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  • 1.
    Björkman, Mats P.
    et al.
    Dept Plant & Environm Sci, Univ Gothenburg, Gothenburg, Sweden.
    Morgner, Elke
    Fac Biosci Fisheries & Econ, Dept Arctic & Marine Biol, Univ Tromso, Tromso, Norway; Dept Arctic Biol, Univ Ctr Svalbard, Longyearbyen, Norway.
    Cooper, Elisabeth J.
    Fac Biosci Fisheries & Econ, Dept Arctic & Marine Biol, Univ Tromso, Tromso, Norway.
    Elberling, Bo
    Univ Ctr Svalbard, Longyearbyen, Norway; Dept Geog & Geol, Univ Copenhagen, Copenhagen, Denmark.
    Klemedtsson, Leif
    Dept Plant & Environm Sci, Univ Gothenburg, Gothenburg, Sweden.
    Björk, Robert G.
    Örebro University, School of Science and Technology. Dept Plant & Environm Sci, Univ Gothenburg, Gothenburg, Sweden.
    Winter carbon dioxide effluxes from Arctic ecosystems: an overview and comparison of methodologies2010In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 24, p. GB3010-Article in journal (Refereed)
    Abstract [en]

    The winter CO(2) efflux from subnivean environments is an important component of annual C budgets in Arctic ecosystems and consequently makes prediction and estimations of winter processes as well as incorporations of these processes into existing models important. Several methods have been used for estimating winter CO(2) effluxes involving different assumptions about the snowpack, all aiming to quantify CO(2) production. Here, four different methods are compared and discussed: (1) measurements with a chamber on the snow surface, F(snow), (2) chamber measurements directly on the soil, F(soil), after snow removal, (3) diffusion measurements, F(2-point), within the snowpack, and (4) a trace gas technique, F(SF6), with multiple gas sampling within the snowpack. According to measurements collected from shallow and deep snow cover in High Arctic Svalbard and subarctic Sweden during the winter of 2007-2008, the four methods differ by up to two orders of magnitude in their estimates of total winter emissions. The highest mean winter CO(2) effluxes, 7.7-216.8 mg CO(2) m(-2) h(-1), were observed using F(soil) and the lowest values, 0.8-12.6 mg CO(2) m(-2) h(-1), using F(SF6). The F(snow) and F(2-point) methods were both within the lower range, 2.1-15.1 and 6.8-11.2 mg CO(2) m(-2) h(-1), respectively. These differences result not only from using contrasting methods but also from the differences in the assumptions within the methods when quantifying CO(2) production and effluxes to the atmosphere. Because snow can act as a barrier to CO(2), F(soil) is assumed to measure soil production, whereas F(SF6), F(snow), and F(2-point) are considered better approaches for quantifying exchange processes between the soil, snow, and the atmosphere. This study indicates that estimates of winter CO(2) emissions may vary more as a result of the method used than as a result of the actual variation in soil CO(2) production or release. This is a major concern, especially when CO(2) efflux data are used in climate models or in carbon budget calculations, thus highlighting the need for further development and validation of accurate and appropriate techniques.

  • 2. Hagedorn, Frank
    et al.
    van Hees, Patrick A. W.
    Örebro University, Department of Natural Sciences.
    Handa, I. Tanya
    Haettenschwiler, Stephan
    Elevated atmospheric CO(2) fuels leaching of old dissolved organic matter at the alpine treeline2008In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 22, no 2, p. GB2004-Article in journal (Refereed)
    Abstract [en]

    Dissolved organic matter (DOM), the mobile form of soil organic matter (SOM), plays an important role in soil C cycling and in nutrient transport. We investigated the effects of 5 years of CO(2) enrichment (370 versus 570 mu mol CO(2) mol(-1)) on DOM dynamics at the alpine treeline, including the analysis of fast-cycling components such as low molecular weight organic acids (LMWOAs), dissolved organic carbon (DOC) biodegradability, and the decomposition of (14)C-labeled oxalate. Concentrations of DOC in canopy throughfall were 20% higher at elevated CO(2), probably driven by higher carbohydrate concentrations in leaves. In the organic soil layer, 5 years of CO(2) enrichment increased water-extractable organic C by 17% and soil solution DOC at 5 cm depth by 20%. The (13)C tracing of recently assimilated CO(2) revealed that the input of recent plant-derived C (< 15% of total DOC) was smaller than the CO(2)-induced increase in DOC. This strongly suggests that CO(2) enrichment enhanced the mobilization of native DOC, which is supported by significant increases in dissolved organic nitrogen (DON). We mainly attribute these increases to a stimulated microbial activity as indicated by higher basal and soil respiration rates (+27%). The (14)C-labeled oxalate was more rapidly mineralized from high CO(2) soils. The concentrations of LMWOAs, but also those of "hydrophilic'' DOC and biodegradable DOC (6% of total DOC), were, however, not affected by elevated CO(2), suggesting that production and consumption of "labile'' DOC were in balance. In summary, our data suggest that 5 years of CO(2) enrichment speeded up the cycling of "labile'' DOM and SOM in a late successional treeline ecosystem and increased the mobilization of older DOM through a stimulated microbial activity. Such a "priming effect'' implies that elevated CO(2) can accelerate the turnover of native SOM, and thus, it may induce increasing losses of old C from thick organic layers.

  • 3.
    Soerensen, A. L.
    et al.
    Department of Environmental Science and Analytical Chemistry, Stockholm University, Stockholm, Sweden.
    Schartup, A. T.
    John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge MA, USA.
    Skrobonja, A.
    Department of Chemistry, Umeå University, Umeå, Sweden.
    Bouchet, S.
    Now at D-USYS Department, ETH Zürich, Zürich, Switzerland; Institut des Sciences Analytiques et de Physico-chimie pour l ’ Environnement et les Materiaux, CNRS/UNIV PAU & PAYS ADOUR, Pau, France.
    Amouroux, D.
    Institut des Sciences Analytiques et de Physico-chimie pour l ’ Environnement et les Materiaux, CNRS/UNIV PAU & PAYS ADOUR, Pau, France.
    Liem-Nguyen, Van
    Örebro University, School of Science and Technology. Department of Chemistry, Umeå University, Umeå, Sweden.
    Björn, E.
    Department of Chemistry, Umeå University, Umeå, Sweden.
    Deciphering the Role of Water Column Redoxclines on Methylmercury Cycling Using Speciation Modeling and Observations From the Baltic Sea2018In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 32, no 10, p. 1498-1513Article in journal (Refereed)
    Abstract [en]

    Oxygen-depleted areas are spreading in coastal and offshore waters worldwide, but the implication for production and bioaccumulation of neurotoxic methylmercury (MeHg) is uncertain. We combined observations from six cruises in the Baltic Sea with speciation modeling and incubation experiments to gain insights into mercury (Hg) dynamics in oxygen depleted systems. We then developed a conceptual model describing the main drivers of Hg speciation, fluxes, and transformations in water columns with steep redox gradients. MeHg concentrations were 2-6 and 30-55 times higher in hypoxic and anoxic than in normoxic water, respectively, while only 1-3 and 1-2 times higher for total Hg (THg). We systematically detected divalent inorganic Hg (Hg-II) methylation in anoxic water but rarely in other waters. In anoxic water, high concentrations of dissolved sulfide cause formation of dissolved species of Hg-II: HgS2H(aq)- and Hg (SH)(2)(0)((aq)). This prolongs the lifetime and increases the reservoir of Hg-II readily available for methylation, driving the high MeHg concentrations in anoxic zones. In the hypoxic zone and at the hypoxic-anoxic interface, Hg concentrations, partitioning, and speciation are all highly dynamic due to processes linked to the iron and sulfur cycles. This causes a large variability in bioavailability of Hg, and thereby MeHg concentrations, in these zones. We find that zooplankton in the summertime are exposed to 2-6 times higher MeHg concentrations in hypoxic than in normoxic water. The current spread of hypoxic zones in coastal systems worldwide could thus cause an increase in the MeHg exposure of food webs.

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