When we talk about solutions to the current climate and global environmental crisis, one vital component is often overlooked: soils.
Billions of years ago, the first soils paved the way for life on land. Now, this unassuming terrain beneath our feet underpins a multitrillion-dollar, global agricultural industry and provides food for billions of humans, as well as countless wild and domestic species.
Soil is central to our relationship with the land and plays a vital role in nearly every key Earth system process responsible for keeping conditions on our planet stable. Civilization, humanity, and other terrestrial life are all dependent on good, ongoing soil health for their survival.
Soil forms the foundation of all terrestrial ecosystems, wild and cultivated. The growth of crops like lettuce, pictured here, depends on the geology, climate and biology of the soil. Image by Julie via Flickr (CC BY-NC-SA 2.0).
Healthy soils are biodiversity hotspots, providing a home to diverse communities of bacteria, fungi and invertebrates, many of which are beneficial, and even vital, to plants. Soils hold 80% of all the carbon stored on land, making them key to meeting global greenhouse gas emissions targets. And they are important to the freshwater cycle, storing water and filtering out pollutants.
But soils are in global crisis. An estimated 75 billion metric tons of soil is lost each year due to erosion from arable land, according to the U.N.’s Food and Agriculture Organization (FAO). In the process, large amounts of greenhouse gases are released, and vital ecosystem services are diminished.
The planetary boundaries framework, first proposed by Johan Rockström in 2009, offers a guide of how close we may be to environmental thresholds that could push Earth into a new, and far less habitable state. According to the most recent updated analysis in 2015, we are now living in the “danger zone” for four of the nine planetary boundaries: we risk catastrophic overshoot of proposed limits for climate change, biodiversity, land-use change, and biogeochemical flows (the planet’s nitrogen and phosphorus cycles).
All four are intimately linked to soil health, which, in turn depends on climate and geology, and on the ecological communities that live within and upon the soil. Put bluntly: humanity’s future very much depends on soil health.
Into the wildly biodiverse rhizospherePlants, while nurtured by sunlight, water and carbon dioxide, also rely on nutrients like nitrogen and phosphorus, which are taken up through their roots. The thin zone of soil around plant roots, known as the rhizosphere, is a hub of constant chemical and microbial activity, vital to soil health.
“You can see the vast amount of carbon that’s stored in plants [above ground] and the action of leaves capturing energy from the sun, and all of that is driven by what’s [invisible and] happening below the ground through the root system,” explains Paul Hallett, a soil physicist at the U.K.’s University of Aberdeen.
Plants secrete acids that enter soils, that then liberate nitrogen and phosphorus for them to absorb. Paraphrasing the 18th-century agriculturalist Jethro Tull, Hallet describes the rhizosphere as “basically the guts of the plant inverted into the soil.” “The plant is secreting acids just like your stomach does,” he says.
In the rhizosphere, plants, fungi, archaea and bacteria meet to exchange, or compete for, nutrients. Each gram of soil contains a mindboggling microscopic ocean of diversity, including as many as 50,000 different species of microorganism, making soils one of the most biodiverse ecosystems on Earth.
Bacteria play important roles here, breaking down organic matter, freeing nutrients from solid rock, and grabbing nitrogen from the air. In addition, some fungi form mutually beneficial associations with plant roots, called mycorrhiza, through which they exchange carbon and minerals.
“To some extent, the plant is mining the soil of its nutrients,” Hallett says, but “at the same time, that carbon that’s being captured [by the plant] from the atmosphere is being released through the roots, interacting with soil particles and with soil organisms, and then those work to aggregate the soil, trap carbon, and stabilize the [soil] structure.” As much as one-fifth of the carbon captured by plants during photosynthesis is released into the soil through their roots and their mycorrhizae.
Healthy soils: A vast carbon storehouseThe enormity of soil organic matter allows for lots of carbon storage potential, making soils an important ally in our efforts to curb climate change. But this is a double-edged sword: soils, because they do store vast sums of carbon, could also be a huge source of greenhouse gas emissions, if managed poorly.
“Somewhere on the scale of 3,000 billion tons of carbon are held in soils globally, with at least 1,500 billion tons in the top meter of soil,” says Jonathan Sanderman, a soil carbon scientist at the Woodwell Climate Research Center in Massachusetts, U.S. Compare that to 600 billion tons of carbon held in all land vegetation, and you gain a sense of the importance of soils for a stable climate.
Soil organisms represent 25% of all biodiversity on Earth and form the foundation of entire ecosystems. Earthworms are particularly important as they help break down and incorporate decaying organic matter into the soil. Their movement through the soil creates a complex soil structure important for water retention and nutrient availability. Top image of earthworm by Luukyi via Flickr (CC BY-NC-SA 2.0). Bottom image of springtail by Thomas Shahan via iNaturalist (CC BY-NC-ND 4.0).
Like plants, soil microbes release some of the carbon they consume into the atmosphere as a byproduct of cellular respiration (this is the main source of soil carbon emissions), and the rest is returned to the soil when the microbes die. New inputs of organic matter decomposing in soil tend to be passed from microbe to microbe until almost all the carbon has been emitted as CO2.
If carbon is to be sequestered in soils for decades or longer, in order to curb global warming, it must not be consumed by soil microbes. This can happen if the carbon becomes chemically bound to soil particles, or if carbon-hungry soil microbes are absent or inactive. Water-logged soils, for example, are oxygen-deprived — a toxic condition for many microbes, so this ultra-wet earth tends to hold on to carbon far longer. Acidic, salty and frozen soils also tend to trap carbon for longer periods.
Scotland’s “flow country” is home to Europe’s largest blanket bog, with its vast store of ancient carbon. Centuries of mismanagement combined with accelerating climate change are degrading this precious ecosystem, but conservation efforts are underway to protect it. For example, the Royal Society for the Protection of Birds (RSPB) manages a 21,000-hectare (52,000-acre) restoration site in Forsinard, pictured here. Image by Banco de Imágenes Geológicas via Flickr (CC BY-NC-SA 2.0).
Over the past two decades, researchers have traced the dynamic journey traveled by carbon through the living and non-living components of the soil — a journey that can last anywhere from a few hours to centuries.
“Long-held hypotheses on the stability of soil carbon have been thrown out the door,” Sanderman says. The prevailing view now is that “microbial access to organic matter is the dominate control on the long-term fate of carbon entering the soil.”
Microbes can remain dormant in soils for decades, even centuries, and only ramp up metabolic activity when the right conditions arrive. But human activities are changing soil conditions, risking the rousing of dormant microbes that may be starting to snack on a buffet of ancient, stored soil carbon — which, once released, finds its way into the atmosphere to humanity’s detriment.
The Arctic is feeling the brunt of climate change impacts, with temperatures in this region increasing at nearly twice the global average. Many frozen landscapes are beginning to thaw, rousing dormant microbes and releasing ancient carbon stores. Image by NPS Climate Change Response via Flickr (CC BY 2.0).
Lost soil carbon wreaking havoc
In a 2017 study, Sanderman and colleagues calculated that more than 110 billion tons of soil carbon were lost over the last 200 years as a result of agriculture. “Carbon from vegetation that would otherwise senesce and slowly decompose is being exported as the harvested product,” feeding billions of people, he explains. Put simply, we are eating, rather than replenishing, our soil carbon reserves.
This agriculture-driven trend in soil carbon release is being exacerbated by climate change, particularly warming. One California study found that an increase in soil temperatures of 4° Celsius (7.2° Fahrenheit) — roughly the rise predicted for this region by 2100 — could lead to a 35% increase in carbon emissions, largely due to increased soil microbial activity.
Carbon stores are unevenly distributed across the globe. Current estimates suggest that more than 110 billion tons of soil carbon had been lost over the past 200 years as a result of unsustainable agriculture. This visualisation of past and present soil carbon uses data from Soils Revealed, a joint project by The Nature Conservancy, Cornell University, the International Soil Reference and Information Centre (ISRIC) and Woodwell Climate Research Center, that aims to provide better decision-making tools for land managers and policy makers. Images courtesy of Soils Revealed.
Soil has the potential to become an ally or an enemy in humanity’s mission to mitigate climate change as these maps from the Soils Revealed project show, restorative management practices such as planting cover crops, using green manure and reducing tillage could store large amounts of atmospheric carbon beneath our feet by 2038; on the other hand, continuing with conventional agricultural methods, converting more forested area to agriculture and allowing grasslands to degrade could release huge stores of soil carbon, exacerbating climate change.
In more northerly latitudes, climate change is advancing faster, rapidly melting permafrost — soils that have been frozen for millennia — and awakening dormant microbes to access and release soil-trapped carbon.
The total Arctic permafrost store is estimated at between 1,300 and 1,600 petagrams (1.4 to 1.75 trillion tons) — roughly half the global soil carbon stock. Experts say that between 5 and 15% of that carbon could be released over the next 80 years, resulting in more warming (leading to more soil carbon releases, in a vicious circle). Once lost, soil carbon stores like these are very slow to recover.
An estimated 260 metric gigatons (286 billion tons) of this hard-to-recover carbon is trapped in soils worldwide, stored in peatlands, marshes, mangroves, primary forests and other carbon-rich ecosystems.
If we fail to protect these ancient reserves, the emissions will become an ineradicable mark against humanity on the climate change scorecard.
Scientists can collect near-continuous measurements of soil carbon emissions in controlled experiments using automated systems like Skyline, pictured here at the UK Centre for Ecology and Hydrology (UKCEH) in Lancaster, U.K. Poor land management and climate change are increasing emissions of greenhouse gases such as carbon dioxide, nitrous oxide, and methane from soils. UKCEH researchers simulated prolonged summer floods like those brought on by escalating climate change, and found changes in the soil microbiome and increases in methane emissions, particularly in intensively managed grassland soils. Image by Simon Oakley/UKCEH.
Conserving soil carbon stores also protects humanity
Soils will only become humanity’s ally, rather than an enemy, in the fight against climate change if we take action now to protect ancient soil carbon stores and restore degraded ecosystems and farmlands. Experts say these actions alone could carry one-quarter of the climate-mitigation load — part of a suite of vital nature-based climate change solutions.
“We need to do a much better job protecting high carbon density natural ecosystems, such as tropical peatlands, from further development,” Sanderman says.
Fortunately, because many of the most carbon-rich soils remaining on Earth coincide with biodiversity hotspots, protecting those ecosystems could offer a win-win for the climate, biodiversity, humanity, and our need to stay within safe planetary boundaries.
Likewise, because so many agricultural lands have been so severely degraded, their restoration could offer a golden opportunity to mitigate greenhouse gas emissions. Put simply: improving soil quality traps more carbon. A hectare of well-managed agricultural soil can sequester around 0.3 tons of carbon per year. Scaling up, and taking into account available land area, good soil management could remove as much as 3 billion tons of CO2 from the atmosphere annually, just under a tenth of current global emissions.
“This scale of carbon drawdown is precisely why there is so much interest in soil carbon [as a nature-based climate solution], but there are enormous barriers to getting to widescale adoption,” warns Sanderman.
At present, intensive agribusiness practices are having immense adverse impacts on soil health, and putting at least four planetary boundaries at risk of overshoot. Heavy machinery compacts soil, crushing its internal structure and reducing water-holding capacity (impacting the freshwater planetary boundary). Plowing rips apart mycorrhizal fungi, releasing more carbon into the atmosphere (impacting the climate change boundary) and increasing reliance on synthetic petrochemical fertilizers. Overuse of these fertilizers leads to increased nitrous oxide emissions, polluting waterways and threatening aquatic life (impacting the biodiversity planetary boundary), and also destabilizing Earth’s nitrogen cycle (a fourth planetary boundary).
“Agricultural fertilizer use (including manure) is important to growing food … but if just 5% of the fertilizer applied ends up in nearby streams and rivers, it can have disastrous effects for aquatic communities,” says Elena Bennett, a professor in sustainability science at McGill University in Canada who contributed to the 2015 assessment of the planetary boundary for freshwater consumption.
We must address this multitude of pressures on soils if we are to restore degraded agricultural lands — an urgent necessity if we are to effectively confront the global climate emergency, while continuing to provide food security for a growing human population.
Heavy farm machinery can compact agricultural soils, reducing their water-holding capacity and leaving surrounding areas vulnerable to flash flooding. The loss of soil structure also leaves it more vulnerable to erosion and harms soil micro-organisms. Image by Werktuigendagen Oudenaarde via Flickr (CC BY-SA 2.0).
S.O.S.: Save our soils
In 1937, U.S. President Franklin Roosevelt wrote a letter to state governors, declaring, “A nation that destroys its soil destroys itself.” There is significant historical evidence to back him up. After analyzing past civilizations, “multiple scholars have concluded that mismanagement of soils was the proximal cause,” of many of their downfalls, says Sanderman. The Maya of Latin America and the Sumerians of Mesopotamia provide two stark examples.
Deteriorating soil health is already having detrimental impacts on modern lives and livelihoods. In some of the world’s most arid regions, for example, worsening droughts triggered by climate change have combined with land-use change and unsustainable irrigation practices to shift once-productive ecosystems into desert. An astonishing 12 million hectares (30 million acres) are lost to desertification each year, impacting the lives of more than 500 million people.
“Desertification due to climate change and land management is a globally-critical soil tipping point,” Sanderman says.
In very arid regions such as parts of West Africa, farmers are trapped in a vicious cycle: droughts dry the soil, causing a thick crust to form on the surface, which then acts as a barrier when rains finally arrive, leading to flash floods that erode precious topsoil and the vital nutrients it contains. For example, in Mauritania, pictured, crop failures due to drought have led to a major food crisis affecting more than 700,000 people. Image by Oxfam International via Flickr (CC BY-NC-ND 2.0).
Land degradation is estimated to cost humanity 10% of its annual global gross product. Combined with climate change, soil degradation could reduce crop yields planetwide by 10%, and up to 50% in some regions, by 2050 according to a 2018 analysis by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES).
The good news: restoring degraded land could reap benefits up to 10 times higher than the cost, the report concluded.
Fortunately, there are as many potential soil solutions as there are sources of soil degradation. Some rely on new technologies; others involve revamping industrial agribusiness to maintain more of an ecosystem’s natural functions, thereby promoting healthy soils.
One example is agroforestry, which combines a balance of trees, crops and pasture to produce a diverse mix of grains, fruits, nuts, animal products, even medicines. Although this method tends not to be as productive in the short term as intensive industrial petrochemical-driven monocultures, it offers farmers greater security against crop failure and can rebuild soil nutrients and structure. In Tajikistan, for example, agroforestry is being successfully employed to restore soils degraded by decades of overgrazing.
There are also small interventions that fit more comfortably into our existing food-production systems. For instance, carefully managed crop rotations that incorporate a variety of plants that promote a range of soil benefits can prevent yield decline — a common problem faced by farmers who may see monoculture yields fall by up to 25% in the second and third years of harvesting and reseeding.
In a similar vein, some farmers have started planting thin strips of beneficial plants, including grasses and clover, along cropland edges, in hopes that the soil health gained beneath those green perimeters will extend out across entire fields. And it seems to be working: planting strips of prairie grasses alongside Iowa agricultural land reduced soil erosion twentyfold; strips of grass and clover planted beside crops in the U.K. improved soil quality and increased earthworm numbers in just one year.
Planting thin strips of soil-boosting plants like grasses and clover at the edge of agricultural fields can improve soil health. Research led by Jonathan Leake at the University of Sheffield revealed that grass-clover strips, left, measurably improved soil structure after just three years. Farmers hope that the benefits of these strips will extend into the rest of the field, boosting crop yields. Image courtesy of Jonathan Martlew and Jonathan Leake.
Revitalizing the rhizosphere
Simply reducing the degree to which we intervene with soils, physically and chemically, can produce impressive positive results. An example: delicate networks of mycorrhizal fungi are easily disrupted by plowing and are slow to regenerate, so some farmers are trying to minimize this disruption.
Conservation tillage (low- or no-till farming methods that minimize plowing), “can improve erosion control, which then leads to improved crop yield [which] will also reduce the amount of nutrient runoff, helping to preserve good water quality,” Bennett explains. ”Knowing that those interactions exist, we can try to take advantage of them for important win-wins.”
One way to minimize plowing is to plant perennial crop varieties that don’t need reseeding after every harvest. For example, Thinopyrum intermedium, a domesticated, perennial wild wheatgrass, can produce crops year after year without plowing.
Longer-lived food plants also tend to create larger, more extensive root networks, improving soil structure, protecting against erosion and enhancing drought resilience. By maintaining farm vegetation cover throughout the year, perennial plants also help sequester more carbon and maintain important climate-regulating processes such as evapotranspiration.
Currently, crop breeders are focused on short-term economic considerations like yield and water-use efficiency, but they could also turn their attention to traits that can nurture a healthier rhizosphere, Hallett says. For instance, plant breeders could select for varieties with longer roots, or more root hairs.
Root hairs “enter into very small pore spaces, and then they can capture nutrients like phosphorus, and they can capture water more effectively,” he explains.
Selecting for crops with longer root hairs could improve drought resilience and reduce nitrogen fertilizer use — two valuable immediate benefits to farmers — as well as store carbon and improve soil structure.
An overhaul of the global food industryAll of these solutions hold promise, but a scaled-up makeover of long-established industrial agribusiness will be no easy feat. And it’s not only large-scale agribusiness that needs to make a sea change. Relatively small, family-owned farms represent 75% of the world’s agricultural land, meaning that smallholder buy-in is critically important to achieving widespread soil health improvement.
“There are at least half a billion people involved with agriculture globally, so it is an enormous effort to get this many people to change the way they are conducting their business,” Sanderman says.
A recent report by the Microbiology Society recommended major government policy incentives to prioritize soil health, including a new food-labeling standard identifying products raised on well-managed soils. A sweeping reform of agricultural subsidies to promote not only higher yields, but also favor best practices that boost ecosystem services, could help expedite a much-needed overhaul of the world’s food and agriculture systems, experts say.
Soils vary hugely as a result of climate, geology, vegetation and human activities. This photo montage of soil samples, collected as part of research led by Rob Griffiths at the UK Centre for Ecology and Hydrology (UKCEH), shows the variability in soil characteristics present across different sites, just within the United Kingdom. Image by Rob Griffiths.
In truth, every nation on Earth needs to reframe its food-growing practices. To succeed, any global push toward sustainable soil management must be built on well-tailored regional strategies. “I don’t think there’s a recipe book [for soil management] that applies everywhere,” Hallett says.
Hallett is part of an international, interdisciplinary research team at China’s Sunjia Critical Zone Observatory, seeking to understand how local geology, hydrology, meteorology and soil science can explain the severe erosion and nutrient leaching prevalent in that region’s red soils. “Having this more holistic understanding of how these environments have formed is critical,” Hallett says.
The research team has, for example, discovered that in red soils like those found in China, up to 90% of soil nitrogen needed for crop growth lies deeper than 2 meters (6 feet) down and so is inaccessible to most plants — a result of decades of fertilizer overuse.
“We’re using that information now to create environmental decision-support tools that have both benefits for the environment and to the farmer through reduced fertilizer cost,” Hallett says.
In this case, and others around the globe, we already know many of the required remedies when it comes to soil health. But knowledge and action aren’t simultaneous. There is an inevitable lag between discovering effective solutions and enacting new practices to boost ecosystem services.
Hence the urgency: we must act immediately, at scale, if we are to leverage soil ecosystems in the fight against catastrophic global environmental change.
Square farming. Currently, crop breeders are focused on short-term economic considerations like yield and water-use efficiency, but they could also turn their attention to traits that can nurture a healthier rhizosphere. Image by Oregon Department of Agriculture via Flickr (CC BY-NC-ND 2.0).
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Banner image of a tractor used for harvesting peanuts Public Domain Pictures (CC0 1.0).
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