Monitoring soil biodiversity with citizen science in Europe: current initiatives, challenges and opportunities

To achieve the large-scale monitoring of biodiversity, the inclusion of multiple actors is required, notably of citizen scientists. Citizen science programs not only enlarge the spatial and temporal coverage across taxonomic groups, but they also have strong societal impacts with increased knowledge and empowerment of the participants. As in traditional ecological research, the vast majority of participatory biodiversity monitoring is focused on aboveground diversity and non-invertebrate, non soil-related taxa. Here, we performed the first synthesis of citizen science projects targeting soil biodiversity at the European scale. Combining a classic literature review on Web of Science and an online search using keywords in different languages, we identified more than 40 projects, in addition to national monitoring schemes led by natural history societies. The projects take place in at least 12 countries spanning an important latitudinal gradient from Spain to Norway, although the majority of the projects are from the United Kingdom and France. Taxa studied are only large-size animals (macrofauna) of different trophic groups (detritivore, predator, herbivore) with an overall strong focus on earthworms. An important diversity in terms of protocols and type of data collected show the adaptability potential of citizen science to different audiences and their level of expertise to ensure the best knowledge production. In the future, main challenges are the need for interoperability between projects, improved taxonomic coverage, as well as increasing the integration of citizen science methods and data into classic research programs and studies. Citizen science will contribute importantly to soil biodiversity monitoring and raising awareness on soil life that is extremely poorly represented in conservation programs and legislations.