To extinguish or not to extinguish: The role of forest fire in nature and soil resilience

3.1 Temperatures

Temperature is the intensity of the heat present in a substance that is exposed to fire. It can be expressed according to a comparative scale. In soil, this is referred to as both diurnal and seasonal variations of the created climate(Simard, 1991). The heated biomass transports the temperature in the shrubs and grasses into the soil. The quantity and the period of exposure determine the severity of the impact caused by forest fire, both positively and negatively. The effect of temperatures on the soil impacts soil's physical, chemical, and biological components. The physiological impact begins to be established at the 40–70 degrees range, and this is due to the quick protein degradation that causes plant tissue death. Roots become dehydrated at soil temperatures of 48 to 54 degrees. (Chandler et al., 1983). The different elements in soil have different threshold temperatures of volatilizations. However, the by-product of all the phases of fire has an impact on belowground sustainability. Heat transfer to the organic and mineral horizons of the soil cause intense effect on the soil biomolecules, physical, chemical, and biological processes essential in maintaining soil sustainability (DeBano et al., 1998). The time is taken for the combustion process, and the transfer of heat is highly varied. Thus, the extent of the effects of the fire is equally variable (Simard, 1991). The research has indicated that the low impact combustion on the different terrains worldwide can promote herbaceous flora, essential in maintaining the belowground soil sustainability (DeBano et al., 1998).

4.1 Cellulose and pectin

Cellulose and pectin together form 50% of dry plant mass. Most of the plant char for measuring forest fire is derived from the cellulose and pectin components. On high temperature exposed, significant observable structural changes occur with a weight loss of approximately 10% (Díaz-Raviña et al., 1996). A small amount of glucose in the cellulose is hydrolyzable from the char that is formed after combustion. However, when cellulose is exposed to 270 degrees, pyrolysis reaction starts, and substantial physical and chemical changes occur (Leisted, 2014). New molecular structural changes are observed at much higher temperatures like phenol, aromatic, aliphatic, and aryl carbon. At 250 degrees, the crystalline structure of sugar is still observed; also, glucans start to appear. Above 300-degree temperature, aromatic hydrocarbon and furan are the main product of pyrolysis. Fig. 4 shows pyrolysis reactions of the cellulose molecule.

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Fig

Pyrolysis of cellulose (Zheng et al., 2016).

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The changes at high temperatures result from nonvolatile anhydroglucose cores that serve as aldol-condensation sites for organic compounds emitted inside the char during combustion. They end up working as molecular attachments that hold tightly together larger nonvolatile residues. Eventually, a polymer is formed that establishes itself gradually by trapping down small reactive pyrolysis products. The further inference made by Boon (1994) is that at a temperature greater than 250 degrees, polyaromatic structures were formed, and it contained furanoid, hydroxy aromatics, different types of carbonyl, and carboxyl groups, and also a significant amount of unsaturated hydrocarbon side chains (Boon et al., 1994). Further, at more severe heating, the furan polymer is observed to disproportionation through the loss of CO and CO₂ that eventually led to the creation of aromatic polymer. In another similar experiment conducted by McGrath, he applied low temperature (30–300 degrees) to D-glucose and sucrose instead of using a higher temperature. He obtained pyrolyzed product furans, pyranones, anhydrosugars, and 5-hydroxymethylfurfural along with cellulose [24]. However, only at a higher temperature of 300 to 600 degrees, polycyclic aromatic hydrocarbons were detected (Fig. 5). The addition of oxygen to the combustion process reduced the number of polycyclic aromatic hydrocarbons emitted from the char, even at higher 400 to 600 degrees temperature. More complex condensed polycyclic aromatic hydrocarbons structures are formed through what McGrath indicates as a Diels-Alder reaction (McGrath et al., 2003).

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Fig. 5

Examples of polycyclic aromatic hydrocarbons (PAHs).

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On the other hand, pectin contains approximately 10 to 15% plant dry weight and is a water-soluble galacturonic-glycan. However, a gradual drop is observed in the amount of both the oxidative and non-oxidative pectin. At temperatures higher than 350 degrees, there is a significant drop by 20% of the initial note oxidative pectin substrate. Furthermore, when oxygen is introduced to the pectin, they were fully oxidized at higher 550 degrees (Waymack et al., 2004). NMR image shows that oxidized and non-oxidized char look virtually identical (Wang et al., 2016).

4.2 Lignin

Lignin is a complex natural noncarbohydrate polymer composed of phenylpropanoid subunits present in the plant vascular tissues and algae. Lignin provides strength and rigidity to wood and barks (Ekielski and Mishra, 2020; Prasad et al., 2021); it is recalcitrant, making it difficult to degrade. The lignin percentage in wood differs from 20% to 40%, whereas it goes down to 4% in grasses (Mishra et al., 2016). They make most of the wood and barks for lending stability to the plants, and fundamentally they do not get easily degraded. Phenylpropanol is a common polymer linked together by ether bonds and C–C bonds (Lüdemann and Nimz, 1973). Many volatile organic compounds are present in wood (lignocellulosic biomass). During a forest fire, these volatile organic compounds get exposed to heat and cause the emission of various gases. Lignin gets exposed to high temperatures that produce volatile gases. Exposing lignin to heat produces char and volatile products that are majorly substituted with methoxyphenyl.

According to Sharma et al. (2004), the highest char is produced at 500 to 600 degrees. However, the pyrolyzing of the pure lignin under the anoxic conditions at 450 degrees and 750 degrees yield 40% to 60%char, respectively (Sharma et al., 2004). Coke is formed when a volatile compound undergoes a condensation reaction. Such responses are absent under anoxic pyrolysis. Pyrolysis at 350C and 550C produce the same amount of char. However, at 550 degrees, only 20% could be collected as char, commonly observed through both procedures. Infrared and Solid-state nuclear magnetic resonance spectroscopy observation revealed that chemical alteration of lignin takes place due to dehydration at low temperatures. Despite there being some minimal decarboxylation occur at the high temperatures, the aromatic ring remains intact. The observation by Sharma et al. (2004) indicated that sodium and potassium in the reactions led to the enhancement of the de-volatilization but, on the other end, reduces the overall yield of char(Sharma et al., 2004).

4.3 Amino acid

Amino acids polymerize to form a protein molecule. When proteins are exposed to high temperatures, it decomposes through a reaction referred to as systematic and random depolymerization reaction. Thermal cyclization of amino acids occurs at high temperatures, and cyclic dipeptides are formed, scientifically referred to as diketopiperazines (Reeves Iii and Francis, 1998). The pyrolysis of amino acids leads to the formation of cyclic oligomers, and water is lost due to the nucleophilic attack of the N-terminal amino group on the C-terminal carbonyl group. The pyrolysis of polypeptides results in decarboxylation, deamination, lower dipeptides, and subsequent dehydration to give 2,5-diketopiperazine molecules. Eventually, in the reaction, ammonium is lost through the aliphatic side chains' homolysis to end up with a cyclic product (Chiavari and Galletti, 1992) (Fig. 6).

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Fig. 6

The primary mechanisms for amino acid thermal breakdown (Britt et al., 2004).

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Cyclic products are formed at a higher temperature (840 degrees) due to the pyrolysis of aromatic hydrocarbons containing proteins, phenols, and their N-containing analogs (nitriles and anilines) (Knicker, 2007).

4.3.1

4.3.1 Maillard reaction

The Maillard reaction entails a complex set of chemical reactions between amino acids and reducing sugars, usually requiring heat. This reaction is responsible for the long-term stabilization of organic nitrogen into the soil.

Carbinolamine is formed because of a nucleophilic attack on the glucose molecule. Carbinolamine then dehydrates to give Schiff bases. Schiff base forms Amadori product after a series of slow rearrangement reactions. A mixture of many isoforms of Amadori products is formed during the Millard reaction Fig. 7.

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Fig. 7

Chemistry of Maillard Reaction and Amadori rearrangement (Li et al., 2014).

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4.4 Free Lipids

The lipid component of soil contains a large and diversified collection of hydrophobic compounds like fatty acids and more complex compounds such as terpenes, polynuclear hydrocarbons, chlorophyll, fats, waxes, sterols, and resins (Braids and Miller, 1975). Together with the waxes, Lipids are the significant contributors to soil water repellency (Stevenson, 1994). Lipids present in the soil undergo major structural alteration due to fire, lipid polymerase into the kerogen-like matter. This could be the main reason for the water repellant nature of burn soil. Fire-affected soil shows a lower percentage of lipid.

In contrast, burn soil has six times more lipid fraction than unburn soil, most likely due to the release of organic compounds from burning biomass. The main difference in the molecular arrangement of lipid before and after the fire is the arrangement of alkyl compounds. The lipid composition of burn soil shows a higher percentage of low molecular weight homologs units. The resin acid shows the dominance of dehydroabietic and secodehydroabietic acids and less presence of pimaric acid. Extensive study is required to understand the chemical composition of soil organic matter up to the molecular level(Schmidt and Noack, 2000).

4.5 Organic matter content

Most of the soil nutrients reside on the top surface of the soil, i.e., O or A horizon (Harrison et al., 2011). Soil organic matter is the main source of Nitrogen phosphorous Sulphur and an essential source of NH₄, potassium, Calcium, Magnesium, Copper, iron, Manganese, and Zinc in soil (Lehmann et al., 2003). However, soil organic matter is most affected by fire (DeBano et al., 1998). The combustion process is often incomplete; it gives rise to pyrogenic organic matter and many high heterogeneous aromatic materials. The fire level happens to be highly variable due to different accumulative factors (Ye et al., 2017). These factors include the fire type: an underground or above-ground fire, how intense the fire was, and the slope. The observed effects will range directly from the extreme destruction of the organic matter to the addition of a surface layer that contributes to approximately 30% of surface input. It does not take after the forest fire for a sharp decrease in the organic matter of some soil to be observed. Gradually, it is accelerated by the notable change in the physic-chemical properties, such as water repellency and the temporal removal of the herbaceous insulating layer (Chandler et al., 1983). However, researchers have indicated that carbon–nitrogen ratio in the soil after a fire is way lower than the original fire, depending on the intensity of the cause fire. After a fire, the soil texture becomes coarser due to stable aggregates (Ulery and Graham, 1993).

4.6 Humic acids

Humic acid is usually an extract of the humic substances that form major organic constituents of soil. It is created by the microbial degradation of the matter contained in decaying dead plants such as charcoal. Heating this soil component will lead to an overall increase in the abundance of the humus fraction (Almendros et al., 2003). The fractions with a more thermolabile character have a more advanced effect. If this fraction possesses free organic matter and non-humified matter, it may contain a weak association with the mineral fraction. Humic acid has two polar functional groups, a phenolic group, and a carboxylic group. It cannot be dissolved in water in its acidic state. Thus, it is used as a soil supplement with agriculture experts(Liu et al., 2018). If the soils are consistently affected by a fire, the humus in the soil will correspond to stuffing up the black carbon.

4.7 Black carbon

The term black carbon is the term that is preferred by most literature personnel to refer to charcoal and other compounds of carbon such as elemental carbon and pyrogenic carbon(Doerr et al., 2018). In common, it is a black material that is usually emitted from different sources that burn fossil fuel. The combustion process of these fossil fuels and vegetation must be incomplete to allow carbon creation. The intensity of studies and analysis done for the black carbon has increased notably in the geochemical and biological studies on the soil (Wagner et al., 2018). This is because of the realization of the magnitude of its importance on different types of biogeochemical processes. Black charcoal is a ubiquitous material with varying forms of organizations that would range from low-rank coal and the likes to that of anthracite and coal resembling material (Kuhlbusch, 1998). The black carbon mainly formed in forest fire consists of randomly oriented layers of graphitic material pushed tightly together (Schmidt and Noack, 2000). However, in any case, this material, made of graphite, is rather an entirely amorphous, substantial alkyl domain and a sustainable oxygen content.

5.1 Soil color and texture

The effects of fire on the color and texture are differently noticed in different conditions. For instance, the soil that gets burnt severely under concentrated fuel is highly noticeable as opposed to that soil moderately burnt. When the temperatures get to higher degrees, the soil changes to red due to the Fe-oxides transformation and completely eradicates organic matter(Ulery and Graham, 1993). However, in low temperatures or when the soil is exposed to moderate fire, its color changes to black or grey due to the ash content (Neary et al., 2005). However, at low temperatures that range from 300 to 600 degrees, the Munsell hues change to yellower. This is due to the drastic decrease in values and chromas (Ngole-Jeme, 2019). Spatially accumulating thermally produced iron oxides can be detected easily in the areas where the fire has affected a massive area. In the severely burned soil, the entire underlying layer gets blackened and thick up to 15 cm and causes an overall reduction of the Munsell value.

The impact of fire on soil differs depending on the kind of soil. Some soils such as sand, silt, and clay have a higher heat tolerance and usually are not affected by combustion unless directly exposed to temperatures higher than 400 degrees at the mineral soil surface. On the other end, clay holds minimal resistance to temperatures, and change is observable starting from 300 degrees. This is because the clay hydration and clay lattice structure break down (Beyers et al., 2005).

5.2 Soil pH and bulk density

Another physical aspect that gets affected is soil pH. Research indicated a general increase in the soil pH after the forest fire (Aref et al., 2011). At considerably high temperatures, there is rather more increase in the levels of pH. This can be attributed to the increased ash level, which holds higher acidity levels (Schafer, 2010). The other aspect affected is the bulk density, which involves the amount of dry soil per unit bulk volume and is expressed on the S.I unit of g/cm3. It is crucial concerning porosity. Porosity in the soil is pore volume in a soil sample divided by the total volume of the bulk sample. The collapsing of aggregates and blockage of spaces by ash and scattered clay particles increases the soil bulk density after a forest fire. The relation between the increase in soil bulk density and the combusted soil is that the bulk density increases alongside the ash depth (Cerda, 2009).

5.3 Soil aggregation

Soil aggregate stability refers to the flexibility of soil structure in response to applied external mechanical forces. It plays a critical role in re-establishing soil structure and function. Soil burning causes a significant decrease in the clay content of the soil. As a result, increase in sand-size course particles this initiates aggregation of finer particles into coarse particle. Some studies also showed no sign of an increase in the soil particle size (García-Corona et al., 2004). Soil aggregation is mainly affected by high-intensity fire; generally, low-intensity fires do not produce many notable changes. After a high-intensity fire, partial or complete soil combustion changes soil aggregation and has a different fluvial behavior than unburn soil (Mataix-Solera et al., 2011). The study by Katrin Burri suggests that Revegetation improves soil aggregate stability by speeding up vegetation growth and boosting soil formation processes (Burri et al., 2009). There is no regular pattern in soil aggregate stability of burnt soil. Stability may decrease, increase or remain the same. Table 2 summarizes some past studies showing the aggregate response to wildfire. Soil aggregation response to wildfire from different studies is summarized in Table 2.