The Pierrelaye case study: transforming a legacy of contamination into a phytomanagement laboratory
From the late 19th century until the early 21st century, domestic raw wastewater containing organic matter and salts was deposited in the Pierrelaye agricultural plain, leading to contamination of the site with trace elements such as lead (Pb), copper (Cu), zinc (Zn) and cadmium (Cd). Intensive market gardening (maraîchage) further contributed to the long-term degradation of soil quality in the area. Data from the BIOSYSMO project revealed the presence of PAHs and pesticides residues in the soil at concentrations up to ten times higher than in nearby non-irrigated reference soils.
At the Pierrelaye site, pHYBi is building on past phytomanagement initiatives by working with existing poplar plantations as part of a circular approach. These plantations, established 15–20 years old, represent a unique long-term phytomanagement setting.
The project focuses on monitoring and optimising the system already in place, including the valorisation of poplar biomass through lignocellulosic fractionation pathways aligned with circular bioeconomy principles. In parallel, pHYBi evaluates how these long-term field conditions shape remediation trajectories over time and provides a basis for testing novel poplar hybrids with improved performance under suboptimal soil conditions, benchmarked against controlled experiments.
The site also offers a valuable reservoir of soil microorganisms adapted to these challenging conditions. By studying soil–plant–microbe interactions, including endophytic fungi, using both culture-dependent and culture-independent approaches, pHYBi aims to better understand their role in poplar performance and resilience in marginal soils.
Overview: different campaigns track soil evolution at Pierrelaye
During the project, three field monitoring campaigns are carried out at the Pierrelaye site to track how soils and vegetation evolve under long-term phytomanagement.
The first campaign, conducted last year, focused on the physicochemical characterization of both topsoil and subsoil layers. While trace elements are typically concentrated in topsoil, the presence of 15–20-year-old poplar plantations raises an important question: what happens at depth when roots gradually explore and modify subsoil conditions over time?
Subsoil compartments are still poorly understood, yet they may play a key role in long-term system functioning, both as a potential zone for trace element redistribution and as a habitat for microbial communities that remain largely unexplored.
This baseline survey, based on LUCAS sampling, included key soil properties (pH, texture, organic carbon, etc) as well as measurements of total and bioavailable trace elements, providing a reference point for following how the system evolves.
Recent activities: biological indicators and dedometric measurements
During this year’s campaign, the coordinators of Work Package 2, Humberto Castillo-González and Michel Chalot, together with Nicolas Garneret (research assistant engineer) and Enguerrand Denoux (M1 Master’s student completing a 10-week internship within pHYBi), focused on screening for biological indicators and monitoring trees to better understand ongoing phytoremediation processes at the site.
Soil samples were collected for microbial activity and community structure, including assessment of mycorrhizal colonization, providing insight into belowground biological functioning. At the same time, dendrometric measurements were carried out using a laser-based device to characterize growth. Leaf, bark, and wood samples were also collected for chemical analyses, allowing us to track trace-element uptake and accumulation within plant tissues.
To support plantation-scale monitoring, drone imagery was acquired across the approximately 6-hectare plantation. However, the complexity of the canopy structure made data acquisition challenging, leading to further exploration of complementary remote sensing approaches.
What the latest monitoring reveals about fungi, poplars and soil recovery at Pierrelaye
The latest monitoring campaign at the Pierrelaye site highlights how long-term phytomanagement, past land use, and current poplar plantations interact within a complex soil–plant system.
- The site is characterised by coarse, sandy, near-neutral soils, which strongly control water movement, nutrient availability, and trace element behaviour.
- One notable finding concerns the legacy of mycorrhizal inoculation carried out around 15 years ago. Ectomycorrhizal structures are visible on some poplar roots as white, star-like branching patterns. Are these the same fungal partners introduced 15 years ago, or new ones that have taken over in the meantime? That’s still under investigation, stay tuned.
- Among the several poplar varieties present at the plantation, Skado was selected within pHYBi for further focus and valorisation. One of its particular features is its high leaf turnover, which contributes to limiting spontaneous vegetation. Clear differences in vegetation structure are observed between planted and unmanaged areas
- Phytotoxicity tests show that soils from historically managed plots support higher germination and improved root elongation compared to untreated soils. In parallel, bioavailable trace element concentrations are generally lower in managed areas, consistent with long-term changes in metal behaviour, including reduced mobility in soils and potential transfer into plant biomass
- Finally, poplar trees and their rhizosphere act as active microbial reservoirs, supporting diverse microbial communities. Metagenomic data further show that this biological activity extends into deeper soil layers, confirming that subsoil compartments are far from inert.