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In the first months after the fire, the soil is very fragile due to the loss of vegetation and modifications of the agents that intervene in the formation of the aggregates.

The aggregation of particles is key to understanding the functioning of the soil. Fire affects aggregation by degrading the biological part, although low intensity fires do not leave obvious signs of their passage. Scientists have developed ingenious methods to evaluate the quality of aggregates.

The soil is the skin that surrounds the solid part of the Earth. It is also where air, water, organic matter and minerals come into contact. And where the living and the inert world coexist and manage the biogeochemical cycles. Nutrients, water, and even the chemical composition of the atmosphere depend on the functioning of the soil.

The soil is made up of organic and inorganic particles. A good soil structure favours many pores where air and water circulate and favours a better exchange of elements with the roots of the plants. If you have a soil with good aggregates you have a fertile soil and with little or no loss of materials. If the structure is poor there is an increase in runoff and loss of soil.

The aggregates of the soil tell us about the health of the edaphic ecosystem. For this reason, scientists have used them as indicators of the impact of some disturbances, and in the case of fire we know a lot about the effect of forest fires thanks to the aggregates.

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Effect of fire on soil aggregates

By aggregate we understand the grouping of individual mineral particles (sand, silt and clay) and organic particles that form the soil. When the individual particles are grouped, they take on the appearance of larger particles, which are called aggregates. So that the structure of the soil develops and, therefore, aggregates are formed, the presence of agents that act as cement between the particles is essential.

The aggregation is dominated by two important phenomena such as flocculation and foundation. Flocculation is due to electrokinetic phenomena, that is, it occurs when negatively charged particles are joined by a particle with an opposite charge. The foundation, on the other hand, consists in the mutual bonding of flocculated particles by the action of varied materials or substances. These agents can be both physical, chemical and biological. For example, the calcium cation (Ca2 +) induces clay flocculation. In particular, the divalent cations associated with clays such as calcium, iron and aluminium favour the formation of aggregates, while sodium favours dispersion.

However, in addition to the abiotic cementing agents, there are biological ones. The roots of the plants hold together the soil particles and help to add them through the secretion of organic compounds that act by binding inorganic substances. In addition, they provide compounds that are used by microorganisms that also participate in the formation of aggregates, mainly fungal hyphae. Do not forget that most of the animals that live in the soil (microfauna, mesofauna and macrofauna) also participate in the formation of the aggregates. For example, gastropods secrete mucilaginous substances that act by grinding the soil particles, while annelid species, such as the earthworm (Lumbricus terrestris), can form stable solid aggregates through a variety of mechanisms of stabilization of the excretions they produce. Ingest soil particles and organic matter.

The stability of the aggregates is the resistance they have to maintain their structure when subjected to external forces such as the impact of raindrops. The formation of aggregates plays a significant role in physical and biogeochemical processes, since it affects the movement and storage of water, aeration, biological activity, vegetation growth and erosion, among others. For example, maintaining the structure is important to have a good system of pores that allow a good infiltration, avoid erosive processes, allow good plant nutrition and a good exchange of gases between the soil and the atmosphere.

The stability of aggregates is considered a physical property, but, since it depends as much on the chemistry as on the biology of the soil, many researchers consider it an integrating and indicative property of soil health. The stability of the aggregates is a synthetic parameter that summarises the state of health of the ecosystem, and in which the perturbations experienced are reflected. That is why it is so valuable to study the effects of fire on the edaphic ecosystem.

Fire and aggregates

Fires have effects on the physical, chemical and biological properties of the soil, which generally depend on the severity of the fire. The changes can be short or long term, they can even be permanent or irreversible depending on the soil and the severity of the fire, and of the management after the passage of the fire. In addition, after fires, due to the loss of vegetation cover and leaf litter, structural stability becomes a key feature in water management and the loss of both nutrients and soil materials.

The impact of the raindrops after the fire can break the aggregates, so that the finer fractions clog the pores, modifying the porosity. The response of aggregates to the passage of fire is complex to evaluate. In the scientific literature we find numerous studies in which it is said that the stability of aggregates decreases.

In other studies, it has been observed that thermal fusions that occur in certain types of soils with a significant proportion of oxides and iron and aluminium hydroxides in their clay fraction can increase the stability of the aggregates. In addition, the presence of hydrophobic substances generated during combustion can produce an increase in the stability of aggregates, since they act as a thin film that partially or totally surrounds the aggregate, and therefore, also act as cementing substances. For all these reasons, the stability of aggregates is a synthetic parameter that allows to quantify the effect of the fire on the quality of the soil.

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