlo-Erba, NA 1500 series II, Thermo Scientific). Prior to 14C 131  

analysis (see above), soil samples were combusted to CO2 in pre-combusted, evacuated 132  

quartz tubes with cupric oxide for 2 hours at 900°C. 133  

Estimating summer ecosystem respiration 134  

To obtain gap-filled estimates of summer Reco, we used the relationship between 135  

respiration and temperature30, represented by an Arrhenius type equation [2]: 136  

137  

! = !!!!!

(!!!!) [2], 138  

139  

where T (K) is the average daily temperature, Eo= 308.56 K is the activation energy, and 140  

Page 7: High Arctic wetting reduces permafrost carbon feedbacks to climate warming

  7  

To= 227.13 K. The fitting parameter A is obtained from the data set. Gap-filled values of 141  

daily Reco (R) were then calculated using the average daily temperature for the missing 142  

days. 143  

For the NEE measurements (Table 1), due to failure of one of the automated 144  

chambers we could not obtain flux data for the bare soil plot in the +4°C treatment in 145  

2011. For the bare soil plots, NEE was well correlated with Reco across the treatments and 146  

years, hence we estimated the missing flux (NEE bare soil, +4°C, 2011, see Table 1) 147  

from a regression using bare soil NEE and Reco data (y = 0.997x +1.77, R² = 0.87). 148  

Uncertainty estimates were determined by error propagation for this and all other fluxes 149  

listed in Table 1. 150  

Estimating contributions of deep, old C pools to ecosystem respiration 151  

We used a two-pool mixing model29 to estimate the contributions from older 152  

permafrost soil C and young, surface soil C to ecosystem CO2 emissions: 153  

154  

!!"# = !!"# + !!"# [3], 155  

!!"#×∆!"!!"# = !!"#×∆!"!!"# + !!"#×∆!"!!"# [4], 156  

157  

where Reco, Rold and Rnew are total ecosystem respiration, decomposition of older, 158  

belowground permafrost C (aka ‘old C loss’), and new C, reflecting plant respiration and 159  

decomposition of young C exuded from plant roots or leached from the litter layer, 160  

respectively. We approximated Δ14Ceco as the summertime average 14C content of Reco, 161  

Δ14Cold as the average (2010-12) 14C content of pore space CO2 from 60-90 cm depth 162  

(Supplementary Fig. 1), reflecting in situ decomposition of older soil C below the rooting 163  

Page 8: High Arctic wetting reduces permafrost carbon feedbacks to climate warming

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zone, and Δ14Cnew as the 14C content of litter to represent young, surface soil C (see Table 164  

S2). But while all soil layers contribute to the overall mix of ages coming out at the 165  

surface (i.e. using a multi-layer mixing model would be better than using a two-pool 166  

mixing model) equation 4 cannot be solved for more than two pools. 167  

We assumed that the mean 14C content of the belowground sources did not change 168  

between years. Our dataset does not allow us to discern CO2 emissions of microbes 169  

associated with the rhizosphere or decomposing litter from those of plant roots, as they 170  

have similar 14C contents. Using CO2 emissions from bare areas as a proxy for microbial 171  

CO2 respiration in vegetated areas (about 15% of Reco) yielded unrealistic (negative) 172  

contributions from older, belowground permafrost C pools. Flux estimates and 173  

uncertainties were calculated separately for each surface cover (vegetated, bare), 174  

treatment (control, +4°C, W, +4°C × W) and year (2010, 2011, 2012). Results were then 175  

averaged by surface cover and year, as we did not detect significant differences in the 14C 176  

content of Reco between treatments (Fig. 2a). Finally, in order to calculate the landscape 177  

emission of old C [5], we multiplied the cumulative Reco of the entire summer by the % of 178  

old C, calculated with the model described above: 179  

180  

!"#$%&"'(  !"#  !!"# = (!!"#!"#×%!!"#

!"# + !!"#!"#$×%!!"#!"#$%)/100   [5] 181  

182  

where Reco is g C m-2. 183  

184  

Page 9: High Arctic wetting reduces permafrost carbon feedbacks to climate warming

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Supplementary Figures 185  

Supplementary Figure 1. Radiocarbon content (per mill) along the soil profile showing 186  

the different treatments (C, +4°C, W & +4°C × W) in bare (a-c) and vegetated plots (d-f) 187  

for 2010, 2011 and 2012 [dashed lines represent 14C content of ambient air at the time of collection]. 188  

189   190   191   192   193   194   195   196   197   198   199   200   201   202   203   204   205  

Page 10: High Arctic wetting reduces permafrost carbon feedbacks to climate warming

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Tables 206   207  Supplementary Table 1. Cumulative precipitation (rain) and air temperature (average 208  

±SD) for the measurement period from Thule airport (THU). 209  

210   211   212   213   214   215   216   217   218   1Snow-free period 219   220   221   222  

Rainfall Tot (mm) Average T (°C)

2010 2011 2012 2010 2011 2012

May 5.8 9.9 53.1 -1.9 (3.7) -4.7 (4.4) -3.8 (3.7) June 5.1 0.3 34.5 2.9 (2.8) 3.6 (3.0) 3.6 (3.3) July 48.0 19.6 50.0 6.6 (1.8) 8.7 (2.0) 7.6 (1.6) Aug 25.9 6.9 23.6 6.9 (1.5) 6.0 (2.2) 4.6 (3.3) June-Aug1 84.8 36.7 161.29 5.3 (2.8) 6.1 (3.1) 5.3 (3.3)

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Supplementary Table 2. Concentration (C), 14C content (Δ14C) and age of bulk soil 223  

organic carbon, and estimated soil bulk density6 (BD) and carbon density (CD) as a 224  

function of depth in a high arctic semi-desert soil under vegetation (column 3: 225  

average±SE, columns 4-5: ±analytical error). [Bulk density values are estimated from samples of 226  

genetic soil horizons]. 227  

228   229   230   231   232   233   234   235   n.m. = not measured, *permafrost 236   237   238   239  

240  

UCIAMS# Depth C Δ14C 14C Age BD6 CD cm % bulk soil per mill yrs BP g soil cm-3 kg C m-2

127892 litter 41.31 (0.60) 65.6 (2.5) modern n.m. n.m. 102946 20 0.33 (n.m.) -185.1 (1.1) 1585 (15) 1.57 0.26 102947 30 0.21 (0.02) -288.6 (1.0) 2675 (15) 1.42 0.15 102948 50 0.22 (n.m.) -315.9 (1.0) 2990 (15) 1.42 0.16 102949 60 0.11 (<0.01) -497.2 (0.8) 5465 (15) 1.73 0.10 102950 80 0.06 (0.01) -733.8 (0.7) 10570 (25) 1.73 0.05 102951 110* 0.06 (n.m.) -811.5 (0.6) 13345 (30) 1.73 0.05