As the world’s human population continues to rise and is expected to reach 8.6 billion by 2030 (UN 2017), it is paramount to respect and protect natural resources, including soil, water, air, minerals, and biodiversity that support life on Earth, including humanity. Current rates of consumption and inadequate management of resources are putting unprecedented pressure on global systems and it is estimated that one to six billion hectares (up to 30%) of land has been degraded globally [1
]. Land degradation negatively affects 3.2 billion people, threatens sustained human well-being and is a major contributor to climate change and biodiversity loss [2
]. Global initiatives to meet these challenges include the UN 2030 Agenda for Sustainable Development (Figure 1
) and the UN Convention on Biodiversity’s Strategic Plan for 2020 (Table 1
). Broadly, these agendas address areas to improve human life and environmental sustainability, rely on the participation of all countries and stakeholders, and will require innovative, timely, and interdisciplinary approaches [3
Soils are central to supporting natural systems and human well-being [4
] (Figure 2
), yet to date soil biodiversity—the diversity of life in soil which drives ecosystems, sustains life aboveground, and maintains healthy landscapes—has remained largely overlooked in global agendas. For example, the term ‘soil biodiversity’ does not appear in any UN documentation while forests, wetlands, rivers, and drylands have received specific attention to their benefit. Soil-dwelling organisms, including bacteria, fungi, nematodes, earthworms, moles, and even plant roots, contribute the majority of living biomass on Earth [5
] and represent more than 25% of all described species [6
], not to mention the genetic diversity represented within these species. The activity and complex interactions of soil organisms provides the backbone for many ecosystem functions, including nutrient cycling, pathogen control, water infiltration, foundations to food webs, and supporting agroecosystems (Figure 2
). Our understanding of the critical connections between soil biodiversity and sustainability are rapidly progressing [8
]. The time has come to incorporate this knowledge to bolster global actions and create a more holistic sustainability agenda that can simultaneously address biodiversity loss, climate change, and land degradation.
Like most of the resources humans rely on, soils and soil biodiversity are under threat by land degradation, climate change, pollution, urbanization, and over-use and misuse [11
]. Soils are a finite, non-renewable resource because they cannot be replenished within a human lifespan [12
]. The formation of soils relies on a complicated balance between time, climate, topography, the underlying parent material, and of course organisms [13
]. Therefore, timing—the swiftness in which we act to protect soils—is crucial. Several global efforts have recognized the urgency with which we must act. For example, the Global Soil Biodiversity Initiative launched in 2011 to bring together researchers and policy makers to integrate the knowledge we are gaining with actions for a sustainable future. Other agencies have also started to include soil biodiversity in their consideration of soils, including the Global Soil Partnership, UN Food and Agriculture Organization, the Intergovernmental Platform on Biodiversity and Ecosystem Services (IPBES), and in 2019 the UN Convention on Biological Diversity requested a global report on soil biodiversity [14
Prior to these recent efforts, integration of soil for nature and global sustainability largely focused on soil’s physical and chemical properties with little consideration for biodiversity [15
]. But, it is now accepted that including soil biota explicitly alongside soil abiotic factors in land management assessments can better serve sustainability goals than consideration of soil abiotic properties alone [8
]. This holistic view of soils has gained traction from landowners, managers, and agencies and makes clear that diverse stakeholders are eager to protect this critical resource using interdisciplinary and sustainably focused methods. Despite this energy, more must be done to recognize and integrate the role of soil biodiversity in building a sustainable future. For example, the majority of soil biodiversity research examines diversity at a community level, across species and trophic levels; however, diversity within species is a critical component of biodiversity which has been all but ignored in soil habitats. Here, in an effort to guide global agendas and identify synergies between multiple sectors (research, users, public, and policy), we present (1) a synthesis of current research: Covering ways soil biodiversity can contribute to the global sustainability agenda; (2) global change and pressures that threaten soil biodiversity; (3) actions that will support conservation of soil biodiversity, while also affecting sustainability and biodiversity goals and targets; and (4) gaps in knowledge in linking soil biodiversity to sustainability.
3. Threats to Soil Biodiversity
Despite the important role soil biodiversity plays across ecosystems, soil organisms face many of the same threats as aboveground organisms and receive far less research, media attention, and legal protection [35
]. Habitat loss from land-use change, climate change, and invasive species, both above- and belowground, are as challenging for soil organisms as they are for terrestrial and aquatic organisms.
Habitat loss is the primary threat to soil biota. Agriculture is the largest driver of habitat loss and biodiversity declines globally [2
], including land conversion to agricultural use and management practices within agroecosystems. Conversion of Amazonian forest to agricultural land-use results in the homogenizations of soil bacterial communities and loss of soil fungal diversity [75
] as well as reductions in macrofauna [77
]. Agricultural fields support smaller and less diverse soil communities than forests and grasslands [17
] and agricultural intensification further reduces soil biodiversity, particularly larger bodied organisms (e.g., invertebrates) [78
]. Even reduced tillage systems typically host less soil biodiversity than natural ecosystems, as shown by Domínguez et al. [80
] in a study of grasslands and agroecosystems in Argentina. In addition to agricultural land use changes, urbanization and suburbanization leads to the destruction of soil habitats through building construction and pavement which results in soil sealing [81
Habitat quality can be degraded through pollution, including excessive nutrient inputs, and invasive species. Heavy metal pollution can shift communities to become dominated by a few taxa that can tolerate, or even thrive with, high levels of chemical inputs with corresponding decreases in taxa abundant in unpolluted soils [82
]. Increased N inputs, from atmospheric deposition or from direct fertilizer application, is also a form of pollution and can shift soil bacterial communities, decreasing Acidobacteria
and increasing Actinobacteria
, and decrease overall microbial activity [84
]. Habitat quality can also be impacted by invasive species. Invasive plants can alter belowground communities through release of exudates toxic to some soil organisms, such as arbuscular mycorrhizal fungi, changes in N-cycling, such as invasive legumes, shifts in fire frequency and intensity, and/or variations in plant litter and root inputs [85
]. In some cases, invasive plants increase diversity and abundance of soil organisms, particularly when invasive plants result in increased litter and root inputs [86
]. In other situations, soil organisms can be invasive species, reducing plant fitness and animal diversity and abundance [87
]. Invasive earthworms in northern North America negatively impact native soil fauna, alter C and nutrient cycles [88
], and affect plant community composition [89
]. Enough research exists to know soil habitats face multiple threats, from direct and indirect inputs of pollutants and nutrients, to shifts in aboveground communities, including exotic species. However, habitat loss and degradation are not the only threats to soil biodiversity.
Climate change is the paramount challenge of our generation, and soil biota are impacted like all other life on Earth. Climate change includes a suite of environmental changes including atmospheric concentration of greenhouse gases, namely CO2
, precipitation quantity and frequency, and temperature. Globally, these variables are predicted to shift in different directions and magnitudes, and as such, understanding how soil biodiversity responds to climate change requires deep understanding of both soil biodiversity in biomes, predicted impacts of climate change, and how soil taxa populations and communities may likely respond. A meta-analysis of soil microbial community studies found soil fungal abundance responses to elevated CO2
varied by taxon and ecosystem [90
]. Altered precipitation regimes impact insects, both above- and belowground. In an Australian grassland, summer drought periods corresponded with increased aboveground insect plant predator populations the following spring, but no changes in belowground invertebrates [91
]. Decreased precipitation increased root-feeding nematode populations in mesic and semi-arid grasslands by decreasing predator nematodes, but this pattern was not observed in xeric grasslands where nematode communities are adapted to prolonged drought periods [92
]. Temperature impacts soil communities in unique ways as well. Increased temperatures impact soil biota physiologically, as some taxa have a very narrow temperature range for optimal functioning and others can tolerate a broader range. In a study of springtails (Collembola) [93
], an exotic species was more tolerant of higher temperatures than native springtail species, potentially increasing the risk that exotic invasive species could usurp native soil communities as climates change. Given the numerous interacting consequences of global climate change and the hyper-diversity of soil communities, there are many uncertainties in understanding climate change impacts on soil biodiversity.
5. Knowledge Gaps
Global knowledge of soil biodiversity distribution lags markedly behind aboveground knowledge. New observations and understandings of soil-dwelling taxa and their habitats consistently come to light. Initiatives to bring together global datasets have created maps that show gaps in biogeography, biomass, and function, and species distributions of nematodes [143
], bacteria [144
], and fungi [145
]. These synthesis efforts have highlighted knowledge gaps in distributions of soil taxa persist. Given the greater uncertainty around global distribution of soil biodiversity compared with aboveground biodiversity, analysis has shown only 37% of areas with the highest levels of both above- and belowground biodiversity overlapped [146
]. More research is needed, particularly in the Global South, which is highly under-represented in the scientific literature, to refine these maps and build confidence needed to shape habitat conservation with soil biota in mind. Knowledge of soil microbial communities has grown exponentially in recent decades with the advent of molecular tools, but knowledge of many soil invertebrates has slowed as taxonomic expertise dwindles at many academic institutions. Understanding and appreciating soil biodiversity within taxa is a major knowledge gap. Soil organisms are estimated to have much higher proportions of undescribed species than larger terrestrial and aquatic taxa, which makes it challenging to evaluate which species may be in need of conservation and how best to prioritize conservation efforts [35
]. Furthermore, lagging understanding genetic diversity within soil biota, both fauna and microbial could slow discovery of pharmaceuticals [22
] as well as capacity for soil communities to respond to changing climate. Understanding of soil organisms, both microbial and faunal, detailed enough to determine conservation status (e.g., common/rare/threatened/endangered) is needed to catch-up to aboveground knowledge and inform action.
Linking soil biodiversity with ecosystem services and functions is a crucial area in which to build information. We know soil organisms are central to nutrient cycling driving crop and livestock production, but we lack clear understanding of which organisms or communities are most directly involved. This leads to ambiguity around recommending actions for producers to leverage soil biology for sustainable agriculture. Water quantity and quality is a top concern for agriculture and humanity. Linking soil biodiversity impacts on water movement and quality at the watershed scale, perhaps through modeling, and scaling those findings up to landscape-scale actions appropriate for addressing current and developing water concerns is a priority. Scaling up soil biodiversity knowledge for global climate action is another key challenge. Recent work is advancing understanding of how soil biology can influence carbon cycling and next steps involve including biodiversity in climate change models to improve predictions [67
Uncertainties also exist around threats to soil biodiversity and solutions to protect soil organisms and their contribution to sustainability. Enough research exists to know soil habitats face multiple threats, from direct and indirect inputs of pollutants and nutrients, to shifts in above-ground communities, including exotic species. However, many questions remain as to exactly how these factors impact understudied taxa like protists and enchytraeids. There is also a scarcity of soil biodiversity knowledge from certain habitat types, like urban areas, which have long been underappreciated by ecologists and soil experts. Even in well-studied systems, like agricultural production, there are knowledge blind-spots. Most research, including soil biodiversity research, has focused on industrial row-cropping practices with very little research on small-scale subsistence farming, which makes up a large portion of global agriculture. Increasing research in these areas will improve our understanding and increase expert confidence in recommending actions to protect soil biodiversity and leverage its functioning for global sustainability. We recommend additional research to build bridges between soil biodiversity expertise and real-world solutions for a sustainable future, and we also believe there is a need to act now, both to protect soil biodiversity and to advance sustainable development agendas.
Soil biodiversity knowledge and research is moving beyond academic circles and being used to support solutions to biodiversity loss, local (water quality, food security), regional (land degradation), and global (climate change) challenges. Managing soils, as the vibrant living systems they are, provides a new perspective for integrated actions and solutions. Soil organisms, microscopic and macroscopic, support all ecosystems: Cycling energy and nutrients to support plant and animal growth in terrestrial systems and maintaining nutrient balances in water, thereby affecting aquatic organisms and ecosystem health. The ways in which soil biodiversity interfaces with multiple ecosystem functions makes it a natural focus for advancing a holistic global sustainability agenda. Soil biodiversity is at the heart of natural solutions for climate, biodiversity, and humanity, including protecting natural areas, restoring degraded ecosystems, employing sustainable agricultural practices, and adapting urban areas for nature and people. As we work toward a sustainable future, let us not overlook the critical and diverse asset, right beneath our feet.