Ninth International Conference on Permafrost ... - IARC Research

The Biocomplexity Manipulation Experiment: Effect of Water Table Drop on CH 4and CO 2Fluxes in the Alaskan Arctic at the Barrow Environmental ObservatoryDonatella ZonaGlobal Change Research Group, San Diego State University, San Diego, CAWalter C. OechelGlobal Change Research Group, San Diego State University, San Diego, CAIntroductionThe arctic tundra contains more than 191.8 Pg C as soilorganic matter (Post et al. 1982). Increasingly, this carbonis, or is at risk of being, released to the atmosphere asCO 2(Oechel et al. 1993, Oechel et al. 1994) and/or CH 4(Vourlitis & Oechel 1993). However, predictions of futurerates of release of CO 2and CH 4flux, following changes intemperature, moisture, and other variables associated withclimate change, are uncertain. To predict with confidencefuture CO 2and CH 4releases to the atmosphere, it isnecessary to understand the controls on net CO 2and CH 4fluxes. The patterns and controls on net ecosystem CO 2andCH 4fluxes are complex and non-linear. Warming and dryingof the tundra can result in increased net CO 2emissions fromthe Arctic to the atmosphere. However, areas that becomewarmer and remain wet, or become wetter, may be larger netemitters of CH 4to the atmosphere.Results and discussionAccording to our study, water table does not have aconsistent impact on methane and carbon dioxide flux, andin certain conditions, lower water table is related to highermethane efflux and does not affect carbon dioxide flux.This result is connected to the importance of other factorslike thaw depth, soil temperature, and soil moisture moreimportant than water table early in the season. Later in theseason, water table depth becomes more important, and weobserved higher methane effluxes from the site where thewater table was higher. During this period, the CO 2fluxesappear to be very similar at both sites. This unexpected resultis contrary to most of the past studies that showed increasedsoil respiration and CO 2release with decrease in watertable and increased aeration status of the soil. A possibleexplanation could be connected to the characteristics of thevegetation in our study site. Even with a water table drop,below surface mosses are able to hold the water and maintainan anaerobic environment. In other words, the two siteswith substantially different soil moisture and water tabledepth are both characterized by largely anaerobic soils. Asa consequence, soil respiration is restricted to the shallowersoil layers, and it is not influenced by the difference in thawdepth or water table between the two sites. In our study site,mosses are major components of the vegetation representingmore than 80% of the biomass.The differential response of the water table drop on CO 2and CH 4fluxes is probably due to the differential importanceof shallower aerobic versus deeper anaerobic soil layers onthe emission of the two gases. Methanogensis probablyoccurs in deeper soil layers, so a deeper thaw depth couldsignificantly increase methane production (as we observedin the north site in late season), while shallower soil layersare the ones mainly responsible for the aerobic respiration.AcknowledgmentsWe thank Robert Clement, University of Edinburgh forhaving written the code for the data reduction; Joe Verfailleand Hiroki Ikawa for field and technical assistance; RobRhew, Steven Hastings, and Rommel Zulueta for insightand advice; Douglas Deutschman for helpful advice on thestatistics analysis; the Barrow Arctic Science Consortium(BASC) and Glenn Sheehan for logistic support. This workwas funded by the Biocomplexity Program of the NationalScience Foundation (award number OPP 0421588).ReferencesOechel, W.C., Cowles, S., Grulke, N., Hastings, S.J.,Lawrence, B., Prudhomme, T., Riechers, G., Strain,B., Tissue, D. & Vourlitis, G.L. 1994. Transient natureof CO 2fertilization in Arctic tundra. Nature 371: 500-503.Oechel, W.C., Hastings, S.J., Vourlitis, G.L., Jenkins, M.,Riechers, G. & Grulke, N. 1993. Recent Change ofArctic tundra ecosystems from a net carbon dioxidesink to a source. Nature 361: 520-523.Post, W.M., Emanuel, W.R., Zinke, P.J. & Stangenberger,A.G. 1982. Soil carbon pools and world life zones.Nature 298: 156-159.Vourlitis, G.L., Oechel, W.C., Hastings, S.J. & Jenkins, M.A.1993. The effect of soil moisture and thaw depth onCH 4flux from wet coastal tundra ecosystem on thenorth slope of Alaska. Chemosphere 26: 329-338.365