Elucidating sources and pathways of dissolved organic carbon in a small, forested catchment – A qualitative assessment of stream, soil and shallow groundwater

Dissolved organic carbon (DOC) constitutes the biggest portion of carbon that is exported from soils. During the last decades, widespread increases in DOC concentrations of surface waters have been observed, affecting ecosystem functioning and drinking water treatment. However, the hydrological controls on DOC mobilization are still not completely understood.We sampled two different topographical positions within a headwater catchment in the Bavarian Forest National Park: at a steep hillslope (880 m.a.s.l.) and in a flat and wide riparian zone (770 m.a.s.l.). By using piezometers, pore water samplers (peepers) and in-stream spectrometric devices we measured DOC concentrations as well as DOC absorbance (A254/A365 and SUVA254) and fluorescence characteristics (fluorescence and freshness indices) in soil water, shallow ground water and stream water in order to gain insights into the DOC source areas during base-flow and during precipitation events.High DOC concentrations (up to 80 mg L-1) were found in soil water from cascading sequences of small ponds in the flat downstream part of the catchment that fill up temporarily. The increase of in-stream DOC concentrations during events was accompanied by changing DOC characteristics at both locations, for example increasing freshness index values. As the freshness index values were approaching the values found in the DOC-rich ponds in the riparian zone, these ponds seem to be important DOC sources during events. Our preliminary results point to a change of flow pathways during events.

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Temperature controls production but hydrology regulates export of dissolved organic carbon at the catchment scale

Hydrology and Earth System Sciences ◽

10.5194/hess-24-945-2020 ◽

2020 ◽

Vol 24 (2) ◽

pp. 945-966 ◽

Cited By ~ 8

Author(s):

Hang Wen ◽

Julia Perdrial ◽

Benjamin W. Abbott ◽

Susana Bernal ◽

Rémi Dupas ◽

...

Keyword(s):

Organic Carbon ◽

Dissolved Organic Carbon ◽

Groundwater Flow ◽

Soil Water ◽

Reactive Transport ◽

Stream Water ◽

Catchment Scale ◽

High Discharge ◽

Doc Production ◽

Doc Concentration

Abstract. Lateral carbon flux through river networks is an important and poorly understood component of the global carbon budget. This work investigates how temperature and hydrology control the production and export of dissolved organic carbon (DOC) in the Susquehanna Shale Hills Critical Zone Observatory in Pennsylvania, USA. Using field measurements of daily stream discharge, evapotranspiration, and stream DOC concentration, we calibrated the catchment-scale biogeochemical reactive transport model BioRT-Flux-PIHM (Biogeochemical Reactive Transport–Flux–Penn State Integrated Hydrologic Model, BFP), which met the satisfactory standard of a Nash–Sutcliffe efficiency (NSE) value greater than 0.5. We used the calibrated model to estimate and compare the daily DOC production rates (Rp; the sum of the local DOC production rates in individual grid cells) and export rate (Re; the product of the concentration and discharge at the stream outlet, or load). Results showed that daily Rp varied by less than an order of magnitude, primarily depending on seasonal temperature. In contrast, daily Re varied by more than 3 orders of magnitude and was strongly associated with variation in discharge and hydrological connectivity. In summer, high temperature and evapotranspiration dried and disconnected hillslopes from the stream, driving Rp to its maximum but Re to its minimum. During this period, the stream only exported DOC from the organic-poor groundwater and from organic-rich soil water in the swales bordering the stream. The DOC produced accumulated in hillslopes and was later flushed out during the wet and cold period (winter and spring) when Re peaked as the stream reconnected with uphill and Rp reached its minimum. The model reproduced the observed concentration–discharge (C–Q) relationship characterized by an unusual flushing–dilution pattern with maximum concentrations at intermediate discharge, indicating three end-members of source waters. A sensitivity analysis indicated that this nonlinearity was caused by shifts in the relative contribution of different source waters to the stream under different flow conditions. At low discharge, stream water reflected the chemistry of organic-poor groundwater; at intermediate discharge, stream water was dominated by the organic-rich soil water from swales; at high discharge, the stream reflected uphill soil water with an intermediate DOC concentration. This pattern persisted regardless of the DOC production rate as long as the contribution of deeper groundwater flow remained low (<18 % of the streamflow). When groundwater flow increased above 18 %, comparable amounts of groundwater and swale soil water mixed in the stream and masked the high DOC concentration from swales. In that case, the C–Q patterns switched to a flushing-only pattern with increasing DOC concentration at high discharge. These results depict a conceptual model that the catchment serves as a producer and storage reservoir for DOC under hot and dry conditions and transitions into a DOC exporter under wet and cold conditions. This study also illustrates how different controls on DOC production and export – temperature and hydrological flow paths, respectively – can create temporal asynchrony at the catchment scale. Future warming and increasing hydrological extremes could accentuate this asynchrony, with DOC production occurring primarily during dry periods and lateral export of DOC dominating in major storm events.

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Temporal changes in photoreactivity of dissolved organic carbon and implications for aquatic carbon fluxes from peatlands

Biogeosciences ◽

10.5194/bg-14-1793-2017 ◽

2017 ◽

Vol 14 (7) ◽

pp. 1793-1809 ◽

Cited By ~ 8

Author(s):

Amy E. Pickard ◽

Kate V. Heal ◽

Andrew R. McLeod ◽

Kerry J. Dinsmore

Keyword(s):

Organic Carbon ◽

Dissolved Organic Carbon ◽

Water Samples ◽

Stream Water ◽

Direct Conversion ◽

Aquatic Systems ◽

Residence Times ◽

Terrestrial Environment ◽

Rainfall Events ◽

Fluorescence Characteristics

Abstract. Aquatic systems draining peatland catchments receive a high loading of dissolved organic carbon (DOC) from the surrounding terrestrial environment. Whilst photo-processing is known to be an important process in the transformation of aquatic DOC, the drivers of temporal variability in this pathway are less well understood. In this study, 8 h laboratory irradiation experiments were conducted on water samples collected from two contrasting peatland aquatic systems in Scotland: a peatland stream and a reservoir in a catchment with high percentage peat cover. Samples were collected monthly at both sites from May 2014 to May 2015 and from the stream system during two rainfall events. DOC concentrations, absorbance properties and fluorescence characteristics were measured to investigate characteristics of the photochemically labile fraction of DOC. CO2 and CO produced by irradiation were also measured to determine gaseous photoproduction and intrinsic sample photoreactivity. Significant variation was seen in the photoreactivity of DOC between the two systems, with total irradiation-induced changes typically 2 orders of magnitude greater at the high-DOC stream site. This is attributed to longer water residence times in the reservoir rendering a higher proportion of the DOC recalcitrant to photo-processing. During the experimental irradiation, 7 % of DOC in the stream water samples was photochemically reactive and direct conversion to CO2 accounted for 46 % of the measured DOC loss. Rainfall events were identified as important in replenishing photoreactive material in the stream, with lignin phenol data indicating mobilisation of fresh DOC derived from woody vegetation in the upper catchment. This study shows that peatland catchments produce significant volumes of aromatic DOC and that photoreactivity of this DOC is greatest in headwater streams; however, an improved understanding of water residence times and DOC input–output along the source to sea aquatic pathway is required to determine the fate of peatland carbon.

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Riparian zone processes and soil water total organic carbon (TOC): implications for spatial variability, upscaling and carbon exports

Biogeosciences Discussions ◽

10.5194/bgd-9-3031-2012 ◽

2012 ◽

Vol 9 (3) ◽

pp. 3031-3069

Author(s):

T. Grabs ◽

K. H. Bishop ◽

H. Laudon ◽

S. W. Lyon ◽

J. Seibert

Keyword(s):

Organic Carbon ◽

Spatial Variability ◽

Total Organic Carbon ◽

Soil Water ◽

Riparian Zone ◽

Stream Water ◽

Chemical Transformations ◽

Topographic Wetness Index ◽

Integration Model ◽

Groundwater Levels

Abstract. Groundwater flowing from hillslopes through riparian (near stream) soils often undergoes chemical transformations that can substantially influence stream water chemistry. We used landscape analysis to predict total organic carbon (TOC) concentrations profiles and groundwater levels measured in the riparian zone (RZ) of a 67 km2 catchment in Sweden. TOC exported from 13 riparian soil profiles was then estimated based on the riparian flow-concentration integration model (RIM). Much of the observed spatial variability of riparian TOC concentrations in this system could be predicted from groundwater levels and the topographic wetness index (TWI). Organic riparian peat soils in forested areas emerged as hotspots exporting large amounts of TOC. Exports were subject to considerable temporal variations caused by a combination of variable flow conditions and changing soil water TOC concentrations. From more mineral riparian gley soils, on the other hand, only small amounts with relatively time-invariant concentrations were exported. Organic and mineral soils in RZs constitute a heterogeneous landscape mosaic that controls much of the spatial variability of stream water TOC. We developed an empirical regression-model based on the TWI to move beyond the plot scale to predict spatially variable riparian TOC concentration profiles for RZs underlain by glacial till.

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Long cold winters give higher stream water dissolved organic carbon (DOC) concentrations during snowmelt

Biogeosciences Discussions ◽

10.5194/bgd-7-4857-2010 ◽

2010 ◽

Vol 7 (3) ◽

pp. 4857-4886 ◽

Cited By ~ 4

Author(s):

A. Ågren ◽

M. Haei ◽

S. Köhler ◽

K. Bishop ◽

H. Laudon

Keyword(s):

Organic Carbon ◽

Dissolved Organic Carbon ◽

Soil Water ◽

Stream Water ◽

Climatic Conditions ◽

Spring Flood ◽

Soil Frost ◽

Winter Climate ◽

Doc Concentration ◽

Preceding Winter

Abstract. We show that long cold winters enhanced the stream water dissolved organic carbon (DOC) concentrations during the following spring flood. Using a 15 year stream record from a boreal catchment, we demonstrate that the interannual variation in DOC concentrations during spring flood was related to the discharge, and winter climate. That discharge is important for DOC concentration agrees with previous studies. By controlling for discharge we could detect that the winter climatic conditions during the preceding winter affected the soil water DOC concentrations, which in turn affected the concentrations in the stream. The results from the stream time-series were also supported by a riparian soil frost experiment, which showed that a long period of soil frost promoted high DOC concentrations in the soil water.

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Regulation of stream water dissolved organic carbon (DOC) concentrations during snowmelt; the role of discharge, winter climate and memory effects

Biogeosciences ◽

10.5194/bg-7-2901-2010 ◽

2010 ◽

Vol 7 (9) ◽

pp. 2901-2913 ◽

Cited By ~ 57

Author(s):

A. Ågren ◽

M. Haei ◽

S. J. Köhler ◽

K. Bishop ◽

H. Laudon

Keyword(s):

Organic Carbon ◽

Dissolved Organic Carbon ◽

Soil Water ◽

Annual Variation ◽

Stream Water ◽

Memory Effects ◽

Winter Climate ◽

Snowmelt Period ◽

Doc Export

Abstract. Using a 15 year stream record from a northern boreal catchment, we demonstrate that the inter-annual variation in dissolved organic carbon (DOC) concentrations during snowmelt was related to discharge, winter climate and previous DOC export. A short and intense snowmelt gave higher stream water DOC concentrations, as did long winters, while a high previous DOC export during the antecedent summer and autumn resulted in lower concentrations during the following spring. By removing the effect of discharge we could detect that the length of winter affected the modeled soil water DOC concentrations during the following snowmelt period, which in turn affected the concentrations in the stream. Winter climate explained more of the stream water DOC variations than previous DOC export during the antecedent summer and autumn.

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From soil water to surface water – how the riparian zone controls element transport from a boreal forest to a stream

Biogeosciences ◽

10.5194/bg-14-3001-2017 ◽

2017 ◽

Vol 14 (12) ◽

pp. 3001-3014 ◽

Cited By ~ 23

Author(s):

Fredrik Lidman ◽

Åsa Boily ◽

Hjalmar Laudon ◽

Stephan J. Köhler

Keyword(s):

Organic Matter ◽

Organic Carbon ◽

Soil Water ◽

Riparian Zone ◽

Stream Water ◽

Major And Trace Elements ◽

Organic Soils ◽

Terrestrial Environment ◽

Riparian Soils ◽

Mineral Soils

Abstract. Boreal headwaters are often lined by strips of highly organic soils, which are the last terrestrial environment to leave an imprint on discharging groundwater before it enters a stream. Because these riparian soils are so different from the Podzol soils that dominate much of the boreal landscape, they are known to have a major impact on the biogeochemistry of important elements such as C, N, P and Fe and the transfer of these elements from terrestrial to aquatic ecosystems. For most elements, however, the role of the riparian zone has remained unclear, although it should be expected that the mobility of many elements is affected by changes in, for example, pH, redox potential and concentration of organic carbon as they are transported through the riparian zone. Therefore, soil water and groundwater was sampled at different depths along a 22 m hillslope transect in the Krycklan catchment in northern Sweden using soil lysimeters and analysed for a large number of major and trace elements (Al, As, B, Ba, Ca, Cd, Cl, Co, Cr, Cs, Cu, Fe, K, La, Li, Mg, Mn, Na, Ni, Pb, Rb, Se, Si, Sr, Th, Ti, U, V, Zn, Zr) and other parameters such as sulfate and total organic carbon (TOC). The results showed that the concentrations of most investigated elements increased substantially (up to 60 times) as the water flowed from the uphill mineral soils and into the riparian zone, largely as a result of higher TOC concentrations. The stream water concentrations of these elements were typically somewhat lower than in the riparian zone, but still considerably higher than in the uphill mineral soils, which suggests that riparian soils have a decisive impact on the water quality of boreal streams. The degree of enrichment in the riparian zone for different elements could be linked to the affinity for organic matter, indicating that the pattern with strongly elevated concentrations in riparian soils is typical for organophilic substances. One likely explanation is that the solubility of many organophilic elements increases as a result of the higher concentrations of TOC in the riparian zone. Elements with low or modest affinity for organic matter (e.g. Na, Cl, K, Mg and Ca) occurred in similar or lower concentrations in the riparian zone. Despite the elevated concentrations of many elements in riparian soil water and groundwater, no increase in the concentrations in biota could be observed (bilberry leaves and spruce shoots).

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Riparian zone hydrology and soil water total organic carbon (TOC): implications for spatial variability and upscaling of lateral riparian TOC exports

Biogeosciences ◽

10.5194/bg-9-3901-2012 ◽

2012 ◽

Vol 9 (10) ◽

pp. 3901-3916 ◽

Cited By ~ 95

Author(s):

T. Grabs ◽

K. Bishop ◽

H. Laudon ◽

S. W. Lyon ◽

J. Seibert

Keyword(s):

Organic Carbon ◽

Spatial Variability ◽

Total Organic Carbon ◽

Soil Water ◽

Riparian Zone ◽

Stream Water ◽

Concentration Profiles ◽

Chemical Transformations ◽

Topographic Wetness Index ◽

Groundwater Levels

Abstract. Groundwater flowing from hillslopes through riparian (near-stream) soils often undergoes chemical transformations that can substantially influence stream water chemistry. We used landscape analysis to predict total organic carbon (TOC) concentration profiles and groundwater levels measured in the riparian zone (RZ) of a 67 km2 catchment in Sweden. TOC exported laterally from 13 riparian soil profiles was then estimated based on the riparian flow–concentration integration model (RIM). Much of the observed spatial variability of riparian TOC concentrations in this system could be predicted from groundwater levels and the topographic wetness index (TWI). Organic riparian peat soils in forested areas emerged as hotspots exporting large amounts of TOC. These TOC fluxes were subject to considerable temporal variations caused by a combination of variable flow conditions and changing soil water TOC concentrations. Mineral riparian gley soils, on the other hand, were related to rather small TOC export rates and were characterized by relatively time-invariant TOC concentration profiles. Organic and mineral soils in RZs constitute a heterogeneous landscape mosaic that potentially controls much of the spatial variability of stream water TOC. We developed an empirical regression model based on the TWI to move beyond the plot scale and to predict spatially variable riparian TOC concentration profiles for RZs underlain by glacial till.

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