Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 183

the rainy season between December and July, whereas the yearly rainfall is
more evenly distributed at Manaus (Otzen, 1992). Parent materials of all
soils are Tertiary sediments. For this study, we investigated five Terra Preta
sites in comparison with nearby Oxisols. Sand-rich soils (sites 1-3) are
located near Manaus and clay-rich substrates (sites 4 and 5) were found in
the north of Manaus and Santarém, respectively. Location and general
characteristics of the investigated soils are described by Glaser (1999).
Black carbon was analysed in the fine earth, in particle size fractions
(2000-250 m,

clay), and in density fractions (<2gcm −3 , light fraction; 2.0-2.4 g cm −3 ,
medium fraction; > 2.4 g cm −3 , heavy fraction) of Terra Preta soils and
surrounding Oxisols with a novel technique using benzenecarboxylic acids
(BCAs) as molecular markers for black carbon (Glaser et al ., 1998). The
analytical procedure includes acid digestion, oxidation, sample clean up,
derivatization and gas chromatography. BCAs were not produced upon
oxidation of model humic substances.

µ m, coarse sand; 250-20 µ m, fine sand; 20-2 µ m, silt; < 2 µ

Results and Discussion

The SOM of Terra Preta consisted of up to 35% of black carbon, some-
times down to 80 cm soil depth, whereas in the Oxisols only the topsoil
contained considerable amounts of black carbon (up to 14% of TOC). In
both Terra Preta soils and Oxisols, most of the organic carbon was located
in the silt and clay fractions (Fig. 3.11.1). Terra Preta soils contained
significantly higher concentrations of black carbon in the fine sand and
silt fractions (not shown here). In Terra Preta soils, a major part of
black carbon is located in the inert carbon pool of the silt fraction. The
contribution of black carbon in the clay fraction increased with increasing
soil depth (Fig. 3.11.1), indicating transport by illuviation. SEM of the
clay fraction showed aggregated amorphous material representing
organomineral complexes besides dark wood-like particles larger than
10 m (Fig. 3.11.2). The investigation of the elemental composition of

the surface of these particles by EDX gave O : C ratios smaller than 0.5,
indicating black carbon. Cellulose would have a theoretical O : C ratio of
0.88 (Stoffyn-Egli et al ., 1997). Due to the low density of black carbon,
particle size fractionation leaves particulate black carbon > 2

µ

µ m in the clay

fraction (Fig. 3.11.2, spot 5).
Density fractionation, on the other hand, allowed the separation and
the subsequent characterization of particulate black carbon in the light
fraction (< 2.0 g cm −3 ), whereas organomineral complexed black carbon
was isolated in the medium fraction (2.0-2.4 g cm −3 ) and black carbon
coatings on minerals in the heavy fraction (d > 2.4 g cm −3 ). Investigation of
black carbon in density fractions of soils from 0-10 cm and 30-40 cm
depth (Fig. 3.11.3) in combination with SEM and EDX (not shown here)