After a fire, the soil sparks interest

Jorge Mataix-Solera in a soil profile in fire-affected area in Guimaraes, Portugal. Photo: A. Cerdà

In 2015, thousands of wildfires burned across the world. Over 20,000 square kilometers have burned in Indonesia this year after annual, and perhaps some illegal, burning of peatland became unmanageable. In the United States, a combination of extended drought and high winds has led to over 37,000 square kilometers catching fire. And in Australia, a massive bushfire caused by lightning, dry conditions and a heatwave continues to burn.

Fire is a natural part of almost every ecosystem. It can clear away dead and decaying materials and make way for new growth. There are even some plants that exhibit pyriscence, releasing their seeds only in response to a fire. While the effect of fire on flora is well studied less is know about how fire effects the living material beneath vegetation, the soil.

Dr. Jorge Mataix-Solera has been studying fire’s effect on soil for more than two decades. In 1994, when Jorge was considering what subject to pursue for his PhD, thousands of hectares of land caught fire around his home-city of Alicante in eastern Spain. His father, a professional soil scientist, had ‘taught him to love the soil’ and now the circumstances in Alicante provided him the motivation (and grant money) to pursue his degree.

Jorge has traveled to Israel, Wales, United States, Australia, and Slovakia to investigate fire’s effect on soil. Fire only interacts with the top few centimeters of soil, but it can have enormous repercussions for the soil properties within those few centimeters. Depending on the severity of the fire, soil organic matter can be burned off, which in turn effects the quantity and quality of the soil microbial community. A fire can also alter soil structure, which can lead to less water infiltration and more erosion.

Another post-fire trend that has interested researchers is water repellency in soils. Water repellency can contribute to erosion and water runoff, the latter of which when paired with a lack of vegetation can lead to catastrophic flooding. Water repellency, as Jorge points out, doesn’t happen ‘in a uniform layer: it’s patchy.’ In the past, this inconsistency was primarily explained by the varying temperatures achieved by the soil during a fire, or ‘the type of vegetation which excretes different organic compounds’ that coat soil particles and repel water. But thanks in part to Jorge’s work, another explanation based on the materials that make up the soil can be included.

The soil has four basic ingredients, and certain ingredients repel water more than others. Soil is composed of minerals, organic matter, water and air. We have already mentioned that some organic compounds can repel water, and the same is true for organic matter. The minerals fraction – from biggest to smallest – is sand, silt and clay. Sand particles, which are coarse and have a low surface area, need very little organic compounds to coat them. Clay particles, on the other hand, are tiny and have over 1,000 times the surface area of sand. They need a lot of organic compounds to coat them in order to become water repellent. As Jorge states, ‘if there is a little bit more clay that may make the difference between if a drop stays on the soil surface or infiltrates.’

In the 21st century the number of large and uncontrollable fires – fires that can burn off organic matter and change the ability of water to infiltrate the soil – has increased on all vegetated continents. This surge in fire frequency and intensity is due to many factors. In some regions a higher proportion of climatic extremes such as drought has led to drier conditions and an abundance of parched vegetation that is ready to spark. Humans moving into previously uninhabited areas is also an issue as naturally occurring fires, which previously might have been allowed to burn, are extinguished to preserve the new inhabitants’ homes. When a fire does ignite, the glut of vegetative fuel that has accumulated over the years can lead to fires with greater intensity and higher potential to become uncontrollable.

After these destructive fires, it is extremely important to protect the soil. Post-fire management can be in some situations, says Jorge, ‘worse than the fire.’ ‘In many cases the land managers want to remove the dead trees. And if they do this when the soil is very fragile – in the first few months after a fire just when some vegetation is just starting to grow – it can degrade the soil.’

During the International Year of Soils, Jorge has been taking citizens and students from fire-affected areas in Spain to his study sites. They work together to set up plots with different types of management. One plot might have mulch applied while another is compacted as if heavy machinery had been used to remove the trees. Then they use a rainfall simulator to measure which plot has the highest amount of water runoff and erosion. ‘And really, the difference was amazing,’ says Jorge. The mulched plots had little water runoff, and what did runoff was crystal clear. Almost all the water ran off the compacted plot and the water was muddy with eroded soil.

Jorge hopes that experiences like this will encourage citizens to value the soil. He likes the term ‘geoderma’ to describe soil, from the Greek for ‘earth’ (geo) and ‘skin’ (derma). It helps people to understand ‘that the soil is not meters and meters and meters, we have only centimeters of soil. And we must take care of it.’





Gully developed after post-fire management of salvage logging (removing trees) and rainfall of 50 mm in a few hours. Pictured: researchers from the University Miguel Hernández. Photo: J. Mataix-Solera
An activity with children using a rain simulator in a fire–affected area. Sierra de Mariola, Alicante, Spain. Photo: J. Mataix-Solera
An activity with children using a rain simulator in a fire–affected area. Sierra de Mariola, Alicante, Spain. Photo: J. Mataix-Solera
An activity with children using a rain simulator in a fire-affected area. Plot with straw mulch treatment. Sierra de Mariola, Alicante, Spain. Photo: J. Mataix-Solera
Water over an extremely water repellent soil that was burned in Pinoso, Alicante, Spain. Photo: J. Mataix-Solera
Researcher Katka Chrenkova installing a plot in fire-affected area before the first soil sampling. Montgó Natural Park. Alicante, Spain. Photo: J. Mataix-Solera

The views expressed here belong to the speaker and do not necessarily represent FAO’s views, positions, strategies or opinions.

Submitted by Catherine DeLong





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mipijan kkoup 07-08-23 08:31
After a fire, the soil becomes intriguing and arouses interest.

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Bellot Twig 02-08-23 12:12
Your article brilliantly highlighted how wildfires can lead to significant soil erosion, nutrient loss, and even alter microbial communities beneath the surface. I was particularly fascinated by your explanation of the chemical changes that occur in the soil due to extreme heat.
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Thank you for sharing this information and your experience!
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