Copyright © 2013 Elsevier. NOTICE: This is the author’s version of a work accepted for publication by Elsevier. Changes resulting from the publishing process, including peer review, editing, corrections, structural formatting and other quality control mechanisms, may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Soil Biology and Biochemistry, Vol. 63, pp. 1 – 4  DOI: http://dx.doi.org/10.1016/j.soilbio.2013.03.011In arctic ecosystems, winter soil respiration can contribute substantially to annual CO2 release, yet the source of this C is not clear. We analysed the 14C content of C released from plant-free plots in mountain birch forest and tundra-heath. Winter-respired CO2 was found to be a similar age (tundra) or older (forest) than C released during the previous autumn. Overall, our study demonstrates that the decomposition of older C can continue during the winter, in these two contrasting arctic ecosystems

Garnett, Mark H.

Hartley, IP

Hopkins, DW

Sommerkorn, M

Wookey, PA

Soil Biology and Biochemistry

Open Research Exeter

  1 THIS MANUSCRIPT HAS BEEN PUBLISHED IN SOIL BIOLOGY AND 1 BIOCHEMISTRY 2  3 The published version can be viewed at: 4  5 http://www.journals.elsevier.com/soil-biology-and-biochemistry/ 6  7  8 9   2 The age of CO2 released from soils in contrasting ecosystems during the arctic winter 10  11 Short communication 12 Date: 10th February 2013 13 Pages: 11 14 Tables: 1 15 Figures: 2 16  17 Iain P. Hartleya,b,*, Mark H. Garnettc, Martin Sommerkornd, David W. Hopkinse, Philip 18 A. Wookeya,f 19  20 aSchool of Biological & Environmental Sciences, University of Stirling, Stirling, 21 FK9 4LA, UK 22 bCurrent address: Geography, College of Life and Environmental Sciences, University 23 of Exeter, Amory Building, Rennes Drive Exeter EX4 4RJ, Tel: +44 1392 724362;  24 Fax: +44 1392 723342; Email: I.Hartley@exeter.ac.uk 25 cNERC Radiocarbon Facility, Scottish Enterprise Technology Park, Rankine Avenue, 26 East Kilbride, Glasgow G75 0QF, UK, Email: M.Garnett@nercrcl.gla.ac.uk 27 dThe James Hutton Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK 28 eSchool of Life Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK, Email: 29 David.Hopkins@hw.ac.uk 30 fCurrent address: Department of Geography, University of Sheffield, Sheffield, 31 S10 2TN, UK, Email: P.Wookey@sheffield.ac.uk 32  33 *Corresponding author34   3 Abstract 35  36 In arctic ecosystems, winter soil respiration can contribute substantially to annual CO2 37 release, yet the source of this C is not clear. We analysed the 14C content of C released 38 from plant-free plots in mountain birch forest and tundra-heath. Winter-respired CO2 39 was found to be a similar age (tundra) or older (forest) than C released during the 40 previous autumn. Overall, our study demonstrates that the decomposition of older C can 41 continue during the winter, in these two contrasting arctic ecosystems.  42  43 Key words: 14CO2, passive sampling, mountain birch, radiocarbon, tree-line, tundra-44 heath, winter respiration 45  46 Arctic soils contain globally significant C stores (Post et al., 1982; Ping et al., 2008). As 47 these areas are warming rapidly (AMAP, 2012), C may be lost if decomposition rates 48 increase in response (Davidson and Janssens, 2006). There is growing recognition that 49 the CO2 released during the long winters in high-latitude/altitude ecosystems can 50 represent a substantial proportion (up to 30%) of annual respiration (Elberling, 2007; 51 Williams et al., 2009), but, for practical reasons, flux measurements are biased towards 52 the growing season. Furthermore, debate continues as to whether the source of the CO2 53 released during the winter is similar to that released during the summer, or is derived 54 mainly from recently-fixed, labile C (Grogan et al., 2001; Grogan and Jonasson, 2005; 55 Nobrega and Grogan, 2007). Importantly, climate change has greater potential to affect 56 rates of winter respiration in the long term, either positively or negatively, if there is a 57 substantial contribution from the large reserves of older SOM, than if most of the 58 respired CO2 is derived from small labile C pools (Jones et al., 2005; Hartley and 59 Ineson, 2008). Here, by undertaking the first 14C analyses of CO2 released from soils 60   4 during the arctic winter, we investigated whether the decomposition of older SOM 61 continues during the winter. 62  The study took place in mountain birch forest (68°19'35''N, 18°50'00''E; 63 elevation ~520 m) and tundra-heath (68°18'07''N, 18°51'16''E; elevation ~710 m), near 64 Abisko, northern Sweden. We collected samples of CO2 released over the 2007-2008 65 winter from three non-vegetated plots in each ecosystem. To allow only soil-respired 66 CO2 to be collected, the plots were clipped and trenched in late summer 2006. In the 67 centre of each plot, 7-cm tall collars were sealed to the surface using putty, with 68 respiration rates and 14CO2 contents being monitored during the 2007 growing season 69 (Hartley et al. 2012). To collect winter-respired CO2, we developed a new technique 70 using molecular sieve cartridges (MSCs) to collect passively (by diffusion) 71 representative samples of CO2 over extended time periods (Garnett et al., 2009). 72  In order to minimise chamber height and potential effects on snow lie, lids were 73 directly placed on top of the collars. MSCs were then connected through the collar sides 74 via auto-shut-off Quick Couplings™ (Colder Products Company, USA), with 75 hydrophobic filters (Accurel PP V8/2 HF, Membrana GmbH, Germany) attached inside 76 each collar to prevent liquid water passing into the MSCs (Fig. 1). For protection, the 77 MSCs were then placed inside foam insulation, PVC pipes and guttering. The lids were 78 closed on 16th-17th September 2007 and the MSCs connected on 20th September. MSCs 79 were recovered on 23rd-24th May 2008. Air samples were collected at the tundra-heath 80 site on 20th September 2007 and 24th May 2008, by pumping air through MSCs for 81 approximately 60 minutes.  82  Soil temperatures at 5 cm were monitored throughout the winter (thermistor 83 probes and CR10x datalogger, Campbell Scientific, Leics, UK). After MSC collection, 84 soil temperatures at 2, 5 and 8 cm depth (digital thermometer, E.T.I. Ltd., West Sussex, 85   5 UK) and soil moisture at 6 cm (Theta probe: ML2, Delta-T Devices, Cambridge, UK) 86 were measured inside and outside the collars.  87  All 13C and 14C analyses were performed on CO2 recovered from the MSCs 88 using established procedures (Hardie et al., 2005). Following convention, 14C results 89 were normalised to a δ13C value of -25 ‰ and expressed as %modern (Stuiver and 90 Polach, 1977). Because collars were not inserted into the soil, it was not possible to 91 avoid some atmospheric contamination. Samples were corrected for atmospheric 92 contamination using the approach of Hartley et al. (2012), after accounting for the 4 ‰ 93 13C fractionation associated with passive sampling (see Garnett et al., 2009). 94  After MSC collection, on the tundra, both soil moisture and temperature were 95 near identical inside and outside the collars. In the forest, temperatures at 5 and 8 cm 96 were 0.6-0.7oC higher within the collar, but there was no significant effect on soil 97 moisture. Therefore, the chambers appeared to have little effect on the soil physical 98 environment. During the winter, temperature at 5 cm was greater in the forest (mean: 99 0.07oC, range: -3.02oC to 6.94oC) than the tundra (mean: -1.73oC, range: -7.18oC to 100 4.66oC). Warmer winter temperatures can increase winter CO2 production, and thus 101 influence annual carbon balances (Grogan and Jonasson, 2006; Nobrega and Grogan, 102 2007; Sullivan, 2010), potentially contributing to the lower C storage in forest than 103 tundra soils. However, it should be emphasised that previous research at the current 104 field sites (Hartley et al., 2012), as well as studies at lower latitudes (Mitchel et al., 105 2007), have identified the important role plant-soil interactions and priming play in 106 controlling soil carbon storage in forest-heath transitions. 107  Consistent with warmer temperatures increasing winter respiraton, our 108 molecular sieves collected more CO2 in the forest than tundra (means of 102.3 ml and 109 71.8 ml, respectively). However, the amount of CO2 collected on our MSCs depends on 110 the average CO2 concentration within the chamber (Garnett et al., 2009). This is 111   6 controlled not only by respiration rates, but also by rates of exchange between the 112 atmosphere and the headspace, which may have been greater on the tundra due to higher 113 wind speeds and reduced snow cover; atmospheric contamination was greater in tundra 114 samples (Table 1). Therefore, volumes collected cannot be translated directly into 115 respiration rates.  116  On the tundra, the 14C content of the CO2 respired during the winter was similar 117 to that collected the previous September (Fig. 2), while in the birch forest, the 14C 118 content of winter-respired CO2 was significantly greater than at any point during the 119 growing season. Mean residence time modelling, based on soil 14C measurements, 120 indicated that C fixed before the 1950s should contribute only a small proportion to 121 total CO2 release (Hartley et al., 2012). Therefore, the increase in the 14C content of the 122 winter-respired CO2 in the birch forest indicates that more 'older' C, enriched in 14C 123 from 20th century nuclear weapons testing, was being released (Fig. 2). This was 124 possibly caused by the gradual loss of recently-fixed, labile C which would have been 125 relatively 14C-depleted (atmospheric CO2 = 105.15 %modern). This process may have 126 been more pronounced in the forest due to both the smaller soil C stocks and the greater 127 inputs of contemporary C associated with the higher plant productivity (Hartley et al. 128 2012). Overall, our results indicate that the decomposition of decade-old SOM can 129 continue during the protracted arctic winter in both forest and tundra ecosystems 130 (Table 1). The fact that such C can be released during the Arctic winter makes it 131 possible for changes in winter conditions to affect substantially the C balance of arctic 132 ecosystems. Finally, in the future, comparative analyses of the CO2 released from both 133 plant-free and vegetated plots, would help further identify the sources of winter-respired 134 CO2 in intact Arctic ecosystems. 135  136  137   7 Acknowledgements 138 This work was carried out within the UK Natural Environment Research Council 139 (NERC) funded Arctic Biosphere Atmosphere Coupling at Multiple Scales (ABACUS, 140 www.abacus-ipy.org) project (a contribution to International Polar Year 2007e2008). 141 The additional 14C analyses carried out for this paper were funded through an award 142 from the NERC Radiocarbon Facility steering committee (reference number 143 1281.0408). We are also very grateful for the help of the staff at the Abisko Scientific 144 Research Station.We thank Jonathan Evans (CEH Wallingford) for collecting the long-145 term soil temperature data. 146  147 References 148  149 AMAP, 2011. Snow, Water, Ice and Permafrost in the Arctic (SWIPA): Climate 150 Change and the Cryosphere. Arctic Monitoring and Assessment Programme (AMAP), 151 Oslo, Norway. 152  153 Davidson, E.A., Janssens, I.A., 2006. Temperature sensitivity of soil carbon 154 decomposition and feedbacks to climate change. Nature 440, 165–173. 155  156 Elberling, B., 2007. Annual soil CO2 effluxes in the High Arctic: the role of snow 157 thickness and vegetation type. Soil Biology and Biochemistry 39, 646–654. 158  159 Garnett, M.H., Hartley, I.P., Hopkins, D.W., Sommerkorn, M., Wookey, P.A., 2009. A 160 passive sampling method for radiocarbon analysis of soil respiration using molecular 161 sieve. Soil Biology and Biochemistry 41, 1450-1456. 162  163   8 Grogan, P., Illeris, L., Micelsen, A., Jonasson, S., 2001. Respiration of recently-fixed 164 plant carbon dominates mid-winter ecosystem CO2 production in sub-arctic heath 165 tundra. Climatic Change 50, 129–142. 166  167 Grogan, P., Jonasson, S., 2005. Temperature and substrate controls on intra-annual 168 variation in ecosystem respiration in two subarctic vegetation types. Global Change 169 Biology 11, 465–475. 170  171 Grogan, P., Jonasson, S., 2006. Ecosystem CO2 production during winter in a Swedish 172 subarctic region: the relative importance of climate and vegetation type. Global Change 173 Biology 12, 1479–1495. 174  175 Hardie, S.M.L., Garnett, M.H., Fallick, A.E., Rowland, A.P., Ostle, N.J., 2005. Carbon 176 dioxide capture using a zeolite molecular sieve sampling system for isotopic studies 177 (13C and 14C) of respiration. Radiocarbon 47, 441-451. 178  179 Hartley, I.P., Garnett, M.H., Sommerkorn, S., Hopkins, D.W., Fletcher, B.J., Sloan, 180 V.L., Phoenix, G.K., Wookey, P.A., 2012. A potential loss of carbon associated with 181 greater plant growth in the European Arctic. Nature Climate Change, 2, 875-879.  182  183 Hartley, I.P., Ineson, P., 2008. 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Deeper snow enhances winter respiration from both 201 plant-associated and bulk soil carbon pools in birch hummock tundra. Ecosystems 10, 202 419–431. 203  204 Ping, C.L., Michaelson, G.J., Jorgenson, M.T., Kimble, J.M., Epstein, H., Romanovsky, 205 V.E., Walker, D.A., 2008. High stocks of soil organic carbon in the North American 206 Arctic region. Nature Geoscience 1, 615–619. 207  208 Post, W.M., Emanuel, W.R., Zinke, P.J., Stangenberger, A.G., 1982. Soil carbon pools 209 and world life zones. Nature 298, 156–159. 210  211 Stuiver, M., Polach, H.A., 1977. Reporting of 14C data. Radiocarbon 19, 355-363. 212  213   10 Sullivan PF (2010) Snow distribution, soil temperature and late winter CO2 efflux from 214 soils near the Arctic treeline in northwest Alaska. Biogeochemistry 99, 65–77. 215  216 Williams, M.W., Helmig, D., Blanken, P., 2009. White on green: under-snow microbial 217 processes and trace gas fluxes through snow, Niwot Ridge, Colorado Front Range. 218 Biogeochemistry 95, 1–12. 219  220  221   11 Table 1 The 14C content and 13C values of samples collected from the tundra-heath and birch forest, with associated measurement uncertainty (±1σ) and radiocarbon laboratory codes. The 13C values have been corrected for fractionation during passive sampling (Garnett et al., 2009). The 14C content of the respired CO2 was calculated after correction for contamination with atmospheric CO2 based on the average 13C value of the atmospheric CO2 at the site, and the 13C of pure samples of respired CO2 collected from monoliths incubated on site in closed containers (see Hartley et al., 2012). The average age of the respired CO2 (relative to the sampling date) was calculated from its bomb-14C concentration by reference to records of direct atmospheric 14CO2 measurements (Levin et al., 2008).                          Collected CO2  Monolith Atmospheric CO2 Atmospheric Respired CO2  Site 14C (%modern) Measured13C Corrected13C Lab code 13C ratio14C (%modern) 13C ratio Fraction 14C (%modern) Age (years)                                  Tundra 108.30±0.51 -24.4±0.1 -20.4 SUERC-19528 -26.60 105.16 -8.8 0.350 109.98 9 Tundra 108.19±0.48 -25.3±0.1 -21.3 SUERC-19529 -26.60 105.16 -8.8 0.300 109.49 8 Tundra 108.61±0.51 -24.4±0.1 -20.4 SUERC-19532 -26.60 105.16 -8.8 0.349 110.46 10            Forest 108.70±0.51 -24.0±0.1 -20.0 SUERC-19533 -26.83 105.16 -8.8 0.380 110.86 11 Forest 109.67±0.49 -26.5±0.1 -22.5 SUERC-19534 -26.83 105.16 -8.8 0.243 111.12 11 Forest 110.25±0.52 -26.2±0.1 -22.2 SUERC-19535 -26.83 105.16 -8.8 0.259 112.04 13                            12 Figure legends  Fig. 1. Photographs showing the installation of one of the systems for passively sampling soil respiration during the arctic winter. Panel (a) shows the hydrophobic filter inside the collar cover, prior to lid being attached. Panel (b) shows the MSC cartridge being attached, before the clips were removed, while panel (c) shows the final arrangement after the cartridge has been covered in insulating foam, protected inside pipe and plastic guttering, pegged in place, taped up and surrounded by stones. Although the sampling system was only 7 cm tall, the stones were arranged to smooth out the vertical profile and minimise any impact on snow drifting patterns.   Fig. 2. The bars indicate the 14C content of the CO2 respired from two sites during the winter. Mean values ±1SE are shown (n = 3). The 14C contents of the CO2 released during the previous growing season are also indicated (May/June, light grey line; July, dark grey line; September, black line; see also Hartley et al., 2012). The dashed line indicates the 14C content of the CO2 in the contemporary atmosphere (~105.16% modern. On the tundra-heath, the 14C content of the winter respired CO2 did not significantly differ from the growing season measurements made in July and September [All statistical tests were carried out using the SPSS version 16 (SPSS Science, Birmingham, UK)]. In contrast, in the birch forest, the '*' indicates that the winter respired CO2 was significantly enriched in 14C compared with all growing season measurements (P < 0.05, repeated measures ANOVA). The winter respired CO2 was also significantly more enriched in 14C in the birch forest compared with the tundra-heath (P < 0.05, t-test).   13 Fig. 1. (a)(b) (c)    14 Fig. 2. Tundra-heath Birch forest14C content of respired CO2 (%modern)105106107108109110111112 *  

The age of CO2 released from soils in contrasting ecosystems during the arctic winter

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