Labile Organic Molecules, the "Secret Weapon" of Biochar?
From "Shifting Paradigms on Biochar: Micro/Nano-structures and Soluble Components are Responsible for its Plant-Growth Promoting Ability”, S Joseph, ER Graber, C Chia, P Munroe, S Donne, T Thomas , Nielsen S, C Marjo, H Rutlidge, G X Pan, Xiaorong Fan, P Taylor , A Rawal, J Hook, Carbon Management, Vol 4, No. 3 (2013).
Like mineral components, labile organic molecules (LOM) of both low and high molecular weight that accumulate at biochar surfaces and condense in pores during biochar production have the potential to influence multiple processes across the soil-rhizosphere-plant continuum. This is for a number of separate and interrelated reasons. Many LOM compounds, known to be phytotoxic or biocidal at high concentrations, could have hormonal effects at low concentrations, for example, inducing plant resistance to stress [7,42]. Another way in which LOM can indirectly affect plant responses is by changing the structure of the soil microbial community, perhaps by providing carbon compounds that only specific members of the microbial consortium can utilize, or by being toxic to weaker members of the consortium . Biochar-induced changes in soil microbial community structure could result in widespread changes in microbial functioning in the soil, thus affecting both nutrient cycling and soil gas emissions [44-47]. Biochar additions can increase microbial diversity in the soil and encourage beneficial soil microorganisms  which can directly enhance plant growth and induce systemic resistance.
While some LOM compounds are phytotoxic, others may induce seed germination. Karrikans, for instance, which are formed during pyrolysis and combustion of biomass, are known to promote seed germination and are believed to contribute to the germination bursts that frequently follow forest fires . Other compounds formed during biomass pyrolysis may also play yet undetermined roles in plant growth. In particular, biochar macromolecular LOM are similar in character to humic substances , which have been long known to positively impact seed germination, root initiation, plant nutrition, and total plant biomass .
Chemical assays measuring the ability of aqueous extracts of biochars to reduce metals revealed that the soluble fraction indeed has substantial ability to reduce oxidized metals, with extracts of low HTT biochars being much more redox active than extracts of high HTT biochars . Aqueous extracts of low and high HTT biochars also reduced and solubilized Mn and Fe from different soils over a wide range of pH values , with the extract of the lower HTT biochar, having a greater variety and concentration of soluble reducing agents, solubilizing more Mn and Fe than the extract of the higher HTT biochar. In the studied systems, the dissolved organic matter (DOM) fraction, in particular phenolic compounds, was proposed to be responsible for the main part of the reducing capacity, which was on a par with the reducing capacities of various humic and fulvic substances.
The implications of these results for the role of biochar in a wide range of chemical and biological redox-mediated reactions in the soil could be wide-reaching . Redox-related processes in soil include microbial electron shuttling, nutrient cycling, free radical scavenging, pollutant degradation, and contaminant mobilization or immobilization. Redox reactions mediated by biochar-derived chemicals could play a role in abiotic formation of humic structures in soils, resulting in increased soil organic carbon content and improved soil aggregation. Redox active structures could serve as electron shuttles between bacterial cells and Fe(III)-bearing minerals , taking part in bacterial reduction and immobilization of metals. Since oxidation of biochar surfaces leads to continued release of redox-active, acidic and phenolic organics of both low and high molecular weight [52,53], as well as to sustained release of various soluble inorganic species , biochar is expected to continue to participate in redox reactions as it ages in the soil.
Besides expediting redox reactions, many LOM compounds are ligands possessing multiple carboxylate, phenol, alcohol or enol groups [42,49], known to form stable metal-organic complexes with metals having different oxidation states. Formation of water-soluble metal–organic complexes can increase the concentration of metals in the aqueous phase and their bioavailability. This effect is well known for root exudates and humic substances. For instance, chelation of Fe and Zn by humic substances enhanced the metals’ solubility in nutrient solution and improved the growth of melons, soybean, and ryegrass . Formation of water-insoluble metal-organic complexes can also occur, resulting in an increase in soil organic matter content. When oxidized by Mn oxides, o- and p-polyhydroxyphenols precipitate as polymeric humic-like substances that are effective chelators of Al [56,57].
I would suggest that people read the following references that look at the value of the labile organic fraction in biochars. In particular Ellen Graber and her colleagues have found that the organic molecules that are readily available on biochars produced at 450C help induce resistance to plant disease. Also read Light et al about the germination benefits of oreganic molecules produced in smoke.
Graber ER, Meller-Harel Y, Kolton M et al. Biochar impact on development and productivity of pepper and tomato grown in fertigated soilless media. Plant Soil, 337, 481-496 (2010).
Kolton M, Meller Harel Y, Pasternak Z, Graber ER, Elad Y, Cytryn E. Impact of biochar application to soil on the root-associated bacterial community structure of fully developed greenhouse pepper plants. Applied and Environmental Microbiology, 77, 4924 - 4930
Light ME, Burger BV, Van Staden J. Formation of a seed germination promoter from carbohydrates and amino acids. J. Agric. Food Chem., 53(15), 5936-5942 (2005).
Lin Y, Munroe P, Joseph S, Henderson R, Ziolkowski A. Water extractable organic carbon in untreated and chemical treated biochars. Chemosphere, 87(2), 151-157 (2011).
50. Graber ER, Tsechansky L, Lew B, Cohen E. Reducing Capacity of Water Extracts of Biochars and their Solubilization of Soil Mn and Fe Eur. J. Soil Sci., in press (2013).
51. Lovley DR, Fraga JL, Blunt-Harris EL, Hayes LA, Phillips EJP, Coates JD. Humic substances as a mediator for microbially catalyzed metal reduction. Acta Hydrochimica et Hydrobiologica, 26(3), 152-157 (1998).
52. Abiven S, Hengartner P, Schneider MPW, Singh N, Schmidt MWI. Pyrogenic carbon soluble fraction is larger and more aromatic in aged charcoal than in fresh charcoal. Soil Biology and Biochemistry, 43(7), 1615-1617 (2011).