‘A poor man’s rainforest’: why we need to stop treating soil like dirt

Hidden under our feet is a miniature landscape made up of tunnels, caves and decaying matter. Soil is where a quarter of the species on our planet are believed to live and in this dark, quiet, damp world, death feeds life. Rotting leaves, fruits, plants and organisms are folded into the soil and burped out as something new.

Good soil structure provides many nooks and crannies that house organisms, which, in turn, create an environment that suits them, directly altering – and improving – the structure of soil. Like a collective of tiny chemists, they keep soils healthy and productive by passing nutrients between them, either by collaborating or killing each other.

Complex food webs move nutrients around the system, generating healthy soils that provide goods and services for humanity. Goods include food, fibre and clean water. Services include regulation of the carbon and nitrogen cycles, nutrient recycling, water storage, regulation of disease and detoxification of pollutants.

Despite all this and the fact that soil is at the heart of our existence, we know very little about it. We do know, however, that this fertile skin has been damaged by intensive farming, pollution, deforestation and global heating. A third of the planet’s land is severely degraded and 24bn tons of fertile soil are lost every year through intensive farming alone, according to a UN-backed study, the Global Land Outlook.

The future of our soil hangs in the balance as the UN prepares for the first Global Symposium on Soil Biodiversity, with scientists warning that soil degradation is as important as the climate crisis and destruction of the natural world above ground.

Earthworms – ecosystem engineers

Earthworms are the unsung heroes of soil biodiversity. As they burrow and feed underground, they break down organic matter, which is then passed along the conveyor belt to smaller organisms. Worm holes create crucial porous structures for water and air to travel through. Deep-burrowing earthworms – known as the ecosystem engineers – can dig tunnels up to two metres deep and dramatically increase soil fertility. In the Arctic invasive earthworms are actually making the soil too fertile.

Earthworms break down organic matter as they feed and burrow, passing it on to other organisms.
Earthworms break down organic matter as they feed and burrow, passing it on to other organisms. Photograph: Duncan Mcewan/naturepl.com

Charles Darwin’s final book, published in 1881, praised the work of earthworms. “It may be doubted if there are any other animals which have played such an important part in the history of the world as these lowly organised creatures,” Darwin wrote.

Millions of other species of soil organisms exist but only a fraction have been identified, and most are too small to see. Soil macrofauna (creatures larger than 2mm) include mammals such as moles, mice and rabbits, which build dens in soil, as well as smaller creatures such as centipedes, woodlice, snails and slugs. Soil mesofauna (creatures smaller than 2mm) include springtails, mites, nematodes and tardigrades. Then there are the microfauna – bacteria, fungi and algae. One teaspoon of healthy soil can contain up to a billion bacteria and more than 1km of fungi.

It may be doubted if there are any other animals which have played such an important part in the history of the world

Charles Darwin on the earthworm

“We are always looking for aliens elsewhere, but we cannot protect the aliens under our feet. These really are aliens – if you look at them you’ll see they are movie-type organisms,” says Carlos António Guerra, a soil ecologist at the German Centre for Integrative Biodiversity Research.

If you’re talking about the health of soil, you’re not just talking about how it facilitates agriculture, but the health of an entire system, says soil scientist Felicity Crotty, speaking at the Oxford Real Farming Conference in January. “Soils are often referred to as a poor man’s tropical rainforests. This is due to the abundance and diversity of life within the soil … These organisms are the ones that are driving the decomposition and nutrient cycling within the soil.”

Soil bacteria seen with a coloured scanning electron micrograph (SEM). Bacteria in the soil are directly linked to nutrient recycling especially carbon, nitrogen, phosphorus and sulphur.
Soil bacteria, seen here with a coloured scanning electron micrograph (SEM), are linked to nutrient recycling, especially carbon, nitrogen, phosphorus and sulphur. Photograph: SPL

Yet, despite providing all these benefits, this complex tangle of life isn’t included in most food webs. “Soil scientists often emphasise the physics, the chemistry, without the soil biology part, but without the biology [soil] is just an inert substrate,” says Crotty.

Canaries in the coalmine

Soil is the top layer of the Earth’s crust and is composed of a mixture of water, gases, minerals and organic matter. Soil considered “good” for agriculture is about 50% solid, 25% air and 25% water. If soil is too compacted, less air and water pass through, which inhibits root growth and overall productivity.

Topsoil is where 95% of the planet’s food is grown and, like rainforests, is extremely fragile; it takes more than 100 years to build 5mm of soil, and moments to destroy it, according to the Royal Society.

Unfortunately, humans have been treating soil like dirt, losing it 50 to 100 times faster than we are able to rebuild it.

Soils that are poor in biodiversity are more fragile because they have lost the structure and all those connections that keep particles together. This means they are more likely to be dispersed by strong winds, or washed away in flooding. “Losing the soil is an extreme example of land degradation, but land degradation starts earlier when you start losing diversity, or functions, or other things related to soil organisms,” says Guerra.

Brazilian farmer and dog walk through deforested area of Amazon in Rondonia state, Brazil.
A deforested area of the Amazon rainforest in Rondonia state, Brazil. Deforestation, along with intensive farming, global heating and pollution is damaging the planet’s soils. Photograph: Carl de Souza/AFP/Getty Images

Soils hold twice as much carbon as the atmosphere, and when soil degrades, the carbon is released. In the 30 years from 1978, the soil in the UK’s croplands lost 10% of the carbon it could store.

“Like ‘canaries in the coalmine’, when soil organisms begin to disappear, ecosystems will soon start to underperform, potentially hindering their vital functions for humankind,” researchers, led by Guerra, wrote in a paper published in Science that urged policymakers to take account of soil when considering conservation priorities.

“If we do not protect soils for the next generations, future above-ground biodiversity and food production cannot be guaranteed,” the 29 soil scientists warned.

Monitoring soil biodiversity

Historically, soil has been left out of wider debates about nature conservation because we know next to nothing about it. “Every soil ecologist in the world does presentations with the same two slides at the beginning – soil biodiversity is very, very important, but we don’t know enough about it,” says Guerra. “In 2018 we got together with the only purpose of working out what it would take to remove that second slide.”

Soils are often referred to as a poor man’s tropical rainforests … due to the abundance and diversity of life within the soil

Felicity Crotty, soil scientist

Authors of the Science paper have created the first global Soil biodiversity observation network to collect and systematically sample data on the condition of soil biodiversity and its functions. The programme will assess soil diversity and soil ecosystem functions in protected and non-protected areas. The goal is to deliver information on the state and trends of soil biodiversity so it can be drawn into policymaking, such as the 2030 European biodiversity strategy.

“We wrote this paper as a grassroots initiative; all of the people involved are investing their own resources to monitor soils. What we are basically saying, is that we are not waiting any more for the politicians to understand that this is a valuable way of investing their money,” says Guerra.

Soil exposed after a landslide.
Soil exposed after a landslide. Photograph: Guido Paradisi/Alamy

Soil biodiversity must be considered when creating policies for nature’s protection because biodiversity below ground seems to require different things to terrestrial biodiversity, researchers say. Soil-focused biodiversity strategies would include better management of dead wood, for example, and environmental compensation schemes that specifically protect areas of unique soil biodiversity.

“We aim for a future where the conservation value of giant earthworms or endemic fungi is recognised and their ecology is properly protected by nature conservation measures,” the researchers say.

Generally, soils that support natural ecosystems have the greatest diversity. In agricultural landscapes, crop rotation, planting cover crops, hedges and ley strips are believed to increase soil fertility. Less intensive, rotational grazing systems, and reducing the use of chemical fertilisers also improve soil fertility. The no-till movement, which started in the US, is taking off around the world.

Protecting soil structure is expected to be included in the UK’s new farming subsidy system, which pays farmers for delivering public goods, as it shifts away from the common agricultural policy. And there are signs the world is waking up to the importance of soil; 2015 was declared the International Year of Soils and the UN increasingly stresses the importance of soils to future food security.

Soil erosion in Maasai heartlands in Tanzania.
Soil erosion in Maasai heartlands, Tanzania. Photograph: Carey Marks/Plymouth University

The clock is ticking. A UN-backed report in 2018, found that land degradation is already undermining the wellbeing of two-fifths of humanity, and urgent action is needed to reverse this trend. Its authors warn there is a strong association between land degradation, migration and political instability, and as the global population rises this problem will become more intractable.

“While soil biodiversity deserves to have a value in itself, if policymakers don’t see soils for what they are – a rich and diverse world – we will quickly see ourselves in a situation where both the subsidies to farmers and the price of our food will systematically increase,” says Guerra. “The depletion of soil organic matter will make the food system less resilient and less adaptable to changes in climate.”

Find more age of extinction coverage here, and follow biodiversity reporters Phoebe Weston and Patrick Greenfield on Twitter for all the latest news and features

Original author: Phoebe Weston
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