Soil Organisms as Bioindicators: Assessing Soil Health and Ecological Risk

Soil organisms are fundamental facilitators of numerous biological processes. Due to their high sensitivity to various stressors, these organisms are widely employed as bioindicators to assess soil quality. Several soil mesofauna and macrofauna groups, including Collembola (springtails), earthworms, and Acari (mites), serve as biodiversity indicators for monitoring changes in below-ground biodiversity. Their utility is recognized in multiple monitoring programmes, as they reflect alterations caused by soil management or land-use changes.

These programmes commonly operate on the reference condition approach. This principle involves comparing the community at an impacted site to that of a reference site within the same region, with similar pedological, land-use, and climatic characteristics. This method is central to many national bioindication schemes, which use shifts in soil fauna communities to monitor soil health. The approach ensures assessments are ecologically relevant and site-specific.

A parallel methodology is applied in Site-specific Ecological Risk Assessment schemes. Here, changes in community composition and species richness at contaminated sites are compared to reference sites. This constitutes the Ecological Line of Evidence (ELoE), one of three complementary lines within the Triad approach. The Triad integrates the ELoE with the Chemical Line of Evidence (ChLoE) and the Ecotoxicological Line of Evidence (EcLoE) for a comprehensive risk evaluation.

The acute sensitivity of soil fauna to chemical contamination makes them excellent environmental indicators. Species such as springtails, earthworms, enchytraeids, mites, and beetles are used to assess the ecotoxicological potential of contaminated soils and evaluate risks from pesticides, industrial chemicals, and wastes. Standardized ecotoxicological tests measure effects on individuals, populations, or communities, with some tests being legally mandated for chemical authorizations, such as within the European Union.

Soil organisms inhabit distinct vertical strata, which is critical for accurate risk assessment. While many mesofauna reside in the top 5 cm, endogeic earthworms concentrate activity in the upper 20 cm, and anecic earthworms inhabit galleries up to 2 meters deep. Defining ecologically relevant exposure scenarios for these strata is essential for precise pesticide risk evaluation. This understanding of living strata also informs the development of a Soil Ecoregion map for Europe, which would integrate soil properties, land-use, climate, and the potential functional composition of soil communities.

Application One: Biodiversity Indicators and Monitoring Programmes. The depletion of soil biodiversity is a major threat to soil quality and ecosystem services in the EU. Consequently, developing operational indicators has been a priority. The ENVASSO project established a series of ready-to-use soil biodiversity indicators for monitoring programmes. Selection was based on three criteria: availability of a standardized methodology, complementarity to other indicators, and ease of interpreting results for both scientific and policy audiences.

Application Two: The Biological Indicator for Soil Quality (BISQ). The Netherlands employs the Biological Indicator for Soil Quality (BISQ) system within a national monitoring network. It uses 25 integrated indicators, including biotic parameters (e.g., nematode and earthworm communities), functional parameters (e.g., microbial respiration), and abiotic parameters. The core principle is comparing measured values at a site to reference values from corresponding undisturbed sites. Results are integrated into a radar histogram, where deviations from 100% indicate the degree of disturbance, providing a holistic assessment of soil sustainability.

Application Three: The Triad Approach for Risk Assessment. The Triad methodology synergizes three Lines of Evidence (LoE). The Chemical Line of Evidence (ChLoE) measures contaminant concentrations against limits. The Ecotoxicological Line of Evidence (EcLoE) involves toxicity tests on soil fauna using site samples. The Ecological Line of Evidence (ELoE) collects field data on species richness and composition. Applied in a tiered approach, it begins with simple screening tests (e.g., earthworm survival) and proceeds to detailed evaluations (e.g., reproduction tests, fauna surveys) if initial results are inconsistent.

Application Four: Ecotoxicological Testing Methodologies. Ecotoxicological tests with soil fauna form a battery of increasing complexity. Standard laboratory tests often use artificial soil (OECD soil), though natural soils are now preferred for greater ecological relevance. Tests measure effects on individuals (survival, growth) or populations (reproduction). Higher-tier semi-field tests, like mesocosm studies, use natural soils to evaluate community-level effects, such as changes in species composition, offering increased ecological realism for final risk characterization.