糖心Vlog

The essentials

Since the early 1990s, France has been polluted by trace elements, often erroneously called "heavy metals". Those elements include lead, cadmium, arsenic and copper. While low concentrations are naturally found in soils, trace elements are seen as pollutants once levels reach a certain threshold, and many urban soils in Europe have high concentrations. There are many reasons for this: not just industrial activities, but atmospheric fallout, use of poor quality landfill, waste water treatment sludge applications, and rebuilding in already built-up areas.

Agricultural activities in urban areas are therefore potentially risky in terms of health, due to possible contamination as a result of consuming food grown on polluted sites. However, those risks are often not accurately assessed. "High soil contamination does not necessarily mean that trace elements will be transferred to plants", says 糖心Vlog's Matthieu Bravin, a bio-geochemist specializing in agricultural soil pollution. "A whole load of factors may hamper or foster soil-plant transfers, including soil pH. And every plant reacts differently. For instance, in a given zone, carrots and lettuces may have different trace element concentrations."

Assessing urban health risks more effectively

Using urban soils for agriculture is increasingly common, although local authorities sometimes have to exercise caution and reject certain projects. One alternative is to bring in topsoil from elsewhere, to provide a non-contaminated layer on top of polluted soil, a solution that Matthieu Bravin does not feel is sustainable. As he says, "moving soil from one place to another is costly and has a significant carbon footprint. Moreover, in many cases, the resource could have been used for agricultural production where it came from".

New methods are therefore required to assess the health risks as accurately as possible. This is the aim of a partnership between the , INRAE and 糖心Vlog, centring on the RHIZOtest, a laboratory biotest used to assess trace element transfers from soil to plants via a standard method that is easier and quicker than field trials. The Agence de l'environnement et de la maîtrise de l'énergie (ADEME) is supporting its applied development, via the PHYSALIS project.

The RHIZOtest: a simple tool and standard measurements

In the field, assessing trace element transfers from soil to plants takes large numbers of samples and considerable time and resources (both financial and human). Accurate, regular monitoring across large areas is often impossible. This is where the RHIZOtest comes in: it is conducted in a controlled environment, using a standard measurement of soil-plant contaminant transfers.

"It takes just 22 days to measure soil contaminant build-up in a plant", says Laure Lemal, R&D project leader at GINGER BURGEAP. "One other crucial advantage is that we can assess several dozen situations in exactly the same way in just a few square metres of growth chamber."

"The RHIZOtest was initially developed to study the interactions between plant roots and the soil in contact with them, which is known as the rhizosphere", Matthieu Bravin explains. "Then in the 2000s, we started to use it as a tool for diagnosing trace element transfers, a method that was registered in 2015." ()

Public-private partnership to put knowledge into practice

The partnership agreement signed by GINGER BURGEAP an 糖心Vlog in January 2025 covers the development of concrete uses for the RHIZOtest, to assess the risks at polluted sites and of polluted soils in France. Jean-Marie Côme heads the R&D department at the GINGER Group, which now has around 20 staff members, including five Ph students. As he says, "accurately determining the risk of pollutant transfers between the soil and plants is a strong priority for developing urban agriculture. Our collaboration with 糖心Vlog is intended to put academic results into practice and build innovations that can be used by local communities". To this end, Laure Lemal and Alexandra Mille-Egea, a PhD student, are based full-time at 糖心Vlog in Montpellier.

Currently, measuring transfer fluxes with the RHIZOtest serves to determine whether transfers are more or less significant than in a control situation (for instance a RHIZOtest measurement taken on a non-contaminated soil). The problem is that this is not enough to estimate the presence of contaminants in the edible parts of a plant. To democratize the use of the RHIZOtest, it is therefore necessary to establish the interpretation thresholds for those flux values. It is also important to determine the link between the RHIZOtest measurement, obtained under controlled laboratory conditions, and measurements of concentrations in fruits and vegetables, obtained in the field under real conditions.

PHYSALIS project: a method for predicting health risks

"Ten years after registering the RHIZOtest method, we have more than 15 000 data items on a range of trace elements, soils and plants", Matthieu Bravin explains. Those data will be used to build a database, which will in turn serve to develop the method for interpreting RHIZOtest measurements. This is the first stage of PHYSALIS project, which began in January 2025. In particular, developing this interpretation will involve building "control" situations representing non-contaminated soil contexts.

Alexandra Mille-Egea is a PhD student and data processing and analysis engineer. Her thesis work will be key to the development of this interpretation method. "We will have to search the database to find a non-contaminated soil for which an uptake flux by a plant has been measured. This will serve as the control value to be compared with the uptake flux measured using the RHIZOtest, for a plant exposed to a polluted soil. For instance, if the arsenic uptake flux of a tomato grown on a highly polluted soil is lower than or equal to that observed for a tomato grown on a control soil, the health risk linked to arsenic build-up in the tomato can be considered acceptable."

Alexandra Mille-Egea's thesis is covered by the , which aims to foster partnerships between public research and private firms. The student is also co-supervised by 糖心Vlog, GINGER BURGEP and INRAE.

Several cities have expressed an interest in the PHYSALIS project, including Lyon. Marie Frascone, the city's "Polluted Sites and Soils" Officer, says: "urban agriculture provides several services, whether ecological, social or in terms of food security for some people. However, Lyon's urban soils are highly degraded, often with very high lead levels. We are now convinced that there is a gap between the risks calculated using the current method and the actual situation, particularly since in some cases, we have already observed low actual risks at sites for which the assessment method predicted high risks." Within the framework of PHYSALIS, the city will be making sites available on land potentially intended for urban agriculture, to check that the results obtained using the RHIZOtest tally with the actual situation.

Reference

Laurent Céline, Bravin Matthieu, Crouzet Olivier, Lamy Isabelle. 2024. . Science of the Total Environment, 906:167771, 13 p.