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Sökning: L773:0886 6236 OR L773:1944 9224 > (2015-2019)

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1.
  • Alfredsson, H., et al. (författare)
  • Estimated storage of amorphous silica in soils of the circum-Arctic tundra region
  • 2016
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 30:3, s. 479-500
  • Tidskriftsartikel (refereegranskat)abstract
    • We investigated the vertical distribution, storage, landscape partitioning, and spatial variability of soil amorphous silica (ASi) at four different sites underlain by continuous permafrost and representative of mountainous and lowland tundra, in the circum-Arctic region. Based on a larger set of data, we present the first estimate of the ASi soil reservoir (0-1 m depth) in circum-Arctic tundra terrain. At all sites, the vertical distribution of ASi concentrations followed the pattern of either (1) declining concentrations with depth (most common) or (2) increasing/maximum concentrations with depth. Our results suggest that a set of processes, including biological control, solifluction and other slope processes, cryoturbation, and formation of inorganic precipitates influence vertical distributions of ASi in permafrost terrain, with the capacity to retain stored ASi on millennial timescales. At the four study sites, areal ASi storage (0-1 m) is generally higher in graminoid tundra compared to wetlands. Our circum-Arctic upscaling estimates, based on both vegetation and soil classification separately, suggest a storage amounting to 219 ± 28 and 274 ± 33 Tmol Si, respectively, of which at least 30% is stored in permafrost. This estimate would account for about 3% of the global soil ASi storage while occupying an equal portion of the global land area. This result does not support the hypothesis that the circum-Arctic tundra soil ASi reservoir contains relatively higher amounts of ASi than other biomes globally as demonstrated for carbon. Nevertheless, climate warming has the potential to significantly alter ASi storage and terrestrial Si cycling in the Arctic.
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2.
  • Björnerås, C., et al. (författare)
  • Widespread Increases in Iron Concentration in European and North American Freshwaters
  • 2017
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 31:10, s. 1488-1500
  • Tidskriftsartikel (refereegranskat)abstract
    • Recent reports of increasing iron (Fe) concentrations in freshwaters are of concern, given the fundamental role of Fe in biogeochemical processes. Still, little is known about the frequency and geographical distribution of Fe trends or about the underlying drivers. We analyzed temporal trends of Fe concentrations across 340 water bodies distributed over 10 countries in northern Europe and North America in order to gain a clearer understanding of where, to what extent, and why Fe concentrations are on the rise. We found that Fe concentrations have significantly increased in 28% of sites, and decreased in 4%, with most positive trends located in northern Europe. Regions with rising Fe concentrations tend to coincide with those with organic carbon (OC) increases. Fe and OC increases may not be directly mechanistically linked, but may nevertheless be responding to common regional-scale drivers such as declining sulfur deposition or hydrological changes. A role of hydrological factors was supported by covarying trends in Fe and dissolved silica, as these elements tend to stem from similar soil depths. A positive relationship between Fe increases and conifer cover suggests that changing land use and expanded forestry could have contributed to enhanced Fe export, although increases were also observed in nonforested areas. We conclude that the phenomenon of increasing Fe concentrations is widespread, especially in northern Europe, with potentially significant implications for wider ecosystem biogeochemistry, and for the current browning of freshwaters.
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3.
  • Bröder, Lisa, et al. (författare)
  • Quantifying Degradative Loss of Terrigenous Organic Carbon in Surface Sediments Across the Laptev and East Siberian Sea
  • 2019
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 33:1, s. 85-99
  • Tidskriftsartikel (refereegranskat)abstract
    • Ongoing permafrost thaw in the Arctic may remobilize large amounts of old organic matter. Upon transport to the Siberian shelf seas, this material may be degraded and released to the atmosphere, exported off-shelf, or buried in the sediments. While our understanding of the fate of permafrost-derived organic matter in shelf waters is improving, poor constraints remain regarding degradation in sediments. Here we use an extensive data set of organic carbon concentrations and isotopes (n=109) to inventory terrigenous organic carbon (terrOC) in surficial sediments of the Laptev and East Siberian Seas (LS + ESS). Of these similar to 2.7 Tg terrOC about 55% appear resistant to degradation on a millennial timescale. A first-order degradation rate constant of 1.5 kyr(-1) is derived by combining a previously established relationship between water depth and cross-shelf sediment-terrOC transport time with mineral-associated terrOC loadings. This yields a terrOC degradation flux of similar to 1.7Gg/year from surficial sediments during cross-shelf transport, which is orders of magnitude lower than earlier estimates for degradation fluxes of dissolved and particulate terrOC in the water column of the LS + ESS. The difference is mainly due to the low degradation rate constant of sedimentary terrOC, likely caused by a combination of factors: (i) the lower availability of oxygen in the sediments compared to fully oxygenated waters, (ii) the stabilizing role of terrOC-mineral associations, and (iii) the higher proportion of material that is intrinsically recalcitrant due to its chemical/molecular structure in sediments. Sequestration of permafrost-released terrOC in shelf sediments may thereby attenuate the otherwise expected permafrost carbon-climate feedback. Plain language summary Frozen soils in the Arctic contain large amounts of old organic matter. With ongoing climate change this previously freeze-locked carbon storage becomes vulnerable to transport and decay. Upon delivery to the shallow nearshore seas, it may either be directly degraded to carbon dioxide or methane and thereby fuel further warming or get buried and stored in sediments on the sea floor. Our understanding of the fate of carbon released from permafrost soils is increasing, yet uncertainties remain regarding its degradation in the sediment. Here we constrain how much land-derived organic carbon is deposited in the top layer of the sediment (the part that is prone to transport and exposed to oxygen-stimulated degradation) in the Laptev and East Siberian Seas. We find that more than half of this stock likely resists degradation, while the rest decays relatively slowly. Therefore, the amount of carbon released annually from degradation in surface sediments is much smaller than what was found to be emitted from overlying waters in earlier studies. We suspect that this difference is caused by a combination of mechanisms hindering degradation in sediments and thus conclude that the burial of land-derived carbon may help to dampen the climate impact of thawing permafrost.
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4.
  • Burdett, Heidi L., et al. (författare)
  • Coralline algae as a globally significant pool of marine dimethylated sulfur
  • 2015
  • Ingår i: Global Biogeochemical Cycles. - : Blackwell Publishing. - 0886-6236 .- 1944-9224. ; 29:10, s. 1845-1853
  • Tidskriftsartikel (refereegranskat)abstract
    • Marine algae are key sources of the biogenic sulfur compound dimethylsulphoniopropionate (DMSP), a vital component of the marine sulfur cycle. Autotrophic ecosystem engineers such as red coralline algae support highly diverse and biogeochemically active ecosystems and are known to be high DMSP producers, but their importance in the global marine sulfur cycle has not yet been appreciated. Using a global sampling approach, we show that red coralline algae are a globally significant pool of DMSP in the oceans, estimated to be similar to 110x10(12) moles worldwide during the summer months. Latitude was a major driver of observed regional-scale variations, with peaks in polar and tropical climate regimes, reflecting the varied cellular functions for DMSP (e.g., as a cryoprotectant and antioxidant). A temperate coralline algal bed was investigated in more detail to also identify local-scale temporal variations. Here, water column DMSP was driven by water temperature, and to a lesser extent, cloud cover; two factors which are also vital in controlling coralline algal growth. This study demonstrates that coralline algae harbor a large pool of dimethylated sulfur, thereby playing a significant role in both the sulfur and carbon marine biogeochemical cycles. However, coralline algal habitats are severely threatened by projected climate change; a loss of this habitat may thus detrimentally impact oceanic sulfur and carbon biogeochemical cycling.
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5.
  • Carstensen, Jacob, et al. (författare)
  • Long-Term and Seasonal Trends in Estuarine and Coastal Carbonate Systems
  • 2018
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 32:3, s. 497-513
  • Tidskriftsartikel (refereegranskat)abstract
    • Coastal pH and total alkalinity are regulated by a diverse range of local processes superimposed on global trends of warming and ocean acidification, yet few studies have investigated the relative importance of different processes for coastal acidification. We describe long-term (1972-2016) and seasonal trends in the carbonate system of three Danish coastal systems demonstrating that hydrological modification, changes in nutrient inputs from land, and presence/absence of calcifiers can drastically alter carbonate chemistry. Total alkalinity was mainly governed by conservative mixing of freshwater (0.73-5.17mmolkg(-1)) with outer boundary concentrations (similar to 2-2.4mmolkg(-1)), modulated seasonally and spatially (similar to 0.1-0.2mmolkg(-1)) by calcifiers. Nitrate assimilation by primary production, denitrification, and sulfate reduction increased total alkalinity by almost 0.6mmolkg(-1) in the most eutrophic system during a period without calcifiers. Trends in pH ranged from -0.0088year(-1) to 0.021year(-1), the more extreme of these mainly driven by salinity changes in a sluice-controlled lagoon. Temperature increased 0.05 degrees Cyr(-1) across all three systems, which directly accounted for a pH decrease of 0.0008year(-1). Accounting for mixing, salinity, and temperature effects on dissociation and solubility constants, the resulting pH decline (0.0040year(-1)) was about twice the ocean trend, emphasizing the effect of nutrient management on primary production and coastal acidification. Coastal pCO(2) increased similar to 4 times more rapidly than ocean rates, enhancing CO2 emissions to the atmosphere. Indeed, coastal systems undergo more drastic changes than the ocean and coastal acidification trends are substantially enhanced from nutrient reductions to address coastal eutrophication.
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6.
  • Catalán, Núria, 1985-, et al. (författare)
  • Behind the Scenes : mechanisms Regulating Climatic Patterns of Dissolved Organic Carbon Uptake in Headwater Streams
  • 2018
  • Ingår i: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236 .- 1944-9224. ; 32:10, s. 1528-1541
  • Tidskriftsartikel (refereegranskat)abstract
    • Large variability in dissolved organic carbon (DOC) uptake rates has been reported for headwater streams, but the causes of this variability are still not well understood. Here we assessed acetate uptake rates across 11 European streams comprising different ecoregions by using whole-reach pulse acetate additions. We evaluated the main climatic and biogeochemical drivers of acetate uptake during two seasonal periods. Our results show a minor influence of sampling periods but a strong effect of climate and dissolved organic matter (DOM) composition on acetate uptake. In particular, mean annual precipitation explained half of the variability of the acetate uptake velocities (Vf(Acetate)) across streams. Temperate streams presented the lowest Vf(Acetate), together with humic-like DOM and the highest stream respiration rates. In contrast, higher Vf(Acetate) were found in semiarid streams, with protein-like DOM, indicating a dominance of reactive, labile compounds. This, together with lower stream respiration rates and molar ratios of DOC to nitrate, suggests a strong C limitation in semiarid streams, likely due to reduced inputs from the catchment. Overall, this study highlights the interplay of climate and DOM composition and its relevance to understand the biogeochemical mechanisms controlling DOC uptake in streams. Plain Language Summary Headwater streams receive and degrade organic carbon and nutrients from the surrounding catchments. That degradation can be assessed by measuring the uptake of simple compounds of carbon or nitrogen such as acetate or nitrate. Here we determine the variability in acetate and nitrate uptake rates across headwater streams and elucidate the mechanisms behind that variability. The balance between nutrients, the composition of the organic materials present in the streams, and the climatic background is at interplay.
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7.
  • Choi, J. H., et al. (författare)
  • Influence of Biogenic Organics on the Chemical Composition of Arctic Aerosols
  • 2019
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 33:10, s. 1238-1250
  • Tidskriftsartikel (refereegranskat)abstract
    • We use an ultrahigh-resolution 15-T Fourier transform ion cyclotron resonance mass spectrometer to elucidate the compositional changes in Arctic organic aerosols collected at Ny-angstrom lesund, Svalbard, in May 2015. The Fourier transform ion cyclotron resonance mass spectrometer analysis of airborne organic matter provided information on the molecular compositions of aerosol particles collected during the Arctic spring period. The air mass transport history, combined with satellite-derived geographical information and chlorophyll concentration data, revealed that the molecular compositions of organic aerosols drastically differed depending on the origin of the potential source region. The protein and lignin compound populations contributed more than 70% of the total intensity of assigned molecules when the air masses mainly passed over the ocean region. Interestingly, the intensity of microbe-derived organics (protein and carbohydrate compounds) was positively correlated with the air mass exposure to phytoplankton biomass proxied as chlorophyll. Furthermore, the intensities of lignin and unsaturated hydrocarbon compounds, typically derived from terrestrial vegetation, increased with an increase in the advection time of the air mass over the ocean domain. These results suggest that the accumulation of dissolved biogenic organics in the Arctic Ocean possibly derived from both phytoplankton and terrestrial vegetation could significantly influence the chemical properties of Arctic organic aerosols during a productive spring period. The interpretation of molecular changes in organic aerosols using an ultrahigh-resolution mass spectrometer could provide deep insight for understanding organic aerosols in the atmosphere over the Arctic and the relationship of organic aerosols with biogeochemical processes in terms of aerosol formation and environmental changes.
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8.
  • Fatichi, Simone, et al. (författare)
  • A Mechanistic Model of Microbially Mediated Soil Biogeochemical Processes : A Reality Check
  • 2019
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 33:6, s. 620-648
  • Tidskriftsartikel (refereegranskat)abstract
    • Present gaps in the representation of key soil biogeochemical processes such as the partitioning of soil organic carbon among functional components, microbial biomass and diversity, and the coupling of carbon and nutrient cycles present a challenge to improving the reliability of projected soil carbon dynamics. We introduce a new soil biogeochemistry module linked with a well-tested terrestrial biosphere model T&C. The module explicitly distinguishes functional soil organic carbon components. Extracellular enzymes and microbial pools are differentiated based on the functional roles of bacteria, saprotrophic, and mycorrhizal fungi. Soil macrofauna is also represented. The model resolves the cycles of nitrogen, phosphorus, and potassium. Model simulations for 20 sites compared favorably with global patterns of litter and soil stoichiometry, microbial and macrofaunal biomass relations with soil organic carbon, soil respiration, and nutrient mineralization rates. Long-term responses to bare fallow and nitrogen addition experiments were also in agreement with observations. Some discrepancies between predictions and observations are appreciable in the response to litter manipulation. Upon successful model reproduction of observed general trends, we assessed patterns associated with the carbon cycle that were challenging to address empirically. Despite large site-to-site variability, fine root, fungal, bacteria, and macrofaunal respiration account for 33%, 40%, 24%, and 3% on average of total belowground respiration, respectively. Simulated root exudation and carbon export to mycorrhizal fungi represent on average about 13% of plant net primary productivity. These results offer mechanistic and general estimates of microbial biomass and its contribution to respiration fluxes and to soil organic matter dynamics.
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9.
  • Feng, Xiaojuan, et al. (författare)
  • Multimolecular tracers of terrestrial carbon transfer across the pan-Arctic : C-14 characteristics of sedimentary carbon components and their environmental controls
  • 2015
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 29:11, s. 1855-1873
  • Tidskriftsartikel (refereegranskat)abstract
    • Distinguishing the sources, ages, and fate of various terrestrial organic carbon (OC) pools mobilized from heterogeneous Arctic landscapes is key to assessing climatic impacts on the fluvial release of carbon from permafrost. Through molecular C-14 measurements, including novel analyses of suberin- and/or cutin-derived diacids (DAs) and hydroxy fatty acids (FAs), we compared the radiocarbon characteristics of a comprehensive suite of terrestrial markers (including plant wax lipids, cutin, suberin, lignin, and hydroxy phenols) in the sedimentary particles from nine major arctic and subarctic rivers in order to establish a benchmark assessment of the mobilization patterns of terrestrial OC pools across the pan-Arctic. Terrestrial lipids, including suberin-derived longer-chain DAs (C-24,C-26,C-28), plant wax FAs (C(24,26,2)8), and n-alkanes (C-27,C-29,C-31), incorporated significant inputs of aged carbon, presumably from deeper soil horizons. Mobilization and translocation of these old terrestrial carbon components was dependent on nonlinear processes associated with permafrost distributions. By contrast, shorter-chain (C-16,C-18) DAs and lignin phenols (as well as hydroxy phenols in rivers outside eastern Eurasian Arctic) were much more enriched in C-14, suggesting incorporation of relatively young carbon supplied by runoff processes from recent vegetation debris and surface layers. Furthermore, the radiocarbon content of terrestrial markers is heavily influenced by specific OC sources and degradation status. Overall, multitracer molecular C-14 analysis sheds new light on the mobilization of terrestrial OC from arctic watersheds. Our findings of distinct ages for various terrestrial carbon components may aid in elucidating fate of different terrestrial OC pools in the face of increasing arctic permafrost thaw.
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10.
  • Fetzel, T., et al. (författare)
  • Quantification of uncertainties in global grazing systems assessment
  • 2017
  • Ingår i: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236 .- 1944-9224. ; 31:7, s. 1089-1102
  • Tidskriftsartikel (refereegranskat)abstract
    • Livestock systems play a key role in global sustainability challenges like food security and climate change, yet many unknowns and large uncertainties prevail. We present a systematic, spatially explicit assessment of uncertainties related to grazing intensity (GI), a key metric for assessing ecological impacts of grazing, by combining existing data sets on (a) grazing feed intake, (b) the spatial distribution of livestock, (c) the extent of grazing land, and (d) its net primary productivity (NPP). An analysis of the resulting 96 maps implies that on average 15% of the grazing land NPP is consumed by livestock. GI is low in most of the world's grazing lands, but hotspots of very high GI prevail in 1% of the total grazing area. The agreement between GI maps is good on one fifth of the world's grazing area, while on the remainder, it is low to very low. Largest uncertainties are found in global drylands and where grazing land bears trees (e.g., the Amazon basin or the Taiga belt). In some regions like India or Western Europe, massive uncertainties even result in GI > 100% estimates. Our sensitivity analysis indicates that the input data for NPP, animal distribution, and grazing area contribute about equally to the total variability in GI maps, while grazing feed intake is a less critical variable. We argue that a general improvement in quality of the available global level data sets is a precondition for improving the understanding of the role of livestock systems in the context of global environmental change or food security. Plain Language Summary Livestock systems play a key role in global sustainability challenges like food security and climate change, yet many unknowns and large uncertainties prevail. We present a systematic assessment of uncertainties related to the intensity of grazing, a key metric for assessing ecological impacts of grazing. We combine existing data sets on (a) grazing feed intake, (b) the spatial distribution of livestock, (c) the extent of grazing land, and (d) the biomass available for grazing. Our results show that most grasslands are used with low intensity, but hotspots of high intensity prevail on 1% of the global grazing area, mainly located in drylands and where grazing land bears trees. The agreement between all maps is good on one fifth of the global grazing area, while on the remainder, it is low to very low. Our sensitivity analysis indicates that the input data for available biomass, animal distribution, and grazing area contribute about equally to the total variability of our maps, while grazing feed intake is a less critical variable. We argue that a general improvement in quality of the available data sets is a precondition for improving the understanding of livestock systems in the context of global environmental change or food security.
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11.
  • Fisher, Rebecca E., et al. (författare)
  • Measurement of the C-13 isotopic signature of methane emissions from northern European wetlands
  • 2017
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 31:3, s. 605-623
  • Tidskriftsartikel (refereegranskat)abstract
    • Isotopic data provide powerful constraints on regional and global methane emissions and their source profiles. However, inverse modeling of spatially resolved methane flux is currently constrained by a lack of information on the variability of source isotopic signatures. In this study, isotopic signatures of emissions in the Fennoscandian Arctic have been determined in chambers over wetland, in the air 0.3 to 3m above the wetland surface and by aircraft sampling from 100m above wetlands up to the stratosphere. Overall, the methane flux to atmosphere has a coherent delta C-13 isotopic signature of -71 +/- 1%, measured in situ on the ground in wetlands. This is in close agreement with delta C-13 isotopic signatures of local and regional methane increments measured by aircraft campaigns flying through air masses containing elevated methane mole fractions. In contrast, results from wetlands in Canadian boreal forest farther south gave isotopic signatures of -67 +/- 1%. Wetland emissions dominate the local methane source measured over the European Arctic in summer. Chamber measurements demonstrate a highly variable methane flux and isotopic signature, but the results from air sampling within wetland areas show that emissions mix rapidly immediately above the wetland surface and methane emissions reaching the wider atmosphere do indeed have strongly coherent C isotope signatures. The study suggests that for boreal wetlands (>60 degrees N) global and regional modeling can use an isotopic signature of -71 parts per thousand to apportion sources more accurately, but there is much need for further measurements over other wetlands regions to verify this.
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12.
  • Fransner, Filippa, et al. (författare)
  • Tracing terrestrial DOC in the Baltic Sea - a 3-D model study
  • 2016
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 30:2, s. 134-148
  • Tidskriftsartikel (refereegranskat)abstract
    • The fate of terrestrial organic matter brought to the coastal seas by rivers, and its role in the global carbon cycle, are still not very well known. Here the degradation rate of terrestrial dissolved organic carbon (DOCter) is studied in the Baltic Sea, a subarctic semi-enclosed sea, by releasing it as a tracer in a 3-D circulation model and applying linear decay constants. A good agreement with available observational data is obtained by parameterizing the degradation in two rather different ways; one by applying a decay time on the order of 10 years to the whole pool of DOCter, and one by dividing the DOCter into one refractory pool and one pool subject to a decay time on the order of 1 year. The choice of parameterization has a significant effect on where in the Baltic Sea the removal takes place, which can be of importance when modeling the full carbon cycle and the CO2 exchange with the atmosphere. In both cases the biogeochemical decay operates on time scales less than the water residence time. Therefore only a minor fraction of the DOCter reaches the North Sea, whereas approximately 80% is removed by internal sinks within the Baltic Sea. This further implies that DOCter mineralization is an important link in land-sea-atmosphere cycling of carbon in coastal- and shelf seas that are heavily influenced by riverine DOC.
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13.
  • Gottselig, N., et al. (författare)
  • Elemental Composition of Natural Nanoparticles and Fine Colloids in European Forest Stream Waters and Their Role as Phosphorus Carriers
  • 2017
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 31:10, s. 1592-1607
  • Tidskriftsartikel (refereegranskat)abstract
    • Biogeochemical cycling of elements largely occurs in dissolved state, but many elements may also be bound to natural nanoparticles (NNP, 1-100 nm) and fine colloids (100-450 nm). We examined the hypothesis that the size and composition of stream water NNP and colloids vary systematically across Europe. To test this hypothesis, 96 stream water samples were simultaneously collected in 26 forested headwater catchments along two transects across Europe. Three size fractions (~1-20 nm, >20-60 nm, and >60 nm) of NNP and fine colloids were identified with Field Flow Fractionation coupled to inductively coupled plasma mass spectrometry and an organic carbon detector. The results showed that NNP and fine colloids constituted between 2 ± 5% (Si) and 53 ± 21% (Fe; mean ± SD) of total element concentrations, indicating a substantial contribution of particles to element transport in these European streams, especially for P and Fe. The particulate contents of Fe, Al, and organic C were correlated to their total element concentrations, but those of particulate Si, Mn, P, and Ca were not. The fine colloidal fractions >60 nm were dominated by clay minerals across all sites. The resulting element patterns of NNP <60 nm changed from North to South Europe from Fe- to Ca-dominated particles, along with associated changes in acidity, forest type, and dominant lithology.
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14.
  • Humborg, Christoph, et al. (författare)
  • Sea-air exchange patterns along the central and outer East Siberian Arctic Shelf as inferred from continuous CO2, stable isotope, and bulk chemistry measurements
  • 2017
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 31:7, s. 1173-1191
  • Tidskriftsartikel (refereegranskat)abstract
    • This large-scale quasi-synoptic study gives a comprehensive picture of sea-air CO 2 fluxes during the melt season in the central and outer Laptev Sea (LS) and East Siberian Sea (ESS). During a 7 week cruise we compiled a continuous record of both surface water and air CO 2 concentrations, in total 76,892 measurements. Overall, the central and outer parts of the ESAS constituted a sink for CO 2 , and we estimate a median uptake of 9.4 g C m -2 yr -1 or 6.6 Tg C yr -1 . Our results suggest that while the ESS and shelf break waters adjacent to the LS and ESS are net autotrophic systems, the LS is a net heterotrophic system. CO 2 sea-air fluxes for the LS were 4.7 g C m -2 yr -1 , and for the ESS we estimate an uptake of 7.2 g C m -2 yr -1 . Isotopic composition of dissolved inorganic carbon (δ 13 C DIC and δ 13 C CO2 ) in the water column indicates that the LS is depleted in δ 13 C DIC compared to the Arctic Ocean (ArcO) and ESS with an offset of 0.5‰ which can be explained by mixing of δ 13 C DIC -depleted riverine waters and 4.0 Tg yr -1 respiration of OC ter ; only a minor part (0.72 Tg yr -1 ) of this respired OC ter is exchanged with the atmosphere. Property-mixing diagrams of total organic carbon and isotope ratio (δ 13 C SPE-DOC ) versus dissolved organic carbon (DOC) concentration diagram indicate conservative and nonconservative mixing in the LS and ESS, respectively. We suggest land-derived particulate organic carbon from coastal erosion as an additional significant source for the depleted δ 13 C DIC .
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15.
  • Isles, Peter D. F., et al. (författare)
  • Recent Synchronous Declines in DIN:TP in Swedish Lakes
  • 2018
  • Ingår i: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236 .- 1944-9224. ; 32:2, s. 208-225
  • Tidskriftsartikel (refereegranskat)abstract
    • Declining atmospheric nitrogen (N) deposition in northern Europe and parts of North America, coupled with ongoing changes in climate, has the potential to alter the nutrient limitation status of freshwater ecosystems. In this study we compared time series data of atmospheric N deposition, air temperature, and precipitation with corresponding estimates of dissolved inorganic nitrogen (DIN), total phosphorus (TP), DIN: TP, and total organic carbon from 78 headwater streams and 95 nutrient-poor lakes in Sweden from 1998 to 2013 to assess trends in, and potential drivers of, lake N:P ratios. We found that trends in nutrients were variable at the scale of individual lakes but were highly synchronous at the regional scale, suggesting underlying control by broad-scale environmental drivers mediated by site-specific characteristics. Widespread declines in lake DIN throughout Sweden were correlated with declines in atmospheric N deposition, particularly in northern areas. TP did not have strong directional trends, but interannual variability was synchronous at regional scales, implying that broad-scale climate drivers were affecting these trends. Overall, we observed a significant decline in DIN:TP throughout Sweden over the monitoring period. At the beginning of the study period, 32% of lakes were N limited and 45% colimited by N and P. Proportions increased to 63% of lakes N limited and 20% colimited by N and P at the end of the study period. These results suggest that N limitation is likely to become more widespread in subarctic and boreal areas of Europe in the future if recent trends continue. Plain Language Summary This article examines the way in which changes in the amount of nitrogen from the atmosphere being delivered to lakes (as a result of fossil fuel combustion) are interacting with global climate change to affect nutrient availability in Swedish lakes. Nitrogen can act as fertilizer in lakes, supporting increased growth of algae and aquatic plants. The amount of nitrogen relative to other important elements such as phosphorus can help to determine which groups of plants and algae dominate lake ecosystems, as well as how much living biomass lakes can sustain. We find that declines in atmospheric deposition of nitrogen, which have resulted from the adoption of policies controlling emissions from fossil fuel combustion, have caused declines in nitrogen concentrations in lakes throughout Sweden. This has changed the balance of nitrogen and phosphorus, which may result in changes to the structure of lake biological communities. At the same time, variability in climate also has subtle but widespread affects on lake nutrient concentrations, suggesting that the availability of nutrients in lakes at northern latitudes is likely to change in the future as the climate warms.
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16.
  • Karlsson, Emma, et al. (författare)
  • Different sources and degradation state of dissolved, particulate, and sedimentary organic matter along the Eurasian Arctic coastal margin
  • 2016
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 30:6, s. 898-919
  • Tidskriftsartikel (refereegranskat)abstract
    • Thawing Arctic permafrost causes massive fluvial and erosional releases of dissolved and particulate organic carbon (DOC and POC) to coastal waters. Here we investigate how different sources and degradation of remobilized terrestrial carbon may affect large-scale carbon cycling, by comparing molecular and dual-isotope composition of waterborne high molecular weight DOC (>1kD, aka colloidal OC), POC, and sedimentary OC (SOC) across the East Siberian Arctic Shelves. Lignin phenol fingerprints demonstrate a longitudinal trend in relative contribution of terrestrial sources to coastal OC. Wax lipids and cutins were not detected in colloidal organic carbon (COC), in contrast to POC and SOC, suggesting that different terrestrial carbon pools partition into different aquatic carrier phases. The C-14 signal suggests overwhelmingly contemporary sources for COC, while POC and SOC are dominated by old C from Ice Complex Deposit (ICD) permafrost. Monte Carlo source apportionment (C-13, C-14) constrained that COC was dominated by terrestrial OC from topsoil permafrost (65%) and marine plankton (25%) with smaller contribution ICD and other older permafrost stocks (9%). This distribution is likely a result of inherent compositional matrix differences, possibly driven by organomineral associations. Modern OC found suspended in the surface water may be more exposed to degradation, in contrast to older OC that preferentially settles to the seafloor where it may be degraded on a longer timescale. The different sources which partition into DOC, POC, and SOC appear to have vastly different fates along the Eurasian Arctic coastal margin and may possibly respond on different timescales to climate change.
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17.
  • Klaus, Marcus, et al. (författare)
  • Evaluations of Climate and Land Management Effects on Lake Carbon Cycling Need to Account Temporal Variability in CO2 Concentration
  • 2019
  • Ingår i: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236 .- 1944-9224. ; 33:3, s. 243-265
  • Tidskriftsartikel (refereegranskat)abstract
    • Carbon dioxide (CO2) concentrations in lakes vary strongly over time. This variability is rarely captured by environmental monitoring but is crucial for accurately assessing the magnitude of lake CO2 emissions. However, it is unknown to what extent temporal variability needs to be captured to understand important drivers of lake carbon cycling such as climate and land management. We used environmental monitoring data of Swedish forest lakes collected in autumn (n = 439) and throughout the whole open water season (n = 22) from a wet and a dry year to assess temporal variability in effects of climate and forestry on CO2 concentrations across lakes. Effects differed depending on the season and year sampled. According to cross-lake comparisons based on autumn data, CO2 concentrations increased with annual mean air temperature (dry year) or catchment forest productivity (wet year) but were not related to colored dissolved organic matter concentrations. In contrast, open water-season averaged CO2 concentrations were similar across temperature and productivity gradients but increased with colored dissolved organic matter. These contradictions resulted from scale mismatches in input data, lead to weak explanatory power (R-2 = 9-32%), and were consistent across published data from 79 temperate, boreal, and arctic lakes. In a global survey of 144 published studies, we identified a trade-off between temporal and spatial coverage of CO2 sampling. This trade-off clearly determines which conclusions are drawn from landscape-scale CO(2 )assessments. Accurate evaluations of the effects of climate and land management require spatially and temporally representative data that can be provided by emerging sensor technologies and forms of collaborative sampling.
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18.
  • Kronberg, Rose-Marie, et al. (författare)
  • Forest harvest contribution to Boreal freshwater methyl mercury load
  • 2016
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 30:6, s. 825-843
  • Tidskriftsartikel (refereegranskat)abstract
    • Effects of Boreal forest harvest on mercury (Hg) and methyl mercury (MeHg) soil pools and export by stream runoff were quantified by comparing 10 reference watersheds (REFs) covered by >80year old Norway spruce (Picea abies Karst.) forests with 10 similar watersheds subjected to clear-cutting (CCs). While total Hg soil storage did not change, MeHg pools increased seven times (p=0.006) in the organic topsoil 2 years after clear-cutting. In undulating terrain, situated above the postglacial marine limit (ML) of the ancient Baltic Sea, the mass ratio between flux-weighted MeHg and dissolved organic carbon (MeHg/DOC) in stream runoff increased 1.8 times (p<0.004) as a consequence of forest harvest. When recalculated to 100% clear-cutting of the watershed, the annual MeHg stream export increased 3.8 times (p=0.047). Below the ML, where the terrain was flatter, neither the MeHg/DOC ratio nor the annual export of MeHg differed between REFs and CCs, likely because of the larger contribution of MeHg exported from peaty soils and small wetlands. The most robust measure, MeHg/DOC, was used to calculate MeHg loadings to Boreal headwaters. If the forest harvest effect lasts 10years, clear-cutting increases MeHg runoff by 12-20% in Sweden and 2% in the Boreal zone as a whole. In Sweden, having intensely managed forests, 37% and 56% of MeHg are exported from peatlands and forest soils, respectively, and forest clear-cutting is adding another 6.6%. In the Boreal zone as a whole peatlands and forests soils contribute with 53% and 46%, respectively, and clear-cutting is estimated to add another 1.0%. An expected rapid increase in Boreal forest harvest and disturbance urge for inclusion of land use effects in mercury biogeochemical cycling models at different scales.
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19.
  • Lauerwald, R., et al. (författare)
  • Natural lakes are a minor global source of N2O to the atmosphere
  • 2019
  • Ingår i: Global Biogeochemical Cycles. - : Wiley-Blackwell. - 0886-6236 .- 1944-9224. ; 33:12, s. 1564-1581
  • Tidskriftsartikel (refereegranskat)abstract
    • Natural lakes and reservoirs are important, yet not well constrained sources of greenhouse gasses to the atmosphere. In particular for N2O emissions, a huge variability is observed in the few, observation‐driven flux estimates that have been published so far. Recently, a process‐based, spatially explicit model has been used to estimate global N2O emissions from more than 6,000 reservoirs based on nitrogen (N) and phosphorous inflows and water residence time. Here, we extend the model to a dataset of 1.4 million standing water bodies comprising natural lakes and reservoirs. For validation, we normalized the simulated N2O emissions by the surface area of each water body and compared them against regional averages of N2O emission rates taken from the literature or estimated based on observed N2O concentrations. We estimate that natural lakes and reservoirs together emit 4.5±2.9 Gmol N2O‐N yr‐1 globally. Our global scale estimate falls in the far lower end of existing, observation‐driven estimates. Natural lakes contribute only about half of this flux, although they contribute 91% of the total surface area of standing water bodies. Hence, the mean N2O emission rates per surface area are substantially lower for natural lakes than for reservoirs with 0.8±0.5 mmol N m‐2yr‐1 vs. 9.6±6.0 mmol N m‐2yr‐1, respectively. This finding can be explained by on average lower external N inputs to natural lakes. We conclude that upscaling based estimates, which do not distinguish natural lakes from reservoirs, are prone to important biases.
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20.
  • Lundin, Lars (författare)
  • Quantifying Carbon and Nutrient Input From Litterfall in European Forests Using Field Observations and Modeling
  • 2018
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 32, s. 784-798
  • Tidskriftsartikel (refereegranskat)abstract
    • Litterfall is a major, yet poorly studied, process within forest ecosystems globally. It is important for carbon dynamics, edaphic communities, and maintaining site fertility. Reliable information on the carbon and nutrient input from litterfall, provided by litter traps, is relevant to a wide audience including policy makers and soil scientists. We used litterfall observations of 320 plots from the pan-European forest monitoring network of the International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests to quantify litterfall fluxes. Eight litterfall models were evaluated (four using climate information and four using biomass abundance). We scaled up our results to the total European forest area and quantified the contribution of litterfall to the forest carbon cycle using net primary production aggregated by bioregions (north, central, and south) and by forest types (conifers and broadleaves). The 1,604 analyzed annual litterfall observations indicated an average carbon input of 224gCm(-2)year(-1) (annual nutrient inputs 4.49gN, 0.32gP, and 1.05gKm(-2)), representing a substantial percentage of net primary production from 36% in north Europe to 32% in central Europe. The annual turnover of carbon and nutrient in broadleaf canopies was larger than for conifers. The evaluated models provide large-scale litterfall predictions with a bias less than 10%. Each year litterfall in European forests transfers 351TgC, 8.2TgN, 0.6TgP, and 1.9TgK to the forest floor. The performance of litterfall models may be improved by including foliage biomass and proxies for forest management.Plain Language Summary All plants shed parts of their biomass periodically. This litterfall is important for transferring carbon and nutrients from the canopy back to the soil. Every year in European forests about one third of carbon produced via photosynthesis falls to the forest floor as litterfall. Broadleaved trees shed more carbon and nutrients than conifers. Models can be used to quantify the average flux across Europe, but need improvement to be applicable at fine scales.
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21.
  • Martens, Jannik, et al. (författare)
  • Remobilization of Old Permafrost Carbon to Chukchi Sea Sediments During the End of the Last Deglaciation
  • 2019
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 33:1, s. 2-14
  • Tidskriftsartikel (refereegranskat)abstract
    • Climate warming is expected to destabilize permafrost carbon (PF-C) by thaw-erosion and deepening of the seasonally thawed active layer and thereby promote PF-C mineralization to CO2 and CH4. A similar PF-C remobilization might have contributed to the increase in atmospheric CO2 during deglacial warming after the last glacial maximum. Using carbon isotopes and terrestrial biomarkers (Delta C-14, delta C-13, and lignin phenols), this study quantifies deposition of terrestrial carbon originating from permafrost in sediments from the Chukchi Sea (core SWERUS-L2-4-PC1). The sediment core reconstructs remobilization of permafrost carbon during the late Allerod warm period starting at 13,000 cal years before present (BP), the Younger Dryas, and the early Holocene warming until 11,000 cal years BP and compares this period with the late Holocene, from 3,650 years BP until present. Dual-carbon-isotope-based source apportionment demonstrates that Ice Complex Deposit-ice- and carbon-rich permafrost from the late Pleistocene (also referred to as Yedoma)-was the dominant source of organic carbon (66 +/- 8%; mean +/- standard deviation) to sediments during the end of the deglaciation, with fluxes more than twice as high (8.0 +/- 4.6 g.m(-2).year(-1)) as in the late Holocene (3.1 +/- 1.0 g.m(-2).year(-1)). These results are consistent with late deglacial PF-C remobilization observed in a Laptev Sea record, yet in contrast with PF-C sources, which at that location were dominated by active layer material from the Lena River watershed. Release of dormant PF-C from erosion of coastal permafrost during the end of the last deglaciation indicates vulnerability of Ice Complex Deposit in response to future warming and sea level changes.
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22.
  • Mason, Robert P., et al. (författare)
  • The Global Marine Selenium Cycle : Insights From Measurements and Modeling
  • 2018
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 32:12, s. 1720-1737
  • Tidskriftsartikel (refereegranskat)abstract
    • Anthropogenic activities have increased the selenium (Se) concentration in the biosphere, but the overall impact on the ocean has not been examined. While Se is an essential nutrient for microorganisms, there is little information on the impact of biological processes on the concentration and speciation of Se in the ocean. Additionally, other factors controlling the distribution and concentration of Se species are poorly understood. Here we present data gathered in the subtropical Pacific Ocean during a cruise in 2011, and we used these field data and the literature, as well as laboratory photochemical experiments examining the stability and degradation of inorganic Se (both Se (IV) and Se (VI)) and dimethyl selenide, to further constrain the cycling of Se in the upper ocean. We also developed a multibox model for the biosphere to examine the impact of anthropogenic emissions on the concentration and distribution of Se in the ocean. The model concurs with the field data indicating that the Se concentration has increased in the upper ocean waters over the past 30 years. Our observational studies and model results suggest that Se (VI) is taken up by phytoplankton in the surface ocean, in contrast to the results of laboratory culture experiments. In conclusion, while anthropogenic inputs have markedly increased Se in the atmosphere (42%) and net deposition to the ocean (38%) and terrestrial landscape (41%), the impact on Se in the ocean is small (3% increase in the upper ocean). This minimal response reflects its long marine residence time. Plain Language Summary We measured selenium in water and particles during a cruise in the Pacific Ocean and use this data along with laboratory photochemcial experiments to examine the formation and degradation of the various Se forms in the upper ocean. We also developed a box model to examine how anthropogenic activities have changed the Se concentration throughout the ocean the potential future impacts and to highlight areas of Se biogeochemistry that need further study.
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23.
  • McCrackin, Michelle L., et al. (författare)
  • A Century of Legacy Phosphorus Dynamics in a Large Drainage Basin
  • 2018
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 32:7, s. 1107-1122
  • Tidskriftsartikel (refereegranskat)abstract
    • There is growing evidence that the release of phosphorus (P) from legacy stores can frustrate efforts to reduce P loading to surface water from sources such as agriculture and human sewage. Less is known, however, about the magnitude and residence times of these legacy pools. Here we constructed a budget of net anthropogenic P inputs to the Baltic Sea drainage basin and developed a three-parameter, two-box model to describe the movement of anthropogenic P though temporary (mobile) and long-term (stable) storage pools. Phosphorus entered the sea as direct coastal effluent discharge and via rapid transport and slow, legacy pathways. The model reproduced past waterborne P loads and suggested an similar to 30-year residence time in the mobile pool. Between 1900 and 2013, 17 and 27 Mt P has accumulated in the mobile and stable pools, respectively. Phosphorus inputs to the sea have halved since the 1980s due to improvements in coastal sewage treatment and reductions associated with the rapid transport pathway. After decades of accumulation, the system appears to have shifted to a depletion phase; absent further reductions in net anthropogenic P input, future waterborne loads could decrease. Presently, losses from the mobile pool contribute nearly half of P loads, suggesting that it will be difficult to achieve substantial near-term reductions. However, there is still potential to make progress toward eutrophication management goals by addressing rapid transport pathways, such as overland flow, as well as mobile stores, such as cropland with large soil-P reserves.
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24.
  • McGuire, A. David, et al. (författare)
  • Variability in the sensitivity among model simulations of permafrost and carbon dynamics in the permafrost region between 1960 and 2009
  • 2016
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 30:7, s. 1015-1037
  • Tidskriftsartikel (refereegranskat)abstract
    • A significant portion of the large amount of carbon (C) currently stored in soils of the permafrost region in the Northern Hemisphere has the potential to be emitted as the greenhouse gases CO2 and CH4 under a warmer climate. In this study we evaluated the variability in the sensitivity of permafrost and C in recent decades among land surface model simulations over the permafrost region between 1960 and 2009. The 15 model simulations all predict a loss of near-surface permafrost (within 3m) area over the region, but there are large differences in the magnitude of the simulated rates of loss among the models (0.2 to 58.8x10(3)km(2)yr(-1)). Sensitivity simulations indicated that changes in air temperature largely explained changes in permafrost area, although interactions among changes in other environmental variables also played a role. All of the models indicate that both vegetation and soil C storage together have increased by 156 to 954TgCyr(-1) between 1960 and 2009 over the permafrost region even though model analyses indicate that warming alone would decrease soil C storage. Increases in gross primary production (GPP) largely explain the simulated increases in vegetation and soil C. The sensitivity of GPP to increases in atmospheric CO2 was the dominant cause of increases in GPP across the models, but comparison of simulated GPP trends across the 1982-2009 period with that of a global GPP data set indicates that all of the models overestimate the trend in GPP. Disturbance also appears to be an important factor affecting C storage, as models that consider disturbance had lower increases in C storage than models that did not consider disturbance. To improve the modeling of C in the permafrost region, there is the need for the modeling community to standardize structural representation of permafrost and carbon dynamics among models that are used to evaluate the permafrost C feedback and for the modeling and observational communities to jointly develop data sets and methodologies to more effectively benchmark models.
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25.
  • Nydahl, Anna, et al. (författare)
  • No long-term trends in pCO2 despite increasing organic carbon concentrations in boreal lakes, streams and rivers
  • 2017
  • Ingår i: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236 .- 1944-9224. ; 31:6, s. 985-995
  • Tidskriftsartikel (refereegranskat)abstract
    • Concentrations of dissolved organic carbon (DOC) from terrestrial sources have been increasing in freshwaters across large parts of the boreal region. According to results from large-scale field and detailed laboratory studies, such a DOC increase could potentially stimulate carbon dioxide (CO2) production, subsequently increasing the partial pressure of CO2 (pCO2) in freshwaters. However, the response of pCO2 to the presently observed long-term increase in DOC in freshwaters is still unknown. Here we tested whether the commonly found spatial DOC-pCO2 relationship is also valid on a temporal scale. Analyzing time series of water chemical data from 71 lakes, 30 streams, and 4 river mouths distributed across all of Sweden over a 17 year period, we observed significant DOC concentration increases in 39 lakes, 15 streams, and 4 river mouths. Significant pCO2 increases were, however, only observed in six of these 58 waters, indicating that long-term DOC increases in Swedish waters are disconnected from temporal pCO2 trends. We suggest that the uncoupling of trends in DOC concentration and pCO2 are a result of increased surface water runoff. When surface water runoff increases, there is likely less CO2 relative to DOC imported from soils into waters due to a changed balance between surface and groundwater flow. Additionally, increased surface water runoff causes faster water flushing through the landscape giving less time for in situ CO2 production in freshwaters. We conclude that pCO2 is presently not following DOC concentration trends, which has important implications for modeling future CO2 emissions from boreal waters.
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26.
  • Röhr, Maria Emilia, et al. (författare)
  • Blue Carbon Storage Capacity of Temperate Eelgrass (Zostera marina) Meadows
  • 2018
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 32:10, s. 1457-1475
  • Tidskriftsartikel (refereegranskat)abstract
    • Despite the importance of coastal ecosystems for the global carbon budgets, knowledge of their carbon storage capacity and the factors driving variability in storage capacity is still limited. Here we provide an estimate on the magnitude and variability of carbon stocks within a widely distributed marine foundation species throughout its distribution area in temperate Northern Hemisphere. We sampled 54 eelgrass (Zostera marina) meadows, spread across eight ocean margins and 36° of latitude, to determine abiotic and biotic factors influencing organic carbon (Corg) stocks in Zostera marina sediments. The Corg stocks (integrated over 25-cm depth) showed a large variability and ranged from 318 to 26,523gC/m2 with an average of 2,721gC/m2. The projected Corg stocks obtained by extrapolating over the top 1m of sediment ranged between 23.1 and 351.7MgC/ha, which is in line with estimates for other seagrasses and other blue carbon ecosystems. Most of the variation in Corg stocks was explained by five environmental variables (sediment mud content, dry density and degree of sorting, and salinity and water depth), while plant attributes such as biomass and shoot density were less important to Corg stocks. Carbon isotopic signatures indicated that at most sites <50% of the sediment carbon is derived from seagrass, which is lower than reported previously for seagrass meadows. The high spatial carbon storage variability urges caution in extrapolating carbon storage capacity between geographical areas as well as within and between seagrass species.
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27.
  • Salvadó, Joan A., et al. (författare)
  • Release of Black Carbon From Thawing Permafrost Estimated by Sequestration Fluxes in the East Siberian Arctic Shelf Recipient
  • 2017
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 31:10, s. 1501-1515
  • Tidskriftsartikel (refereegranskat)abstract
    • Black carbon (BC) plays an important role in carbon burial in marine sediments globally. Yet the sequestration of BC in the Arctic Ocean is poorly understood. Here we assess the concentrations, fluxes, and sources of soot BC (SBC)-the most refractory component of BC-in sediments from the East Siberian Arctic Shelf (ESAS), the World's largest shelf sea system. SBC concentrations in the contemporary shelf sediments range from 0.1 to 2.1 mg g(-1) dw, corresponding to 2-12% of total organic carbon. The Pb-210-derived fluxes of SBC (0.42-11 g m(-2) yr(-1)) are higher or in the same range as fluxes reported for marine surface sediments closer to anthropogenic emissions. The total burial flux of SBC in the ESAS (similar to 4,000 Gg yr(-1)) illustrates the great importance of this Arctic shelf in marine sequestration of SBC. The radiocarbon signal of the SBC shows more depleted yet also more uniform signatures (-721 to -896%; average of -774 +/- 62%) than of the non-SBC pool (-304 to -728%; average of -491 +/- 163%), suggesting that SBC is coming from an, on average, 5,900 +/- 300 years older and more specific source than the non-SBC pool. We estimate that the atmospheric BC input to the ESAS is negligible (similar to 0.6% of the SBC burial flux). Statistical source apportionment modeling suggests that the ESAS sedimentary SBC is remobilized by thawing of two permafrost carbon (PF/C) systems: surface soil permafrost (topsoil/PF; 25 +/- 8%) and Pleistocene ice complex deposits (ICD/PF; 75 +/- 8%). The SBC contribution to the total mobilized permafrost carbon (PF/C) increases with increasing distance from the coast (from 5 to 14%), indicating that the SBC is more recalcitrant than other forms of translocated PF/C. These results elucidate for the first time the key role of permafrost thaw in the transport of SBC to the Arctic Ocean. With ongoing global warming, these findings have implications for the biogeochemical carbon cycle, increasing the size of this refractory carbon pool in the Arctic Ocean.
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28.
  • Soerensen, Anne L., et al. (författare)
  • A mass budget for mercury and methylmercury in the Arctic Ocean
  • 2016
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 30:4, s. 560-575
  • Tidskriftsartikel (refereegranskat)abstract
    • Elevated biological concentrations of methylmercury (MeHg), a bioaccumulative neurotoxin, are observed throughout the Arctic Ocean, but major sources and degradation pathways in seawater are not well understood. We develop a mass budget for mercury species in the Arctic Ocean based on available data since 2004 and discuss implications and uncertainties. Our calculations show that high total mercury (Hg) in Arctic seawater relative to other basins reflect large freshwater inputs and sea ice cover that inhibits losses through evasion. We find that most net MeHg production (20Mga(-1)) occurs in the subsurface ocean (20-200m). There it is converted to dimethylmercury (Me2Hg: 17Mga(-1)), which diffuses to the polar mixed layer and evades to the atmosphere (14Mga(-1)). Me2Hg has a short atmospheric lifetime and rapidly degrades back to MeHg. We postulate that most evaded Me2Hg is redeposited as MeHg and that atmospheric deposition is the largest net MeHg source (8Mga(-1)) to the biologically productive surface ocean. MeHg concentrations in Arctic Ocean seawater are elevated compared to lower latitudes. Riverine MeHg inputs account for approximately 15% of inputs to the surface ocean (2.5Mga(-1)) but greater importance in the future is likely given increasing freshwater discharges and permafrost melt. This may offset potential declines driven by increasing evasion from ice-free surface waters. Geochemical model simulations illustrate that for the most biologically relevant regions of the ocean, regulatory actions that decrease Hg inputs have the capacity to rapidly affect aquatic Hg concentrations.
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29.
  • Soerensen, Anne L., et al. (författare)
  • Deciphering the Role of Water Column Redoxclines on Methylmercury Cycling Using Speciation Modeling and Observations From the Baltic Sea
  • 2018
  • Ingår i: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236 .- 1944-9224. ; 32:10, s. 1498-1513
  • Tidskriftsartikel (refereegranskat)abstract
    • 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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30.
  • Teutschbein, Claudia, 1985-, et al. (författare)
  • Future Riverine Inorganic Nitrogen Load to the Baltic Sea From Sweden : An Ensemble Approach to Assessing Climate Change Effects
  • 2017
  • Ingår i: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236 .- 1944-9224. ; 31:11, s. 1674-1701
  • Tidskriftsartikel (refereegranskat)abstract
    • The dramatic increase of bioreactive nitrogen entering the Earth’s ecosystems continues toattract growing attention. Increasingly large quantities of inorganic nitrogen are flushed from land towater, accelerating freshwater, and marine eutrophication. Multiple, interacting, and potentiallycountervailing drivers control the future hydrologic export of inorganic nitrogen. In this paper, we attempt toresolve these land-water interactions across boreal/hemiboreal Sweden in the face of a changing climatewith help of a versatile modeling framework to maximize the information value of existing measurementtime series. We combined 6,962 spatially distributed water chemistry observations spread over 31 years withdaily streamflow and air temperature records. An ensemble of climate model projections, hydrologicalsimulations, and several parameter parsimonious regression models was employed to project future riverineinorganic nitrogen dynamics across Sweden. The median predicted increase in total inorganic nitrogenexport from Sweden (2061–2090) due to climate change was 14% (interquartile range 0–29%), based on theensemble of 7,500 different predictions for each study site. The overall export as well as the seasonal patternof inorganic nitrogen loads in a future climate are mostly influenced by longer growing seasons and morewinter flow, which offset the expected decline in spring flood. The predicted increase in inorganic nitrogenloading due to climate change means that the political efforts for reducing anthropogenic nitrogen inputsneed to be increased if ambitions for reducing the eutrophication of the Baltic Sea are to be achieved.
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31.
  • von Schiller, D., et al. (författare)
  • Sediment Respiration Pulses in Intermittent Rivers and Ephemeral Streams
  • 2019
  • Ingår i: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236 .- 1944-9224. ; 33:10, s. 1251-1263
  • Tidskriftsartikel (refereegranskat)abstract
    • Intermittent rivers and ephemeral streams (IRES) may represent over half the global stream network, but their contribution to respiration and carbon dioxide (CO2) emissions is largely undetermined. In particular, little is known about the variability and drivers of respiration in IRES sediments upon rewetting, which could result in large pulses of CO2. We present a global study examining sediments from 200 dry IRES reaches spanning multiple biomes. Results from standardized assays show that mean respiration increased 32-fold to 66-fold upon sediment rewetting. Structural equation modeling indicates that this response was driven by sediment texture and organic matter quantity and quality, which, in turn, were influenced by climate, land use, and riparian plant cover. Our estimates suggest that respiration pulses resulting from rewetting of IRES sediments could contribute significantly to annual CO2 emissions from the global stream network, with a single respiration pulse potentially increasing emission by 0.2-0.7%. As the spatial and temporal extent of IRES increases globally, our results highlight the importance of recognizing the influence of wetting-drying cycles on respiration and CO2 emissions in stream networks.
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32.
  • Wild, Birgit, et al. (författare)
  • Microbial nitrogen dynamics in organic and mineral soil horizons along a latitudinal transect in western Siberia
  • 2015
  • Ingår i: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236 .- 1944-9224. ; 29:5, s. 567-582
  • Tidskriftsartikel (refereegranskat)abstract
    • Soil N availability is constrained by the breakdown of N-containing polymers such as proteins to oligopeptides and amino acids that can be taken up by plants and microorganisms. Excess N is released from microbial cells as ammonium (N mineralization), which in turn can serve as substrate for nitrification. According to stoichiometric theory, N mineralization and nitrification are expected to increase in relation to protein depolymerization with decreasing N limitation, and thus from higher to lower latitudes and from topsoils to subsoils. To test these hypotheses, we compared gross rates of protein depolymerization, N mineralization and nitrification (determined using N-15 pool dilution assays) in organic topsoil, mineral topsoil, and mineral subsoil of seven ecosystems along a latitudinal transect in western Siberia, from tundra (67 degrees N) to steppe (54 degrees N). The investigated ecosystems differed strongly in N transformation rates, with highest protein depolymerization and N mineralization rates in middle and southern taiga. All N transformation rates decreased with soil depth following the decrease in organic matter content. Related to protein depolymerization, N mineralization and nitrification were significantly higher in mineral than in organic horizons, supporting a decrease in microbial N limitation with depth. In contrast, we did not find indications for a decrease in microbial N limitation from arctic to temperate ecosystems along the transect. Our findings thus challenge the perception of ubiquitous N limitation at high latitudes, but suggest a transition from N to C limitation of microorganisms with soil depth, even in high-latitude systems such as tundra and boreal forest.
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33.
  • Wilson, Samuel T., et al. (författare)
  • Short-term variability in euphotic zone biogeochemistry and primary productivity at Station ALOHA : A case study of summer 2012
  • 2015
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 29:8, s. 1145-1164
  • Tidskriftsartikel (refereegranskat)abstract
    • Time-series observations are critical to understand the structure, function, and dynamics of marine ecosystems. The Hawaii Ocean Time-series program has maintained near-monthly sampling at Station ALOHA (22°45′N, 158°00′W) in the oligotrophic North Pacific Subtropical Gyre (NPSG) since 1988 and has identified ecosystem variability over seasonal to interannual timescales. To further extend the temporal resolution of these near-monthly time-series observations, an extensive field campaign was conducted during July-September 2012 at Station ALOHA with near-daily sampling of upper water-column biogeochemistry, phytoplankton abundance, and activity. The resulting data set provided biogeochemical measurements at high temporal resolution and documents two important events at Station ALOHA: (1) a prolonged period of low productivity when net community production in the mixed layer shifted to a net heterotrophic state and (2) detection of a distinct sea-surface salinity minimum feature which was prominent in the upper water column (0-50 m) for a period of approximately 30 days. The shipboard observations during July-September 2012 were supplemented with in situ measurements provided by Seagliders, profiling floats, and remote satellite observations that together revealed the extent of the low productivity and the sea-surface salinity minimum feature in the NPSG.
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34.
  • Winton, V.H.L., et al. (författare)
  • Multiple sources of soluble atmospheric iron to Antarctic waters
  • 2016
  • Ingår i: Global Biogeochemical Cycles. - : John Wiley & Sons. - 0886-6236 .- 1944-9224. ; 30:3, s. 421-437
  • Tidskriftsartikel (refereegranskat)abstract
    • The Ross Sea, Antarctica, is a highly productive region of the Southern Ocean. Significant new sources of iron (Fe) are required to sustain phytoplankton blooms in the austral summer. Atmospheric deposition is one potential source. The fractional solubility of Fe is an important variable determining Fe availability for biological uptake. To constrain aerosol Fe inputs to the Ross Sea region, fractional solubility of Fe was analyzed in a snow pit from Roosevelt Island, eastern Ross Sea. In addition, aluminum, dust, and refractory black carbon (rBC) concentrations were analyzed, to determine the contribution of mineral dust and combustion sources to the supply of aerosol Fe. We estimate exceptionally high dissolved Fe (dFe) flux of 1.2 × 10−6 g m−2 y−1 and total dissolvable Fe flux of 140 × 10−6 g m−2 y−1 for 2011/2012. Deposition of dust, Fe, Al, and rBC occurs primarily during spring-summer. The observed background fractional Fe solubility of ~0.7% is consistent with a mineral dust source. Radiogenic isotopic ratios and particle size distribution of dust indicates that the site is influenced by local and remote sources. In 2011/2012 summer, relatively high dFe concentrations paralleled both mineral dust and rBC deposition. Around half of the annual aerosol Fe deposition occurred in the austral summer phytoplankton growth season; however, the fractional Fe solubility was low. Our results suggest that the seasonality of dFe deposition can vary and should be considered on longer glacial-interglacial timescales.
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35.
  • Zdanowicz, Christian M., et al. (författare)
  • Historical variations of mercury stable isotope ratios in arctic glacier firn and ice cores
  • 2016
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 30:9, s. 1324-1347
  • Tidskriftsartikel (refereegranskat)abstract
    • The concentration and isotopic composition of mercury (Hg) were determined in glacier core samples from Canadian Arctic ice caps dating from pre-industrial to recent time (early 21st century). Mean Hg levels increased from ≤ 0.2 ng L-1 in pre-industrial time to ~0.8-1.2 ng L-1 in the modern industrial era (last ~200 years). Hg accumulated on Arctic ice caps has D199Hg and D201Hg that are higher (~-1 to 2.9 ‰) than previously reported for Arctic snow (mostly < -1 ‰) impacted by atmospheric Hg depletion events, suggesting that these events contribute little to Hg accumulation on ice caps. The range of d202Hg, D199Hg and D201Hg in glacier cores overlaps with that of Arctic Hg0(g) and of seawater in Baffin Bay, but also with that of mid-latitude precipitation and industrial Hg sources, including coal and Hg ores. A core from Agassiz ice cap (80.7 °N) shows a ~+1 ‰ shift in d202Hg over the 19th-20th centuries that could reflect changes in the isotopic composition of the atmospheric Hg pool in the High Arctic in response to growing industrial emissions at lower latitudes. This study is the first ever to report on historical variations of Hg stable isotope ratios in Arctic ice cores. Results could help constrain future modeling efforts of the global Hg biogeochemical cycle and the atmosphere's response to changing Hg emissions, past and future.
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36.
  • Doughty, Christopher E., et al. (författare)
  • Source and sink carbon dynamics and carbon allocation in the Amazon basin
  • 2015
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236. ; 29:5, s. 645-655
  • Tidskriftsartikel (refereegranskat)abstract
    • Changes to the carbon cycle in tropical forests could affect global climate, but predicting such changes has been previously limited by lack of field-based data. Here we show seasonal cycles of the complete carbon cycle for 14, 1ha intensive carbon cycling plots which we separate into three regions: humid lowland, highlands, and dry lowlands. Our data highlight three trends: (1) there is differing seasonality of total net primary productivity (NPP) with the highlands and dry lowlands peaking in the dry season and the humid lowland sites peaking in the wet season, (2) seasonal reductions in wood NPP are not driven by reductions in total NPP but by carbon during the dry season being preferentially allocated toward either roots or canopy NPP, and (3) there is a temporal decoupling between total photosynthesis and total carbon usage (plant carbon expenditure). This decoupling indicates the presence of nonstructural carbohydrates which may allow growth and carbon to be allocated when it is most ecologically beneficial rather than when it is most environmentally available.
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37.
  • Girardin, Cécile A J, et al. (författare)
  • Seasonal trends of Amazonian rainforest phenology, net primary productivity, and carbon allocation
  • 2016
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236. ; 30:5, s. 700-715
  • Tidskriftsartikel (refereegranskat)abstract
    • The seasonality of solar irradiance and precipitation may regulate seasonal variations in tropical forests carbon cycling. Controversy remains over their importance as drivers of seasonal dynamics of net primary productivity in tropical forests. We use ground data from nine lowland Amazonian forest plots collected over 3 years to quantify the monthly primary productivity (NPP) of leaves, reproductive material, woody material, and fine roots over an annual cycle. We distinguish between forests that do not experience substantial seasonal moisture stress (“humid sites”) and forests that experience a stronger dry season (“dry sites”). We find that forests from both precipitation regimes maximize leaf NPP over the drier season, with a peak in production in August at both humid (mean 0.39 ± 0.03 Mg C ha−1 month−1 in July, n = 4) and dry sites (mean 0.49 ± 0.03 Mg C ha−1 month−1 in September, n = 8). We identify two distinct seasonal carbon allocation patterns (the allocation of NPP to a specific organ such as wood leaves or fine roots divided by total NPP). The forests monitored in the present study show evidence of either (i) constant allocation to roots and a seasonal trade-off between leaf and woody material or (ii) constant allocation to wood and a seasonal trade-off between roots and leaves. Finally, we find strong evidence of synchronized flowering at the end of the dry season in both precipitation regimes. Flower production reaches a maximum of 0.047 ± 0.013 and 0.031 ± 0.004 Mg C ha−1 month−1 in November, in humid and dry sites, respectively. Fruitfall production was staggered throughout the year, probably reflecting the high variation in varying times to development and loss of fruit among species.
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38.
  • Luo, Yiqi, et al. (författare)
  • Toward more realistic projections of soil carbon dynamics by Earth system models
  • 2016
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236. ; 30:1, s. 40-56
  • Tidskriftsartikel (refereegranskat)abstract
    • Soil carbon (C) is a critical component of Earth system models (ESMs), and its diverse representations are a major source of the large spread across models in the terrestrial C sink from the third to fifth assessment reports of the Intergovernmental Panel on Climate Change (IPCC). Improving soil C projections is of a high priority for Earth system modeling in the future IPCC and other assessments. To achieve this goal, we suggest that (1) model structures should reflect real-world processes, (2) parameters should be calibrated to match model outputs with observations, and (3) external forcing variables should accurately prescribe the environmental conditions that soils experience. First, most soil C cycle models simulate C input from litter production and C release through decomposition. The latter process has traditionally been represented by first-order decay functions, regulated primarily by temperature, moisture, litter quality, and soil texture. While this formulation well captures macroscopic soil organic C (SOC) dynamics, better understanding is needed of their underlying mechanisms as related to microbial processes, depth-dependent environmental controls, and other processes that strongly affect soil C dynamics. Second, incomplete use of observations in model parameterization is a major cause of bias in soil C projections from ESMs. Optimal parameter calibration with both pool- and flux-based data sets through data assimilation is among the highest priorities for near-term research to reduce biases among ESMs. Third, external variables are represented inconsistently among ESMs, leading to differences in modeled soil C dynamics. We recommend the implementation of traceability analyses to identify how external variables and model parameterizations influence SOC dynamics in different ESMs. Overall, projections of the terrestrial C sink can be substantially improved when reliable data sets are available to select the most representative model structure, constrain parameters, and prescribe forcing fields.
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39.
  • Peng, Shushi, et al. (författare)
  • Benchmarking the seasonal cycle of CO2 fluxes simulated by terrestrial ecosystem models
  • 2015
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236. ; 29:1, s. 46-64
  • Tidskriftsartikel (refereegranskat)abstract
    • We evaluated the seasonality of CO2 fluxes simulated by nine terrestrial ecosystem models of the TRENDY project against (1) the seasonal cycle of gross primary production (GPP) and net ecosystem exchange (NEE) measured at flux tower sites over different biomes, (2) gridded monthly Model Tree Ensembles-estimated GPP (MTE-GPP) and MTE-NEE obtained by interpolating many flux tower measurements with a machine-learning algorithm, (3) atmospheric CO2 mole fraction measurements at surface sites, and (4) CO2 total columns (X-CO2) measurements from the Total Carbon Column Observing Network (TCCON). For comparison with atmospheric CO2 measurements, the LMDZ4 transport model was run with time-varying CO2 fluxes of each model as surface boundary conditions. Seven out of the nine models overestimate the seasonal amplitude of GPP and produce a too early start in spring at most flux sites. Despite their positive bias for GPP, the nine models underestimate NEE at most flux sites and in the Northern Hemisphere compared with MTE-NEE. Comparison with surface atmospheric CO2 measurements confirms that most models underestimate the seasonal amplitude of NEE in the Northern Hemisphere (except CLM4C and SDGVM). Comparison with TCCON data also shows that the seasonal amplitude of X-CO2 is underestimated by more than 10% for seven out of the nine models (except for CLM4C and SDGVM) and that the MTE-NEE product is closer to the TCCON data using LMDZ4. From CO2 columns measured routinely at 10 TCCON sites, the constrained amplitude of NEE over the Northern Hemisphere is of 1.60.4 gC m(-2)d(-1), which translates into a net CO2 uptake during the carbon uptake period in the Northern Hemisphere of 7.92.0 PgC yr(-1).
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40.
  • Rinne, Janne, et al. (författare)
  • Temporal Variation of Ecosystem Scale Methane Emission From a Boreal Fen in Relation to Temperature, Water Table Position, and Carbon Dioxide Fluxes
  • 2018
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236. ; 32:7, s. 1087-1106
  • Tidskriftsartikel (refereegranskat)abstract
    • We have analyzed decade-long methane flux data set from a boreal fen, Siikaneva, together with data on environmental parameters and carbon dioxide exchange. The methane flux showed seasonal cycle but no systematic diel cycle. The highest fluxes were observed in July–August with average value of 73 nmol m−2 s−1. Wintertime fluxes were small but positive, with January–March average of 6.7 nmol m−2 s−1. Daily average methane emission correlated best with peat temperatures at 20–35 cm depths. The second highest correlation was with gross primary production (GPP). The best correspondence between emission algorithm and measured fluxes was found for a variable-slope generalized linear model (r2 = 0.89) with peat temperature at 35 cm depth and GPP as explanatory variables, slopes varying between years. The homogeneity of slope approach indicated that seasonal variation explained 79% of the sum of squares variation of daily average methane emission, the interannual variation in explanatory factors 7.0%, functional change 5.3%, and random variation 9.1%. Significant correlation between interannual variability of growing season methane emission and that of GPP indicates that on interannual time scales GPP controls methane emission variability, crucially for development of process-based methane emission models. Annual methane emission ranged from 6.0 to 14 gC m−2 and was 2.7 ± 0.4% of annual GPP. Over 10-year period methane emission was 18% of net ecosystem exchange as carbon. The weak relation of methane emission to water table position indicates that space-to-time analogy, used to extrapolate spatial chamber data in time, may not be applicable in seasonal time scales.
  •  
41.
  • Rütting, Tobias, 1977, et al. (författare)
  • Leaky nitrogen cycle in pristine African montane rainforest soil
  • 2015
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236. ; 29:10, s. 1754-1762
  • Tidskriftsartikel (refereegranskat)abstract
    • Many pristine humid tropical forests show simultaneously high nitrogen (N) richness and sustained loss of bioavailable N forms. To better understand this apparent upregulation of the N cycle in tropical forests, process-based understanding of soil N transformations, in geographically diverse locations, remains paramount. Field-based evidence is limited and entirely lacking for humid tropical forests on the African continent. This study aimed at filling both knowledge gaps by monitoring N losses and by conducting an in situ 15N labeling experiment in the Nyungwe tropical montane forest in Rwanda. Here we show that this tropical forest shows high nitrate (NO3−) leaching losses, confirming findings from other parts of the world. Gross N transformation rates point to an open soil N cycle with mineralized N nitrified rather than retained via immobilization; gross immobilization of NH4+ and NO3− combined accounted for 37% of gross mineralization, and plant N uptake is dominated by ammonium (NH4+). This study provided new process understanding of soil N cycling in humid tropical forests and added geographically independent evidence that humid tropical forests are characterized by soil N dynamics and N inputs sustaining bioavailable N loss.
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42.
  • Ulfsbo, Adam, 1985, et al. (författare)
  • Rapid Changes in Anthropogenic Carbon Storage and Ocean Acidification in the Intermediate Layers of the Eurasian Arctic Ocean: 1996-2015
  • 2018
  • Ingår i: Global Biogeochemical Cycles. - : American Geophysical Union (AGU). - 0886-6236. ; 32:9, s. 1254-1275
  • Tidskriftsartikel (refereegranskat)abstract
    • The extended multiple linear regression technique is used to determine changes in anthropogenic carbon in the intermediate layers of the Eurasian Basin based on occupations from four cruises between 1996 and 2015. The results show a significant increase in basin-wide anthropogenic carbon storage in the Nansen Basin (0.44-0.73 +/- 0.14 mol Cm-2.year(-1)) and the Amundsen Basin (0.63-1.04 +/- 0.09 mol C.m(-2).year(-1)). Over the last two decades, inferred changes in ocean acidification (0.020-0.055 pH units) and calcium carbonate desaturation (0.05-0.18 units) are pronounced and rapid. These results, together with results from carbonate-dynamic box model simulations and I-129 tracer distribution simulations, suggest that the accumulation of anthropogenic carbon in the intermediate layers of the Eurasian Basin are consistent with increasing concentrations of anthropogenic carbon in source waters of Atlantic origin entering the Arctic Ocean followed by interior transport. The dissimilar distributions of anthropogenic carbon in the interior Nansen and Amundsen Basins are likely due to differences in the lateral ventilation of the intermediate layers by the return flows and ramifications of the boundary current along the topographic boundaries in the Eurasian Basin.
  •  
43.
  • Walker, Anthony P., et al. (författare)
  • Predicting long-term carbon sequestration in response to CO2 enrichment: How and why do current ecosystem models differ?
  • 2015
  • Ingår i: Global Biogeochemical Cycles. - 0886-6236. ; 29:4, s. 476-495
  • Tidskriftsartikel (refereegranskat)abstract
    • Large uncertainty exists in model projections of the land carbon (C) sink response to increasing atmospheric CO2. Free-Air CO2 Enrichment (FACE) experiments lasting a decade or more have investigated ecosystem responses to a step change in atmospheric CO2 concentration. To interpret FACE results in the context of gradual increases in atmospheric CO2 over decades to centuries, we used a suite of seven models to simulate the Duke and Oak Ridge FACE experiments extended for 300 years of CO2 enrichment. We also determine key modeling assumptions that drive divergent projections of terrestrial C uptake and evaluate whether these assumptions can be constrained by experimental evidence. All models simulated increased terrestrial C pools resulting from CO2 enrichment, though there was substantial variability in quasi-equilibrium C sequestration and rates of change. In two of two models that assume that plant nitrogen (N) uptake is solely a function of soil N supply, the net primary production response to elevated CO2 became progressively N limited. In four of five models that assume that N uptake is a function of both soil N supply and plant N demand, elevated CO2 led to reduced ecosystem N losses and thus progressively relaxed nitrogen limitation. Many allocation assumptions resulted in increased wood allocation relative to leaves and roots which reduced the vegetation turnover rate and increased C sequestration. In addition, self-thinning assumptions had a substantial impact on C sequestration in two models. Accurate representation of N process dynamics (in particular N uptake), allocation, and forest self-thinning is key to minimizing uncertainty in projections of future C sequestration in response to elevated atmospheric CO2.
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