Biogeosciences www.biogeosciences.net 1726-4170 1726-4189 5 5 2008 10.5194/bg-5-1457-2008 http://www.biogeosciences.net/5/1457/2008/ http://www.biogeosciences.net/5/1457/2008/bg-5-1457-2008.html http://www.biogeosciences.net/5/1457/2008/bg-5-1457-2008.pdf 1457 1473 2008-10-28 Fluxes and ¹³C isotopic composition of dissolved carbon and pathways of methanogenesis in a fen soil exposed to experimental drought K.-H. Knorr kh.knorr@uni-bayreuth.de B. Glaser C. Blodau Limnological Research Station & Department of Hydrology, University of Bayreuth, Universitaetsstrasse 30, 95440 Bayreuth, Germany Department of Soil Physics, University of Bayreuth, Universitaetsstrasse 30, 95440 Bayreuth, Germany Peatlands contain a carbon stock of global concern and significantly contribute to the global methane burden. The impact of drought and rewetting on carbon cycling in peatland ecosystems is thus currently debated. We studied the impact of experimental drought and rewetting on intact monoliths from a temperate fen over a period of ~300 days, using a permanently wet treatment and two treatments undergoing drought for 50 days. In one of the mesocosms, vegetation had been removed. Net production of CH₄ was calculated from mass balances in the peat and emission using static chamber measurements. Results were compared to ¹³C isotope budgets of CO₂ and CH₄ and energy yields of acetoclastic and hydrogenotrophic methanogenesis. Drought retarded methane production after rewetting for days to weeks and promoted methanotrophic activity. Based on isotope and flux budgets, aerobic soil respiration contributed 32–96% in the wet treatment and 86–99% in the other treatments. Drying and rewetting did not shift methanogenic pathways according to δ¹³C ratios of CH₄ and CO₂. Although δ¹³C ratios indicated a prevalence of hydrogenotrophic methanogenesis, free energies of this process were small and often positive on the horizon scale. This suggests that methane was produced very locally. Fresh plant-derived carbon input apparently supported respiration in the rhizosphere and sustained methanogenesis in the unsaturated zone, according to a ¹³C-CO₂ labelling experiment. The study documents that drying and rewetting in a rich fen soil may have little effect on methanogenic pathways, but result in rapid shifts between methanogenesis and methanotrophy. Such shifts may be promoted by roots and soil heterogeneity, as hydrogenotrophic methanogenesis occurred locally even when conditions were not conducive for this process in the bulk peat. Achtnich, C., Bak, F., and Conrad, R.: Competition for electron-donors among nitrate reducers, ferric iron reducers, sulfate reducers, and methanogens in anoxic paddy soil, Biol. Fertil. Soils, 19, 65–72, 1995. Aerts, R. and Ludwig, F.: Water-table changes and nutritional status affect trace gas emissions from laboratory columns of peatland soils, Soil Biol. Biochem., 29, 1691–1698, 1997. Beer, J. and Blodau, C.: Transport and thermodynamics constrain belowground carbon turnover in a northern peatland, Geochim. Cosmochim. Acta, 71, 2989–3002, 2007. Belyea, L. R. and Malmer, N.: Carbon sequestration in peatland: Patterns and mechanisms of response to climate change, Global Change Biol., 10, 1043–1052, 2004. Blodau, C. and Moore, T. R.: Experimental response of peatland carbon dynamics to a water table fluctuation, Aquat. Sci., 65, 47–62, 2003a. Blodau, C. and Moore, T. R.: Micro-scale CO₂ and CH₄ dynamics in a peat soil during a water fluctuation and sulfate pulse, Soil Biol. Biochem., 35, 535–547, 2003b. Blodau, C., Roulet, N. T., Heitmann, T., Stewart, H., Beer, J., Lafleur, P., and Moore, T. R.: Belowground carbon turnover in a temperate ombrotrophic bog, Glob. Biogeochem. Cycles, 21, GB1021, doi:10.1029/2005GB002659, 2007. Boehme, S. E., Blair, N. E., Chanton, J. P., and Martens, C. S.: A mass balance of C-13 and C-12 in an organic-rich methane-producing marine sediment, Geochim. Cosmochim. Acta, 60, 3835–3848, 1996. Bousquet, P., Ciais, P., Miller, J. B., Dlugokencky, E. J., Hauglustaine, D. A., Prigent, C., Van der Werf, G. R., Peylin, P., Brunke, E. G., Carouge, C., Langenfelds, R. L., Lathiere, J., Papa, F., Ramonet, M., Schmidt, M., Steele, L. P., Tyler, S. C., and White, J.: Contribution of anthropogenic and natural sources to atmospheric methane variability, Nature, 443, 439–443, 2006. Chanton, J. P.: The effect of gas transport on the isotope signature of methane in wetlands, Org. Geochem., 36, 753–768, 2005. Chasar, L. S., Chanton, J. P., Glaser, P. H., and Siegel, D. I.: Methane concentration and stable isotope distribution as evidence of rhizospheric processes: Comparison of a fen and bog in the glacial lake agassiz peatland complex, Ann. Bot.-London, 86, 655–663, 2000. Chimner, R. A. and Cooper, D. J.: Influence of water table levels on CO₂ emissions in a colorado subalpine fen: An in situ microcosm study, Soil Biol. Biochem., 35, 345–351, 2003. Coles, J. R. P. and Yavitt, J. B.: Linking belowground carbon allocation to anaerobic CH₄ and CO₂ production in a forested peatland, new york state, Geomicrobiol. J., 21, 445–455, 2004. Conrad, R.: Quantification of methanogenic pathways using stable carbon isotopic signatures: A review and a proposal, Org. Geochem., 36, 739–752, 2005. Dettling, M. D., Yavitt, J. B., and Zinder, S. H.: Control of organic carbon mineralization by alternative electron acceptors in four peatlands, central new york state, USA, Wetlands, 26, 917–927, 2006. Fierer, N. and Schimel, J. P.: A proposed mechanism for the pulse in carbon dioxide production commonly observed following the rapid rewetting of a dry soil, Soil Sci. Soc. Amer. J., 67, 798–805, 2003. Freeman, C., Nevison, G. B., Kang, H., Hughes, S., Reynolds, B., and Hudson, J. A.: Contrasted effects of simulated drought on the production and oxidation of methane in a mid-wales wetland, Soil Biol. Biochem., 34, 61–67, 2002. Granberg, G., Mikkela, C., Sundh, I., Svensson, B. H., and Nilsson, M.: Sources of spatial variation in methane emission from mires in northern sweden: A mechanistic approach in statistical modelling, Global Biogeochem. Cy., 11, 135–150, 1997. Gu, B. H., Schelske, C. L., and Hodell, D. A.: Extreme C-13 enrichments in a shallow hypereutrophic lake: Implications for carbon cycling, Limnol. Oceanogr., 49, 1152–1159, 2004. Hoehler, T. M., Alperin, M. J., Albert, D. B., and Martens, C. S.: Apparent minimum free energy requirements for methanogenic archaea and sulfate-reducing bacteria in an anoxic marine sediment, FEMS Microbiol. Ecol., 38, 33–41, 2001. Hornibrook, E. R. C., Longstaffe, F. J., and Fyfe, W. S.: Evolution of stable carbon isotope compositions for methane and carbon dioxide in freshwater wetlands and other anaerobic environments, Geochim. Cosmochim. Acta, 64, 1013–1027, 2000a. Hornibrook, E. R. C., Longstaffe, F. J., and Fyfe, W. S.: Factors influencing stable isotope ratios in CH₄ and CO₂ within subenvironments of freshwater wetlands: Implications for delta-signatures of emissions, Isot. Environ. Healt. S., 36, 151–176, 2000b. Hornibrook, E. R. C., Longstaffe, F. J., Fyfe, W. S., and Bloom, Y.: Carbon-isotope ratios and carbon, nitrogen and sulfur abundances in flora and soil organic matter from a temperate-zone bog and marsh, Geochem. J., 34, 237–245, 2000c. IPCC: Climate change 2001, 3rd assessment report, Intergovernmenmental Panel on Climate Change, Geneva, 2001. Jin, Y. and Jury, W. A.: Characterizing the dependence of gas diffusion coefficient on soil properties, Soil Sci. Soc. Amer. J., 60, 66–71, 1996. Joabsson, A. and Christensen, T. R.: Methane emissions from wetlands and their relationship with vascular plants: An arctic example, Global Change Biol., 7, 919–932, 2001. Kammann, C., Grunhage, L., and Jager, H. J.: A new sampling technique to monitor concentrations of CH₄, N₂O and CO₂ in air at well-defined depths in soils with varied water potential, Eur. J. Soil Sci., 52, 297–303, 2001. King, J. Y. and Reeburgh, W. S.: A pulse-labeling experiment to determine the contribution of recent plant photosynthates to net methane emission in arctic wet sedge tundra, Soil Biol. Biochem., 34, 173–180, 2002. Knorr, K. H., Oosterwoud, M., and Blodau, C.: Experimental drought alters rates of soil respiration and methanogenesis but not carbon exchange in soil of a temperate fen, Soil Biol. Biochem., 40, 1781–1791, doi:1710.1016/j.soilbio.2008.1703.1019, 2008. Lafleur, P. M., Moore, T. R., Roulet, N. T., and Frolking, S.: Ecosystem respiration in a cool temperate bog depends on peat temperature but not water table, Ecosystems, 8, 619–629, 2005. Laiho, R.: Decomposition in peatlands: Reconciling seemingly contrasting results on the impacts of lowered water levels, Soil Biol. Biochem., 38, 2011–2024, 2006. Lansdown, J. M., Quay, P. D., and King, S. L.: CH₄ production via CO₂ reduction in a temperate bog: A source of $^13$C-depleted CH₄, Geochim. Cosmochim. Acta, 56, 3493–3503, 1992. Lerman, A.: Geochemical processes – water and sediment environments, Krieger Publishing Company, Inc., Malabar, Florida, 1988. Lovley, D. R. and Goodwin, S.: Hydrogen concentrations as an indicator of the predominant terminal electron-accepting reactions in aquatic sediments, Geochim. Cosmochim. Acta, 52, 2993–3003, 1988. Mainiero, R. and Kazda, M.: Effects of carex rostrata on soil oxygen in relation to soil moisture, Plant Soil, 270, 311–320, 2005. Mikaloff Fletcher, S. E., Tans, P. P., Bruhwiler, L. M., Miller, J. B., and Heimann, M.: CH₄ sources estimated from atmospheric observations of CH₄ and its C-13/C-12 isotopic ratios: 1. Inverse modeling of source processes, Global Biogeochem. Cy., 18, GB4004, doi:10.1029/2004GB002223, 2004. Moore, P. D.: The future of cool temperate bogs, Environ. Conserv., 29, 3–20, 2002. Nordstrom, D. K. and Munoz, J. L.: Geochemical thermodynamics, second ed., Blackwell Scientific Publications, 493 pp., 1994. Paul, S., Kusel, K., and Alewell, C.: Reduction processes in forest wetlands: Tracking down heterogeneity of source/sink functions with a combination of methods, Soil Biol. Biochem., 38, 1028–1039, 2006. Penning, H., Plugge, C. M., Galand, P. E., and Conrad, R.: Variation of carbon isotope fractionation in hydrogenotrophic methanogenic microbial cultures and environmental samples at different energy status, Global Change Biol., 11, 2103–2113, 2005. Peters, V. and Conrad, R.: Sequential reduction processes and initiation of CH₄ production upon flooding of oxic upland soils, Soil Biol. Biochem., 28, 371–382, 1996. Popp, T. J., Chanton, J. P., Whiting, G. J., and Grant, N.: Methane stable isotope distribution at a carex dominated fen in north central alberta, Global Biogeochem. Cy., 13, 1063–1077, 1999. Roden, E. E. and Wetzel, R. G.: Organic carbon oxidation and suppression of methane production by microbial Fe(III) oxide reduction in vegetated and unvegetated freshwater wetland sediments, Limnol. Oceanogr., 41, 1733–1748, 1996. Roulet, N., Moore, T., Bubier, J., and Lafleur, P.: Northern fens - methane flux and climatic-change, Tellus B, 44, 100–105, 1992. Sander, R.: Compilation of henry's law constants for inorganic and organic species of potential importance in environmental chemistry (version 3), available online: http://www.henrys-law.org, 1999. Shannon, R. D. and White, J. R.: A three year study of controls in methane emissions from two michigan peatlands, Biogeochemistry, 27, 35–60, 1994. Smemo, K. A. and Yavitt, J. B.: A multi-year perspective on methane cycling in a shallow peat fen in central new york state, USA, Wetlands, 26, 20–29, 2006. Strom, L., Ekberg, A., Mastepanov, M., and Christensen, T. R.: The effect of vascular plants on carbon turnover and methane emissions from a tundra wetland, Global Change Biol., 9, 1185–1192, 2003. Stumm, W. and Morgan, J. J.: Aquatic chemistry – chemical equilibria and rates in natural waters, Environ. Sci. Technol., edited by: Schnoor, J. L. and Zehnder, A., Wiley-Interscience, New York, 1996. Updegraff, K., Bridgham, S. D., Pastor, J., Weishampel, P., and Harth, C.: Response of CO₂ and CH₄ emissions from peatlands to warming and water table manipulation, Ecol. Appl., 11, 311–326, 2001. van Bodegom, P. M., Scholten, J. C. M., and Stams, A. J. M.: Direct inhibition of methanogenesis by ferric iron, FEMS Microbiol. Ecol., 49, 261–268, 2004. Wachinger, G., Fiedler, S., Zepp, K., Gattinger, A., Sommer, M., and Roth, K.: Variability of soil methane production on the micro-scale: Spatial association with hot spots of organic material and archaeal populations, Soil Biol. Biochem., 32, 1121–1130, 2000. Waldron, S., Hall, A. J., and Fallick, A. E.: Enigmatic stable isotope dynamics of deep peat methane, Global Biogeochem. Cy., 13, 93–100, 1999. Whiticar, M. J.: Carbon and hydrogen isotope systematics of bacterial formation and oxidation of methane, Chem. Geol., 161, 291–314, 1999. Whiting, G. J. and Chanton, J. P.: Primary production control of methane emission from wetlands, Nature, 364, 794–795, 1993. Yavitt, J. B. and Seidmann-Zager, M.: Methanogenic conditions in northern peat soils, Geomicrobiol. J., 23, 119–127, 2006.