Fire Effects on the Biological and Chemical Properties of Soil Organic Matter

Biological properties

Fire appears to increase the amount and biodegradation rate of readily decomposable soil organic matter while simultaneously increasing the resistance of the stable portion of soil organic matter.  In laboratory mineralization assays of wildfire-impacted soils, Fernández et al. (1999) observed increases in the amounts and decomposition rate constants of labile (available) carbon (C) as well as decreases in the decomposition rates of the resistant portion of the C pool. The increased decomposition activity that immediately follows fire is likely a result of increased levels of readily decomposable C as well as increased pH, the conversion of nutrients to soluble forms, increased soil temperature, and increased water availability to microbes due to lessened plant water demands. The increased decomposition activity in recently burned soils is viewed as an important nutrient conservation mechanism, as it leads to microbial retention of nutrients that might otherwise be lost from the soil (Woodmansee and Wallach, 1981). The fire-related decreases in the decomposition rates of resistant carbon probably stem from the conversion of humus to “black carbon.”

Contrasting effects of prescribed fire on nitrogen mineralization (release of ammonium or nitrate from organic matter) have been reported. In a southeastern Missouri oak-hickory forest under long-term burning treatment, Vance and Henderson (1984) demonstrated that soils from burned areas were persistently lower in both extractable inorganic N (ammonium plus nitrate) and N mineralization than unburned areas.  They attributed the decreased nitrogen availability to poor substrate quality (perhaps aromatic nitrogen as noted below). Similarly, Bell and Binkley (1989) found increased N immobilization and decreased N mineralization in regularly burned loblolly forests of the South Carolina coastal plain. The high C:N ratios in the burned soils were proposed as an explanation, as high values of this ratio favor N immobilization. Notably, Bell and Binkley (1989) took their samples after a relatively severe burn. In contrast, Schoch and Binkley (1986) found increased N mineralization from the forest floor and increased N levels in foliage following a low-intensity prescribed burn in a previously unburned loblolly forest in the North Carolina piedmont. The apparent conflict between the results of Schoch and Binkley (1986) and other studies may be related to long-term burning effects, differences in C:N ratios, or differences in fire severity. It is of both economic and ecological interest to determine the conditions under which fire might expected to create a “pulse” of plant-available N. For more information, see Fire Effects on Soil Nutrients.

Chemical properties

Heating soil organic matter in the laboratory produces rapid losses of carbohydrates and proteins and eventually produces residues rich in aromatic compounds (Almendros et al. 1992, 2003). The aromatic compounds formed include aromatic forms of nitrogen (Almendros et al. 2003; Knicker et al. 1996), which may be responsible for the reduced N availability observed in some burned areas. Because N is the major limiting nutrient in most forests (Fisher and Binkley, 2000), the relationship between the chemical structure of N and N availability in burned systems is a pressing research need. Hydrophobic polymers are also formed upon heating, and these are probably responsible for the soil hydrophobicity that is observed after fire (Almendros et al., 1992).

The effects of fire on soil organic matter chemistry in soils subjected to repeated burning in the field have been less consistent than effects observed in the laboratory. Nevertheless, useful results have been obtained for specific ecosystems. For example, Guinto et al. (1999) observed an elevated ratio of alkyl carbon (i.e., lipids and waxes) to O-alkyl carbon (carbohydrates) in a burned sclerophyll forest of Australia. This ratio, which is a measure of the degree of humification, had a strong negative correlation with N mineralization in the field. Golchin et al. (1997) contrasted the soil organic matter from an annually burned grassland on volcanic soil with the organic matter of nearby formerly burned grasslands that were reverting to forests. The annually burned grassland had more aromatic and less alkyl carbon than the forested areas. These findings are consistent with other studies that have highlighted the role of fire-generated aromatic compounds in soil genesis (See Charcoal). For a detailed review of fires effect on soil organic matter chemistry, see Gonzalez-Perez et al. (2004)