Tom Miles

THE NATURE, DISTRIBUTION, AND IMPACT OF CHARCOAL IN SOILS. J. O. Skjemstad, L. J. Janik,and L. R. Spouncer, CSIRO, Land and Water, PMB 2, Glen Osmond, South Australia 5064. 1998


We have developed two techniques for the isolation and estimation of soil charcoal. The first technique, which relies on high energy photo-oxidation, [1] can be used for both isolation and estimation but is slow and expensive. The second method utilises infrared spectroscopy and can be used for the rapid quantification of
charcoal carbon in soils [2].

Scanning electron microscopy shows that the highly aryl charcoal residues from photo-oxidation have a plant-like morphology. These charcoal particles can vary considerably in size and morphology and range from plant-like fragments in the 5-40 mm particle range to sub-micron particles which are often platey in nature and can be difficult to distinguish morphologically from clay. Virtually all of this finely divided charcoal appears to result from the burning of grasses and understorey vegetation in savannas and open woodlands.

In Australia, burning of grasslands has been wide spread for millennia and regular burning is an essential component for the functioning of some ecosystems. Soils from all states in Australia and covering a wide range of environments have been separated and analysed for their charcoal content through photooxidation and NMR analysis. All soils have a significant proportion of carbon remaining after photooxidation and in most cases, charcoal makes a major contribution to this fraction, often exceeding 80%.
Although nearly all soils studied contain measurable amounts of charcoal, certain soil types always contain significant amounts. For example, soils that are dark in
colour but contain low amounts of organic carbon (<3%) invariably contain a high proportion of charcoal. It would appear that finely-divided charcoal is significant
in determining soil colour, at least in soils thathave a low or moderate iron content. Soils formed from alluvium with moderate or high clay contents also
often contain significant amounts of charcoal. It is postulated that finely divided charcoal materials are mobile, behaving in a manner similar to clay and silt
and hence accumulate in the same locations where clay and silt materials accumulate. Charcoal is therefore often found in high concentrations and to some depth in soils such as Vertisols, soils formed on riverine plains and deltas and in marine coastal sediments. The distribution of charcoal both laterally and vertically in soils is highly variable and appears to reflect the amount of above ground biomass susceptible to fire and burning frequency, clay content as well as erosional and illuvial processes as described above.

The charcoal material in soils forms the basis of the inert or highly passive pools and has a significant influence on the dynamics of soil organic carbon. Under exploitive cultivation, soils high in charcoal appear to resist organic carbon decline, while those which are low, decline more rapidly [3]. Exploitive cultivation
therefore, often results in an increasing trend in the aromaticity of the organic matter as more labile soil carbon pools decompose leaving the highly aromatic charcoal largely intact. As a result, effective soil organic carbon modelling can only be achieved if the charcoal content of the soil can be estimated reliably.

References: [1] Skjemstad, J. O. et al. 1996. Aust.
J. Soil Res. 34, 251-271. [2] Janik, L. J. et al. 1998.
Aust. J. Exp. Ag. 38, 681-696. [3] Skjemstad, J. O. et
al. 1998. Aust. J. Exp. Ag. 38, 667-680.

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