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High sulfate concentration enhances iron mobilization from organic soil to water

  • Caroline Björnerås
  • Martin Škerlep
  • Dimitrios Floudas
  • Per Persson
  • Emma S. Kritzberg
Publishing year: 2019-09
Language: English
Pages: 245-259
Publication/Series: Biogeochemistry
Volume: 144
Issue: 3
Document type: Journal article
Publisher: Springer

Abstract english

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.


  • Soil Science
  • Boreal soil
  • Brownification
  • Dissolved organic carbon
  • Iron biogeochemistry
  • Jaapia ochroleuca
  • Reduced atmospheric S deposition


  • MICCS - Molecular Interactions Controlling soil Carbon Sequestration
  • Molecular Ecology and Evolution Lab
  • ISSN: 0168-2563
Emma Kritzberg
E-mail: emma [dot] kritzberg [at] biol [dot] lu [dot] se

Senior lecturer

Aquatic ecology

+46 46 222 40 79



Research group

Aquatic Ecology


Doctoral students and postdocs

PhD students, main supervisor

PhD students, assistant supervisor

Post docs

Johanna Sjöstedt