PhD Position in Rhizosphere Biogeochemistry: Soil genesis and Root Exudation, and the impact on Free-Living Nitrogen Fixers and Molybdenum

The PhD candidate will be based at KU Leuven, Department of Earth and Environmental Sciences, Division of Soil and Water Management, in the SPIRAL lab led by Prof. Jean-Thomas Cornelis. The group studies how microscale soil processes drive macroscale ecosystem properties, with a focus on soil–plant feedbacks, biogeochemical cycles and ecosystem resistance, resilience and restoration. We work across diverse soils and eco-cultural contexts, and we value multi-disciplinary, collaborative and respectful science.

SPIRAL stands for Soil Genesis and Plant Interactions: Reciprocal feedbacks Across Landscapes. The group was founded at the University of British Columbia (Canada) as the SoilRes3 Lab and Prof. Cornelis has just moved to KU Leuven; read more about their research and philosophy at

Unit website

Project

Nitrogen limits plant productivity in many terrestrial ecosystems. Free-living (asymbiotic) nitrogen fixers in the rhizosphere can supply a significant amount of nitrogen, without relying on symbiosis or synthetic fertiliser. However, nitrogen fixation is energetically very costly, and the nitrogenase enzyme needs molybdenum (Mo) as a co-factor, a trace element that is often poorly available in soils.

This project tests the idea that the way plants can actively steer this process through their root exudates is highly dependent on soil-forming processes. We hypothesise that soil-plant stoichiometry influences (1) the root release of easily degradable carbohydrates that fuel free-living N fixers as an energy source, and (2) carboxylates (e.g. citrate, malate, oxalate) that mobilise Mo bound to soil minerals and make it available for nitrogenase. Together, these mechanisms may create a positive feedback loop: more belowground plant carbon fuel free-living N fixers while supplying Mo needed for the enzymatic factory. Soil water conditions are expected to modulate each step of this loop.

By linking root exudation, Mo biogeochemistry and microbial N fixation, the project aims to understand how plants and soils co-regulate nitrogen inputs to terrestrial ecosystems, with implications for sustainable agriculture and ecosystem restoration.

As a PhD researcher, you will investigate the plant–soil–water–microbe interactions that control free-living N fixation in the rhizosphere. More specifically, you will:

• Quantify root exudation (carbohydrates and carboxylates) under contrasting soil and water conditions, using rhizoboxes, mesocosms and/or field experiments
• Measure free-living N fixation rates (e.g. 15N2 incubations, acetylene reduction assay) and link them to the abundance and activity of diazotrophs (e.g. nifH gene markers)
• Study how carboxylates mobilise Mo (and associated Fe) from soil minerals, using batch experiments, soil fractionation and advanced (isotope) geochemical tools
• Investigate how soil moisture and drying–rewetting cycles affect exudation, Mo availability and N fixation
• Integrate these results into a conceptual and/or quantitative model of the plant–soil positive feedback loop
• Publish your results in international journals, present them at conferences, and contribute to the supervision of master’s students

Profile

Required

• A master’s degree in Bioscience Engineering, Soil Science, Environmental Sciences, Earth Sciences, Biology, Chemistry or a related field
• A strong background in soil science and biogeochemistry
• A strong interest in soil–plant–water interactions and feedbacks in terrestrial ecosystems
• Enthusiasm for laboratory and experimental work
• Good written and spoken English
• Curiosity, critical thinking, and the ability to work both independently and in a team

Assets

• Experience with plant growth experiments, rhizosphere sampling or root exudate collection
• Experience with soil chemical analyses (e.g. ICP-MS, chromatography) or stable isotopes
• Familiarity with soil microbiology
• Skills in data analysis (e.g. R or Python)

You don’t tick every box? Please apply anyway. Skills can be learned, and we value curiosity and motivation as much as experience.

Offer

• We offer a competitive full-time PhD scholarship for 1 year (renewable for another 3 years) in accordance with KU Leuven's doctoral salary scale.
• Supervision by Prof. Jean-Thomas Cornelis
• A stimulating, international and multi-disciplinary research environment, with access to state-of-the-art laboratory facilities
• Flexible work organisation that leaves room for creativity and new ideas
• Opportunities for training, conference participation and international collaboration
• Membership of the KU Leuven Doctoral School and its training programme
• KU Leuven offers additional benefits in terms of health insurance, public transport at reduced rates, free bicycle use, access to university sports facilities, childcare opportunities – among others. You can find more information on working conditions at KU Leuven on the below-mentioned website.

Department of Earth and Environmental Sciences

Division of Soil & Water Management

Working conditions at KU Leuven

Arenberg Doctoral School

Interested?

For more information please contact Prof. dr. Jean-Thomas Cornelis, mail: jean-thomas.cornelis@kuleuven.be.

We warmly encourage you to reach out to current and former lab members to hear about our research environment.

Please include a CV, a motivation letter (max. 1 page), transcripts of your bachelor’s and master’s degrees, and the contact details of two referees.

You can apply for this job no later than November 16, 2026 via the online application tool

KU Leuven strives for an inclusive, respectful and socially safe environment. We embrace diversity among individuals and groups as an asset. Open dialogue and differences in perspective are essential for an ambitious research and educational environment. In our commitment to equal opportunity, we recognize the consequences of historical inequalities. We do not accept any form of discrimination based on, but not limited to, gender identity and expression, sexual orientation, age, ethnic or national background, skin colour, religious and philosophical diversity, neurodivergence, employment disability, health, or socioeconomic status. For questions about accessibility or support offered, we are happy to assist you at this email address.

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