Autor:innen:
Jan Jagode | Institute of Soil Science, Leibniz Universität Hannover | Germany
Hamed Kashi | Institute for Plant Nutrition and Soil Science, Christian-Albrechts-University Kiel | Germany
Dr. Jannis Florian Carstens | Institute of Soil Science, Leibniz Universität Hannover | Germany
Jianyu Tao | Institute for Plant Nutrition and Soil Science, Christian-Albrechts-University Kiel | Germany
Dörthe Holthusen | Federal Institute for Hydrology, Department U3 | Germany
Dr. Jana Carus | Federal Institute for Hydrology, Department U3 | Germany
Dr. Elmar Fuchs | Federal Institute for Hydrology, Department U3 | Germany
Dr. Heiner Fleige | Institute for Plant Nutrition and Soil Science, Christian-Albrechts-University Kiel | Germany
Prof. Dr. Sandra Spielvogel | Institute for Plant Nutrition and Soil Science, Christian-Albrechts-University Kiel | Germany
Prof. Dr. Georg Guggenberger | Institute of Soil Science, Leibniz Universität Hannover | Germany
Under oxidizing conditions, Fe(III) oxides constitute one of the most important types of inorganic binding agents for organic matter (OM) stabilization. However, the fluctuating redox conditions in -intertidal transition zones such as brackish marshes in estuarine ecosystems call into question if Fe(III) oxides can function as binding agents, or if Fe(II) minerals can act in a analogous way for OM stabilization. Therefore, we aimed to investigate how the fluctuating redox conditions in the field affected Fe(II) and Fe(III) minerals as inorganic binding agents for OM.
Membrane cylinders with defined Fe mineral/clay coated quartz sand fillings were inserted along a depth gradient in a soil profile in a brackish marsh of the tidel River Elbe. These included (1) Fe(III) oxide, (2) Fe(II) carbonate, and (3) Fe(II) sulfide, which were incubated in the field for 6 months. In an additional experiment, 13C labeled OM sorbed onto the Fe mineral coated sands and particulate OM was further added to the membrane cylinders to study preservation and exchange of OM with surrounding soil. Fe mineralogy was examined via a sequential Fe extraction scheme. Organic carbon (OC) and 13C isotope ratio were measured via EA IRMS.
Fe oxides without OM addition showed no transformation, even in the anoxic zone, whereas with OM addition, a notable shift to Fe carbonates occurred. Fe sulfides displayed nearly total absence of pyrite in both experiments, instead displaying a shift to ascorbic acid-extractable Fe and oxalate extractable Fe suggesting (at least partial) oxidation to Fe(II/III) minerals or Fe(III) oxides. Fe carbonate without OM addition transformed to low- and high crystalline Fe oxides, though Fe carbonate partially persisted even in the oxic zone for the 6-month duration. In contrast, upon OM addition only a minor fraction of Fe carbonate transformed to crystalline Fe oxides, instead shifting to ferrihydrite under oxic conditions and a major portion of Fe(II) minerals persisting in the anoxic zone (possibly as siderite). Fe sulfides and Fe carbonates had almost the same OC range than Fe oxides. The preservation of sorbed 13C-labelled OM decreased with soil depth and in the order Fe oxide > Fe carbonate > Fe sulfide. This suggests exchange of sorbed OM with OM from the surrounding soil, possibly during the redox-sensitive Fe mineral transformation. Both Fe(II) & Fe(II) minerals seem to be able to stabilize OM under fluctuating redox conditions.