Earth's fragile skin: why soils are so important to life and climate
Last week, authors of a UN report by the Food and Agriculture Organisation (FAO), alerted about the future of global soils and the need to consider urgent action to halt degradation and a shift away from intensive agriculture. Fertile soil is being lost at the rate of 24bn tonnes a year and a third of the planet’s land is now severely degraded. Compiling the state of knowledge on soil biodiversity, the 300 scientists consider the worsening state of soils as critical as the climate crisis and loss of biodiversity, but also part of the solution:
‘Soil biodiversity could constitute, if an enabling environment is built, a real nature-based solution to most of the problems humanity is facing today, from the field to the global scale.’
Why it matters. With all life contained inside a very thin layer - a few kilometres large only - around the Earth’s mineral crust, soils play the role of a ‘living skin’ for land. Thousands of years are needed to form them, but unfortunately intensive farming, forest destruction, pollution can destroy them in just a few decades.
A quarter of all animal species on the planet live below us. Comprising fungi, bacteria, archaea, protists and others, the organisms in this community drive the carbon and nutrient cycling that regulate soil fertility, atmospheric composition and the climate. They are in dire need of protection.
Soils are a particularly neglected element of the climate system. As the second largest carbon sink after the oceans, they store as much carbon as all plants above ground. And according to the Crowther Lab at the ETH Zurich, healthier soils could store more carbon and become critical in either tackling or aggravating the climate emergency.
The link between soils and climate. Carbon is either fixed or released from soils, depending the activity of the soil organisms and conditions.
Carbon is fixed into soils through the transformation of plant and animal detritus. During this degradation of organic matter, some soil organisms emit CO2 into the atmosphere through their breathing and others like bacteria and archaea fix carbon by using atmospheric CO2 as their energy source. By feeding on bacteria, fungi and plant material, nematodes fulfil a key role: they regulate the activity of other organisms in the soil food web, and influence the rate at which organic matter is processed in the soil.
Beyond this direct role in the carbon cycle, soil organisms are also critical for efforts to reduce overall greenhouse gas (GHG) emissions from agriculture. Globally, agricultural ecosystems contribute 10 to 12 per cent of all direct anthropogenic GHG emissions each year, with an estimated 38 per cent resulting from soil nitrous oxide emissions and 11 percent from methane in rice cultivation. Soil microorganisms are involved in every step of nitrogen and carbon transformations that yield these greenhouse gases, and sustainable management of the soil environment to minimise emissions must become a key objective.
Warming soils lose carbon. Researchers like Crowther in Zurich went further and asked themselves how global warming might change the activity of soil microbes.
And, despite uncertainty around results and need for more research, they found a trend which they published in 2019: warming soils lose carbon, and the more carbon a site holds, the more it loses. A 1°C rise in global temperature could cause microbes to release about 55 billion tons of soil carbon by 2050, accelerating global warming by up to 17%.
The result could be like adding emissions from a country like the United States to the atmosphere, on top of other tipping points already identified such as the release of methane from the permafrost and the declining albedo effect of melting ice sheets at the poles.
A map of soil degradation. The main causes of damage to soils are intensive agriculture, with excessive use of fertilisers, pesticides and antibiotics killing soil organisms and leaving it prone to erosion. The destruction of forests and natural habitats to create farmland also degrades soil, particularly affecting the symbiotic fungi that are important in helping trees and plants grow. Droughts and wildfires - increased by global heating - could also play a part. All continents are affected, as the map from UNEP indicates below .
The shift to agroecology. Solutions include protection of existing healthy soils from damage and restoration of degraded soils by growing a diverse range of plants. But above all, a radical shift towards more sustainable agro-ecological practices and agroforestry is required, with the change of food consumption habits that go with it.
Meat production covers 80 per cent of the world’s farmland today and is the main driver of deforestation and erosion. On the other hand, halting the loss and degradation of natural systems and promoting their restoration have the potential to contribute over one-third of the total climate change mitigation scientists say is required by 2030.
The bottomline: a fatal imbalance to adress. With Earth systems all inter-related, acknowledging the role of soils and its microorganisms in the fine balance between biological life, the chemical environment and climate, needs much more immediate attention. Food production and consumption patterns combined with a continuously growing demand in energy are leading humanity towards a breaking point, and dangerous feedback loops are in the process of being triggered. In the face of such grave concerns, the recent launch on 3 December of the European Union Soil Observatory within the Green New Deal adopted is no coincidence, and could hopefully contribute to a future policy acceleration at European level.