3.1 State and trends of soil biodiversity

Describing the state and trends of soil biodiversity within a national assessment of soil biodiversity is an important step to provide a comprehensive picture of the current state of soil biodiversity, its spatial distribution and changes over time, which in turn can provide a basis for developing policy and governance strategies to conserve and sustainably manage soil biodiversity. While this spatial and temporal characterization may be easier for some groups (e.g., for earthworms), others (e.g., bacteria) may require special consideration of the relevant scale and characteristics to be represented. It would also help to identify the critical gaps in local knowledge and understanding of soil biodiversity, such as the role of microbial communities and the interactions between above- and below-ground biodiversity, but also support the establishment of national distribution maps of soil organisms and monitoring systems that target specific areas or are closing important knowledge gaps. While such information may be seen as a mere mapping of current knowledge, it is crucial for researchers, policy-makers, land managers and other stakeholders that aim to improve the knowledge base on soil biodiversity and functions in their country or region. Therefore, national assessments should consider (i) presenting the distribution of soil biodiversity groups as well as the main ecosystem functions and services associated with them; (ii) highlighting the trends in distribution, abundance and community composition of particularly threatened groups; and (iii) identifying vulnerable areas of strong soil biodiversity loss and gaps in research, including taxonomic experts.

3.2 Direct drivers of soil biodiversity, ecosystem functions and services

Drivers of biodiversity change can be differentiated based on their characteristics (biological, chemical and physical), their direction of effect (positive or negative), their mode of function (e.g., toxicity vs. change in resource availability or energy requirements; Baas et al., 2010) or their specificity (i.e., whether they are specific to one taxon, have consistent effects across taxa or if they induce cascades across soil taxa and functional groups) (Rillig et al., 2021). Given that driver attribution and general ranking (i.e., in terms of importance) may substantially vary across countries and environmental conditions, as well as given that different groups of organisms vary in their responses to environmental change, detailed information on multiple taxa and traits at the national level is urgently needed to better understand and predict threats to the different facets of soil biodiversity (FAO et al., 2020). Therefore, a national assessment should provide evidence of their impacts and an outlook of potential or effective ecological, social and economic consequences. Special attention may be given to co-occurring drivers of soil biodiversity that can have synergistic effects (Beaumelle et al., 2021; Krause et al., 2023; Rillig et al., 2019), where the consequences of multiple stressors can intensify each other and thus have more pronounced effects than one would expect to find based on single-driver studies (Rillig et al., 2019; Thakur et al., 2020). For example, the effects of increasing temperature were shown to be minor under ambient water conditions, but detrimental to soil biological activity under drought (Thakur et al., 2018). Therefore, when assessing the drivers of changes in soil biodiversity, ecosystem functions and services it is important to (i) describe the distribution of specific drivers, including their intensity and frequency (when relevant); (ii) identify areas where these drivers accumulate, including their nature; and (iii) their potential trends based on socioeconomic development pathways.

3.3 Indirect drivers of soil biodiversity, ecosystem functions and services

How soils are managed is driven by higher-level factors related to economic forces, policies, consumption patterns, land use, culture and tradition. Given the multiplicity of factors that affect soil biodiversity in an indirect way as well as the scarcity of policies directly focusing on the protection of soils (Bartkowski et al., 2021), let alone soil biodiversity (Köninger et al., 2022), it is essential that national assessments of soil biodiversity adopt a broad view and include as broad a set of factors as possible. This may be challenging due to the scarcity of dedicated research on indirect drivers of soil (biodiversity) change in social and political sciences, especially beyond agriculture. Because of that, it may be necessary for soil biodiversity assessments to rely on indirect evidence, for example, related to the drivers of changes in land use or management that are known to have effects on soil biodiversity (e.g., consequences of the common agricultural policy on tillage or pesticide application). Furthermore, it is essential that assessments consider indirect drivers of soil biodiversity change across sectors, including land-use sectors (e.g., agriculture, food production and forestry), relevant policies (agricultural policy, energy policy, settlement policies and regional planning, etc.) and relevant value chains (e.g., food from farm through retail to consumption) (Bartkowski & Bartke, 2018; Gütschow et al., 2021). An increasingly important role can be expected by the impact of education and social media (e.g., the recent rise in veganism may substantially impact animal husbandry and any associated land use practice).

3.4 Links between soil biodiversity and ecosystem functions

The interactions of different coexisting soil organisms with their abiotic and biotic environment drive a multitude of different ecosystem functions and affect their ecological state (Bardgett & van der Putten, 2014; Bender et al., 2014; Geisen et al., 2019; Wagg et al., 2014). Experiments have shown that the diversity of soil organisms is critical for ecosystem functions like soil formation, organic matter decomposition (Bradford et al., 2002, 2014; Hättenschwiler et al., 2005; Heemsbergen et al., 2004), carbon cycling (Nielsen et al., 2011), nutrient dynamics (Sheehan et al., 2006), soil water dynamics, soil aggregation and aeration (Lehmann et al., 2017), plant growth (Eisenhauer et al., 2018; van Groenigen et al., 2014) and plant community succession (De Deyn et al., 2003). However, the composition and roles of biological soil communities vary across environmental contexts, requiring assessments of context-dependent relationships between biodiversity and ecosystem functioning. This nuanced view of the effects of soil biodiversity on multiple ecosystem functions in different contexts is essential for a national assessment as it provides local evidence on the context-dependency of such relations. It can also serve to highlight the positive or negative effects of local land use and/or management practices and support the identification of regional best practices. In a larger context, with the future global availability of national soil biodiversity assessments, these contextual examples can also be used to support an international framework of good practices to support ecosystem multifunctionality in soils from different land-use types. In fact, when establishing these links between soil biodiversity and ecosystem functions in such an assessment, several considerations should be taken to capture different local land use and soil types and the national environmental gradient. For instance, linking experimental results to observational studies and/or remote sensing generated data (Wellmann et al., 2020) allows to better understand these causal relationships. When local relationships are missing it is important to highlight such gaps (e.g., geographical, environmental or land-use type) to give room for the next assessment to fill them and also to give clear directions to new research topics that may need to be developed.

3.5 Soil-related ecosystem services

Although soils are a central component of ecosystems, and soil-related ecosystem services are essential for human well-being, there is a plethora of soil-related ecosystem services with almost no standardization or specific classification (Paul et al., 2021). Soil biodiversity influences numerous soil-related ecosystem services, but most of the regulatory influences of soil biodiversity on soil-related ecosystem services are only theoretically known. More importantly, the lack of scientific evidence is obvious for the links between soil and soil biodiversity and human health (Banerjee & van der Heijden, 2023; Brevik & Sauer, 2015). National assessments of soil biodiversity may thus highlight that the analysis of causal relations between soil biodiversity and ecosystem services is complex, given the national combination of biotic and abiotic characteristics (e.g., pore volume, humus and other C content and nutrient content) as well as land management measures (Bethwell et al., 2021) that shape ecosystems and, therefore, the supply of ecosystem services. Especially cultural ecosystem services related to the perceptions, values, indirect effects and benefits, for example, the contribution of soil biodiversity to landscape aesthetics, are more difficult to assess. Considering these aspects, a national assessment of soil biodiversity should focus on first identifying the relevant ecosystem services provided by soils using available data and literature (Paul et al., 2021), and second on identifying the distribution, frequency (e.g., ecosystem services supply is known for having temporal and spatial variability) and amount of the supply of the identified services. Together, these aspects will allow for a better identification of the most important benefits and values from ecosystem services. Furthermore, the supply of soil-related ecosystem services within one country might depend on or come from areas beyond national administrative borders (interregional flows), for example, flood regulation/flood risk management (Kleemann et al., 2020), and, therefore, requires monitoring systems across national borders (Koellner et al., 2019; Schröter et al., 2016) including the use of remote sensing techniques (Wellmann et al., 2020).

3.6 Sustainable approaches for soil nature conservation

Given the enormous biological diversity in the soil, it is important to note that conservation policies at multiple scales (e.g., regional, national or global), and more importantly nature conservation actions, have rarely considered soil biodiversity (Cameron et al., 2019; Eisenhauer & Guerra, 2019; Zeiss et al., 2022). This becomes even more apparent in comparison to marine or aboveground terrestrial biodiversity that were frequently and prominently used to underpin the establishment of nature conservation policies (Guerra et al., 2021; Jung et al., 2021). These facts are all the more alarming as the majority of publications on insect decline do not point out that a huge share of aboveground insect species (in particular Diptera, Hymenoptera and Coleoptera) spend most of their life as larvae in soils. Recently, at the global level, countries engaged in the Convention of Biological Diversity have agreed to consider the assessment and sustainable use of soil biodiversity in their national legislation. While this is an important step, it is now essential that national assessments of soil biodiversity provide clear guidelines that highlight the nature conservation potential as well as the vulnerability of local soil communities. As a direct contribution to this aspect, a national assessment should evaluate nature conservation management practices and existing nature protection areas, to assess the existence and feasibility of targeted actions to protect and sustainably manage soil biodiversity and ecosystem functions (Zeiss et al., 2022). This approach allows for the identification of positive and/or potentially negative consequences for soil biodiversity and can aid future research and policy-makers in assessing the effectiveness of different nature management, conservation and restoration practices. As this approach targets nature conservation management plans and their included actions, it also serves as an important mechanism to improve awareness among conservation managers of the importance of local, context-dependent actions to improve soil condition and protect soil biodiversity.

Another aspect to consider is the existence of Red List species within national borders. This requires two parallel approaches: (i) the identification of currently classified Red List species within the country's borders (including their distribution), and (ii) if not existing, the elaboration or support of the identification and assessment of national Red List species. While Red List species are significantly biased (Phillips et al., 2017), recent efforts (e.g., focusing on fungi [Põlme et al., 2020] or myriapods [Karam-Gemael et al., 2020]) show that, when targeted approaches are implemented, a significant amount of information on soil communities can be gathered, and the understanding of their vulnerabilities improved. In addition, since most conservation management practices and nature protection areas are related to a given ecosystem, such as agricultural sites, forests or urban landscapes, we advise that interdisciplinary teams should be formed to aid in the evaluation of such management practices. Ultimately, such an approach can guide future management practices and the establishment of nature protection areas, as well as identify ecosystems, areas or soil organisms that are in need of sustainable management or active conservation and protection.

3.7 Governance options

National assessments of soil biodiversity should not only study the drivers, state and trends as well as potential consequences for human well-being but also critically review the effectiveness of management practices to halt biodiversity loss and propose governance options for biodiversity conservation and sustainable use. We suggest that such a portfolio of governance options should be evidence-based, ecologically effective, economically feasible, target group-oriented, considered legitimate and relevant by societal actors and take into account local and regional as well as national (and international) constraints (Bartkowski et al., 2021). Given that many stakeholders, such as land owners/users, consumers and policy-makers, can indirectly and directly influence soil biodiversity and quality, the development of options for action should not happen in isolation but in close collaboration with stakeholders and with the support and at best a mandate from political decision-makers (Albert et al., 2017). In that context, it is important that the development of options for decision-makers, including land managers, considers the context specificity of its recommendations, which reflects the ecological and societal heterogeneity, and associated trade-offs. For instance, local trade-offs may arise with above-ground processes and related management or nature protection actions. Considering societal heterogeneity and these trade-offs or synergies can improve the effectiveness and acceptance of sustainable options for actions. Citizen science can be a helpful tool in that endeavour. Although a specific list of options for action will vary from country to country as well as over time, we suggest some key options for action that may be discussed in each national assessment of soil biodiversity. Such options include (i) the establishment or long-term support of a nationwide, coordinated and regular soil biodiversity monitoring (including accompanying pedological data); (ii) incentives for site-specific and environmentally friendly soil management, for the diversification of landscape structures (Lausch et al., 2015), and the reduction of land take (including unsealing of sealed soils); (iii) the designation of more protected areas focusing on specific soil organisms and their sustainable management; and (iv) the transfer of knowledge into society to raise awareness for the importance of soil biodiversity. Ultimately, such a target-oriented, integrative and societally inclusive approach can initiate the necessary processes to protect soil biodiversity and sustainably manage soils at the local, regional and national levels.