Soil and ecosystem services: the “brown gold” beneath our feet

by Maria Chiara Rosace, Ciro Gardi ¹ | October 26, 2020

1. Scientific officer at European Food Safety Authority (EFSA).

Having access to safe and healthy food is a fundamental asset for human health and it is certainly clearer now that in order for it to be guaranteed, soils must be equally healthy (read more here: Protect our soil to preserve food safety). However, providing us with food is not the only service soil may offer. There are countless other equally important ecosystem services that deserve to be further explored.

Soil biodiversity: resource or threat

Soil is one of the most complex ecosystems and certainly among the most diverse habitats on the planet. Often referred to as a biodiversity hotspot, it contains an enormous amount and variety of organisms, most of them probably still unknown. The vast majority of these organisms are not a threat to human health and actually provide numerous ecosystem services through the complex interactions developed between organisms within the soil and with the soil itself. It is certainly true that there are microorganisms capable of causing disease in humans. A broad overview of these pathogens is provided in the report published by the JRC (Joint Research Centre) in 2011 (Jeffery and van der Putten, 2011). For example, many different species of bacteria, belonging to the genus Salmonella, can be introduced into the soil with liquid chemical fertilizer on agricultural lands. Salmonellosis is the most common symptomatic infection caused by bacteria of Salmonella type that can be easily contracted by humans through the consumption of vegetables grown in such soils (Jeffery and van der Putten, 2011).
Despite the presence of harmful organisms, soil is home to many other “good” organisms that are non-pathogenic and essential for life. Some are a key resource to produce medicines, including important and well-known antibiotics (Oliver and Gregory, 2014) while others play an essential role to make soil work properly.

What does ecosystem services actually mean? According to the definition provided by the Millennium Ecosystem Assessment, ecosystem services are described as “the multiple benefits provided by ecosystems to mankind” (Millennium Ecosystem Assessment, 2005). These include supply services, such as food and water or regulatory services, such as flood and disease control. Soil organisms are directly involved in the decomposition of organic matter as well as the physical breakdown of litter (Schmidt et al., 2011; Van Veen e Kuiman, 1990). What is litter? Well, it is the outermost layer of soil, composed of branches, leaves, debris in different stages of decomposition. One of the main results of disintegration is that leaves, for example, are converted into fine particles which are then redistributed across the soil by other organisms and used as an energy source. These functions are all necessary for plants to grow and are therefore fundamental for food production. And that is not all. Bacteria, fungi and other soil organisms are involved in soil aggregations and fragmentations processes, in the transport of plant residues and organic matter, in the nutrient cycle and in the fixation of nitrogen into soils, making them available for crops and reducing the need for inorganic fertilisers. Moreover, they are involved in the:

• formation of mycorrhizae (i.e. symbiotic associations between fungi and plant roots, as illustrated in Figure 2);
• formation of pore and aggregate systems, altering the soil structure;
• increase of soil porosity (which facilitates water infiltration and thus reduces erosion);
• plant pest control.

Microorganisms such as bacteria and fungi also play an essential role in the decomposition of synthetic compounds (i.e. pesticides) and other chemicals widely used in conventional agriculture. This essential activity ensures that toxic substances do not accumulate in the soil (Orgiazzi et al., 2016).
The chart in Figure 2 provides an overview of the main ecosystem services offered by soil organisms.

Protecting soil and its biodiversity is therefore fundamental to humans. Soil organisms are small, masterfully adapted and specialised to live in conditions very different from the surface. They are therefore much more susceptible to change and stress than other animals. The occurrence of stressful events, such as ploughing on agricultural soil (Gardi et al., 2013), can lead to the total disappearance of some organisms from the soil system. This might lead to the loss of important ecosystem services (Jeffery and Gardi, 2010) and functionality of the entire ecosystem, resulting in negative effects on both the environment and humans themselves.

Soil as regulator and water cycle purifier

Soils are able to remove contaminants from water. Contaminants are removed from soil through physical capture (when water passes through soil pores), through chemical absorption and through biodegradation performed by soil microorganisms (Helmke and Losco, 2013). Let’s see an example. Chemical constituents in water can be attracted to soil particles through the process of adsorption. Clay minerals present in most soils present a net negative surface charge that causes dissolved cations (positive) to adsorb to the soil matrix (as they say, opposites attract each other!). Heavy metals have a positive charge and are therefore retained by negative soil charges.

Furthermore, soil acts as a reservoir for water, which remains available for plants. Water flows over and through the soil. Thanks to the soil, the underground water flow is better regulated: when water is absorbed or infiltrates the soil, it takes longer to reach watercourses, whose runoff peaks result mitigated, thus reducing the risk of flooding. Soil consumption through construction is one of the main causes of degradation and may lead to an increased risk of flooding and water scarcity (European Union, 2012). When soil is covered with artificial waterproofing materials, such as concrete or asphalt, we talk about soil sealing. Soil sealing has effects on essential ecosystem services, such as food production, water absorption, soil filtering capacity and on biodiversity (European Union, 2012). How? It is a concrete vertical barrier between soil, atmosphere and hydrosphere. ISPRA, the Higher Institute for Environmental Protection and Research, realises the Soil Coverage Map with a high spatial resolution that can be consulted by accessing the links available on this page.

Soil, organic matter and climate change

Healthy soils provide the Earth’s largest carbon deposit. But how can soil contain so much carbon (C)?
Soil carbon levels result from the interactions of various ecosystem processes, including photosynthesis, respiration and decomposition. Carbon is the fundamental constituent of all organic compounds (this means that carbohydrates, lipids, proteins and nucleic acids are primarily made up of carbon). It is released into the soil through plant roots or through the decomposition of plant material by microorganisms. During decomposition and respiration processes, carbon is released back into the atmosphere in the form of carbon dioxide (CO₂). CO₂ is fundamental for many animals and plants’ vital processes, but it is also the main greenhouse gas in the atmosphere of our planet. If sustainably managed, soils can play a key role in climate change mitigation because of their ability to store carbon in the form of organic matter. In order to better understand this point, it is necessary to look more into the carbon cycle, reported in Figure 4. On the other hand, poorly managed soils release more carbon into the atmosphere in the form of carbon dioxide, thus contributing to climate change.

Final considerations: why soil matters

Beyond the emphasis on the negative aspects, it is necessary to be aware of the importance of the different functions performed by our soil, all of which are to some extent vital for human wellbeing. Whether it gives support to food production and agriculture, to plants and animals, or through the role played in water quality and availability and effects on climate change, it is clear that healthy soils are fundamental for the conservation and progress of human life on this planet. What would happen if there were no more fertile soil on Earth? How could we produce the food we need? And above all, how could we manage the growing need for food in relation to the constant increase in the human population? These are rightful questions, with worrying answers, which must lead to new decisions in order to take the necessary steps to protect and safeguard our soils. The European Commission is already doing something, proposing new laws to protect soils and for their sustainable use and supporting the research of the scientific community to shed light on the mysteries that still lie beneath our feet. As scientists, it is also fundamental to share this knowledge with everyone and to make everyone aware of the need to defend our soils starting from our daily actions.
Soil is of priceless value, despite it is difficult to understand this concept. It has been defined “the brown gold”, a definition that encloses what it is and how important it actually is to all of us.

References

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2. FAO (Food and Agriculture Organisation of the United Nations), 2015. Soils help to combat and adapt to climate change by playing a key role in the carbon cycle. Disponibile online, http://www.fao.org/3/a-i4737e.pdf?utm_source=infographics&utm_medium=qrcode&utm_campaign=i4737 [Accessed: 15 Settembre 2020]
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