In conservation theory and practice, biodiversity can be maintained and protected through two general conservation approaches; 1) protec ting areas and species and 2) integrating conservation in use and management of land (Hummel et al. 2019, Niesenbaum 2019). Methods employed may differ among different land-use perspectives.

To contribute to conservation and sustainable management of soil biodiversity, a few initiatives and research networks have been established over the years. Agreements and definitions of the conservation of soil biodiversity were brought to the international agenda by FAO in cooperation with the Convention on Biological Diversity (CBD) with the International Initiative for the Conservation and Sustainable Use of Soil Biodiversity, established in 2002. In 2012, the FAO set up the Global Soil Partnership (GSP) to further increase attention and work on soils, due to their vital importance for food and agriculture. Another important initiative is the Global Soil Biodiversity Initiative (GSBI), a network of scientists, policy and public, that was established in 2011 and has given an international platform for assessing and synthesising knowledge on soil biodiversity.

The collection of data on soil biodiversity is a challenge that is also raised by e.g. the GBIF (Global Biodiversity Information Facility), an international network and data infrastructure funded by the world's governments and aimed at providing open access to data about all types of life on Earth. In addition, the European Cost Action Edaphobase will create the structures, capacities and procedures necessary for expanding the existing data platform on soil fauna (“Edaphobase“) into an open, publicly available data warehouse for Europe-wide soil biodiversity data as well as for developing tools that use and evaluate this data. Additionally, efforts such as the Soil Biodiversity Observation Network (SoilBON) https://www.globalsoilbiodiversity.org/soilbon are aimed toward systematically collecting observational data on soil biotic and abiotic factors worldwide to assess the condition of soil biodiversity and functions with a focus on the effects of protection/conserved status of the land area (Guerra et al. 2021).

One of the major barriers in the capacity to develop effective soil conservation practices and policies is the lack of standardised indicators and methods to collect data to provide baselines and trends in monitoring conservation of soil biodiversity (from now on SB). Different methods used by different research groups to measure the same taxonomic groups and their functions, although acceptable for local conservation purposes, result in incomparable datasets across different regions and temporal scales. Additionally, it remains unknown if, and to what degree, the spatial and temporal resolution of the measurements of environmental parameters are adequate to the actual resolution of SB presence and abundance data thus far (Eisenhauer et al. 2021, Gábor et al. 2022). The question remains, what standard indicators and methods provide substantial information on the levels of soil biodiversity and associated ecosystem functions? (Guerra et al. 2021).

Predictive modeling. Modern statistical analyses such as Scenario Modelling, Species Distribution Modelling (Salako et al. 2023), General Dissimilarity Modelling, and Niche-Space Modelling can be effective ways to predict adequate habitat and select areas for conservation of SB. These can also overcome some sampling bias but cannot be a “silver bullet” due to the small-scale heterogeneity of communities in soil. Success of these techniques will require (1) further collation and harmonisation of observational soil-biodiversity data at an international level (including a paradigm change among researchers and funding agencies regarding open-access data sharing (e.g. Michener 2015, Tedersoo et al. 2021), (2) improved precision in the association between observational SB data and environmental and climate metadata, as well as (3) a strategy to increase capacity-building efforts in these efforts as well as training new taxonomic experts and local stakeholders/practitioners to conduct accurate assessments and use models. How can experts, methods, and models be further developed and honed to provide an effective portfolio of best practices?

Richness vs. Function. What about soil biodiversity needs protecting? In contrast to aboveground life, which is more easily observed and vastly more investigated, the richness and ecosystem functions of soil invertebrate and microbial taxa are still in need of clarification. This leads to the question, what aspect(s) of SB should be the target of conservation? While the overall diversity (species richness) of taxa in soil is important in and of itself (Phillips et al. 2020), the functional aspects of soil faunal and microbial life cannot be lost in the process of protecting taxonomic diversity (Phillips et al. 2020). Active restoration and conservation require attention to this complexity of species diversity and other biodiversity facets (e.g. size variation, life history traits) (Eisenhauer et al. 2021, Guerra et al. 2022Eisenhauer et al. 2021, Guerra et al. 2022) as well as a diversity of functions (Nielsen et al. 2010Nielsen et al. 2010).Maintenance of species richness, community composition, and ecosystem functions are not often synonymous, and investigations into a trait-based approach to SB conservation and restoration are largely lacking (Guerra et al. 2022). Auclerc et al. (2022) summarised the importance of functional-trait approaches to restoration with soil invertebrates, but also detailed critical knowledge gaps. These include:

1. the lack of use of trait-based techniques for restoration and, therefore, a dearth of evidence of its benefits,

2. general lack of knowledge of the roles invertebrates play in ecosystems,

3. a lack of representation of these data in current trait-based databases, and

4. the complex relationships of function to traits (Auclerc et al. 2022)

What conservation methods protect soil biodiversity? Since conservation management and site selection have typically not considered SB and its ecosystem functions, it is still unclear how current conservation affects SB and how to adjust current conservation and restoration practices to positively impact soil biodiversity across the EU and regionally. We know that current site selections and management practices do not generally benefit SB (Zeiss et al. 2022), but potential biodiversity-friendly management options exist that could be "scaled-up" (Barrios et al. 2023). However, regions across Europe must be evaluated for what specific SB communities and associated functions they are capable of supporting. Guerra et al. (2022) showed that, globally, areas that may rank highly in one ecological dimension, such as species richness, may not rank highly in another. This suggests that potential sites for conservation are not equal, nor can they be treated similarly, when considering what areas to conserve and what restoration/conservation practices are effective when targeting SB.

Many soil taxa are simply unknown to science and awaiting description (Orgiazzi et al. 2016) because:

A. soil fauna and microbes are often cryptic and difficult to observe without disturbing their functioning and habitat, and the variance in the diversity of these communities is significant over just millimetres (Rillig et al. 2015);

B. microbial taxa are difficult, sometimes impossible, to isolate and culture with our current methodologies. This is compounded by the differences in methods necessary to detect and quantify different soil organisms due to heterogeneity in their ecologies (ranging from water-related to truly terrestrial species), size classes (ranging from microbes to megafauna), and distribution patterns (White et al. 2020, Decaens 2010, Eisenhauer et al. 2021);

C. for invertebrate taxa, specialised taxonomic expertise is often needed to identify species within groups of soil animals. Expertise in many soil fauna groups is rare, leading to a perpetual cycle of infrequent opportunities for knowledge transfer and a dwindling body of experts;

D. and, lastly, we lack a unified definition of soil biodiversity to use as a basis for policy development and regulatory measures (Rillig et al. 2019, FAO et al. 2020, Orgiazzi 2022).

This lack of understanding of what soil-dwelling taxa exist leads to a fundamental roadblock in pursuit of other information vital to SB conservation, such as their distributions and understanding the ecosystem functions and services they provide to humans and other life on Earth.

We lack critical information on the ecologies of most soil taxa, the habitats in which they dwell, and what drives their distributions (Cameron et al. 2018) to be able to understand how and where conservation can be achieved for different taxonomic groups. In the same wheelhouse as function, ecology includes “how and what” biota interact, procreate, and consume in their environment to drive ecosystem functions and services for human well-being. Studies in the ecology and life histories of soil-dwelling species are time-consuming and detail-oriented undertakings necessary to understand their ecosystem functions and effects on other life, yet they are often considered not innovative enough to be funded. Current knowledge in invertebrate ecology is based on manipulative landscape experiments and some direct observation and mesocosm experiments, the latter two of which are rare research approaches in ecology, but common in biological control. In microbial research, the current methods include molecular methods for identification (i.e. metabarcoding, “shotgun” approaches), which many fewer studies on the functional genes that reveal what different microbes digest and release.

Current understanding of distributional patterns is based on expert knowledge, observational data from landscape gradient studies, and/or available records in museum collections, but these vary in utility. One common issue if the lack necessary environmental and climate metadata to associate taxa to habitat characteristics is missing from publications and essentially non-existent in museum records (Gotelli et al. 2023). While large-scale efforts are underway to coalesce observational data on species’ occurrences in international databases, again the required environmental and climate metadata is often missing in uploaded datasets. Experimental research on the response of soil taxa presence and diversity to environmental predictors is patchy (Phillips et al. 2023), biased towards unrealistic levels of edaphic parameters change and unrepresentative for the tropics (Cameron et al. 2018, Guerra et al. 2020), and not directly comparable across ecosystems.

Drivers. Data and theory on the influence of drivers on small- and broad-scale distributions are widely lacking (Thakur et al. 2020, Eisenhauer et al. 2021), rendering conservation assessments and priorities difficult (Decaens et al. 2006). This includes the drivers of community dissimilarity in soil taxa across ecosystems, along with their uniqueness (e.g., endemic species, specialisation for given habitats). For instance, while disturbed habitats can show high species richness and total densities, these are often caused by generalist species, leading to a homogenization of SB and loss of diversity at the landscape scale in a region or country (Gossner et al. 2016, Delgado-Baquerizo et al. 2021, Guerra et al. 2021, Banerjee et al. 2024). Recent work revealed the ubiquity of complex interactions between multiple co-occurring environmental drivers that could affect distributions or evolutionary tactics (Rillig et al. 2019), yet these are poorly studied. These complexities, including effects of land-use and human pressures, are needed in an integrated evaluation of current practices. Extrapolation of conclusions from agricultural research investigating increasing SB for increased ecosystem function can be a starting point for developing best practices. Long-term studies and experiments focusing on specific techniques, such as dead wood management in forests, recognition of trees as “hot spots” of soil biological activity and encouraging heterogeneous soil habitat through diversification of plant species (Eisenhauer et al. 2018) are needed to understand their direct and indirect effects on SB.

Considering qualitative (functional) attributes of soil life during assessment of site-scale measures will vastly improve conservation of SB at broader scales i.e. landscape scale (Ciobanu et al. 2019, Zeiss et al. 2022). These would inform, specifically, the proportion of species within a local community specifically adapted or specialised to the site/habitat as a first approximation in assessing “intact” habitats for soil health as well as land-use measures and conservation.

The current knowledge on threats and, especially, extinction risks for soil-dwelling biota is little and inconsistent, but vital to knowing where and how to start conserving this diverse biotic group. We have more questions than answers: Which potential aboveground biodiversity threats are also threats to SB? What known threats need redefined thresholds to inform conservation decisions? How do we define threats to SB that may be overlooked in conventional conservation thinking (e.g. intentional foreign microbial inocula)?

Unknow extinction risks. The vulnerability of soil invertebrate and microbial organisms, including rare species, is almost entirely unknown and little progress has been made (the best being Decaens et al. 2008). Bottlenecks to conservation of SB include identifying very rare/threatened, endemic, and vulnerable species and their habitats for protection (Veresoglou et al. 2015).

Currently, abundance and distribution baselines and thresholds for soil organisms comparable to those for above-ground organisms do not exist though they are urgently called for by policy (EEA 2023). “Red Listing” of soil invertebrate organisms is rare (Phillips et al. 2017, Mueller et al. 2022), for one reason, because typical criterion for listing, such as “population size” in a region or country, are inappropriate for organisms in substrate such as soil. Few studies have incorporated IUCN criteria (i.e. IUCN 2022) for identifying threatened or endangered soil species (Marchán and Domínguez 2022, Salako et al. 2023). However, this necessitates answers to some fundamental, yet wholly uninvestigated, questions: What defines rarity for soil taxa? How appropriate for the myriad of soil taxa are local abundance, habitat specificity, and/or geographical distribution in determining rarity? How do we determine susceptibility to extinction for soil biota?

To identify and have threatened species recognized, this requires knowledge of the species and its detailed functional criteria, especially in the case of species that are highly sensitive to climate shift, invasion of exotic species, etc. Moreover, standardised assessment criterion for rare or threatened taxa across the EU as is necessary for European and EU regional efforts of conservation (van der Putten et al. 2023). With these standards, we can identify the taxa at risk, create a preliminary list of what species/OTUs are threatened, and identify conservation practices, concrete management options, and potential sites for conservation. This is critical to predict the fate of soil organisms under global change and ensure their conservation.

A corollary to the identification of rare, threatened, and endemic species is, what are the criteria to designate something as invasive with regards to SB? This has not been taken into consideration, primarily, because the directionality of invasions in soils is difficult to determine, and we are unaware of the identity of most local and invasive soil taxa. It is also unknown what environmental or economic damage 'invasive' organisms can cause to soils and ecosystems, unlike similar studies in, for example, agricultural settings. The two barriers to finding out this information are that:

1. there is little way to track invasion or origin of a present microbial OTU and

2. specifically for microbes, there is no conceptual models to think about what a species is in the way other species are conceptualized.

Increasing the taxonomic information of soil biota communities, starting with that in already vulnerable ecosystems, such as those susceptible to the increasing oscillations in heat and temperature regimes, will be critical to provide the foundation to monitor the influence of soil invasive species on the functioning and stability of our ecosystems.

Conservation of soil biodiversity is a multifaceted process involving, what we expect will be, a multitude of approaches that will benefit the large-scale diversity of soil life across Europe as well as the needs and environments of the regions within Europe. Developing effective ways to conserve and monitor the trends in soil biodiversity across the complex functions of these communities is as important as the communities themselves and should be taken into account in considering their protection. To do this, increasing cooperation between soil ecologists and other disciplines for, as examples, chemical analyses and molecular and morphological identification, continue to be integral to developing a full understanding of the soil biodiversity community. In this regard, one of our responsibilities for the future of soil health and should be plans for current and future monitoring and ‘assessments’ i.e. Soil Biodiversity Observation Network (SoilBON), Global Soil Biodiversity Observatory (GLOSOB) (Eisenhauer et al. 2021, Guerra et al. 2022, Nielsen et al. 2010).