Message #116810

[Bianca]

Wood charcoal



The transformation of biomass into charcoal produces a series of charcoal derivates covered under the name of pyrogenic or black carbon, the composition of which varies; from lightly charred organic matter, up to soot particles rich in graphite formed by recomposition of free radicals (Hedges et al. 2000).

Here, all types of charbonated materials are called charcoal.

By convention, charcoal is considered to be any natural organic matter thermically transformed with
an O/C percentage less than 0.6[16] (smaller values have been suggested).

Because of possible interactions with minerals and organic matter from the soil, it is almost impossible
to identify charcoal with any certainty by determining only the proportion of O/C. The H/C percentage[18] or molecular markers such as benzenepolycarboxylic acid[19], are therefore used as second level
of identification.

Charcoal was added to poor soils, as wood charcoal processed at low temperature and with a limited supply of oxygen (i.e., with smothered fires).

William Woods (University of Kansas, Lawrence), expert on (ancient) abandoned living sites, has measured in Terra preta up to 9% black carbon (against 0.5% in surrounding soils). B. Glaser et al have found up to 70 times more carbon than in surrounding Ferralsols[3], with approximative average values of 50 Mg ha-1 m-1.

Amending the soil with low temperature charcoal produced from a mix of wood and leafy biomass (termed biochar) has been observed to increase the activity of arbuscular mycorrhizal fungi.

Finnish researcher Janna Pietikäinen has tested high porosity materials such as zeolite, activated carbon and charcoal; these tests show – contrary to her expectations - that microbial growth is substantially improved with charcoal. It may be so that these small pieces of charcoal tend to migrate within the soil, providing a habitat for bacteria that decompose the biomass in the surface ground cover. It is theorized that this process may have an essential role in Terra preta soils' self-propagation; a virtuous cycle would be established as the fungus spreads from the charcoal, fixing additional carbon, stabilizing the soil with glomalin, and increasing nutrient availability for nearby plants.  Many other agents contribute, from earthworms to humans and the charring process.

The chemical structure of charcoal in Terra preta soils is characterized with poly-condensed aromatic groups, providing prolonged biological and chemical stability that sustains the fight against microbial degradation; it also provides, after partial oxydation, the highest nutrients retention.

Wood charcoal (but not that from grasses or high cellulose made at low temperature), thus has an internal layer of biological oil condensates that the bacteria consume, and that is similar to cellulose in its effects on microbial growth (Christoph Steiner, EACU 2004).

Charring at high temperature loses that layer and brings little increase in the soil fertility. Glaser et al. (1998 and 2003) and Brodowski et al. (2005) have proved that the formation of condensed aromatic structures depends on the manufacture of charcoal. It is the slow oxidation of charcoal that creates carboxylic groups; these increase the cations' exchange capacity in the soil.

Lehmann et al have studied the nucleus of black carbon particles produced by the biomass. They have found it highly aromatic even after thousands of years in the soil and presenting spectral characteristics of fresh charcoal. Around that nucleus and on the surface of the black carbon particles, there were higher proportions of forms of carboxylic and phenolic Cs spatially and structurally distinct from the particle's nucleus. Analysis of the groups of molecules provides evidences both for the oxydation of the black carbon particle itself, as well as for the adsorption of non-black carbon.

This charcoal is thus decisive for the sustainability aspect of Terra preta soils.

Amendements of Ferrasol with wood charcoal greatly increases vegetal productivity.

Note that agricultural lands have lost in average 50% of their carbon due to the practice of intensive cultivation and other degradations of human origin.

It is important to note that the fresh charcoal must first be “charged” before it can function as a bio
tope.  Several experiments demonstrate that uncharged charcoal can bring a provisional depletion of available nutrients when first put into the soil - until its pores fill up with nutrients. This is overcome
by soaking the charcoal for a few weeks (2 to 4 weeks) in any liquid nutrient (urine, plant tea. ...).