Ninth International Conference on Permafrost ... - IARC Research

Seasonal Sources of Soil Respiration from High Arctic Landscapes Dominated byPolar StripesClaudia I. CzimczikDepartment of Earth System Science, University of California, Irvine, CA 92614, USASusan E. TrumboreDepartment of Earth System Science, University of California, Irvine, CA 92614, USAJeffrey WelkerEnvironment and Natural Resources Institute and Biology Department, University of Alaska AnchorageIntroductionCarbon (C) cycling studies in the Arctic have largelyfocused on Low Arctic wet sedge and tussock tundraecosystems, with studies in the High Arctic only recentlycontributing to our understanding of the complete Pan ArcticC biogeochemistry. It is becoming increasingly clear thatHigh Arctic soils contain significant amounts of soil C thatpreviously have been underestimated by up to an order ofmagnitude (Horwath 2007). In addition, we now recognizethat soil C pools in portions of the High Arctic landscapemay be spatially heterogeneous (horizontally and vertically),especially in areas where pattern ground is extensive such aslandscapes, where polar strips result in vegetated troughs andbarren ridges. In addition, because this region is warming atsome of the highest rates across the entire biome, increasinglevels of microbial decomposition and soil respiration mayprovide an important positive feedback to global warmingthat warrants quantification.We studied the seasonal pattern of CO 2evolved from thesoil surface (soil respiration) and its production within themineral soil to 60 cm depth from a High Arctic prostratedwarf shrub tundra (polar semi-desert) ecosystem innorthwest Greenland as part of a NSF Biocomplexity projecton C cycling in cold, dry ecosystems. We used radiocarbon to(1) partition the net flux of soil respiration into plant-derivedC sources (C cycling on a time scale of days to a few years)and microbial-derived sources (slower cycling C) and (2) toinvestigate whether microorganisms are currently accessingvery old C pools that used to be unavailable under previousclimatic conditions.Material and MethodsStudy siteSamples were taken at two locations near Thule Air Base,northwest Greenland (76°32′N, 68°50′W) throughout thegrowing season from June to August 2007. The landscapeswere dominated by classic polar stripes formed from frostcracking, aeolian in-filling, colonization by higher plants,and freeze thaw dynamics resulting in troughs dominatedby Dryas integrifolia and Salix arctica and ridges that werenon-vegetated. This landscape type represents about onethirdof the entire High Arctic terrestrial land cover.SamplingSoil respiration rates were measured with paired, dynamicchambers (n = 3) in troughs and ridges, together withmeasurements of air and soil temperature. In addition, wemonitored the concentration and isotopic signature (δ 13 C,Δ 14 C) of CO 2within the soil profile to 60 cm depth as well asthe isotopic signature of CO 2in ambient air. Gas fluxes andconcentrations were monitored approximately weekly. Theisotopic signature of CO 2was measured monthly. CO 2wassampled with evacuated canisters.At UC Irvine, CO 2was cryogenically purified, reducedto graphite, and analyzed for its stable isotope signature(IRMS) and radiocarbon content (AMS) (Xu et al. 2007). Theisotopic signature of the main soil respiration sources (rootsand microorganisms) were investigated using laboratoryincubations or freshly-cut roots and intact soil cores. Inaddition, we measured the content and isotopic compositionof C and nitrogen in each core.Results and DiscussionIn vegetated troughs, soil respiration rates peaked in thesummer, when temperatures were highest (Fig. 1). CO 2concentrations in the soil pore space reached a plateau in thesummer. Respiration rates from ridges were low throughoutSoil respiration(mg C m -2 hr -1 )CO 2concentration(ppm)10080604020025002000150010005000Ridge -20 cmRidge -30 cmRidge -60 cmTrough -20 cmDateTroughRidge6/1/07 7/1/07 8/1/07 9/1/07Figure 1. Flux of CO 2from the soil surface (top panel) andconcentration of CO 2in the soil pore space (bottom panel)throughout the growing season.53