BG Biogeosciences BG Biogeosciences 1726-4189 Copernicus Publications Göttingen, Germany 10.5194/bg-12-1257-2015 Positive feedback of elevated CO<sub>2</sub> on soil respiration in late autumn and winter Keidel L. 1 Kammann C. 1 Grünhage L. 1 Moser G. https://orcid.org/0000-0002-0030-2370 1 Müller C. 1 2 Department of Plant Ecology, Justus Liebig University Gießen, Gießen, Germany School of Biology and Environmental Science, University College Dublin, Dublin, Ireland 26 02 2015 12 4 1257 1269 Copyright: © 2015 L. Keidel et al. 2015 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://bg.copernicus.org/articles/12/1257/2015/bg-12-1257-2015.html The full text article is available as a PDF file from https://bg.copernicus.org/articles/12/1257/2015/bg-12-1257-2015.pdf

Soil respiration of terrestrial ecosystems, a major component in the global carbon cycle is affected by elevated atmospheric CO<sub>2</sub> concentrations. However, seasonal differences of feedback effects of elevated CO<sub>2</sub> have rarely been studied. At the Gießen Free-Air CO<sub>2</sub> Enrichment (GiFACE) site, the effects of +20% above ambient CO<sub>2</sub> concentration have been investigated since 1998 in a temperate grassland ecosystem. We defined five distinct annual seasons, with respect to management practices and phenological cycles. For a period of 3 years (2008–2010), weekly measurements of soil respiration were carried out with a survey chamber on vegetation-free subplots. The results revealed a pronounced and repeated increase of soil respiration under elevated CO<sub>2</sub> during late autumn and winter dormancy. Increased CO<sub>2</sub> losses during the autumn season (September–October) were 15.7% higher and during the winter season (November–March) were 17.4% higher compared to respiration from ambient CO<sub>2</sub> plots. <br><br> However, during spring time and summer, which are characterized by strong above- and below-ground plant growth, no significant change in soil respiration was observed at the GiFACE site under elevated CO<sub>2</sub>. This suggests (1) that soil respiration measurements, carried out only during the growing season under elevated CO<sub>2</sub> may underestimate the true soil-respiratory CO<sub>2</sub> loss (i.e. overestimate the C sequestered), and (2) that additional C assimilated by plants during the growing season and transferred below-ground will quickly be lost via enhanced heterotrophic respiration outside the main growing season.

 References Allen, A. S., Andrews, J. A., Finzi, A. C., Matamala, R., Richter, D. D., and Schlesinger, W. H.: Effects of free-air CO<sub>2</sub> enrichment (FACE) on belowground processes in a Pinus taeda forest, Ecol. Appl., 10, 437–448, <a href="http://dx.doi.org/10.2307/2641105">https://doi.org/10.2307/2641105</a>, 2000. Amundson, R.: The carbon budget in soils, Annu. Rev. Earth Pl. Sc., 29, 535–562, 2001. Andrews, J. A. and Schlesinger, W. H.: Soil CO<sub>2</sub> dynamics, acidification, and chemical weathering in a temperate forest with experimental CO<sub>2</sub> enrichment, Global Biogeochem. Cy., 15, 149–162, <a href="http://dx.doi.org/10.1029/2000gb001278">https://doi.org/10.1029/2000gb001278</a>, 2001. Bader, M. K. F. and Körner, C.: No overall stimulation of soil respiration under mature deciduous forest trees after 7 years of CO<sub>2</sub> enrichment, Global Change Biol., 16, 2830–2843, <a href="http://dx.doi.org/10.1111/j.1365-2486.2010.02159.x">https://doi.org/10.1111/j.1365-2486.2010.02159.x</a>, 2010. Batjes, N. H.: Total carbon and nitrogen in the soils of the world, Eur. J. Soil Sci., 47, 151–163, <a href="http://dx.doi.org/10.1111/j.1365-2389.1996.tb01386.x">https://doi.org/10.1111/j.1365-2389.1996.tb01386.x</a>, 1996. Burton, A. J., Pregitzer, K. S., Zogg, G. P., and Zak, D. R.: Latitudinal variation in sugar maple fine root respiration, Can. J. For. Res., 26, 1761–1768, <a href="http://dx.doi.org/10.1139/x26-200">https://doi.org/10.1139/x26-200</a>, 1996. Burton, A. J., Pregitzer, K. S., Zogg, G. P., and Zak, D. R.: Drought reduces root respiration in sugar maple forests, Ecol. Appl., 8, 771–778, <a href="http://dx.doi.org/10.1890/1051-0761(1998)008[0771:drrris]2.0.co;2">https://doi.org/10.1890/1051-0761(1998)008[0771:drrris]2.0.co;2</a>, 1998. Carol Adair, E., Reich, P. B., Trost, J. J., and Hobbie, S. E.: Elevated CO<sub>2</sub> stimulates grassland soil respiration by increasing carbon inputs rather than by enhancing soil moisture, Global Change Biol., 17, 3546–3563, <a href="http://dx.doi.org/10.1111/j.1365-2486.2011.02484.x">https://doi.org/10.1111/j.1365-2486.2011.02484.x</a>, 2011. Chapin III, F. S., Matson, P. A., and Mooney, H. A.: Principles of terrestrial ecosystem ecology, Springer, New York, 436 pp., 2002. Craine, J. M., Wedin, D. A., and Chapin, F. S.: Predominance of ecophysiological controls on soil CO<sub>2</sub> flux in a Minnesota grassland, Plant Soil, 207, 77–86, 1999. Craine, J. M., Wedin, D. A., and Reich, P. B.: The resonse of soil CO<sub>2</sub> flux to changes in atmospheric CO<sub>2</sub>, nitrogen supply and plant diversity, Global Change Biol., 7, 947–953, 2001. Dawes, M. A., Hagedorn, F., Handa, I. T., Streit, K., Ekblad, A., Rixen, C., Korner, C., and Hattenschwiler, S.: An alpine treeline in a carbon dioxide-rich world: synthesis of a nine-year free-air carbon dioxide enrichment study, Oecologia, 171, 623–637, <a href="http://dx.doi.org/10.1007/s00442-012-2576-5">https://doi.org/10.1007/s00442-012-2576-5</a>, 2013. de Graaff, M.-A., Six, J., Harris, D., Blum, H., and van Kessel, C.: Decomposition of soil and plant carbon from pasture systems after 9 years of exposure to elevated CO<sub>2</sub>: impact on C cycling and modeling, Global Change Biol., 10, 1922–1935, <a href="http://dx.doi.org/10.1111/j.1365-2486.2004.00862.x">https://doi.org/10.1111/j.1365-2486.2004.00862.x</a>, 2004. de Graaff, M.-A., Van Groenigen, K.-J., Six, J., Hungate, B. A., and Van Kessel, C.: Interactions between plant growth and soil nutrient cycling under elevated CO<sub>2</sub>: a meta-analysis, Global Change Biol., 12, 2077–2091, 2006. Denef, K., Bubenheim, H., Lenhart, K., Vermeulen, J., Van Cleemput, O., Boeckx, P., and Müller, C.: Community shifts and carbon translocation within metabolically-active rhizosphere microorganisms in grasslands under elevated CO<sub>2</sub>, Biogeosciences, 4, 769–779, <a href="http://dx.doi.org/10.5194/bg-4-769-2007">https://doi.org/10.5194/bg-4-769-2007</a>, 2007. Drigo, B., Kowalchuk, G. A., and van Veen, J. A.: Climate change goes underground: effects of elevated atmospheric CO<sub>2</sub> on microbial community structure and activities in the rhizosphere, Biol. Fertil. Soils, 44, 667–679, <a href="http://dx.doi.org/10.1007/s00374-008-0277-3">https://doi.org/10.1007/s00374-008-0277-3</a>, 2008. Drigo, B., Van Veen, J. A., and Kowalchuk, G. A.: Specific rhizosphere bacterial and fungal groups respond differently to elevated atmospheric CO<sub>2</sub>2, Isme J., 3, 1204–1217, <a href="http://dx.doi.org/10.1038/ismej.2009.65">https://doi.org/10.1038/ismej.2009.65</a>, 2009. Field, C. B., Jackson, R. B., and Mooney, H. A.: Stomatal response to increased CO<sub>2</sub>: implications from the plant to the global scale, Plant Cell Environ., 18, 1214–1225, 1995. Grüters, U., Janze, S., Kammann, C., and Jäger, H.-J.: Plant functional types and elevated CO<sub>2</sub>: a method of scanning for causes of community alteration, J. Appl. Bot. Food Qual., 80, 116–128, 2006. Guntinas, M. E., Gil-Sotres, F., Leiros, M. C., and Trasar-Cepeda, C.: Sensitivity of soil respiration to moisture and temperature, J. Soil Sci. Plant Nutr., 13, 445–461, <a href="http://dx.doi.org/10.4067/s0718-95162013005000035">https://doi.org/10.4067/s0718-95162013005000035</a>, 2013. Heinemeyer, A., Di Bene, C., Lloyd, A. R., Tortorella, D., Baxter, R., Huntley, B., Gelsomino, A., and Ineson, P.: Soil respiration: implications of the plant-soil continuum and respiration chamber collar-insertion depth on measurement and modelling of soil CO<sub>2</sub> efflux rates in three ecosystems, Eur. J. Soil Sci., 62, 82–94, <a href="http://dx.doi.org/10.1111/j.1365-2389.2010.01331.x">https://doi.org/10.1111/j.1365-2389.2010.01331.x</a>, 2011. Hungate, B. A., Chapin III, F. S., Zhong, H., Holland, E. A., and Field, C. B.: Stimulation of grassland nitrogen cycling under carbon dioxide enrichment, Oecologia, 109, 149–153, 1997. Hutchinson, G. L. and Mosier, A. R.: Improved soil cover method for field measurement of nitrous oxide fluxes, Soil Sci. Soc. Am. J., 45, 311–316, 1981. Jackson, R. B., Cook, C. W., Pippen, J. S., and Palmer, S. M.: Increased belowground biomass and soil CO<sub>2</sub> fluxes after a decade of carbon dioxide enrichment in a warm-temperate forest, Ecology, 90, 3352–3366, <a href="http://dx.doi.org/10.1890/08-1609.1">https://doi.org/10.1890/08-1609.1</a>, 2009. Jäger, H.-J., Schmidt, S. W., Kammann, C., Grünhage, L., Müller, C., and Hanewald, K.: The University of Gießen Free-Air Carbon Dioxide Enrichment Study: Description of the experimental site and of a new enrichment system, J. Appl. Bot., 77, 117–127, 2003. Janssens, I. A. and Ceulemans, R.: The response of soil CO<sub>2</sub> efflux under trees grown in elevated atmospheric CO<sub>2</sub>: A literature review, Phyton-Ann. REI Bot., 40, 97–101, 2000. Jastrow, J. D., Miller, R. M., and Owensby, C. E.: Long-term effects of elevated atmospheric CO<sub>2</sub> on below-ground biomass and transformation to soil organic matter in grassland, Plant Soil, 224, 85–97, 2000. Kammann, C., Grünhage, L., Grüters, U., Janze, S., and Jäger, H.-J.: Response of aboveground grassland biomass and soil moisture to moderate long-term CO<sub>2</sub> enrichment, Basic Appl. Ecol., 6, 351–365, 2005. Kammann, C., Müller, C., Grünhage, L., and Jäger, H.-J.: Elevated CO<sub>2</sub> stimulates N<sub>2</sub>O emissions in permanent grassland, Soil Biol. Biochem., 40, 2194–2205, 2008. Keeling, C. D., Chin, J. F. S., and Whorf, T. P.: Increased activity of northern vegetation inferred from atmospheric CO<sub>2</sub> measurements, Nature, 382, 146–149, 1996. King, J. S., Pregitzer, K. S., Zak, D. R., Sober, J., Isebrands, J. G., Dickson, R. E., Hendrey, G. R., and Karnosky, D. F.: Fine-root biomass and fluxes of soil carbon in young stands of paper birch and trembling aspen as affected by elevated atmospheric Co-2 and tropospheric O-3, Oecologia, 128, 237–250, 2001. King, J. S., Hanson, P. J., Bernhardt, E., DeAngelis, P., Norby, R. J., and Pregitzer, K. S.: A multiyear synthesis of soil respiration responses to elevated atmospheric CO<sub>2</sub> from four forest FACE experiments, Global Change Biol., 10, 1027–1042, <a href="http://dx.doi.org/10.1111/j.1529-8817.2003.00789.x">https://doi.org/10.1111/j.1529-8817.2003.00789.x</a>, 2004. Kirschbaum, M. U. F.: Will changes in soil organic carbon act as a positive or negative feedback on global warming?, Biogeochem., 48, 21–51, <a href="http://dx.doi.org/10.1023/a:1006238902976">https://doi.org/10.1023/a:1006238902976</a>, 2000. Klironomos, J. N., Allen, M. F., Rillig, M. C., Piotrowski, J., Makvandi-Nejad, S., Wolfe, B. E., and Powell, J. R.: Abrupt rise in atmospheric CO<sub>2</sub> overestimates communitiy response in a model-plant soil system, Nature, 433, 621–624, 2005. Lagomarsino, A., Lukac, M., Godbold, D. L., Marinari, S., and De Angelis, P.: Drivers of increased soil respiration in a poplar coppice exposed to elevated CO<sub>2</sub>, Plant Soil, 362, 93–106, <a href="http://dx.doi.org/10.1007/s11104-012-1261-0">https://doi.org/10.1007/s11104-012-1261-0</a>, 2013. Lal, R.: Soil carbon sequestration impacts on global climate change and food security, Science, 304, 1623–1627, 2004. Leadley, P. W. and Drake, B. G.: Open top chambers for exposing plant canopies to elevated CO<sub>2</sub> concentration and for measuring net gas-exchange, Vegetatio, 104, 3–15, <a href="http://dx.doi.org/10.1007/bf00048141">https://doi.org/10.1007/bf00048141</a>, 1993. Lenhart, K.: The effects of long-term Free Air CO<sub>2</sub> Enrichment (FACE) on soil aggregation, soil carbon input, and ecosystem CO<sub>2</sub> dynamics in a temperate grassland ecosystem, Department of Plant Ecology, Justus-Liebig University, Gießen, 134 pp., 2008. LI-COR: LI-8100 Instruction Manual, LI-8100 automated soil CO<sub>2</sub> flux system., Li-COR, Inc, Lincoln, NE, USA 68504, 2007. Liu, Q., Edwards, N. T., Post, W. M., Gu, L., Ledford, J., and Lenhart, S.: Temperature-independent diel variation in soil respiration observed from a temperate deciduous forest, Global Change Biol., 12, 2136–2145, 2006. Lloyd, J. and Taylor, J. A.: On the temperature-dependence of soil respiration, Funct. Ecol., 8, 315–323, <a href="http://dx.doi.org/10.2307/2389824">https://doi.org/10.2307/2389824</a>, 1994. Lukac, M., Lagomarsino, A., Moscatelli, M. C., De Angelis, P., Cotrufo, M. F., and Godbold, D. L.: Forest soil carbon cycle under elevated CO<sub>2</sub> – a case of increased throughput?, Forestry, 82, 75–86, <a href="http://dx.doi.org/10.1093/forestry/cpn041">https://doi.org/10.1093/forestry/cpn041</a>, 2009. Luo, Y., Jackson, R. B., Field, C. B., and Mooney, H. A.: Elevated CO<sub>2</sub> increases belowground respiration in California grasslands, Oecologia, 108, 130–137, <a href="http://dx.doi.org/10.1007/bf00333224">https://doi.org/10.1007/bf00333224</a>, 1996. Luo, Y.: Transient ecosystem responses to free-air CO<sub>2</sub> enrichment (FACE): experimental evidence and methods of analysis, New Phytol., 152, 3–8, 2001. Luo, Y., Wu, L., Andrews, J. A., White, L., Matamala, R., Schäfer, K. V. R., and Schlesinger, W. H.: Elevated CO<sub>2</sub> differentiates ecosystem carbon processes: deconvolution analysis of Duke forest data, Ecol. Monogr., 71, 357–376, 2001. Luo, Y. and Zhou, X.: Soil Respiration and the Environment, Academic/Elsevier, San Diego, 328 pp., 2006. Masyagina, O. V. and Koike, T.: Soil Respiration in Model Plantations under Conditions of Elevated CO<sub>2</sub> in the Atmosphere (Hokkaido Island, Japan), Russ. J. Ecol., 43, 24–28, <a href="http://dx.doi.org/10.1134/s1067413611060099">https://doi.org/10.1134/s1067413611060099</a>, 2012. McDermitt, D., Xu, L., Gracia, R., Madsen, R., and Anderson, D.: On equalizing pressure in a soil respiration chamber with pressure in the ambient air under windy conditions, Geophysical Research Abstracts, 7, 05841, 2005. Meine, M.: Charakterisierung und Quantifizierung der mikrobiellen Bodenrespiration eines Grünlandbodens unter erhöhten atmosphärischen CO<sub>2</sub>-Konzentrationen, diploma, Geography, Phillipps-Universität Marburg, Marburg, 101 pp., 2013. Mielnick, P. C. and Dugas, W. A.: Soil CO<sub>2</sub> flux in a tallgrass prairie, Soil Biol. Biochem., 32, 221–228, <a href="http://dx.doi.org/10.1016/S0038-0717(99)00150-9">https://doi.org/10.1016/S0038-0717(99)00150-9</a>, 2000. Monastersky, R.: Global carbon dioxide levels near worrisome milestone, Nature, 497, 13–14, 2013. Moss, R. H., Edmonds, J. A., Hibbard, K. A., Manning, M. R., Rose, S. K., van Vuuren, D. P., Carter, T. R., Emori, S., Kainuma, M., Kram, T., Meehl, G. A., Mitchell, J. F. B., Nakicenovic, N., Riahi, K., Smith, S. J., Stouffer, R. J., Thomson, A. M., Weyant, J. P., and Wilbanks, T. J.: The next generation of scenarios for climate change research and assessment, Nature, 463, 747–756, 2010. Moyano, F. E., Vasilyeva, N., Bouckaert, L., Cook, F., Craine, J., Curiel Yuste, J., Don, A., Epron, D., Formanek, P., Franzluebbers, A., Ilstedt, U., Kätterer, T., Orchard, V., Reichstein, M., Rey, A., Ruamps, L., Subke, J.-A., Thomsen, I. K., and Chenu, C.: The moisture response of soil heterotrophic respiration: interaction with soil properties, Biogeosciences, 9, 1173–1182, <a href="http://dx.doi.org/10.5194/bg-9-1173-2012">https://doi.org/10.5194/bg-9-1173-2012</a>, 2012. Müller, C., Kammann, C., Ottow, J. C. G., and Jäger, H.-J.: Nitrous oxide emission from frozen grassland soil and during thawing periods, Z. Pflanzenern. Bodenk., 166, 46–53, 2003. Müller, C., Stevens, R. J., Laughlin, R. J., and Jäger, H.-J.: Microbial processes and the site of N<sub>2</sub>O production in a temperate grassland soil, Soil Biol. Biochem., 36, 453–461, 2004. Nakayama, F. S., Huluka, G., Kimball, B. A., Lewin, K. F., Nagy, J., and Hendrey, G. R.: Soil carbon dioxide fluxes in natural and CO<sub>2</sub>-enriched systems, Agric. For. Met., 70, 131–140, <a href="http://dx.doi.org/10.1016/0168-1923(94)90052-3">https://doi.org/10.1016/0168-1923(94)90052-3</a>, 1994. Newton, P. C. D., Clark, H., Edwards, G. R., and Ross, D. J.: Experimental confirmation of ecosystem model predictions comparing transient and equilibrium plant responses to elevated atmospheric CO<sub>2</sub>, Ecol. Lett., 4, 344–347, 2001. Niklaus, P. A., Spinnler, D., and Korner, C.: Soil moisture dynamics of calcareous grassland under elevated CO<sub>2</sub>, Oecologia, 117, 201–208, <a href="http://dx.doi.org/10.1007/s004420050649">https://doi.org/10.1007/s004420050649</a>, 1998. Pendall, E., Leavitt, S. W., Brookes, T., Kimball, B. A., Pinter, P. J., Jr, Wall, G. W., LaMorte, R. L., Wechsung, G., Wechsung, F., Adamsen, F., Matthias, A. D., and Thompson, T. L.: Elevated CO<sub>2</sub> stimulates soil respiration in a FACE wheat field, Bas. App. Ecol., 2, 193–201, 2001. Pregitzer, K. S., Burton, A. J., King, J. S., and Zak, D. R.: Soil respiration, root biomass, and root turnover following long-term exposure of northern forests to elevated atmospheric CO<sub>2</sub> and tropospheric O<sub>3</sub>, New Phytol., 180, 153–161, <a href="http://dx.doi.org/10.1111/j.1469-8137.2008.02564.x">https://doi.org/10.1111/j.1469-8137.2008.02564.x</a>, 2008. Raich, J. W. and Potter, C. S.: Global patterns of carbon dioxide emissions from soils, Global Biogeochem. Cy., 9, 23–36, 1995. Raich, J. W. and Schlesinger, W. H.: The global carbon dioxide flux in soil respiration and its relationship to vegetation and climate, Tellus, 44B, 81–99, 1992. Rastetter, E. B., Ryan, M. G., Shaver, G. R., Melillo, J. M., Nadelhoffer, K. J., Hobbie, J. E., and Aber, J. D.: A general biogeochemical model describing the response of the C and N cycles in terrestrial ecosystems to changes in CO<sub>2</sub>, climate, and N deposition, Tree Phys., 9, 101–126, 1991. Raynaud, D. and Barnola, J. M.: An Antarctic ice core reveals atmospheric CO<sub>2</sub> variations over the past few centuries, Nature, 315, 309–311, 1985. Regan, K., Kammann, C., Hartung, K., Lenhart, K., Muller, C., Philippot, L., Kandeler, E., and Marhan, S.: Can differences in microbial abundances help explain enhanced N(2)O emissions in a permanent grassland under elevated atmospheric CO(2)?, Global Change Biol., 17, 3176–3186, <a href="http://dx.doi.org/10.1111/j.1365-2486.2011.02470.x">https://doi.org/10.1111/j.1365-2486.2011.02470.x</a>, 2011. Rodrigo, A., Recous, S., Neel, C., and Mary, B.: Modelling temperature and moisture effects on C-N transformations in soils: comparison of nine models, Ecol. Mod., 102, 325–339, 1997. Rogers, H. H., Runion, G. B., and Krupa, S. V.: Plant responses to atmospheric CO<sub>2</sub> enrichment with emphasis on roots and the rhizosphere, Environ. Pollut., 83, 155–189, 1994. Schils, R. L. M., Kuikman, P., Liski, J., van Oijen, M., Smith, P., Webb, J., Alm, J., Somogyi, Z., van den Akker, J., Billett, M., Emmett, B. A., Evans, C. D., Lindner, M., Palosuo, T., Bellamy, P. H., Jandl, R., and Hiederer, R.: Review of existing information on the interrelations between soil and climate change, Alterra, Wageningen, 208, p. 23, available at: <a href="http://ec.europa.eu/environment/soil/review_en.htm">http://ec.europa.eu/environment/soil/review_en.htm</a>, 2008. Selsted, M. B., van der Linden, L., Ibrom, A., Michelsen, A., Larsen, K. S., Pedersen, J. K., Mikkelsen, T. N., Pilegaard, K., Beier, C., and Ambus, P.: Soil respiration is stimulated by elevated CO<sub>2</sub> and reduced by summer drought: three years of measurements in a multifactor ecosystem manipulation experiment in a temperate heathland (CLIMAITE), Glob. Change Biol., 18, 1216–1230, <a href="http://dx.doi.org/10.1111/j.1365-2486.2011.02634.x">https://doi.org/10.1111/j.1365-2486.2011.02634.x</a>, 2012. Skopp, J., Jawson, M. D., and Doran, J. W.: Steady-State Aerobic Microbial Activity as a Function of Soil Water Content, Soil Sci. Soc. Am. J., 54, 1619–1625, <a href="http://dx.doi.org/10.2136/sssaj1990.03615995005400060018x">https://doi.org/10.2136/sssaj1990.03615995005400060018x</a>, 1990. Soe, A. R. B., Giesemann, A., Anderson, T. H., Weigel, H. J., and Buchmann, N.: Soil respiration under elevated CO<sub>2</sub> and its partitioning into recently assimilated and older carbon sources, Plant Soil, 262, 85–94, <a href="http://dx.doi.org/10.1023/B:PLSO.0000037025.78016.9b">https://doi.org/10.1023/B:PLSO.0000037025.78016.9b</a>, 2004. Soussana, J. F., Fuhrer, J., Jones, M. B., and Van Amstel, A. R.: The greenhouse gas balance of grasslands in Europe, Agric. Ecosyst. Environ., 121, 1–4, 2007. Verburg, P. J., Arnone III, J. A., Obrist, D., Schorran, D. E., Evans, R. D., Leroux-Swarthout, D., Johnson, D. W., Luo, Y., and Coleman, J. S.: Net ecosystem carbon exchange in two experimental grassland ecosystems, Glob. Change Biol., 10, 498–508, 2004. Volk, M. and Niklaus, P. A.: Respiratory carbon loss of calcareous grasslands in winter shows no effects of 4 years' CO<sub>2</sub> enrichment, Funct. Ecol., 16, 162–166, 2002. Wan, S. Q. and Luo, Y. Q.: Substrate regulation of soil respiration in a tallgrass prairie: Results of a clipping and shading experiment, Global Biogeochem. Cy., 17, 1054, <a href="http://dx.doi.org/10.1029/2002gb001971">https://doi.org/10.1029/2002gb001971</a>, 2003. Ward, J. K. and Kelly, J. K.: Scaling up evolutionary responses to elevated CO<sub>2</sub>: lessons from <i>Arabidopsis</i>, Ecol. Lett., 7, 427–440, 2004. Wasshausen, W.: Frühjahrspflege auf dem Grünland: Zehn Punkte beachten, Landwirtschaftsblatt Weser-Ems, 8, 6–8, 1987. Zak, D. R., Pregitzer, K. S., Curtis, P. S., Teeri, J. A., Fogel, R., and Randlett, D. L.: Elevated atmospheric CO<sub>2</sub> and feedback between carbon and nitrogen cycles, Plant Soil, 151, 105–117, 1993. Zak, D. R., Pregitzer, K. S., King, J. S., and Holmes, W. E.: Elevated atmospheric CO<sub>2</sub>, fine roots and the response of soil microorganisms: a review and hypothesis, New Phytol., 147, 201–222, 2000. Zhou, X., Sherry, R. A., An, Y., Wallace, L. L., and Luo, Y.: Main and interactive effects of warming, clipping, and doubled precipitation on soil CO<sub>2</sub> efflux in a grassland ecosystem, Global Biogeochem. Cy., 20, GB1003, https://doi.org/1010.1029/2005GB002526, 2006.