Impact of fires on chromium speciation and bioavailability in tropical agroecosystems (Réunion Island)

Context
Crop fires, a common practice for agricultural land management, release numerous pollutants into the environment. While the atmospheric pollution they cause is well-documented, recent research suggests they may also mobilize toxic trace elements in cultivated soils.
Chromium (Cr) is a striking example. Naturally present in some soils, it is often found in a harmless form (Cr(III)). However, the high temperatures generated by fires can transform it into its toxic and carcinogenic form, hexavalent chromium (Cr(VI)). This transformation potentially threatens the safety of soils, water, and food. Yet, the mechanisms of this fire-induced redox transition, as well as its long-term consequences for agroecosystems, remain poorly studied.

This study is part of the international FIRE project (Induced REdox transitions of toxic metals in the soil-plant-air continuum), which brings together the University of Vienna, working on Cr-contaminated rice fields (India), and two French teams (Cirad and Cerege) studying sugarcane fields cultivated on naturally Cr-rich plots (Réunion Island).

Objectives and Research Questions

This PhD project aims to assess the environmental and health risks associated with the transformation of Cr(III) to Cr(VI) due to fires in tropical agroecosystems. The candidate will seek to answer the following questions:

• How does fire intensity, influenced by crop type and soil properties, impact the formation of Cr(VI)?
• Is soil-to-plant transfer of Cr(VI) observed in cultivated plots subjected to fire?
• What are the mechanisms behind the transformation of Cr(III) from soil minerals to Cr(VI)?

Methodology

To address these questions, the candidate will adopt a multi-scale approach, combining field experiments (Réunion Island) with laboratory analyses, to understand the transformation of Cr(III) to Cr(VI) due to fires.

Field experiments involving agricultural fires will be conducted on experimental plots cultivated on Réunion Island. During these controlled fires, the evolution of soil temperature profiles will be recorded. Sampling and analysis of the various soil horizons affected by the fires will allow the transformation and availability of Cr to be assessed. Similar measurements will be carried out on ash samples collected from the soil surface. On the same plots, sampling of cultivated plants (sugarcane) will quantify Cr uptake and assess the health impact of fires. Finally, a comprehensive characterization of metal speciation in key samples (soils, ashes, and/or plants) will be performed using advanced spectroscopy techniques (XAS, HERFD-XAS, μXRF).

Job location
CEREGE, Aix-en-Provence, France

Required candidate profile
The candidate must :

• Have a scientific background in life sciences or earth sciences, with an option/emphasis in environmental sciences.
• Have strong knowledge in analytical chemistry, geochemistry, or related disciplines.
• Be proficient in English and have strong writing skills.

Experience (coursework or internship) in agronomy and/or skills in plant biology would be a plus.