SUPPLEMENTARY INFORMATION

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1 Supplementary Figure 1 Simplified schematic representation of carbon cycles of widely different time-scales through northern peatlands and potential impacts of climate warming. Carbon cycles through ecosystems (solid arrows) as a result of photosynthesis by autotrophic organisms (e.g. vascular plants, mosses) and respiration of recently assimilated carbon, litter and older soil organic carbon by autotrophic or heterotrophic organisms (e.g. micro-organisms). In peatlands, heterotrophic decomposition of litter and soil organic carbon is much slower than the production of new biomass at the surface. As a result, thick layers of peat have accumulated, the age of which increases with depth. Climate warming may alter (dotted arrows) the fluxes of carbon and thereby alter the sizes of the different organic carbon pools. Here we focus on the effects on the aerobic respiratory fluxes from different organic carbon pools (arrows 1, 2, 3) because of their crucial role in controlling carbon stocks with wideranging residence times in northern peatlands and feedbacks to the CO 2 concentration in 1

2 the atmosphere. In contrast to ecosystems on mineral soils, our data demonstrate that stimulation of total ecosystem respiration rates by experimental climate warming in a sub-arctic peatland was strong (arrows 1, 2, 3) and persistent for at least 8 years. The stimulation resulted from similar responses of both short-term, plant-related respiration processes (arrow 1) and longer-term, heterotrophic respiration (arrows 2, 3). Furthermore, while centuries-to-millennia old peat at greater depth is thought to be stabilised by cold or perennially frozen conditions, anoxia and the poor quality of the remaining material, and its sensitivity to climate warming is therefore debated, our data show that at least 69% of the warming-induced increase in respiration originated from enhanced respiration of subsurface peat reservoirs (25-50 cm deep). The large, longterm carbon stocks at greater depth in northern peatlands are therefore much more sensitive to warming than was thought previously (arrow 3) and are at risk to form a long-lasting positive feedback to climate change. δ 13 C ( ) Depth (cm) Supplementary Figure 2 Depth profile of the carbon isotopic signatures of bulk carbon and respired CO 2 of peat collected in the sub-arctic bog where the climate manipulation experiments were performed. δ 13 C-signatures of respired CO 2 (open 2

3 squares) were determined on seven occasions during a 288-days laboratory incubation of fresh peat samples (equivalent to 3 g oven-dried mass), incubated in 650-ml flasks at 10 C and field capacity. On each occasion, the headspace of each flask was flushed with air at ambient CO 2 concentration for 2 min. and closed with a tight lid with a butyl rubber septum. Headspace air was sampled seven times at regular intervals during the subsequent 24 h into 12-ml septum-capped glass vials (Exetainers, Labco Ltd, High Wycombe, UK), which had been flushed with N 2 and pre-evacuated. Extracted air was replaced with equal amounts of N 2 each time, and measured CO 2 concentrations and δ 13 C-signatures were corrected for this dilution before analyzing the data with Keelingplots 34. δ 13 C-signatures of bulk carbon (black diamonds) were measured of ground initial peat material. Peat cores were collected at the site of the experimental warming study in September 2005 and kept frozen until the start of the incubation 5 months later. Living roots, rhizomes and branches were removed before incubation and bulk carbon analyses. Isotopic composition of bulk carbon was analysed by an elemental analyzer (NC2500, ThermoQuest Italia, Rodana, Italy) coupled on-line to a stable isotope ratio mass spectrometer (Delta plus, ThermoFinnigan, Bremen, Germany). Carbon isotopic composition of sampled CO 2 was analysed within 48h by a gas chromatograph (Gasbench II) coupled to the isotope ratio mass spectrometer; CO 2 concentrations were measured on a gas chromatograph (Hewlett Packard 5890 A, Agilent Technologies, Santa Clara, CA). Error bars: SEM of each depth (n = 4 peat cores). 3

4 Supplementary Methods Effects of the climate manipulations on the environment. On average, the presence of open-top chambers (OTCs) increased daily mean air temperatures by C in spring, by C in summer and, through a passive accumulation of snow resulting in a doubling of the ambient snow cover (max. ~15 cm) but no lengthening of the snow cover duration, by C in winter 19. These effects are well within the range of changes in temperature and snow cover observed during the past decades and projected for the next few decades in northern Europe and the (sub-) arctic in general 16,35. Average effects of OTCs on daily mean temperatures in the upper 20 cm of peat were comparable to those on air temperatures: C in spring, C in summer and C in winter (Supplementary Table 1). OTC-effects on daily maximum temperatures were stronger than effects on daily mean temperatures for both air and soil (Supplementary Table 1, Ref. 19). Soil temperature responses did not differ significantly between depths (repeated measures anovas depth treatment interactions P > 0.10 for all periods except October-April: P > 0.05), nor did daily mean and maximum soil temperatures differ between the long-term and the companion warming treatments (planned orthogonal contrasts: P > 0.10 for all spring and summer periods). Other environmental variables were not consistently affected by the OTC treatments of both experiments. Mean summer soil moisture in the upper 15 cm of peat (832 % of dry weight on average in the summer ambient treatments, 34 % by volume) was lower after 5-6 years in the long-term summer-otc treatments (700 % DW, 26 % by volume; P < 0.05), but not in the first year in the companion experiment OTCs (P > 0.10). Winter snow addition and/or spring warming did not affect soil moisture during the spring and summer (P > 0.10). Soil redox potential was consistently high throughout the spring and summer (between 404 and 433 mv at -5, -15 and -30 cm), slightly lower in the summer warming treatments at -15 cm (335 mv; P < 0.05) and slightly higher in 4

5 the summer warming treatments at -30 cm (520 mv; P < 0.05). As a result of the absence of a true water table and the highly aerobic conditions, no detectable methane emissions were measured in any of the treatments in both spring and summer (ST and MJvdW unpublished data). Maximum active layer depth was not affected by any of the climate manipulations in both experiments (P > 0.05), while during the growing season it was 5.5 cm deeper in the OTC-treatment of the companion experiment only (P < 0.05). Supplementary Table 1 Increases in soil temperatures ( C) at different depths in response to the presence of open-top chambers during the spring, summer or winter season, averaged over 2 years (n = 3 plots per treatment). Season Spring Summer Winter Month May June July Aug Sep Oct-Apr Daily mean temperature -5 cm 0.6** 1.1* 0.7* * -10 cm 0.3** 1.2* 0.6* * -20 cm 0.5** 0.7* 0.5* * Daily maximum temperature -5 cm 1.2* 2.2** 1.6* cm 0.5* 1.8** 1.1* cm 0.6* 0.9** 0.6* Asterisks indicate significance level of OTC-effects (repeated measures anovas with planned orthogonal contrasts, including data for 2 years and three depths): **: P 0.01; *: P 0.05; + : P

6 Calculation of subsurface peat contribution to the increase in respiration rate. We arbitrarily defined two layers in the peat column above the permafrost and estimated the minimum average relative contribution of subsurface (25-50 cm, i.e. the deep part of the active layer) peat respiration to the average total increase in respiration rate in response to the OTC treatments over the four measurement occasions using a set of two endmember mixing models. We considered the total CO 2 efflux in the OTC treatments to be composed of a part similar in strength and isotopic composition to the efflux in the ambient treatments plus the increase in efflux, of which we calculated the average δ 13 C- signature using the mass balance equation: F OTC * δ OTC = F Ambient * δ Ambient + F Increase * δ Increase (1), where F represents the fractional contribution of different flux parts to the average total CO 2 efflux in the OTC treatments and δ the corresponding δ 13 C-signature. Laboratory incubation studies of homogenised soils have consistently shown that warming may decrease δ 13 C-signatures of respired CO 2 by per C due to shifts in decomposition towards more depleted, recalcitrant substrates as a consequence of changes in microbial community 17,36. We therefore estimated a range of expected isotopic compositions of the respired CO 2 from the surface and subsurface peat layers, respectively, using their average measured δ 13 C-signatures of respired CO 2 during laboratory incubation at 10 C (Supplementary Figure 2), corrected for actual field temperatures as follows: δ 0-25 cm = δ 0-25 cm (10) + β * (T 0-25 cm - 10) (2a), δ cm = δ cm (10) + β * (T cm - 10) (2b), where δ 0-25 cm and δ cm are corrected isotopic signatures of respired CO 2 at field temperatures in the surface and subsurface peat layers, respectively, δ 0-25 cm (10) and δ cm(10) are the corresponding isotopic signatures at 10 C in the laboratory, and T 0-25 cm 6

7 and T cm are the average ambient soil temperatures at the two depths observed at the site at the four measurement occasions. A separate set of corrected isotopic signatures of respired CO 2 was calculated for the total and the increase in respiration in the OTC treatments (δ' 0-25 cm and δ' cm ), by increasing the ambient soil temperatures by 1.0 C. The values of the correction factor β were constrained by the literature range given above and by the following set of mass balance equations: F Ambient * δ Ambient = F 0-25 cm (Ambient) * δ 0-25 cm + F cm (Ambient) * δ cm (3a), F Increase * δ Increase = F 0-25 cm (Increase) * δ' 0-25 cm + F cm (Increase) * δ' cm (3b), F OTC * δ OTC = F 0-25 cm (OTC) * δ' 0-25 cm + F cm (OTC) * δ' cm (3c), where the fractional contributions of the fluxes from different depths (F 0-25 cm and F cm) to the different flux parts in the OTC treatments (F Ambient, F Increase and F OTC : all normalised to 1) should all be between 0 and 1. We finally estimated the lower limit of the relative contribution of subsurface peat to the increase in respiration in the OTC treatments by selecting the minimum value of F cm (Increase) through solving equation (3b) with the range of valid β-values based on these constraints. Calculation of global increase in heterotrophic respiration from northern peatlands. We made a crude estimation of the worldwide increase in heterotrophic CO 2 -emissions from northern peatlands in direct response to mild climate warming in the growing season comparable to the scenario simulated in our experiment, as a means to indicate the magnitude of the potential impact only. We multiplied: the global area of undrained, unmined northern peatlands ( m 2 ; Ref. 2) * the global average annual soil respiration rate of northern bogs and mires (94 g C m -2 yr -1 ; Ref. 37) * the fraction of annual respiration in the snow-free season in the arctic and sub-arctic ( ; Refs. 38,39) * a factor to correct for potential over-estimation by using day-time fluxes (0.74; Ref. 40) * the average fractional increase in ecosystem respiration in the 7

8 long-term summer warming treatments presented here (0.515) * the fraction of heterotrophic respiration in peatlands ( ; Refs. 25,41). As we used both the lower and upper limits of the ranges for two of the parameters, this resulted in a range of potential increased global loss of soil organic carbon from northern peatlands as respired CO 2. Supplementary Notes Statistical analyses. The effects of the long-term climate manipulations on ecosystem respiration rates were analysed with repeated measures anovas with planned orthogonal contrasts to allow for direct testing of potential changes in treatment effects over the years (year treatment interaction). Because respiration rates were measured weekly to monthly with a focus on slightly different parts of the spring and summer seasons each year, we first tested for different years and seasons whether the effects of the treatments depended on the period within the season (repeated measures anovas: period treatment interaction), which was never significant (data not shown). Because OTCs were installed on some of the long-term treatments on the transition from spring to summer each year (see Methods section), we calculated average rates for each plot for each season separately and specified two different sets of contrasts for spring and summer. For the spring season (May), when only one of the treatments was warmed by an OTC, we specified the following contrasts: (1) ambient versus summer warming ; (2) the previous two versus summer warming + winter snow addition ; (3) the previous three versus summer warming + winter snow addition + spring warming. For the summer (June-September) the contrasts were specified as follows: (1) summer warming + winter snow addition versus summer warming + winter snow addition + spring warming ; (2) the previous two versus summer warming ; (3) the previous three 8

9 versus ambient. In both cases, contrast (3) was the contrast of primary interest, comparing treatments with and without an OTC present at the time of measurement. OTC-effects on respiration rates in the companion experiment were analysed with separate repeated measures anovas for total ecosystem respiration rates and for its two components. The latter analysis included flux component as a factor to test for differences in OTC-effect between plant-related and heterotrophic respiration (warming flux component interaction). Data of the 2 years were averaged into four periods of approx. 1 month to allow for testing of changes in treatment effects over time (period warming interaction). Effects of the long-term and companion OTCs on isotopic signatures of respired CO 2 were analysed with a repeated measures anova including all four measurement occasions with the following planned orthogonal contrasts: (1) ambient with or without vegetation versus long-term OTC with vegetation and short-term OTC with or without vegetation; (2) long-term OTC with vegetation versus short-term OTC with vegetation. Difference in response to OTCs of isotopic signatures of respired CO 2 between patches with and without vegetation was analysed with a repeated measures anova of the four treatments of the companion experiment (warming flux-type interaction). Normality and homoscedasticity of the data were analysed with Levene s test and visual inspection of residual plots and normal probability plots. Data were lntransformed if needed to improve homoscedasticity. Since analysis of variance is robust to considerable heterogeneity of variances as long as sample sizes are equal 42, we included one period of slightly heteroscedastic R h and R a data and isotopic data in two repeated measures anovas of the companion experiment. For all analyses, block was included if it accounted for a significant part of the variation. 9

10 Additional references. 35. Kohler, J., Brandt, O., Johansson, M. & Callaghan, T. V. A long-term Arctic snow depth record from Abisko, northern Sweden, Polar Research 25, (2006). 36. Andrews, J. A., Matamala, R., Westover, K. M. & Schlesinger, W. H. Temperature effects on the diversity of soil heterotrophs and the δ 13 C of soil-respired CO 2. Soil Biol. Biochem. 32, (2000). 37. Raich, J. W. & Schlesinger, W. H. The global carbon dioxide flux in soil respiration and its relationship to vegetation and climate. Tellus B 44, (1992). 38. Clein, J. S. & Schimel, J. P. Microbial activity of tundra and taiga soils at subzero temperatures. Soil Biol. Biochem. 27, (1995). 39. Grogan, P. & Jonasson, S. Temperature and substrate controls on intra-annual variation in ecosystem respiration in two subarctic vegetation types. Glob. Change Biol. 11, (2005). 40. Silvola, J., Alm, J., Ahlholm, U., Nykänen, H. & Martikainen, P. J. CO 2 fluxes from peat in boreal mires under varying temperature and moisture conditions. J. Ecol. 84, (1996). 41. Crow, S. E. & Wieder, R. K. Sources of CO 2 emission from a northern peatland: root respiration, exudation, and decomposition. Ecology 86, (2005). 42. Zar, J. H. Biostatistical Analysis (Prentice Hall, Englewoodcliffs, NJ, 1999). 10