# Fire-Induced Changes in Soil and Implications on Soil Sorption Capacity and Remediation Methods
## 3 Fire-Induced Changes in Soil Properties and the Implications on Soil Sorption Capacity
### Fire-Induced Changes in Soil Organic Matter Content and the Implications on Soil Sorption Capacity  
According to [PERSON] et al. [112] and [PERSON] et al. [123], organic matter is the single most important factor affecting the sorption of heavy metals, pesticides and other organic compounds in soils. Soil organic matter has several reactive sites ranging from potentially anionic hydroxyls (R-OH) and carboxylic groups (R-COOH), to cationic sulfhydryl (R-SH) and amino groups (R-NH\({}_{2}\)), as well as aromatic (-Ar-) and aliphatic ([-CH\({}_{2}\)-]n) moieties that are the main un-charged, non-polar regions of the soil solid phase. The amount of these moieties in any given soil varies, and so does their surface area and chemical composition, which significantly influence the sorption characteristics of any given soil. Organic matter contribution to soil sorption is more pronounced in sandy soils than in clayey soils, where clay minerals are the main contributors [117]. Changes in organic matter structure and content in soils could reduce or increase soil sorption capacity. During fire events, organic matter undergoes a series of changes which include free vaporization of moisture at 100 \({}^{\circ}\)C, degradation and decomposition of lignin and hemicellulose between 130 \({}^{\circ}\)C and 200 \({}^{\circ}\)C, chemical dehydration of cellulose at 280 \({}^{\circ}\)C, smoldering between 500-600 \({}^{\circ}\)C, and flaming/igniting between 800-1500 \({}^{\circ}\)C [79; 80]. Fires with temperatures of between 250 and 460 \({}^{\circ}\)C could cause volatilization, charring or complete oxidation of soil organic matter. At these temperatures, [PERSON] et al. [81] and [PERSON] [10] indicated that aromatic compounds accumulate at the expense of aliphatic ones. Though soil organic matter content is drastically reduced by wildfires, organic components including free lipids, and humic acids have been shown to increase [82], with new aromatic compounds being formed from the alteration of carbohydrates [83]. Studies by [PERSON] et al. [84] indicate that chain length in lipids is decreased, whereas [PERSON] et al. [81] showed that changes in solubility of humic substances occur. [PERSON] et al. [85] also noted that aromaticity of organic material increases as soil temperatures increase. [PERSON] et al. [81] have described the changes caused by fire on humic substances as:  
* Total removal of external oxygen-containing functional groups, leading to the formation of substances with low solubility,
* Decreased aliphatic chain length of humic acids and other hydrocarbons,
* Increased degree of aromaticity in organic matter,
* Structural changes in humic acids, including polymerization,
* Formation of nitrogen-containing heterocycles
* Formation of black carbon. Burning of organic matter in soils not only produces ash, but pyrogenic carbon (biochar or charcoal) is also produced when plant biomass is exposed to fire at temperatures below 900 \({}^{\circ}\)C in oxygen-limited conditions. This could occur during ground or smoldering fires which take place below the surface of soils that have woody plant roots and high organic matter in the surface layer. The high sorption capacity of charcoal has been acknowledged in studies by [PERSON] et al. [51], [PERSON] et al. [124], [PERSON] et al. [125], and [PERSON] and [PERSON] [126]. [PERSON] et al. [127], for example, reported Pb, Ni, Cd and Cr absorption capacities by biochar to be 2.4-147, 19.2-33.4, 0.3-39.1, and 3.0-123 mg/g, respectively, highlighting the contribution of this material to soil sorption when present. According to [PERSON] et al. [128], as the temperature at which charcoal in soil is formed increases, the absorption capacity of the produced charcoal increases. Fires of high intensity, especially under poorly aerated conditions, could therefore increase soil sorption capacity due to the formation of biochar. This condition is, however, not permanent, because of biochar oxidation over time [129], though [PERSON] et al.