Applicability of soil biodiversity indicators in transnational monitoring schemes

Soil biodiversity is remarkably extensive, encompassing a wide range of organisms from microbes to large invertebrates living in the soil and on the soil surface. While the importance of soil organisms for ecosystem functioning is clear, our understanding of how human actions and global changes impact life in and on the soil remains limited. Additionally, we know little about how shifts in community composition, such as those driven by global change, might influence ecosystem processes. This uncertainty highlights the urgent need for broad-scale monitoring efforts. As part of the Biodiversa+ partnership, we are conducting a pilot study on soil biodiversity to support the development of transnational soil biodiversity monitoring.

For this, we are conducting an assessment of soil biodiversity at various broadleaved and coniferous forest sites across ten countries in and around Europe. In accordance with the SoilBON and SoilBON Food Web protocols, we employ pitfall traps and hand-sorting of soil cores to collect soil macro-invertebrates, which are then morphologically identified to the family level and, where feasible, to the species level. Additionally, bulk soil samples are collected for molecular analysis to determine the richness of bacteria, fungi, and invertebrates, enabling a comparison between traditional morphological identification methods and eDNA-based analyses.

To enhance the clarity of our results for the public and stakeholders, we assess essential biodiversity variables (EBVs) and diversity indicators obtained from both traditional species data and molecular data. Examples for indicators derived from morphological species identification include carabid richness, abundance and biomass as well as abundance and diversity of soil macrofauna. An example for a morphology-based EBV is the community biomass of selected functional groups of terrestrial arthropods. When appropriate primers are selected, molecular methods yield data for a wider range of taxa than traditional methods, for instance, all arthropods regardless of their assignment to a specific size group (mesofauna, macrofauna). Consequently, indicators based on molecular data include arthropod diversity, fungal species richness and fungal community composition. In our study, we examine the suitability of these indicators, as well as their respective strengths and limitations, in the context of soil biodiversity monitoring.