| 1. |
- Hawkes, Jeffrey A., et al.
(författare)
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Regional diversity of complex dissolved organic matter across forested hemiboreal headwater streams
- 2018
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 8:1
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Tidskriftsartikel (refereegranskat)abstract
- Dissolved organic matter (DOM) from soils enters the aquatic environment via headwater streams. Thereafter, it is gradually transformed, removed by sedimentation, and mineralised. Due to the proximity to the terrestrial source and short water residence time, the extent of transformation is minimal in headwaters. DOM has variable composition across inland waters, but the amount of variability in the terrestrial end member is unknown. This gap in knowledge is crucial considering the potential impact large variability would have on modelling DOM degradation. Here, we used a novel liquid chromatography –mass spectrometry method to characterise DOM in 74 randomly selected, forested headwater streams in an 87,000 km2 region of southeast Sweden. We found a large degree of sample similarity across this region, with Bray-Curtis dissimilarity values averaging 8.4 ± 3.0% (mean ± SD). The identified variability could be reduced to two principle coordinates, correlating to varying groundwater flow-paths and regional mean temperature. Our results indicate that despite reproducible effects of groundwater geochemistry and climate, the composition of DOM is remarkably similar across catchments already as it leaves the terrestrial environment, rather than becoming homogeneous as different headwaters and sub-catchments mix.
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| 2. |
- Winterdahl, Mattias, et al.
(författare)
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Riparian Zone Influence on Stream Water Dissolved Organic Carbon Concentrations at the Swedish Integrated Monitoring Sites
- 2011
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Ingår i: Ambio. - : Springer Science and Business Media LLC. - 0044-7447 .- 1654-7209. ; 40:8, s. 920-930
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Tidskriftsartikel (refereegranskat)abstract
- Short-term variability in stream water dissolved organic carbon (DOC) concentrations is controlled by hydrology, climate and atmospheric deposition. Using the Riparian flow-concentration Integration Model (RIM), we evaluated factors controlling stream water DOC in the Swedish Integrated Monitoring (IM) catchments by separating out hydrological effects on stream DOC dynamics. Model residuals were correlated with climate and deposition-related drivers. DOC was most strongly correlated to water flow in the northern catchment (Gammtratten). The southern Aneboda and Kindla catchments had pronounced seasonal DOC signals, which correlated weakly to flow. DOC concentrations at GAyenrdsjon increased, potentially in response to declining acid deposition. Soil temperature correlated strongly with model residuals at all sites. Incorporating soil temperature in RIM improved model performance substantially (20-62% lower median absolute error). According to the simulations, the RIM conceptualization of riparian processes explains between 36% (Kindla) and 61% (Aneboda) of the DOC dynamics at the IM sites.
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| 3. |
- Gustafsson, Jon Petter, 1964-, et al.
(författare)
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Aluminium and base cation chemistry in dynamic acidification models - need for a reappraisal?
- 2018
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Ingår i: SOIL. - : COPERNICUS GESELLSCHAFT MBH. - 2199-3971 .- 2199-398X. ; 4:4, s. 237-250
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Tidskriftsartikel (refereegranskat)abstract
- Long-term simulations of the water composition in acid forest soils require that accurate descriptions of aluminium and base cation chemistry are used. Both weathering rates and soil nutrient availability depend on the concentrations of Al3+, of H+, and of base cations (Ca2+, Mg2+, Na+, and K+). Assessments of the acidification status and base cation availability will depend on the model being used. Here we review in what ways different dynamic soil chemistry models describe the processes governing aluminium and base cation concentrations in the soil water. Furthermore, scenario simulations with the HD-MINTEQ model are used to illustrate the difference between model approaches. The results show that all investigated models provide the same type of response to changes in input water chemistry. Still, for base cations we show that the differences in the magnitude of the response may be considerable depending on whether a cation-exchange equation (Gaines-Thomas, Gapon) or an organic complexation model is used. The former approach, which is used in many currently used models (e.g. MAGIC, ForSAFE), causes stronger pH buffering over a relatively narrow pH range, as compared to state-of-the-art models relying on more advanced descriptions in which organic complexation is important (CHUM, HD-MIN PLQ). As for aluminium, a "fixed" gibbsite constant, as used in MAGIC, SMART/VSD, and ForSAFE, leads to slightly more pH buffering than in the more advanced models that consider both organic complexation and Al(OH)(3) (s) precipitation, but in this case the effect is small. We conclude that the descriptions of acid-base chemistry and base cation binding in models such as MAGIC, SMART/VSD, and ForSAFE are only likely to work satisfactorily in a narrow pH range. If the pH varies greatly over time, the use of modern organic complexation models is preferred over cation-exchange equations.
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| 4. |
- Audet, Joachim, et al.
(författare)
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Forest streams are important sources for nitrous oxide emissions - Nitrous oxide emissions from Swedish streams
- 2020
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Ingår i: Global Change Biology. - : Wiley. - 1354-1013 .- 1365-2486. ; 26, s. 629-641
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Tidskriftsartikel (refereegranskat)abstract
- Streams and river networks are increasingly recognized as significant sources for the greenhouse gas nitrous oxide (N2O). N2O is a transformation product of nitrogenous compounds in soil, sediment and water. Agricultural areas are considered a particular hotspot for emissions because of the large input of nitrogen (N) fertilizers applied on arable land. However, there is little information on N2O emissions from forest streams although they constitute a major part of the total stream network globally. Here, we compiled N2O concentration data from low-order streams (~1,000 observations from 172 stream sites) covering a large geographical gradient in Sweden from the temperate to the boreal zone and representing catchments with various degrees of agriculture and forest coverage. Our results showed that agricultural and forest streams had comparable N2O concentrations of 1.6 +/- 2.1 and 1.3 +/- 1.8 mu g N/L, respectively (mean +/- SD) despite higher total N (TN) concentrations in agricultural streams (1,520 +/- 1,640 vs. 780 +/- 600 mu g N/L). Although clear patterns linking N2O concentrations and environmental variables were difficult to discern, the percent saturation of N2O in the streams was positively correlated with stream concentration of TN and negatively correlated with pH. We speculate that the apparent contradiction between lower TN concentration but similar N2O concentrations in forest streams than in agricultural streams is due to the low pH (<6) in forest soils and streams which affects denitrification and yields higher N2O emissions. An estimate of the N2O emission from low-order streams at the national scale revealed that ~1.8 x 10(9) g N2O-N are emitted annually in Sweden, with forest streams contributing about 80% of the total stream emission. Hence, our results provide evidence that forest streams can act as substantial N2O sources in the landscape with 800 x 10(9) g CO2-eq emitted annually in Sweden, equivalent to 25% of the total N2O emissions from the Swedish agricultural sector.
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| 5. |
- McGivney, Eric, et al.
(författare)
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Assessing the impact of acid rain and forest harvest intensity with the HD-MINTEQ model - soil chemistry of three Swedish conifer sites from 1880 to 2080
- 2019
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Ingår i: SOIL. - : COPERNICUS GESELLSCHAFT MBH. - 2199-3971 .- 2199-398X. ; 5:1, s. 63-77
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Tidskriftsartikel (refereegranskat)abstract
- Forest soils are susceptible to anthropogenic acidification. In the past, acid rain was a major contributor to soil acidification, but, now that atmospheric levels of S have dramatically declined, concern has shifted towards biomass-induced acidification, i.e. decreasing soil solution pH due to tree growth and harvesting events that permanently remove base cations (BCs) from forest stands. We use a novel dynamic model, HD-MINTEQ (Husby Dynamic MINTEQ), to investigate possible long-term impacts of two theoretical future harvesting scenarios in the year 2020, a conventional harvest (CH, which removes stems only), and a whole-tree harvest (WTH, which removes 100 % of the above-ground biomass except for stumps) on soil chemistry and weathering rates at three different Swedish forest sites (Aneboda, Gardsjon, and Kindla). Furthermore, acidification following the harvesting events is compared to the historical acidification that took place during the 20th century due to acid rain. Our results show that historical acidification due to acid rain had a larger impact on pore water chemistry and mineral weathering than tree growth and harvesting, at least if nitrification remained at a low level. However, compared to a no-harvest baseline, WTH and CH significantly impacted soil chemistry. Directly after a harvesting event (CH or WTH), the soil solution pH sharply increased for 5 to 10 years before slowly declining over the remainder of the simulation (until year 2080). WTH acidified soils slightly more than CH, but in certain soil horizons there was practically no difference by the year 2080. Even though the pH in the WTH and CH scenario decreased with time as compared to the no-harvest scenario (NH), they did not drop to the levels observed around the peak of historic acidification (1980-1990), indicating that the pH decrease due to tree growth and harvesting would be less impactful than that of historic atmospheric acidification. Weathering rates differed across locations and horizons in response to historic acidification. In general, the predicted changes in weathering rates were very small, which can be explained by the net effect of decreased pH and increased Al3+, which affected the weathering rate in opposite ways Similarly, weathering rates after the harvesting scenarios in 2020 remained largely unchanged according to the model.
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| 6. |
- Wallin, Marcus, et al.
(författare)
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Representative regional sampling of carbon dioxide and methane concentrations in hemiboreal headwater streams reveal underestimates in less systematic approaches
- 2014
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Ingår i: Global Biogeochemical Cycles. - 0886-6236 .- 1944-9224. ; 28, s. 465-479
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Tidskriftsartikel (refereegranskat)abstract
- Boreal headwater streams have been identified as hot spots for evasion of greenhouse gases (GHGs). This study was the first to systematically determine the concentrations of CO2 and CH4 in hemiboreal headwater streams. The use of a headspace sampling method focusing on GHGs in combination with a statistically representative selection of more than 200 streams across two regions in Sweden was the basis for defining the base flow concentrations of CO2 and CH4. All streams were supersaturated relative to the atmosphere in CO2 and the majority in CH4 for the 82% of streams in which CH4 was detected. The spatial variability in both CO2 and CH4 was high but positively related to total organic carbon, mean annual temperature, and proportion of peatland in the catchment. There were, however, regional differences in the spatial controls, which are something that predictive models need to consider. The data set allowed for comparison between a headspace and an alkalinity-based method for determining CO2. More than 50% of the streams contained no alkalinity which made the alkalinity-based determination of CO2 impossible. In addition, half of the streams with alkalinity had alkalinities low enough (<0.07meqL(-1)) to make the CO2 determination very uncertain. The streams with low pH and no alkalinity contained median CO2 concentrations that were 45% higher than the streams containing alkalinity. Therefore, large-scale generalizations about CO2 in such headwaters will be significantly underestimated if (1) headwaters are underrepresented and (2) the headwaters are sampled but CO2 is calculated from their alkalinity.Key Points 207 statistically selected headwaters were directly sampled for CO2 and CH4 All streams were supersaturated in CO2 and CH4 but with large spatial variation Indirect methods will create bias in large scale data sets of headwater CO2
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| 7. |
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| 8. |
- Conrad, Sarah, et al.
(författare)
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Seasonal Variations of Redox State in Hemiboreal Soils Indicated by Changes of δ56Fe, Sulfate, and Nitrate in Headwater Streams
- 2019
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Ingår i: ACS Earth and Space Chemistry. - : American Chemical Society (ACS). - 2472-3452. ; 3:12, s. 2816-2823
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Tidskriftsartikel (refereegranskat)abstract
- During recent decades, much focus has been put on the iron (Fe) isotope ratios in soils, rivers, and oceans, while studies on the variation in headwater streams are scarce. Here we assess seasonal water chemical data from 104 hemiboreal headwater streams. Between summer and late autumn, decreasing Fe concentrations and simultaneously increasing sulfate and nitrate concentrations suggest a shift from reduced to oxidized conditions in the soils along the main groundwater flow paths. Fe isotope data, obtained from a subpopulation of 16 streams, show low δ56Fe ratios during summer drought, indicating an important influx of reduced groundwater to the streams with primarily Fe(II) as an important Fe source. In total, the δ56Fe data ranged between −0.8 ± 0.1 and 1.8 ± 0.1‰ with the lowest values in summer and maximum δ56Fe ratios in late autumn or spring, indicating an influx of more oxidized, less Fe(II) rich groundwater during those seasons. Local differences in δ56Fe ratios between the headwater streams, seemed to be driven by the different soil redox status of the catchments. The streams with the lowest δ56Fe ratios during summer are characterized by a small share (4.4 ± 6.6%) of wetlands, indicating discharge of reduced groundwater from mainly anoxic, moist, organic-rich mineral soils during drought. Relatively high total organic carbon (TOC) concentrations (2.4 ± 1.1 mM) and low pH (5.2 ± 0.8) may have restricted efficient Fe(II) oxidation in streamwater especially during the late autumn survey. Our results from hemiboreal headwater streams reveal the importance of climatic, pedogenic, and land cover-derived controls on the provenance of stream Fe loads that is likely broadly applicable to similar streams elsewhere.
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| 9. |
- Ekström, Sara M., et al.
(författare)
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Increasing concentrations of iron in surface waters as a consequence of reducing conditions in the catchment area
- 2016
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Ingår i: Journal of Geophysical Research - Biogeosciences. - 2169-8953 .- 2169-8961. ; 121:2, s. 479-493
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Tidskriftsartikel (refereegranskat)abstract
- Recent studies report trends of strongly increasing iron (Fe) concentrations in freshwaters. Since Fe is a key element with a decisive role in the biogeochemical cycling of major elements, it is important to understand the mechanisms behind these trends. We hypothesized that variations in Fe concentration are driven mainly by redox dynamics in hydraulically connected soils. Notably, Fe(III), which is the favored oxidation state except in environments where microbial activity provide strong reducing intensity, has several orders of magnitude lower water solubility than Fe(II). To test our hypothesis, seasonal variation in water chemistry, discharge, and air temperature was studied in three Swedish rivers. Methylmercury and sulfate were used as indicators of seasonal redox changes. Seasonal variability in water chemistry, discharge, and air temperature in the Eman and Lyckeby Rivers implied that the variation in Fe was primarily driven by the prevalence of reducing conditions in the catchment. In general, high Fe concentrations were observed when methylmercury was high and sulfate was low, indicative of reducing conditions. The Fe concentrations showed no or weak relationships with variations in dissolved organic matter concentration and aromaticity. The seasonal variation in Fe concentration of the Ume river was primarily dependent on timing of the snowmelt in high- versus low-altitude areas of the catchment. There were long-term trends of increasing temperature in all catchments and also trends of increasing discharge in the southern rivers, which should increase the probability for anaerobic conditions in space and time and thereby increase Fe transport to the aquatic systems.
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| 10. |
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