Soil: Biodiversity & Restoration
Forest Preserves of Cook County Stewardship Council
13 January 2019
Elizabeth Bach, Nachusa Grasslands, The Nature Conservancy
Soil as a reservoir of biodiversity
Soil is home to more than 25% of total global biodiversity, and the majority of soil biodiversity has not been described by science. Soil-dwelling organisms include microbes, earthworms (annelids), round-worms (nematodes), water bears (tardigrades), springtails (collembola), mites (acari), pillbugs/rolly-pollies (isopods), insects, spiders, burrowing mammals, reptiles, and amphibians. These organisms play critical roles in ecosystems, including tallgrass prairie, savannas, and wetlands. Soil is home to many parts of insect life-cycles, supporting pollinator services and providing an important component of the aboveground food web. Soil organisms cycle nutrients that support plant growth and diversity, through mutualisms and pathogens. The presentation will highlight biological diversity belowground and explore how it is intricately linked to diversity aboveground.
The Global Soil Biodiversity Atlas: www.atlas.globalsoilbiodiversity.org
Bach, E.M. and Wall, D.H. 2018. Trends in Global Biodiversity: Soil Biota and Processes. In:
Dominick A. DellaSala, and Michael I. Goldstein (eds.) The Encyclopedia of the Anthropocene, vol.
3, p. 125-130. Oxford: Elsevier. (attached)
Soil supports ecosystem restoration
Many ecosystem restorations in the Chicago region generally begins with sowing seeds into soil. Soil texture, such as sandy vs. clayey, plays a critical role in shaping plant communities. How does soil biology shape plant community restoration, specifically in tallgrass prairie restoration? This is an emerging field of research and evidence is only beginning to accumulate. We know certain plants require specific soil microbial partners to germinate, such at Eastern Prairie Fringed Orchid (Platanthera leucophaea). Warm-season grasses are more dependent on mycorrhizal fungal partners than cool season grasses. Legumes host nitrogen-fixing bacteria, changing nutrient cycling and in turn changing competition conditions between plant species. Evidence suggests mycorrhizal fungal partners can improve establishment of late-successional prairie plants with higher coefficients of conservatism. Soil biology may even synch with plant phenology, both interacting with plants differently from emergence, flowering, and dormancy and interfacing with plants with different seasonal life histories. The presentation will synthesize the current scientific literature investigating above/below-ground relationships in tallgrass prairie restoration specifically, including caveats and gaps in current knowledge.
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Bach E.M., and Wall D.H. (2018) Trends in Global Biodiversity: Soil Biota and Processes. In: Dominick A. DellaSala, and Michael I. Goldstein (eds.) The Encyclopedia of the Anthropocene, vol. 3, p. 125-130. Oxford: Elsevier.
© 2018 Elsevier Inc. All rights reserved.
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Trends in Global Biodiversity: Soil Biota and Processes
EM Bach and DH Wall, Colorado State University, Fort Collins, CO, United States © 2018 Elsevier Inc. All rights reserved.
Why Does Soil Biodiversity Matter?
Soil is one of the most complex habitats on Earth and hosts an astounding diversity of life. It is estimated that 25%–30% of all species on Earth live in soils for all or part of their lives (Orgiazzi et al., 2016). Biodiversity in soils and its functioning is critical to sustaining life on Earth, including human life. Ecosystem functions from which people derive benefits are considered ecosystem services. Ecosystem services provided by soil organisms include supporting crop and livestock production, housing antibiotics and pathogens, controlling nutrient loads in soils and water, and regulating greenhouse gas cycling and climate change (Bardgett and van der Putten, 2014). People also actively manage soils extensively to maximize ecosystem services like food production. Below we highlight ways that soil biodiversity supports human life, is at risk from human actions, and can be supported by humanity for a healthy future.
What is Soil Biodiversity?
Organisms from all three domains of life, Archaea, Bacteria, and Eukaryota (including protozoa, fungi, invertebrates, vertebrates, and plants), live in soil (Table 1). Soil animals are classically organized by size: microfauna (<0.1 mm), mesofauna (0.1–2 mm), macrofauna (2–20 mm), and megafauna (>20 mm). These organisms interact in soil food webs (Fig. 1), which transfer the essential building blocks of life: carbon, nitrogen, and phosphorous. Recent advances in next-generation sequencing of soil organisms and high-throughput measurements of their activities have deepened scientific understanding of who lives beneath the soil and what they do in ecosystems around the world.
The global distribution of soil biodiversity (Fig. 2) does not appear to follow the pattern seen in plants and animals aboveground. Unlike aboveground organisms, many of whom follow latitudinal gradients and have more diversity in the tropics, there is little evidence that soil biodiversity has a similar pattern (Bardgett and van der Putten, 2014). Global studies of soil biodiversity show that soils are highly diverse, each having a different species composition that is restricted to a specific area or a high degree of endemism (Fierer et al., 2012; Wu et al., 2011). Factors affecting the distribution of soil organisms globally do not necessarily mirror aboveground movement patterns. Soil animals are limited in their ability to move great distances and can exhibit high levels of diversity speciation across small spatial scales, as observed for rotifers (Robeson et al., 2011). Microorganisms have limited ability to move within the soil matrix, but can be easily transported on atmospheric aerosols and within water bodies. However, molecular work indicates environmental factors such as climate and soil conditions, like pH, shape microbial commu- nities to a greater extent than movement (Fierer et al., 2012). Overall, high levels of belowground biodiversity do not necessarily correspond with “hotspots” of aboveground biodiversity, such as the tropics (Fierer et al., 2012; Wu et al., 2011). This means prioritizing conservation efforts to areas with high aboveground biodiversity may not protect soil organisms and the benefits we derive from their interactions and resulting ecosystem functions and services.
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Interactions among soil organisms and the soil habitat result in many ecosystem functions, which in turn provide ecosystem services. Loss of biodiversity breaks connections in the transfer of water, energy (in the form of carbon), and nutrients, thus limiting ecosystem functions and services that sustain life, including human life (Bardgett and van der Putten, 2014). The distinction between ecosystem functions and services is that functions are biological, geochemical, and physical processes and components within an ecosystem, and services are benefits that humans obtain from ecosystems. Ecosystem services were grouped into four broad categories by the Millennium Ecosystem Assessment (2005): provisioning, regulating, supporting, and cultural. Provisioning ecosystem services provide products such as food, fiber, and/or fuel. Regulating services include cycling of water, air, and elements like carbon and nutrients that maintain balance in ecosystems and globally. Supporting services are structures like habitat for living organisms, including humans. Cultural services include areas and items of particular significance to human cultures, for example plant, animal, and mineral products used in ceremony and rituals, sacred landscapes, and products used for cultural identity, such as livelihood, art, jewelry, and clothing. Soils provide key ecosystem services under all four categories.
Soil provisioning services include food production, from crops and livestock, fiber, and biotechnology, like pharmaceutical products and bioremediation of polluted areas. Soil organisms provide food and fiber by supporting plant growth. Decomposition is a key function that directly impacts productivity of ecosystems. When soil organisms decompose dead plant material, they release carbon and nutrients including nitrogen and phosphorous that are essential components of DNA and compose parts of plant cells. Soil organisms use some of these nutrients, but many of them are used by actively growing plants. Together, soil biodiversity and nutrient cycling comprise important components of the ecosystem service, soil fertility. Essentially all soil organisms are involved in decomposition and recycling of nutrients, and reductions in soil biodiversity consistently reduce plant litter decomposition (de Graaff et al., 2015), impacting plant productivity including crop yield (Bender and van der Heijden, 2015). Soil stability and the prevention of erosion is a key provisioning service, protecting buildings and maintaining soil fertility. Roots, animals such as earthworms and macroarthropods, and fungi play key roles in structuring and stabilizing soils (Six et al., 2004). Soil organisms are also involved in biocontrol of agricultural pests and disease. Agricultural management that enhances soil biodiversity may provide similar levels of protection against pests as conventional agrochemicals and produce comparable crop yields, in the same ecosystems (Crowder and Jabbour, 2014). In addition, soil organisms are source of pharmaceutical products like penicillin (from the fungus Penicillium chrysogenum) and tetracyclines (from the bacteria Streptomyces spp.) (Orgiazzi et al., 2016).
Regulating services include climate regulation. Plants pull carbon dioxide out of the atmosphere through photosynthesis and use the carbon for growth of roots, stems, and leaves. When the plant dies, soil organisms use some forms of carbon, nitrogen, and phosphorous in the dead plant material for energy and growth and the remaining carbon can remain in soils for years (Bardgett and van der Putten, 2014). Some of the plant material consumed by earthworms, mites, collembolan, fungi, and bacteria is respired as carbon dioxide or other greenhouse gases like methane and nitrous oxide, completing the carbon and nitrogen cycles (Crowther et al., 2015). Carbon and nitrogen cycling impact water quality, as soil organisms release nitrate, ammonia, and phosphates during decomposition. In most ecosystems, plants and other organisms consume these essential nutrients. Excess nutrients, which can originate from fertilizer application, can exit the soil as rain water moves through on its way to surface waters and groundwater (Bardgett and van der Putten, 2014; Orgiazzi et al., 2016). Surplus nutrients in streams and ponds can lead to rapid growth or blooms of cyanobacteria and algae that use up dissolved oxygen in water, killing fish and other aquatic organisms. Soil biodiversity through regulation of decomposition and transformation of nutrients is a key to supporting plant growth and maintaining nutrient balance in waterways.
Supporting ecosystem services include soil formation and habitat (Orgiazzi et al., 2016). Organisms are involved in all stages of soil formation, from colonizing rock surfaces through the establishment of vegetation and persist in highly weathered soils with limited nutrients. Soil formation and maturation supports not only the organisms living in the soil, but the aboveground organisms that consume plants and insects. Soil structure provided by roots, earthworms, macroarthropods, and fungi is essential habitat for other soil organisms and plays a key role is a soil’s ability to remain a long-term support for aboveground communities (Lavelle et al., 2016). Soil biodiversity even supports aquatic life through their role in regulating water nutrient concentrations (Bardgett and van der Putten, 2014). For example, different species of earthworms can influence water infiltration and retention in soil (Ernst et al., 2009). In addition, increased soil moisture can impact denitrifying bacterial communities and increase their activity, reducing nitrate to nitrogen gas (Orgiazzi et al., 2016). Soil biodiversity is an interconnected support network for all life on earth.
Cultural services include communities and community values centered around working the land like farming or ranching, spirituality, creative expression, and natural capital (Orgiazzi et al., 2016). Many cultures around the world have deeply rooted respect and reverence for their homeland and for soils. Many ancient and historic civilizations built earth mounds for numerous purposes, including burial of the dead, spiritual rituals, and for housing. Humans have used clay soils for millennia to form bowls, pots, jars, cookware, and jewelry for both function and aesthetics. Soils have been used as source of pigments for painting and dying almost from the beginning of human history (Orgiazzi et al., 2016). Furthermore, soils serve as an archeological repository, holding evidence of past human civilizations and livelihoods.
Threats and Interventions
Soil and soil biodiversity is strongly impacted by human activities (Food and Agriculture Organization of the United Nations, 2015). Globally, soil biodiversity is experiencing high risks in many parts of the world, particularly those areas with high human
Ecosystem Functions and Services
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Fig. 3 Map of global threats to soil biodiversity (Orgiazzi et al., 2016).
populations and intensive use (Fig. 3). The top global threats to soil and soil biodiversity all result from human activities, including land use practices and climate change.
Land use change encompasses the three most significant threats to soil at a global scale: erosion, loss of soil carbon, and nutrient imbalance (Food and Agriculture Organization of the United Nations, 2015). Soil erosion occurs naturally to a degree, but human impacted landscapes, especially agricultural lands, are particularly vulnerable due to removal of perennial vegetation, engineered changes in hydrology (e.g., drainage ditches, tiling, and increased impervious surface), and mechanical working of soil (e.g., tillage), which dramatically increases risk of soil erosion by wind and water. Erosion directly impacts soil organisms through loss or relocation of organic matter-rich topsoil. Soil ecosystem engineers like earthworms and termites can either play a role in facilitating soil erosion or stabilizing soil against erosion (Lavelle et al., 2016). Several land use factors influence soil carbon as well. Erosion can redistribute soil carbon or remove it entirely. Land use practices also decrease soil biodiversity and abundance (Tsiafouli et al., 2015), alters soil organisms’ activity, and shifts the rate of organic matter processing and storage (Bardgett and van der Putten, 2014). For example, tillage increases oxygen availability in soil, increasing aerobic respiration and resulting in the respiration of soil organic matter as carbon dioxide and other greenhouse gasses (Crowther et al., 2015). Conversely, restoration of degraded lands can increase soil carbon stocks (Orgiazzi et al., 2016). Nutrient imbalance is also largely driven by agricultural practices. Addition of fertilizer, whether inorganic or organic, greatly impacts soil nutrients, generally increasing nitrogen, phosphorous, and potassium, which are essential for growing crops. However, shifting the abundance of nitrogen, phosphorous, and/or potassium relative to soil carbon can impact soil communities and diversity (Leff et al., 2015). Another key concern with soil nutrient imbalance is the potential for increased nutrient leaching from managed systems, which can negatively impact aquatic life (Orgiazzi et al., 2016). Conversion of forests to agricultural use is a major human-driven shift in land use (Food and Agriculture Organization of the United Nations, 2015). With increasing human populations, especially concentrated in many of the most food insecure regions of the world, it is critical to recognize that action is needed to increase food security in a way that minimizes the risk to soil to protect long- term provisioning ecosystem services, including food production.
Climate change threatens soil biodiversity in many ways (Bardgett and van der Putten, 2014). Increased carbon dioxide can increase microfauna and detritivores, but decrease mesofauna and herbivores, bacteriavores, fungivores, and predators. Changes in temperature and precipitation impact soil communities differently across ecosystems. Increased temperature decreases soil com- munities from cold dry ecosystems, but does not have consistent impacts across ecosystems. Reduced precipitation reduces soil biota in forests, but neither drought nor irrigation had a consistent impact on soil communities in other ecosystems (Blankinship et al., 2011). Interactions among soil biota and microorganisms also impact rates of decomposition and resulting fluxes of carbon dioxide. Crowther et al. (2015) found combining warming and nitrogen addition increased fungal growth and decomposition rates, but only when soil invertebrates were excluded, indicating interactions across trophic levels can mediate climate responses. Furthermore, these responses can lead to feedbacks, where increasing temperature increases respiration of greenhouse gasses which in turn causes further changes in global temperatures (Crowther et al., 2015). Interactions between organisms, climate, and their soil habitat are complex, but this complexity may provide novel insights to improve our scientific understanding of climate change impacts now and into the future. Climate modelers are actively looking to incorporate soil microbial (Xu et al., 2014) and faunal communities (Filser et al., 2016) to improve predictions for future global change.
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Other human activities also contribute to the major threats to soil biodiversity. Soil nutrient contents can be shifted by deposition of nutrients like nitrogen and sulfur, which enter the atmosphere through exhaust from factories, cars, and planes (Crowther et al., 2015). The nutrients can be carried far from the point of entry and be deposited in ecosystems through precipitation or dry deposition. Invasive species introduced by humans include soil organisms, such as European earthworms, which dramatically altered litter decomposition and nutrient cycling in much of North America (Craven et al., 2016). In Europe, the invasion of the New Zealand flatworm threatens native earthworm populations. Invasive plants can also negatively impact soil communities through competition and allelopathy, which in turn negatively impacts native aboveground communities and ecosystem services (Orgiazzi et al., 2016).
There are numerous activities people can do to protect soil biodiversity and sustain the ecosystem services soils provide. Within agroecosystems, management approaches including no-tillage and conservation tillage in agroecosystems can increase soil biodi- versity (microbes and animals) and resulting ecosystem services (Bardgett and van der Putten, 2014; Tsiafouli et al., 2015). Alternative agricultural practices like agroforestry and perennial crops can support larger and more diverse soil communities, including microbes, insects, and mammals, than annual row-crops (Crowder and Jabbour, 2014). These systems may also require fewer fertilizer and pesticide inputs as they integrate soil biological functioning with agricultural services. In some situations, restoration or rehabilitation of degraded ecosystems is possible. Major remediation projects attempt to transform a completely altered ecosystem, such as a mining area, into a functioning ecosystem. Soil biodiversity, including microbes and some animals, can play a direct role in these projects, as many soil bacteria and fungi can utilize heavy metals and other forms of pollution that are toxic to many other plants and animals (Bradshaw, 1997). Soil organisms, including fungi, earthworms, and plants, can be used to stabilize soils and control erosion, as mentioned above. Other forms of restoration may require removing one or more invasive soil animal or plant species or planting a diverse native plant community in agricultural areas. While ecological restoration can be successful, conservation and management of protected natural landscapes is central to preserving soil biodiversity and ecosystem services (Bardgett and van der Putten, 2014; Orgiazzi et al., 2016).
Conclusions and Future Directions
Soil biodiversity provides fertile ground for advancing global sustainability because it integrates so many challenges including climate regulation, water quality, pollution remediation, food and fiber production, and habitat for organisms aboveground and underwater (Bardgett and van der Putten, 2014). Both interest in and knowledge of soil biodiversity is increasing, even as fewer people work directly with the land. Analytical tools of the 21st century enable scientists to learn more than ever before about the amazing diversity of life beneath our feet. Several recent efforts have brought synthesis of soil biodiversity knowledge and the role of soil biodiversity to the attention of policy makers, community leaders, teachers, and students. The United Nations declared 2015 as the International Year of Soils. The first Status of the World’s Soils Report was published at the close of 2015 (Food and Agriculture Organization of the United Nations, 2015), and the first Global Soil Biodiversity Atlas was published in 2016 (Orgiazzi et al., 2016). Through these achievements and new projects they inspire, work continues to enhance our understanding and recognition of the tight connection and the role soils and soil biodiversity plays in the future of all life on Earth, including humanity.
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