Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 184

Fig. 3.11.1. Black carbon in particle size fractions.

gave evidence for a complex dynamics of black carbon in Terra Preta soils.
From 20 to 70% of the total black carbon content was found in the light
fraction even in 30-40 cm soil depth (Fig. 3.11.3), identifying a major part
of black carbon as particulate and therefore chemically and biologically
inert. The occurrence of particulate black carbon in 30-40 cm soil depth
indicates transport of black carbon into deeper soil depth by turbation. In
the clay-rich Terra Preta, however, the major part of black carbon was
located in the medium density fraction and, therefore, was organomineral
complexed. Oxidation on the edges of the polyaromatic core of black
carbon produces carboxylic groups, which are involved in organomineral
complexation, but also contribute exchange sites for cations and increase
the cation exchange capacity (CEC). It was surprising that black carbon
was also found to a minor extent in the heavy fraction. SEM and EDX
showed that black carbon in this fraction was physically embedded within
plaques of iron and aluminium oxides.

Conclusions

Our investigations showed that black carbon plays a central role in the
stability and therefore the sustainability of Terra Preta soils. Due to the