Ninth International Conference on Permafrost ... - IARC Research

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The Contribution of Old Carbon to Respiration from Alaskan Tundra FollowingPermafrost ThawEdward A.G. Schuur, Jason G. Vogel, K. Grace Crummer, Hanna Lee, Koushik DuttaDepartment of Botany, University of Florida, Gainesville, FL 32601-8526, USAIntroductionUp to 1670 Pg of soil carbon (C) has accumulatedin high latitude ecosystems after the retreat of the lastmajor ice sheets. This soil C has until now been largelyprotected from decomposition by cold temperature, waterlogging,and permafrost. Recent studies suggest that, dueto climate warming, these ecosystems may no longer beaccumulating C, and in some cases may be losing stored Cto the atmosphere. We hypothesize that sustained transfersof C to the atmosphere that could cause a significant positivefeedback to climate change will come from old C, whichforms the bulk of the soil pool.Materials and MethodsField sitesWe used radiocarbon (∆ 14 C) measurements of carbondioxide to detect the age of C respired from tussock tundranear Denali National Park, Alaska. These measurementswere made in the Eight Mile Lake Watershed (63°52′42.1ʺN,149°15′12.9ʺW) on the north slope of the Alaska Range.Ground temperature in a borehole has been monitored forseveral decades at this site, before and after the permafrostwas observed to thaw (Osterkamp & Romanovsky 1999).In this watershed, our study has defined three sites thatrepresent differing amounts of disturbance from permafrostthawing based on observations of the vegetation and theborehole measurements: (1) tussock tundra typical of arcticecosystems, dominated by the sedge Eriophorum vaginatumand Sphagnum spp mosses (Minimal thaw); (2) a site nearthe borehole used for permafrost temperatures, where thevegetation composition has been shifting to include moreshrub species, such as Vaccinium uliginosum and Rubuschamaemorus (Moderate thaw); and (3) a site located wherepermafrost melted more than several decades ago, nowlargely dominated by shrub species (Extensive thaw) (Schuuret al. 2007). These three sites are a natural experimentalgradient representing the long-term effects of permafrostthawing on C loss. This thawing has occurred without anyvisible surface disturbance (i.e., fire) and is thought to be aneffect of regional climate warming. We made radiocarbonmeasurements of ecosystem respiration, incubations ofsoil organic matter, and incubations of aboveground andbelowground plant biomass to determine the age and isotopicvalue of C respired from these sites.Radiocarbon field measurementsEcosystem respiration ∆ 14 CO 2measurements weremeasured using a modified dynamic flow chamber systemanalogous to the system used for measuring ecosystem CO 2fluxes. At monthly intervals during the growing season(May–September), dark, 10 L, plastic chambers were placedover collars in the soil surface. To remove backgroundatmospheric air present when the chamber top is fit to thecollar, the air stream is scrubbed with Ascarite. Carbondioxide was scrubbed from the system at a rate similar tothat of soil CO 2efflux until 2–3 chamber volumes of airhad passed through the scrubber, when the CO 2remainingis almost exclusively soil-respired CO 2. The air stream wasthen diverted through a molecular sieve to quantitatively trap~1.0 mg CO 2. In the laboratory, the molecular sieve trapswere heated to 625°C to desorb CO 2. Carbon dioxide wasthen purified and analyzed for δ 13 C and ∆ 14 C.Radiocarbon laboratory measurementsTo estimate the contribution of plant respiration, surfacelitter, and deep, old soil C to surface ∆ 14 CO 2fluxes, weincubated these materials in the laboratory to determine theisotopic composition of evolved CO 2. We collected 2 replicatesoil samples per site down to the mineral soil interface(typically the full August active layer depth) and separatedthe profiles into 5 depth increments (0–5, 5–15, 15–25, 25–35, 35+ cm). The final depth increment was variable becausethe total organic layer thickness varied among samples. Allsamples were split, large roots and stems removed, and thehalves kept intact to preserve the soil structure. Soil coresplits were incubated in separate glass jars at 3°C and 8°Cto determine the temperature sensitivity (expressed asa Q 10value) for respiration from these soil. Rates of CO 2production were then measured daily using an IRGA tomonitor the change in CO 2concentration in the incubationjar headspace over time. Jars were sealed and flushed withmoist CO 2-free air when CO 2concentrations exceed 1%.After 5 days of incubation in the laboratory, the jars werecompletely scrubbed with CO 2-free air, and respired 14 CO 2was allowed to accumulate and was then collected for ∆ 14 Canalysis. Similar incubations were made of aboveground andbelowground plant parts harvested from 5 x 5cm quadratsduring the growing season.Source partitioningWe estimated the relative contribution of the plant and soilcomponents to the R ecoflux using the ∆ 14 C measurementsand a standard statistical modeling approach. The laboratorysoil incubations were combined into surface soil (top 2horizons) and deep soil (lower 3 horizons) components byflux-weighting the soil incubations. To calculate a combinedisotope respiration value for the surface and deep soil, the∆ 14 C values for the layers that were combined were weightedby (1) the relative CO 2flux on a per gram dry soil basis, (2)275
Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tTable 1. Fluxes and isotopes of carbon dioxide from laboratory incubations of soil organic layers at 15°C.SiteSoil LayercmCarbon Fluxµg C gdw -1 hr -1(± SE)Soil Masskgdw m -2(± SE)Average SummerTemperatureºC∆ 14 C‰ (± SE)Minimal 0-5 8.09 (4.21) 3.06 (0.76) 14.0 +101 (7)5-15 6.05 (2.14) 5.13 (1.90) 12.2 +96 (13)15-25 3.07 (1.64) 8.80 (0.79) 4.7 +53 (6)25-35 1.66 (0.23) 14.61 (3.14) 2.0 +48 (1)35+ 1.26 (0.65) 181.86 (46.21) 0.6 -56 (66)Moderate 0-5 7.65 (1.31) 3.80 (0.56) 14.0 +94 (11)5-15 3.47 (1.73) 5.49 (1.01) 12.2 +78 (23)15-25 3.16 (0.71) 10.06 (0.87) 4.7 +35 (10)25-35 1.18 (0.28) 17.37 (0.92) 2.0 +25 (4)35+ 0.38 (0.18) 66.87 (19.27) 0.6 -32 (32)Extensive 0-5 9.88 (3.38) 3.06 (0.76) 14.0 +101 (7)5-15 4.26 (2.00) 5.13 (1.90) 12.2 +96 (13)15-25 2.05 (1.01) 8.80 (0.79) 4.7 +53 (6)25-35 1.98 (0.58) 14.61 (3.14) 2.0 +48 (1)35+ 0.46 (0.13) 181.86 (46.21) 0.6 -56 (66)the relative amount of soil mass in the combined horizons,and (3) by the average field temperature for the horizons(this varied over the season). Plant respiration ∆ 14 C valuesmeasured over the growing season matched the atmosphericvalues in 2004 (data not shown), and were assumed to followthe atmospheric decline.Total R eco∆ 14 CO 2flux is a combination of surface anddeep soil respiration, along with plant respiration. Becausethere is no single solution that describes the contribution of3 unknown sources with a single isotope tracer, a standardstatistical approach yields a range of possible contributionsof the component sources to R eco. Of the sources (plantrespiration, surface soil respiration, deep soil respiration),the contribution of the deep soil is the most clearly defined,as demonstrated by the smallest standard deviation andoverall range. This is a result of the deep soil having a ∆ 14 Cvalue furthest away from the ∆ 14 C value of R eco, thus itscontribution to the total is most constrained. While the deepsoil C was the only source that could bring the R eco∆ 14 Cvalue below that of the current atmosphere,the ∆ 14 C values of plant respiration and surface soil respirationwere more similar and thus could substitute for one another.probable contributions from these different sources toecosystem respiration. Deep soil respiration generallyaveraged between 5–15% of total ecosystem respiration, butreached as high as 40% in some months. When aggregatedacross the growing season, the two sites undergoing moredisturbance from permafrost thaw had on average 2–3 timesthe loss of old, deep C as compared to the least disturbedsite. From this isotope partitioning, we determined that therespiration of old C increases following permafrost thawand contributes towards making these tundra ecosystems netsources of C to the atmosphere.ReferencesOsterkamp, T.E. & Romanovsky, V.E. 1999. Evidence forwarming and thawing of discontinuous permafrost inAlaska. Permafrost and Periglac. Process 10: 17-37.Schuur, E.A.G., Crummer, K.G., Vogel, J.G. & Mack, M.C.2007. Plant species composition and productivityfollowing permafrost thaw and thermokarst inAlaskan tundra. Ecosystems 10: 280-292.Results and ConclusionsOver the study period, ecosystem respiration radiocarbonvalues averaged from +35‰ to +95‰ in different monthsacross sites. For soil incubations, surface soil radiocarbonwas elevated relative both to ecosystem respiration and thecurrent atmospheric radiocarbon value, demonstrating thesignificant contribution from C fixed over the past years toseveral decades (Table 1). The deeper soil, in contrast, hadrespiration isotope values that averaged below zero, reflectingthe significant effect of radioactive decay on the isotopecontent of deeper soil layers. The plant and soil incubationswere combined in a multisource mixing model to determine276
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ContentsPreface ...................
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Mapping and Modeling the Distributi
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Satellite Observations of Frozen Gr
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xiiIce Wedge Thermal Regime in Nort
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xivPermafrost Response to Dynamics
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NICOP SponsorsUniversitiesUniversit
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Deep Permafrost Studies at the Lupi
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Effect of Fire on Pond Dynamics in
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Cryological Status of Russian Soils
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Acoustical Surveys of Methane Plume
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Permafrost Delineation Near Fairban
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Preparatory Work for a Permanent Ge
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A Provisional Soil Map of the Trans
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Martian Permafrost Depths from Orbi
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Time Series Analyses of Active Micr
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Impact of Permafrost Degradation on
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DC Resistivity Soundings Across a P
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Modeling Thermal and Moisture Regim
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A Provisional Permafrost Map of the
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Alpine Permafrost Distribution at M
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Cryogenic Formations of the Caucasu
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Modeling Potential Climatic Change
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A Hypothesis: A Condition of Growth
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Modeled Continual Surface Water Sto
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Freeze/Thaw Properties of Tundra So
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Discontinuous Permafrost Distributi
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Thermal Regime Within an Arctic Was
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Seasonal and Interannual Variabilit
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Twelve-Year Thaw Progression Data f
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Continued Permafrost Warming in Nor
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Landsliding Following Forest Fire o
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A Permafrost Model Incorporating Dy
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Seasonal Sources of Soil Respiratio
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Greenland Permafrost Temperature Si
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The Importance of Snow Cover Evolut
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The Account of Long-Term Air Temper
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The Combined Isotopic Analysis of L
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Adaptating and Managing Nunavik’s
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Human Experience of Cryospheric Cha
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HiRISE Observations of Fractured Mo
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A Soil Freeze-Thaw Model Through th
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Mapping and Modeling the Distributi
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First Results of Ground Surface Tem
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Historical Changes in the Seasonall
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Rock Glaciers in the Kåfjord Area,
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Snowpack Evolution on Permafrost, N
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Climate Change in Permafrost Region
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Maximizing Construction Season in a
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Pleistocene Sand-Wedge, Composite-W
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