Handbook to establish a large-scale soil biodiversity monitoring: the French experience of the RMQS-Biodiversity

International audienceOne quarter of the living beings are located beneath our feet but we know very little of them (FAO, 2020). This statement will perhaps quickly change because the soils are now on the political agenda. For France, soil is mentioned in the Green Pact at European level (Montanarella, 2020) and in the country government's Biodiversity Plan. The law on soil health is also planned for 2023 (Köninger et al., 2022). However, as its biological component remains poorly understood, the indicators used by stakeholders give only a biased view of soil quality (Lehmann et al., 2020). It is therefore urgent to catch up on the knowledge of soil biodiversity in order to establish benchmarks for bioindicators, based on standardised data.In parallel, the French Biodiversity Office working on the linkages between all French terrestrial biodiversity monitorings, pointed out the absence of soil biodiversity monitoring in France.Rather than creating de novo a soil biodiversity monitoring, it was preferred to add biodiversity surveys to the already existing French Soil Quality Monitoring Network (RMQS), hereafter called the RMQS-Biodiversity. The RMQS covers a big part of the French territory (the continental part as the over seas) since 2000. Every year, 180 study sites are sampled. Thus, all the sites are sampled in 10-12 years (Jolivet et al., 2018). The RMQS provides data about all physical and chemical aspects of soils. Regarding the soil biological component, microorganisms and enzymatic activities are also surveyed. By the past, soil fauna was studied on around 100 sites but the experience was not maintained (Imbert et al., 2021). Moreover, a major strength of the RMQS is the network of involved people included the 12 field teams, the coordination team, the funders and data users (researchers and stakeholders).To implement the biological measurements, we gathered a group of experts on soil biodiversity. As meetings go by, five protocols were defined to assess the most exhaustively possible the soil biodiversity taxa and three functions (soil macroporosity, enzymatic activities and organic matter degradation).  Then, the protocols were tested in real conditions on 30 RMQS study sites with the field teams. The duration of each protocol was quoted to clearly assess the costs.We concluded that the biodiversity sampling of 180 RMQS study sites per year, would cost around 1 000 000 euros. We propose five scenarios giving compromises between financial costs and data quality.If the RMQS-Biodiversity is maintained, it would make possible: 1) to advance on the still too partial knowledge of soil biodiversity and its interactions with agricultural practices and 2) based on the knowledge acquired, to develop bioindicators and their benchmarks, in order to accurately assess soil quality, in the context of Soil Health (Lehmann et al., 2020).  A complete soil monitoring, including its three components (physical, chemical and now biological), would thus provide a relevant tool to policy- makers to reach reconciling human activities and soil integrity