Interview — Soil: the living treasure under our feet

What is soil?

Soil is a complex, dynamic and living body, which can be seen as the living skin of Earth. It is composed of mineral and organic components, as well as air and water. In very broad terms, mineral components consist of particles such as sand, silt and clay composed of different chemical components, while organic components derive from living organisms, including plants, bacteria, fungi, fauna and their residues.

Soils are important reservoirs of biodiversity. Around a quarter to a third of all organisms occur in soil. The biodiversity of soil can include organisms ranging from microscopically small bacteria and nematodes, to springtails, mites, millipedes, earthworms, moles and mice. Each of these groups is species rich. For example, in Germany alone, there are 50 different earthworm species that we know of. In fact, the diversity of soil life is often significantly higher than above the ground in the same site. A commonly cited number is that one cubic metre of forest soil can contain up to 2 000 invertebrate species.

What happens in a soil ecosystem?

Soil ecosystems vary substantially, especially at the microhabitat level. The same block of soil contains very diverse habitats — soil surface, below ground bulk soil and pore space — each home to different organisms. For example, most organisms living in soil are very dependent on and live in soil pores. These can be filled with air or water, with different groups of organisms living in each.

There are other ways of looking at soil habitats. For example, there are microscopic boundary layers between soil particles as well as biological hotspots, including the rhizosphere where plant roots are or the drilosphere around earthworm burrows. Spatial scale is also very important.

Yet, all these species in all these microhabitats live together and interact in what we call the soil biome. For example, they can feed on each other or the faecal pellets of one provide nutrients for others. These interactions in the soil biome are essential for soil functions, which in turn provide ecosystem services.

What kind of services does soil provide?

Soil structure and soil organic matter are two of the best-known examples important for ecosystem services. Soil structure is defined by how different particles are assembled in the soil matrix. Soil includes a combination of bigger and smaller aggregates of soil particles, air- and water-filled pores, etc. Soil species can work directly on the soil structure. For example, earthworms through their burrowing activities move things around and thus change the soil structure. Some of these changes can consist of making new pores and closing others, making some parts denser or bringing new food sources for soil organisms. Earthworms are considered ecosystem engineers, as they can really churn up the soil.

The structure of the soil is also a key factor in the water cycle. It plays a role in determining how much water soil can take up and retain, how it purifies it and how this water can feed plants and so on. Imagine if soil could not retain or purify water, what that would mean for agriculture, flooding or our health.

The other example is the nutrient cycle involving how much soil organic matter — i.e. carbon, nitrogen and phosphorus — is taken up and stored in soil. Carbon inputs to soil are all organic and are the basis of the soil food web. Organic compounds, such as leaves and root tips, have to be broken down to simpler compounds by organisms living in soil before they can be used by plants. In a quite complex multi-step process, one after another, different organisms degrade what used to be dead leaves or branches and turn them into inorganic compounds that are suitable to be taken up/used by plants. About 90 % of forest leaf litter is processed by millipedes, earthworms and woodlice. Without these organisms, we would be suffocating in leaf litter.

There are soil bacteria that convert atmospheric nitrogen into mineral nitrogen, which is essential for plant growth. Fungi transport nutrients through the soil from one location to another. All these microbial processes are regulated by the grazing of larger animals feeding on these microbes. We need to see these rich and complex interactions as the essence of a well-functioning system, which then provides us with the ecosystem services mentioned above.

In fact, healthy soils provide us with a wide range of benefits. For example, the nutrient cycle is key to food and fibre production. There are also clear links with the water cycle. When the soil structure is altered or destroyed, the ability of soil to purify, take up and hold water is affected. Compaction or soil sealing, for example, can lead to more flooding.

Soil microbial enzymes are being isolated in laboratories to see how they can be used for industry. For example, these enzymes can replace chemicals in, for example, the paper industry. Similarly, the pharmaceutical industry uses soil bacteria in developing medicines, including penicillin and streptomycin.

Do we know enough about soil biodiversity?

Soil biology is a fairly young field of research. Moreover, soil is a cryptic environment, difficult to observe. Despite this, we tend to underestimate what we know. In Europe, we have a good general understanding of which groups of organisms occur in and which are the main constituent species of soil. We have a fair understanding of what drives biodiversity as well as a basic understanding of how human soil use will affect soil biodiversity. There are many sources of information on soil, including the European Atlas of Soil Biodiversity by the Joint Research Centre and the French Atlas of Soil Bacteria.

However, to monitor change over time, we need time series for soil biodiversity. The time series we have are often for protected natural sites, and there we can see that soil biodiversity is usually maintained and preserved. Furthermore, most of the soil monitoring done at the moment looks only at chemical compounds. Along with contaminants, we also need to monitor other parameters and understand how climate change or different agricultural methods affect soil biodiversity and the various soil functions they drive. There have been many studies across Europe, but knowledge has not been compiled in a way that enables us to establish baselines across Europe.

Soil in general and soil biodiversity in particular are very site specific. Effective measures often need more detailed and site-specific information, not only on biodiversity and species distribution and interactions at a given site, but also on, for instance, the impacts of human activities and climate change at that site.

What are the main threats soil biodiversity faces today?

There are many threats, including contamination linked to our land use practices. For example, pesticides, herbicides and other chemicals linked to agricultural intensification impact species distribution and harm soil biodiversity. Other threats include physical changes such as compaction and soil sealing — covering the soil with artificial surfaces such as concrete or asphalt. Compaction reduces pore space, affecting the species living in pores, while soil sealing cuts off carbon and water input into soil and also reduces the dispersal of species.

Because of its small scale and because it is a relatively slow process, soil species’ dispersal is often ignored. Over longer time-frames, there is actually very active dispersal across the landscape, enabling high levels of soil biodiversity. By reducing landscape-level biodiversity above the ground through monocultures and landscape homogenisation, we also risk losing biodiversity in soil.

Climate change impacts, such as significant changes in precipitation (drought or floods), could also affect soil biodiversity. 2018 was so warm and dry that we observed a 90-95 % reduction in soil invertebrates at some of our field sites. If we consistently reduce species diversity, all these soil activities can be impacted.

What is being done to protect soil in Europe?

There are global and European efforts and initiatives aimed at protecting soil, such as the Global Soil Partnership, as well as EU policies and directives — at least 18 directives by my own estimate, including the common agricultural policy. They address a wide range of areas from reducing pollutant emissions and sustainable land use to awareness raising. The better implementation of these policies and directives would certainly also be a good way forward for soil biodiversity. On the ground, there are many actions that can be taken, such as reducing fertiliser and pesticide use and adopting precision farming for agricultural soil.

Nearly half of the Sustainable Development Goals (SDGs) are linked to soil — from clean water and climate change mitigation to zero hunger — without healthy soil these SGDs will not be achieved.