PhD project offered by the IMPRS-gBGC in July 2026

Coupled Dynamics of Dissolved Organic Matter (DOM) and Organo-Iron-Oxide Associations Under Climate Change: A combined spectroscopic and transport study along a pedological and lithological gradient

Karin Eusterhues , Kai U. Totsche , Gerd Gleixner

Project description

In temperate forest ecosystems, submicron-sized organo-iron-oxide associations are formed as secondary mixed phases during soil development and maturation. These environmental colloids regulate the long-term stabilization of soil organic carbon (SOC). However, their formation, structural stability, and vertical translocation are directly governed by abiotic physicochemical parameters and their coupling with Dissolved Organic Matter (DOM). DOM acts as a highly reactive chemical agent whose molecular architecture (e.g., aromaticity, functional group density, molecular weight) dictates whether it will bind to iron oxyhydroxides via ligand exchange, induce purely chemical (reductive?) dissolution, or alter surface charges to mobilize existing iron colloids. This coupled Fe-DOM network is strictly constrained by the pedological setting (e.g., diagnostic horizons of Spodosols, Cambisols, Stagnosols, and Luvisols) and the underlying bedrock lithology. Pedogenesis establishes vertical gradients in pH, texture, temperature, and moisture, creating distinct genetic horizons (e.g., Ah, E, Bw, Bt, Bhs) with unique geochemical microenvironments. Climate change acts as a physical and thermodynamic disruptor to these systems by altering soil temperature profiles, moisture status, and evaporation rates. Intense drying-wetting cycles and prolonged thermal exposure alter the abiotic desiccation, crystallization, and chemical fragmentation of DOM inputs. Rather than viewing the soil matrix as static, this project focuses entirely on the abiotic, interdependent mechanisms driving the Fe-DOM system. We trace the chemical fractionation of DOM and the mineralogical transformation of iron phases as they cross sharp pedological boundaries, migrating from topsoils through deeply weathered subsoil horizons, down to their eventual export into active creek sediments. This research investigates how bedrock lithology and pedogenic horizons control the abiotic sorption of DOM onto secondary iron phases, and how climate-driven temperature and moisture shifts alter DOM quality and iron oxide crystallinity. It identifies geochemical tipping points during extreme hydrologic events where DOM switches from an aggregate stabilizer to a mobilizing agent for colloid-bound iron, tracing these coupled signatures across the topsoil-subsoil-saprolite-creek continuum. To achieve this, a sampling matrix of topsoil, subsoil, saprolite and creek sediments will be established. Field observations will be paired with controlled laboratory incubations to quantify the formation and transformation of organo-iron-mineral associations transformations. Finally, transport experiments with columns packed with soil material will simulate vertical transport, while spectroscopy -microscopy maps the chemical interfaces across the gradient.

Working group & cooperations

The successful applicant will be affiliated with the Institute of Geosciences at Friedrich Schiller University in Jena (Prof. Dr. Kai Uwe Totsche, Department of Hydrogeology) and the MPI-BGC Biogeochemical Processes Department (apl. Prof. Gerd Gleixner, Molecular Biogeochemistry). Microbial mediated formaton and degradation of the organo-Fe-moneral associations may be studied in cooperation with the Geomicrobiology Group (Prof. Kirsten Küsel) of FSU Jena. The PhD project is tightly linked with the collaborative research project AquaDiva, an ongoing coordinated research project that will grant access to its long-term observation data. Possible Collaborations beyond AquaDiva/IMPRS/Jena Experiment: Synchrotron based XAS-spectromicrosopy

Requirements for the PhD project are

We welcome applications from curious and motivated students from any country who have

A Master’s degree in Soil Science, Geochemistry, Hydrogeology, Mineralogy, Environmental Chemistry, or Environmental Geology.
Sound education in pedology, quantitative soil chemistry, or mineralogy, with a background in soil physics and soil hydrology.
Practical experience with analytical chemistry tools (e.g., fluorescence, chromatography, elemental analyzer), wet-chemical laboratory setups (column percolation), or solid-phase physical/spectroscopic techniques (FTIR, SEM, XRD, XAS) is highly valued.
Good oral and written communication skills in English

The Max Planck Society (MPS) strives for gender equality and diversity. The MPS aims to increase the proportion of women in areas where they are underrepresented. Women are therefore explicitly encouraged to apply. We welcome applications from all fields. The MPS has set itself the goal of employing more severely disabled people. Applications from severely disabled persons are expressly encouraged.

Atomic force (AFM) image recorded in PeakForce QNM mode of a bacterium, a clay platelet, and aggregated Fe oxides deposited on mica. The height image shows topography over a range of 0-117 nm, while the error image emphasizes topgraphic edges on a scale from -1.2 to 1.2. The adhesion image, shown on a scale from 0 to 8 nN, provides additional material contrast: Fe oxides can be distinguished because they display lower adhesion to the AFM tip (darker gray tones) than the bacterium, the clay platelet, and the surrounding mica surface.