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Earthworms (L. terrestris) have dug tunnels through yellow earth and subsequently deposited casts in the top litter layer, thus mixing minerals and plant debris. Photo taken of a laboratory terrarium. © INRA, FAYOLLE Léon

Soil ecotoxicology and agroecology: partners in progress

Developing new bioindicators for use in soil ecotoxicology

Ecotoxicology is focused on developing bioindicators, which are needed to complement tools already available and which will reveal how soil ecosystems are being modified.

By Catherine Foucaud-Scheunemann, translated by Jessica Pearce
Updated on 07/31/2014
Published on 07/03/2014

Lumbricus terrestris. © INRA, Céline Pelosi
Lumbricus terrestris © INRA, Céline Pelosi

Soil makes up the top layer of the earth’s crust; it is also a vibrant ecosystem that is home to highly diverse flora and fauna. Additionally, it is the interface between the atmosphere and the earth’s deeper layers and a medium through which water, gas, and other materials are exchanged; its functions have a particularly important role in agricultural production. Finally, it is a resource that varies across space and that may suffer sometimes irreversible damage.

Given the complex roles of the soil layer, contamination by chemical pollutants can interfere with certain environmental functions, such as the maintenance of biodiversity, the degradation of pollutants, and the cycling of organic matter.

By developing relevant and reliable biological indicators, otherwise known as bioindicators, it is possible to evaluate the risk of organisms being exposed to chemical pollutants while accounting for societal practices that can influence exposure. Such indicators must also be able to characterize the state of an ecosystem and how well it is functioning. Bioindicator development is a goal that INRA researchers are assiduously pursuing.

Evaluating the ecotoxicological impacts of heavy metals on soil organisms

Many surfaces that could normally be cultivated suffer from environmental problems that render them unsuitable to produce food crops. However, they can still be used in other ways.

This is true of the Pierrelaye Plain, which is located to the west of Paris. It spans an area of over 20 km2 that was historically used for farming. During several decades of the last century, however, the plain was exposed to untreated water coming from Paris, whose contents polluted the soil with heavy metals (zinc, lead, copper, and cadmium).

Soil analyses have shown that although the taxonomic diversity of macroinvertebrates living in the plain’s soils has remained largely unaffected by the levels of heavy metals, macroinvertebrate abundance has decreased. However, not all species are equally affected since the impact they experience is dependent on the pathway of exposure. For example, species that consume organic matter, such as earthworms and slugs, are more affected—because they ingest the pollutants—than other species that experience direct contact.

Earthworm species are not equally affected by pesticides

Reducing the use of pesticides is an essential part of plans to make agricultural practices more sustainable. Although reducing pesticide use results in an overall increase in the number of earthworms found in fields, not all species respond in the same way. For instance, Allolobophora chlorotica shows less of a response than Lumbricus terrestris or L. castaneus. Furthermore, not all pesticides have the same effects: insecticides have a greater impact on earthworms than do herbicides or fungicides.

Although risks associated with pesticide use are usually evaluated by examining the compounds’ direct effects on organisms (e.g., diversity or abundance), it is also important to examine how pesticides affect the ecological functions of these organisms.  For instance, earthworms, through their consumption of soil, play a major role in maintaining soil structure. It has been shown that simple tests that estimate the production of casts (earthworm excretions) can help reveal if earthworm activity has declined as a result of the use of certain pesticides.

Establishing new bioindicators

These results suggest alternative ways of determining the effects and outcomes of chemical pollutants found in soils. Instead of focusing on a single species, which does not allow the full impacts and repercussions of chemical pollutants to be detected, it is better to examine a group of organisms with different functional roles that exist at different trophic levels and in different habitat types. One of the major challenges in ecotoxicology is establishing new associated bioindicators and biomarkers.

 In order for a bioindicator to properly reflect the overall condition of a given environment of interest, it should:

  • be scientifically well described (for example, its biology and ecology must be well established, including its diet, reproductive patterns, trophic level, and pollutant exposure pathways);
  • specifically account for different land management methods and types of soil pollution;
  • be tied to or correlated with ecosystem functions;
  • account for physical, chemical, and biological soil properties or processes;
  • be quantitatively dependable (precise, reliable, and robust);
  • be valid (reflect natural variability through a range of responses);
  • be easy to use and affordable (both when sampling and analyzing the samples).

Furthermore, agroecology is uncovering new issues that should be explored by ecotoxicological research with a view to acquiring a better understanding of how chemical compounds behave and affect the living world. For instance, it is important to address the bioavailability of micropollutants as a consequence of their speciation and the ability of mixed substances to act as stressors in not just individual organisms but in whole populations. It is also crucial to take into account the benefits that humans receive from properly functioning ecosystems, that is to say, ecosystem services, and to revisit the procedures by which chemical substances are approved with regards to the structure and functioning of agrosystems.

References

- Hedde M. et al. 2013. Responses of soil macroinvertebrate communities to Miscanthus cropping in different trace metal contaminated soils. Biomass and Bioenergy 55:122.

- Pelosi C. et al. 2013. Reduction of pesticide use can increase earthworm populations in wheat crops in a European temperate region. Agriculture, Ecosystems and Environment 181: 223.

- Hedde M. et al. 2012. Functional traits of soil invertebrates as indicators for exposure to soil disturbance. Environmental Pollution 164: 59.

Capowiez Y. et al. 2010. Earthworm cast production as a new behavioural biomarker for toxicity testing. Environmental Pollution 158: 388.

RESEARCH PROGRAMS DEDICATED TO SOIL ORGANISMS

  • BETSI - The biological and ecological functional traits of soil invertebrates. Relationships between species and environmental factors. The response of soil organisms to environmental factors, and the development of bioindicators. Headed by Mickael Hedde (INRA Versailles-Grignon)  
  • RESACOR - Reconversion of agricultural land with contaminated soils. Headed by: Isabelle Lamy (INRA Versailles-Grignon) 
  • BIO2 - Bioindicators of soil quality; brings together several collaborating institutions, one of which is INRA (Dijon, Provence-Alpes Côte d’Azur, Rennes, and Versailles-Grignon centers).