Charcoal and Its Effect on Soil Properties

Charcoal is a dark-colored, solid carbonaceous material that results from the combustion of organic material. Kuhlbusch (1998) has proposed some useful definitions of charcoal and related materials:

• Charcoal is the carbonaceous residue formed in-place during combustion. It retains the gross anatomical features of the original material.
• Soot is the solid-phase carbonaceous particulate material formed above a fire by gas-to-solid conversion.
• Black carbon is a component of charcoal and soot. BC is highly aromatic, resistant to oxidation at 340 ^oC under pure oxygen, and has a molar H/C ratio of 0.2 or less.

Given the widespread historic occurrence of fire throughout the South (Stanturf et al. 2002), the chemistry of organic matter in many southern soils is likely to have been strongly influenced by the formation of black carbon. Since there is little information available on the importance of charcoal in southern soils, the information presented here originates from other regions.

Soil black carbon content

The BC contents of eight Australian soils of variable char content ranged from near-zero to 30% (mass of BC per mass of total carbon), with results varying widely according to the analytical method employed. Most soils had BC contents between zero and 10% (Schmidt et al. 2001).

Role of charcoal in in soil formation

Charred organic matter is thought to be a source of stable aromatic carbon in many soils, according to a review by Schmidt and Noack (2000). The evidence cited by these authors includes the chemical similarity between humic acids extracted from certain soils and those extracted from charred material, a positive relationship between black carbon content and black color in German chernozemic soils, and the implication of fire as a factor in the genesis of deep black soils in the xeric moisture regime of Northern California. 

Charred organic matter may also play a role in translocating iron (Fe) and aluminum (Al) down the soil profile via organo-metal complexes, which is an important process in the genesis of Spodosols.  Fernández et al. (1997) found that humic extracts from wildfire-impacted soil had much higher levels of bound Fe and Al than those obtained from the soil of a nearby unburned area.

Decomposition of Black Carbon

The decomposition or chemical transformation of black carbon (BC) in ecosystems is likely, but slow.  Bird et al. (1999) studied BC levels in sandy savanna soils of Africa, and concluded that BC had an overall half-life of less than 100 years in surface soil.  Although it was difficult to distinguish between true decomposition and removal by transport, they noted that the charcoal in recently burned areas was hard, black, and of a vitreous luster whereas older charcoal was soft and brown. Zackrisson et al. (1996) noted that young charcoal was more effective than old in mitigating the inhibitory effects of phenolic crowberry (Empetrum spp.) extracts on the germination of European aspen (Populus tremula L.).

Effects of charcoal on plants

Charcoal, when applied to soil in amounts expected after wildfire, enhanced plant nitrogen uptake, altered the competitive balance between plant species (including ericaceous species), and stimulated moss and fern production in a Swedish boreal forest ecosystem. These effects were attributed to charcoal’s ability to bind and deactivate phenolic compounds in the soil (Wardle et al. 1998). There is little or no information on the whether such effects occur in southern ecosystems, which contain numerous ericaceous species.