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  • 1
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    Riparian Microtopography Affects Event‐Driven Stream DOC Concentrations and DOM Quality in a Forested Headwater Catchment (2022)
    Blaurock, Katharina ; Garthen, Phil ; da Silva, Maria P. ; [et al.]
    Details
    Publication Date: 2024-01-19
    Description: The transport of dissolved organic carbon (DOC) from the soils to inland waters plays an important role in the global carbon cycle. Widespread increases in DOC concentrations have been observed in surface waters over the last few decades, affecting carbon balances, ecosystem functioning and drinking water treatment. However, the primary hydrological controls on DOC mobilization are still uncertain. The aim of this study was to investigate the role of microtopography in the riparian zone for DOC export and DOM quality. DOC concentration and DOM quality in the shallow groundwater of a riparian zone and in streamflow in a forested headwater catchment was investigated using fluorescence and absorbance characteristics. We found higher DOC concentrations with a higher aromaticity in the microtopographical depressions, which were influenced by highly dynamic shallow groundwater levels, than in the flat forest soil. As a result of the frequent wet‐dry cycles in the upper soil layers, aromatic DOC accumulated in the shallow groundwater within and below the microtopographical depressions. Rising groundwater levels during precipitation events led to the connection of the microtopographical depressions to the stream, resulting in a change toward more aromatic DOC in the stream. Increasing stream DOC concentrations were accompanied by increasing concentrations of iron and aluminum, suggesting the coupled release of these metals with DOC from the riparian zone. Our results highlight the importance of the interplay between microtopography and groundwater level dynamics in the riparian zone for DOC export from headwater catchments.
    Description: Plain Language Summary: Dissolved organic carbon (DOC) is the result of the continuous breakdown of organic material, such as leaves. It accumulates in the soil and is transported to streams mainly during precipitation events. In this study, we analyzed the shallow groundwater of two differing sites in the Bavarian Forest National Park. Both sites were located close to the stream, but one was characterized by typical forest soil and one by small ponds, which were occasionally filled with water. The site with ponds showed much higher DOC concentrations and the DOC was chemically different from the other site. During a precipitation event, we observed a shift in chemical composition of stream water parameters toward the chemical characteristics found at the site with ponds. Therefore, we conclude that the ponds contribute substantially to DOC mobilization, once they fill with water and get connected to the stream.
    Description: Key Points: This study found small‐scale differences in dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) quality in the riparian zone. Microtopographical depressions were characterized by high DOC concentrations and aromatic DOC. In‐stream DOC concentrations and DOM quality during a precipitation event resembled shallow groundwater below microtopographical depressions.
    Description: Stifterverband http://dx.doi.org/10.13039/501100008384
    Description: Helmholtz Association http://dx.doi.org/10.13039/501100009318
    Description: Rudolf and Helene Glaser Foundation
    Description: European Regional Development Funds
    Description: https://doi.org/10.6084/m9.figshare.19086455
    Description: https://doi.org/10.48758/ufz.12908
    Keywords: ddc:551 ; dissolved organic carbon ; hydrology ; microtopography ; DOM quality
    Language: English
    Type: doc-type:article
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  • 2
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    Iron Exports From Catchments Are Constrained by Redox Status and Topography (2022)
    Details
    Publication Date: 2022-04-04
    Description: Fe(III) hydroxides stabilize organic carbon (OC) and P in soils. Observations of rising stream Fe concentrations are controversially posited to result from a flushing of iron‐rich deeper soil layers or a decrease of competing electron acceptors inhibiting Fe reduction (NO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$ and SO42− $\mathrm{S}{{\mathrm{O}}_{4}}^{2-}$). Here, we argue that catchment topography constrains the release of Fe, OC, and P to streams. We therefore incubated organic topsoil and mineral subsoil and modified the availability of NO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$. We found that Fe leaching was highest in topsoil. Fe, OC, and P released at quantities proportional to their ratios in the source soil. Supply of NO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$ reduced Fe leaching to 18% and increased pore water OC:Fe and P:Fe ratios. Subsoil, however, was an insignificant Fe source (〈0.5%). Here, the leached quantities of Fe, OC and P were highly disproportionate to the soil source with an excess of released OC and P. We tested if experimental findings scale up using data from 88 German catchments representing gradients in NO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$ concentration and topography. Average stream Fe concentrations increased with decreasing NO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$ and were high in catchments with shallow topography where high groundwater levels support reductive processes and topsoils are hydrologically connected to streams; but Fe concentrations were low in catchments with steep topography where flow occurs primarily through subsoils. OC:Fe and P:Fe ratios in the streams similarly varied by NO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$ and topography. This corroborates the findings from the laboratory experiment and suggests that catchment topography and competing electron acceptors constrain the formation of Fe‐reducing conditions and control the release of Fe, OC, and P to streams.
    Description: Plain Language Summary: Iron is the second most abundant metal in the crust; its cycle is tightly connected to those of carbon, oxygen, and sulfur. The oxidized form (FeIII) is almost insoluble, but Fe can be mobilized by complexation or microbial Fe reduction. Both processes depend on availability of organic C. We found that Fe concentrations in streams were constrained by the topography of catchments and NO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$ abundance. Shallower catchments are characterized by higher groundwater tables connecting the organic topsoils efficiently to streams. NO3− $\mathrm{N}{{\mathrm{O}}_{3}}^{-}$ suppresses Fe reduction as a competing electron acceptor to Fe. We conclude that trends in soil wetness or atmospheric N deposition can change the stability of Fe and thus the release of PO43− $\mathrm{P}{{\mathrm{O}}_{4}}^{3-}$ and harmful metals to surface waters.
    Description: Key Points: Organic topsoils leach substantial amounts of Fe when incubated in the absence of NO3, a competing electron acceptor that inhibits Fe reduction. Shallow catchments with fluvially coupled topsoils and low NO3 availability release 200 fold more Fe than steep ones with high NO3 abundance. Catchment topography and NO3 availability explain 62%–64% of the variability of Fe concentration and OC:Fe and P:Fe ratios across 88 streams.
    Description: EFRE‐Europe
    Description: https://doi.org/10.4211/hs.43601618877945c5a46b715aa98db729
    Keywords: ddc:551.9
    Language: English
    Type: doc-type:article
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