Distinguishing Black Carbon from Biogenic Humic Substances in Soil Clay Fractions

Distinguishing Black Carbon from Biogenic Humic Substances in Soil Clay Fractions
Laird, David, Chappell, Mark, Martens, Dean, Wershaw, Robert, Thompson, Michael
Geoderma, October 3, 2007
Interpretive Summary:
Soil organic matter, often called humus, contributes substantially to the quality of soils by stabilizing soil structure and both retaining and helping to cycle plant nutrients. The chemical structure of soil humus is very complex, but has been thought in the past to be a complex polymer with a backbone of aromatic carbon. We developed a method to physically separate aged charcoal from soil organic matter. The charcoal contributed about 8% of the carbon in the three soils we studied. The charcoal was old and was not very available to soil microorganisms. The true humus was the fraction of the soil organic matter left after the charcoal had been extracted. The true humus was young and readily available to soil microorganisms. The true humus had very little aromatic carbon; rather it was a mixture of fatty acids and sugar-like structures. This discovery means that many of the previously proposed models for the structure of soil humus are wrong, because most of the aromatic carbon is in the charcoal not in the true humus. This study will help scientists to understand how soil organic matter is formed and stabilized in soils. Such information is needed to design agricultural management systems that build soil organic matter. Policy makers need to understand that charcoal is a natural component of soils and that adding charcoal to soils will help build soil quality because in the future conservation practices may include adding charcoal to soils.
Technical Abstract:
Most models of soil humic substances include a substantial component of aromatic carbon (C) either as the backbone of humic heteropolymers or as a significant component of supramolecular aggregates of degraded biopolymers. Here we report that most of the aromatic C in the clay fraction of three studied soils was associated with discrete particles (0.2-2 um) of pyrogenic black carbon (BC), which were physically separated with the coarse clay subfraction. The physically separated BC particles contained approximately 60% aromatic C, with the remainder being a mixture of aliphatic, anomeric and carboxylic C. We hypothesize that as BC particles aged in soils their surfaces were oxidized to form carboxylic groups. Further, we suggest that anomeric and aliphatic C accumulated in BC particles either by adsorption of dissolved biogenic compounds from the soil solution or by direct deposition of biogenic materials from microbes living within the BC particles. The biogenic soil organic matter that was physically separated with the medium and fine clay subfractions was dominated by aliphatic, anomeric, and carboxylic groups with little aromatic C and existed as diffuse filamentous clumps and films binding clay particles together. The lack of aromatic C in the biogenic soil organic matter is inconsistent with the heteropolymer model for the formation of humic substances.