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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-308
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-308
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 09 Sep 2019

Submitted as: research article | 09 Sep 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Effects of extraction conditions on the redox properties of soil organic matter (SOM) and its ability to stimulate microbial iron(III) mineral reduction by electron shuttling

Yuge Bai1, Edisson Subdiaga2, Stefan B. Haderlein2, Heike Knicker3, and Andreas Kappler1 Yuge Bai et al.
  • 1Geomicrobiology, Center of Applied Geosciences, University of Tuebingen, Tuebingen, 72074, Germany
  • 2Environmental Mineralogy and Chemistry, Center of Applied Geosciences, University of Tuebingen, Tuebingen, 72074, Germany
  • 3Instituto de Recursos Naturales y Agrobiología de Sevilla, Consejo Superior de Investigaciones Científicas (IRNAS-CSIC), Seville, 41012, Spain

Abstract. Soil organic matter (SOM), including humic substances (HS), is redox-active, can be microbially reduced, and transfers electrons in an abiotic reaction to Fe(III) minerals thus serving as electron shuttle. The standard procedure to extract HS from soil and separate them into humic acids (HA) and fulvic acids (FA) involves alkaline and acidic solutions potentially leading to unwanted changes in SOM chemical and redox properties. To determine the effects of extraction conditions on the redox and electron shuttling properties of SOM extracts, we prepared HS and SOM extracts from a forest soil applying either a combination of 0.1 M NaOH and 6 M HCl, or water (pH 7). Both chemical extractions (NaOH / HCl) and water extractions were done in separate setups under either oxic or anoxic conditions. Furthermore, we applied the NaOH / HCl treatment to a subsample of the water-extracted-SOM. We found that soil extraction with NaOH lead to ca. 100 times more extracted C and the extracted HS had 2–3 times higher electron exchange capacities (EEC) than SOM extracted by water. For water-extracted SOM, anoxic extraction conditions lead to about 7 times more extracted C and 1.5 times higher EEC than under oxic extraction conditions. This difference was probably due to the occurrence of microbial reduction and dissolution of Fe(III) minerals in the soil during the water extraction at neutral pH and the concomitant release of Fe(III) mineral-bound organic matter. NaOH / HCl treatment of the water-extracted SOM lead to 2 times higher EEC values in the HA isolated from the SOM compared to the water-extracted SOM itself, suggesting the chemical treatment with NaOH and HCl caused changes of redox-active functional groups of the extracted organic compounds. Higher EEC of extracts in turn resulted in a higher stimulation of microbial Fe(III) mineral reduction by electron shuttling, i.e. faster initial Fe(III) reduction rates, and in most cases also in higher reduction extents. Our findings suggest that SOM extracted with water at neutral pH should be used to better reflect environmental SOM redox processes in lab experiments and that potential artefacts of the chemical extraction method and anoxic extraction condition need to be considered when evaluating and comparing abiotic and microbial SOM redox processes.

Yuge Bai et al.
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