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.¹
It is theorized that this process may have an essential role in Amazonian 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.² Many other agents contribute, from earthworms to humans and the charring process. The chemical structure of charcoal 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 nutrient retention¹.
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). It is the slow oxidation of charcoal that creates carboxylic groups; these increase the cations' exchange capacity in the soil.
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 biotope. 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 (kelp, worm leachate, urine, plant tea etc)
Organic matter and nutrients Charcoal's porosity brings a better retention of organic matter, of water and of dissolved organic nutrients.
References:
¹ “Prehistorically modified soils of central Amazonia: a model for sustainable agriculture in the twenty-first century”, by Bruno Glaser
² Plant-fungal interactions via Glomalin: A fungal protein that affects soil ecosystem cycling of C, N, P & S. By Jude Maul and Laurie Drinkwater (Cornell University, Ithaca, NY)
