| 1. |
- Björnerås, Caroline, et al.
(författare)
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High sulfate concentration enhances iron mobilization from organic soil to water
- 2019
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Ingår i: Biogeochemistry. - : Springer Science and Business Media LLC. - 0168-2563 .- 1573-515X. ; 144:3, s. 245-259
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Tidskriftsartikel (refereegranskat)abstract
- Widespread increases in iron (Fe) concentrations are contributing to ongoing browning of northern freshwaters, but the driver/s behind the trends are not known. Fe mobilization in soils is known to be controlled by redox conditions, pH, and DOC availability for complexation. Moreover, high sulfate concentrations have been suggested to constrain Fe in transition from soil to water, and declining sulfate deposition to have the opposite effect. We studied the effect of these Fe mobilization barriers in a microcosm experiment, applying high (peak S deposition) and low (present day) sulfate treatments and oxic versus anoxic conditions to boreal (O horizon) soil slurries. We hypothesized that anoxic conditions would favor Fe release. On the contrary we expected high sulfate concentrations to suppress Fe mobility, through FeS formation or by lowering pH and thereby DOC concentrations. Anoxia had positive effects on both Fe and DOC concentrations in solution. Contrasting with our hypothesis, Fe concentrations were enhanced at high sulfate concentrations, i.e. increasing acidity in high sulfate treatments appeared to promote Fe mobilization. Establishment of the basidiomycete fungus Jaapia ochroleuca in the oxic treatments 44 days into the experiment had a major impact on Fe mobilization by increasing total Fe concentrations in solution. Thus, anoxia and acidity, along with fungi mediated mobilization, were important in controlling Fe release from soil to the aqueous phase. While Fe is often assumed to precipitate as Fe(oxy)hydroxides in the transition from anoxic to oxic water in the riparian zone, Fe from anoxic treatments remained in solution after introduction of oxygen. Our results do not support reduced atmospheric S deposition as a driver behind increasing Fe concentrations in boreal freshwaters, but confirm the importance of reducing conditions—which may be enhanced by higher soil temperature and moisture—for mobilization of Fe across the terrestrial-aquatic interphase.
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| 2. |
- Björnerås, Caroline, et al.
(författare)
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Sediment Records Shed Light on Drivers of Decadal Iron Concentration Increase in a Boreal Lake
- 2022
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Ingår i: Journal of Geophysical Research - Biogeosciences. - : American Geophysical Union (AGU). - 2169-8953 .- 2169-8961. ; 127:3
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Tidskriftsartikel (refereegranskat)abstract
- Increasing iron (Fe) concentrations are found in lakes on a wide geographical scale but exact causes are still debated. The observed trends might result from increased Fe loading from the terrestrial catchment, but also from changes in how Fe distributes between the water column and the sediments. To get a better understanding of the causes we investigated whether there has been any change in the sediment formation of Fe sulfides (FeS) as an Fe sink in response to declining atmospheric sulfur (S) deposition during recent decades. For our study, we chose Lake Bolmen in southern Sweden, a lake for which we confirmed that Fe concentrations in the water column have strongly increased along with water color during 1966-2018. Our investigations showed that Fe accumulation and speciation varied independently of S accumulation patterns in the Lake Bolmen sediment record. Thus, we were not able to relate the positive trend in Fe concentrations to reduced FeS binding in the sediments. Furthermore, we found that Fe accumulation rates increased along with lake water Fe concentrations, indicating that increased catchment loading rather than a change in the distribution between the sediments and the water column has driven the increase in Fe concentrations. The increased loading may be due to land-use change in the form of an extensive expansion of coniferous forest during the past century. Altered forest management practices and increased precipitation may have led to enhanced weathering and erosion of organic soil layers under aging coniferous forest.
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| 3. |
- Björnerås, Caroline, et al.
(författare)
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The lake as an iron sink - new insights on the role of iron speciation
- 2021
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Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 584
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Tidskriftsartikel (refereegranskat)abstract
- The solubility and behavior of iron (Fe) in natural waters is tightly linked to Fe speciation, and Fe speciation likely influences how Fe distributes between the water column and sediments. In this study, the function of a lake as an Fe sink, with focus on the role of Fe speciation, was assessed for Lake Bolmen in southern Sweden. We found that a large fraction of the Fe flowing in to the lake was efficiently lost by sedimentation in the lake basin. Fe in inflowing water was a mix of organically complexed mononuclear Fe, Fe-(oxy)hydroxides and Fe-bearing clays, while surface sediments were composed of Fe-(oxy)hydroxides, Fe-bearing clays, Fe-bearing silicates and Fe sulfides. The absence of organically complexed Fe in the surface sediments indicates that the lake is mainly a sink for minerogenic fractions. Furthermore, while lakes are considered to be sinks of Fe, it has been suggested that this function may be impaired by increasing precipitation and consequently shorter water residence time. In this study there were large within- and between-year variations in precipitation and Fe concentrations. However, rather than smaller Fe losses to the sediments during wet years, within-lake losses tended to be larger due to higher loading of Fe from the catchment. Thus, forecasted increases in precipitation may result in enhanced catchment export and Fe loading to lakes, and subsequently enhanced Fe sequestration in sediments.
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| 4. |
- Herzog, Simon David, et al.
(författare)
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Characterization of Iron and Organic Carbon Colloids in Boreal Rivers and Their Fate at High Salinity
- 2020
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Ingår i: Journal of Geophysical Research - Biogeosciences. - 2169-8953. ; 125:4
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Tidskriftsartikel (refereegranskat)abstract
- Riverine colloids are important carriers of macronutrients, trace metals, and pollutants into marine waters. The aim of the current study was to extend the understanding of iron (Fe) and organic carbon (OC) colloids in boreal rivers and their fate at higher salinities. X-ray absorbance spectroscopy (XAS) and dynamic light scattering (DLS) were combined to explore Fe speciation and colloidal characteristics such as size and surface charge and how these are affected at increasing salinity. XAS confirmed the presence of two Fe phases in the river waters—Fe-organic matter (OM) complexes and Fe(oxy)hydroxides. From DLS measurements on filtered and unfiltered samples, three particle size distributions were identified. The smallest particles (10–40 nm) were positively charged and suggested to consist of essentially bare Fe(oxy)hydroxide nanoparticles. The largest particles (300–900 nm) were dominated by Fe(oxy)hydroxides associated with chromophoric molecular matter. An intermediate size distribution (100–200 nm) with a negative surface charge was presumably dominated by OM and containing Fe-OM complexes. Increasing the salinity resulted in a removal of the smallest distribution. Unexpectedly, both the intermediate and largest size distributions were still detected at high salinity. The collective results suggest that Fe(oxy)hydroxides and Fe-OM complexes are both found across the wide size range studied and that colloidal size does not necessarily reflect either Fe speciation or stability toward salinity-induced aggregation. The findings further demonstrate that also particles beyond the typically studied <0.45-μm size range should be considered to fully understand the riverine transport and fate of macronutrients, trace metals, and pollutants.
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| 5. |
- Herzog, Simon David, et al.
(författare)
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Organic iron complexes enhance iron transport capacity along estuarine salinity gradients of Baltic estuaries
- 2020
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Ingår i: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 17:2, s. 331-344
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Tidskriftsartikel (refereegranskat)abstract
- Rivers discharge a notable amount of dissolved Fe (1:5×109 mol yr-1) to coastal waters but are still not considered important sources of bioavailable Fe to open marine waters. The reason is that the vast majority of particular and dissolved riverine Fe is considered to be lost to the sediment due to aggregation during estuarine mixing. Recently, however, several studies demonstrated relatively high stability of riverine Fe to salinity-induced aggregation, and it has been proposed that organically complexed Fe (Fe-OM) can "survive" the salinity gradient, while Fe (oxy)hydroxides are more prone to aggregation and selectively removed. In this study, we directly identified, by X-ray absorption spectroscopy, the occurrence of these two Fe phases across eight boreal rivers draining into the Baltic Sea and confirmed a significant but variable contribution of Fe-OM in relation to Fe (oxy)hydroxides among river mouths. We further found that Fe-OM was more prevalent at high flow conditions in spring than at low flow conditions during autumn and that Fe-OM was more dominant upstream in a catchment than at the river mouth. The stability of Fe to increasing salinity, as assessed by artificial mixing experiments, correlated well to the relative contribution of Fe-OM, confirming that organic complexes promote Fe transport capacity. This study suggests that boreal rivers may provide significant amounts of potentially bioavailable Fe beyond the estuary, due to organic matter complexes.
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| 6. |
- Herzog, Simon D., et al.
(författare)
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Salinity Effects on Iron Speciation in Boreal River Waters
- 2017
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Ingår i: Environmental Science and Technology. - : American Chemical Society (ACS). - 0013-936X .- 1520-5851. ; 51:17, s. 9747-9755
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Tidskriftsartikel (refereegranskat)abstract
- Previous studies report high and increasing iron (Fe) concentrations in boreal river mouths. This Fe has shown relatively high stability to salinity-induced aggregation in estuaries. The aim of this study was to understand how the speciation of Fe affects stability over salinity gradients. For Fe to remain in suspension interactions with organic matter (OM) are fundamental and these interactions can be divided in two dominant phases: organically complexed Fe, and colloidal Fe (oxy)hydroxides, stabilized by surface interactions with OM. The stability of these two Fe phases was tested using mixing experiments with river water and artificial seawater. Fe speciation of river waters and salinity-induced aggregates was determined by synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy. The relative contribution of the two Fe phases varied widely across the sampled rivers. Moreover, we found selective removal of Fe (oxy)hydroxides by aggregation at increasing salinity, while organically complexed Fe was less affected. However, Fe-OM complexes were also found in the aggregates, illustrating that the control of Fe stability is not explained by the prevalence of the respective Fe phases alone. Factors such as colloid size and the chemical composition of the OM may also impact the behavior of Fe species.
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| 7. |
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| 8. |
- Meklesh, Viktoriia, et al.
(författare)
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Characterization of the Colloidal Properties of Dissolved Organic Matter From Forest Soils
- 2022
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Ingår i: Frontiers in Soil Science. - : Frontiers Media SA. - 2673-8619.
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Tidskriftsartikel (refereegranskat)abstract
- Components of dissolved organic matter (DOM) span from sub-nm molecules to colloidal aggregates of several hundred nm. The colloidal fraction is important for the transport of organic matter and associated elements in the environment, and for the stability of DOM constituents with respect to microbial decomposition. This study focuses on the colloidal properties of DOM extracted from spruce forest soils of a chronosequence. The DOM samples were obtained by common water extraction procedures at 21 and 100°C, respectively. We applied an experimental approach combining chemical analysis with light and X-ray scattering techniques that informed on the colloidal size, charge, and structure of DOM. Results showed that two main types of colloids were present: semi-flexible cylinders and fractal aggregates. The cylinders consisted of carbohydrates, presumably hemicelluloses, while the aggregates were a composite material containing a large fraction of carbohydrates together with aliphatics and clay particles. These fractal aggregates dominated the cold-water extracts whereas the strong increase in total organic carbon by hot-water extraction caused a concomitantly strong increase of semi-flexible cylinders, which became the predominant species. Comparison between the chronosequence soils showed that with increasing forest age, the amount of carbon extracted per gram of soil declined and the concentration of the semi-flexible cylinders decreased. Thus, the distribution between the fractal aggregates and cylinders in the forest soil DOM samples depends on the composition of the soil organic matter and the leaching temperature. Changes in this distribution may have important implications for the reactivity and stability of DOM colloids.
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| 9. |
- Škerlep, Martin, et al.
(författare)
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Spruce forest afforestation leading to increased Fe mobilization from soils
- 2022
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Ingår i: Biogeochemistry. - : Springer Science and Business Media LLC. - 0168-2563 .- 1573-515X. ; 157:3, s. 273-290
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Tidskriftsartikel (refereegranskat)abstract
- Increasing exports of Fe and DOC from soils, causing browning of freshwaters, have been reported in recent decades in many regions of the northern hemisphere. Afforestation, and in particular an increase of Norway spruce forest in certain regions, is suggested as a driver behind these trends in water chemistry. In this study, we tested the hypothesis that the gradual accumulation of organic soil layers in spruce forests, and subsequent increase in organic acid concentrations and acidity enhances mobilization of Fe. First generation Norway spruce stands of different ages (35, 61, 90 years) and adjacent arable control plots were selected to represent the effects of aging forest. Soil solutions were sampled from suction lysimeters at two depths (below organic soil layer and in mineral soil) during two years, and analyzed for Fe concentration, Fe speciation (XAS analysis), DOC, metals, major anions and cations. Solution Fe concentrations were significantly higher in shallow soils under older spruce stands (by 5- and 6-fold) than in control plots and the youngest forest. Variation in Fe concentration was best explained by variation in DOC concentration and pH. Moreover, Fe in all soil solutions was present as mononuclear Fe(III)-OM complexes, showing that this phase is dominating Fe translocation. Fe speciation in the soil was also analyzed, and found to be dominated by Fe oxides with minor differences between plots. These results confirmed that Fe mobilization, by Fe(III)-OM complexes, was higher from mature spruce stands, which supports that afforestation with spruce may contribute to rising concentrations of Fe in surface waters.
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