EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Fe(II)-catalyzed transformation of Fe (hydr)oxides in particle-size soil organic matter fractions from amended agricultural soils

Beatrice Giannetta1, Ramona Balint2, Daniel Said-Pullicino1, César Plaza3, Maria Martin1, and Claudio Zaccone4
Beatrice Giannetta et al.
  • 1Department of Agricultural, Forest and Food Sciences, University of Torino, Grugliasco, Italy (
  • 2Institute of Geosciences and Earth Resources, National Research Council of Italy (CNR), Turin, Italy
  • 3Institute of Agricultural Sciences Spanish National Research Council (CSIC), Madrid, Spain
  • 4Department of Biotechnology, University of Verona, Verona, Italy

Redox-driven changes in Fe crystallinity and speciation may affect soil organic matter (SOM) stabilization and carbon (C) turnover, with consequent influence on global terrestrial soil organic carbon (SOC) cycling. Under reducing conditions, increasing concentrations of Fe(II) released in solution from the reductive dissolution of Fe (hydr)oxides may accelerate ferrihydrite transformation, although our understanding of the influence of SOM on these transformations is still lacking. 

Here, we evaluated abiotic Fe(II)-catalyzed mineralogical changes in Fe (hydr)oxides in bulk soils and size-fractionated SOM pools (for comparison, fine silt plus clay, FSi+Cl, and fine sand, FSa) of an agricultural soil, unamended or amended with biochar, municipal solid waste compost, and a combination of both. 

FSa fractions showed the most significant Fe(II)-catalyzed ferrihydrite transformations with the consequent production of well-ordered Fe oxides irrespective of soil amendment, with the only exception being the compost-amended soils. In contrast, poorly crystalline ferrihydrite still constituted ca. 45% of the FSi+Cl fractions of amended soils, confirming the that the higher SOM content in this fraction inhibits atom exchange between aqueous Fe(II) and the solid phase. Building on our knowledge of Fe(II)-catalyzed mineralogical changes in simple systems, our results evidenced that the mechanisms of abiotic Fe mineral transformations in bulk soils depend on Fe mineralogy, organic C content and quality, and organo-mineral associations that exist across particle-size SOM pools. Our results underline that in the fine fractions the increase in SOM due to organic amendments can contribute to limiting abiotic Fe(II)-catalyzed ferrihydrite transformation, while coarser particle-size fractions represent an understudied pool of SOM subjected to Fe mineral transformations. 

How to cite: Giannetta, B., Balint, R., Said-Pullicino, D., Plaza, C., Martin, M., and Zaccone, C.: Fe(II)-catalyzed transformation of Fe (hydr)oxides in particle-size soil organic matter fractions from amended agricultural soils , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14873,, 2020


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  • CC1: Comment on EGU2020-14873, Zhe Zhou, 07 May 2020

    Nice work. I am interested in the way you make the SOM-Fh association. Is it through coprecipitation, coating SOM particle with Fh or adsorbed SOM into Fh? And I would like to ask about your opinion about how SOM inhibited Fe(II)-catalyzed ferrihydrite transformation.



    • AC1: Reply to CC1, Beatrice Giannetta, 07 May 2020

      Dear Zhe, 

      Thank you for your comment! The kinetics of Fe recrystallization and transformation processes under changing redox conditions have been mostly studied in reductionist model systems including DOM, SOM-ferrihydrite precipitates, ferrihydrite-humic acids coprecipitates, freshwater flocs or soil slurries, but rarely in more complex, bulk soils. That is why we studied how Fe phase transformations relate to the quantity and chemistry of SOM under field conditions, across different SOM pools (bulk, fine sand, fine silt and clay). So, we studied the SOM-Fe associations naturally occurring in those fractions.

      In the finer SOM fractions, the higher OM contents, the strong Fe-OM associations, and a possible interaction between Fe oxides and clay minerals were more likely to inhibit Fe atom exchange kinetics.

      Related to the quality of SOM, in the fine sand fraction:

      1) the binding of the added Fe(II) into the carboxyl-rich aromatic groups typical of compost is the most likely explanation for the inhibition of the transformation of ferrihydrite to more crystalline forms.

      2) the presence of magnetite and lepidocrocite in this fine sand fraction amended with biochar was possibly due to the role of its aromatic constituents in mediating electron transfer processes thus favouring the transformation of the Fe(III) oxyhydroxide.