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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NCLIMATE2058
NATURE CLIMATE CHANGE | www.nature.com/natureclimatechange 1
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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
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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
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48
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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
4
(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
5
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
6
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
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
8
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
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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
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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)
11
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