Understanding soil requires knowledge of particle aggregation. Fire affects aggregation by degrading the biological fraction, although low-intensity fire does not leave obvious signs in its wake. Scientists have developed ingenious methods to assess the quality of aggregates.
Soil is like a skin, wrapping the Earth’s solid core. It is also where air, water, organic matter and minerals come into contact. And where the living and the lifeless world unite and mediate biogeochemical cycles. Nutrients, water and even the chemical composition of the atmosphere all depend on soil functioning.
«A good soil structure encourages many pores through which air and water can circulate, and promotes a better exchange of elements with plant roots»
Soil consists of organic and inorganic particles. A good soil structure encourages many pores through which air and water can circulate, and promotes a better absorption of elements by plant roots. Good soil aggregates imply a fertile soil, with little or no loss of matter. Conversely, if the structure is poor there is increased runoff and soil loss.
Soil aggregates indicate the health of the soil ecosystem. Therefore, scientists have used them as indicators of the impact of certain stress factors, and when it comes to fire we can find out a lot about the effect of forest fires thanks to aggregates. The Environmental Soil Science Group at the Universidad Miguel Hernández de Elche has spent the last two decades conducting research into aggregates and how they are affected by fire. Here the most relevant findings are summarised and future challenges discussed.
The effect of fire on soil aggregates
The definition of aggregate is an assemblage of individual mineral (sand, silt and clay) and organic particles that form soil. When individual particles are grouped together, they take on the appearance of larger particles, which are called aggregates. For soil structure to develop and, likewise, to form aggregates, there must be agents that act like cement between the particles. Soil aggregation is dominated by two important phenomena: flocculation and aggregation. Flocculation is due to electro-kinetic phenomena, namely, it occurs when negatively charged particles are held together by a particle with the opposite charge. Aggregation, meanwhile, is the binding of flocculated particles due to the action of different materials or substances. These agents can be physical, chemical and/or biological. For example, calcium cation (Ca2+) induces clay flocculation. In particular, the divalent cations associated with clays –such as calcium, iron and aluminium– favour aggregate formation, while sodium favours dispersion. Both the quantity and quality of organic matter in the soil play a key role in aggregate formation, acting as a metal chelating agent and facilitating the formation of aggregates, such as iron ions (Fe3+) in podzolic soils or calcium in calcareous soils.
However, in addition to abiotic aggregation agents there are also biological ones. Plant roots hold soil particles together and facilitate aggregation through the secretion of organic compounds that act as inorganic bonds. They also provide compounds used by microorganisms that are also involved in the formation of aggregates, mainly fungal hyphae. And let’s not forget that most animals living in the soil (microfauna, mesofauna and macrofauna) are also involved in aggregate formation. For example, gastropods secrete mucilaginous substances that act as soil particle cement, while annelid species, such as the earthworm (Lumbricus terrestris), can form stable solid aggregates through a variety of mechanisms stabilising the excretions produced on ingesting soil particles and organic matter.
«For soil structure to develop and, likewise, to form aggregates, there must be agents that act like cement between the particles»
Aggregate stability refers to the ability of soil aggregates to resist disruption when outside forces –such as the impact of raindrops– are applied. Aggregate formation plays an important role in physical and biogeochemical processes, affecting water movement and storage, aeration, biological activity, vegetation growth and erosion, among other factors. For example, structure maintenance is important in terms of having a good porous system, which enhances infiltration, prevents erosion and facilitates plant nutrition –as well as improving the soil-atmosphere exchange of gases–.
There are many possible methods to study aggregation, which are both resourceful and innovative but, as yet, there is no universally applicable method (Mataix Solera et al., 2010). The choice of a particular method mainly depends upon the disruption mechanism applied. Aggregates are chiefly broken down as a consequence of the mechanical action of water; therefore, methods have been developed to simulate the disintegration forces present under field conditions. Among such methods is one based on knowing the percentage of aggregates that withstand raindrop impact, simulated in the laboratory (Roldán et al., 1994). The percentage of aggregates and size distribution are the parameters most commonly used to indicate soil structure stability and therefore its resistance to external factors.
Aggregate stability is considered a physical property; however, as it depends on soil chemistry and biology, many researchers consider it to be a property both integrative and indicative of soil health. Aggregate stability is a synthetic parameter indicating the status of ecosystem health, and reflecting the disruptions experienced. This is why it is so useful when it comes to studying the effects of fire on the soil ecosystem.
«After a fire, because plant cover and leaf litter are destroyed, structural stability becomes a key feature in water management and affects the loss of both soil components and nutrients»
Fire and aggregates
Fire has effects on the physical, chemical and biological properties of soil, generally depending on the severity of the fire. Changes may be short or long term, they may even be permanent or irreversible, depending on the soil and the severity of the fire, as well as post-fire management. Moreover, after a fire, because plant cover and leaf litter are destroyed, structural stability becomes a key feature in water management and affects the loss of both soil components and nutrients.
The impact of raindrops after a fire can break the aggregates down, so the finer fractions clog the pores, thereby changing soil porosity. It is complex to assess how aggregates respond after being subjected to fire. There are many studies in the scientific literature reporting that post-fire aggregate stability decreases, especially due to the loss of soil organic matter, while –by contrast– there are others (Mataix-Solera and Doerr, 2004; Arcenegui et al., 2008) reporting that it increases for a variety of reasons. Indeed, in low-intensity fires some studies have observed an increase in aggregates because of the growth in organic matter.
Other studies have shown that as a consequence of heat, fusion can occur in certain soil types having a significant proportion of iron and aluminium oxides and hydroxides in the clay fraction, which can increase aggregate stability. Furthermore, the presence of hydrophobic substances generated during combustion can lead to increased aggregate stability by acting as a thin film, partially or completely surrounding the aggregate, and therefore also act as aggregating substances (Arcenegui et al., 2008). Therefore, aggregate stability is a synthetic parameter that is useful to quantify the effect of fire on soil quality.
Arcenegui, V. et al., 2008. «Immediate Effects of Wildfires on Water Repellency and Aggregate Stability in Mediterranean Calcareous Soils». Catena, 74: 219-226.
Mataix-Solera, J. & S. H. Doerr, 2004. «Hydrophobicity and Aggregate Stability in Calcareous Topsoils from Fire-affected Pine Forest in Southeastern Spain». Geoderma, 118: 77-88.
Mataix-Solera, J. et al., 2010. «¿Cómo estudiar la estabilidad de agregados en suelos afectados por incendios? Métodos e interpretación de resultados». In Cerdà, A. & A. Jordán (eds.). Actualización en métodos y técnicas para el estudio de los suelos afectados por incendios forestales. Càtedra de Divulgació de la Ciència. University of València. Valencia.
Roldán, A. et al., 1994. «An Incubation Experiment to Determinate Factors Involving Aggregation Changes in an Arid Soil Receiving Urban Refuse». Soil Biology and Biochemistry, 26: 1699-1707.