Tracing black carbon in soil using SEM/EDX, biomarker analyses, and compound-specific radiocarbon analyses
S. Brodowski (1), P. M. Grootes (2), W. Zech (3), W. Amelung (1)

Mollisols are known to contain stable, black humus components which originate from
charred or coal-derived particles. As such black carbon (BC) significantly affects soil
fertility and interferes with models on soil organic matter dynamics, an accurate prediction of BC input into soils and an elucidation of the mechanisms of BC turnover
is essential. The main aims of this study were (i) to identify the sources of BC in the
Mollisols of the long-term field experiment in Halle and Bad Lauchstädt, Germany,
(ii) to quantify and localize the BC contents in different soil C pools, and (iii) to
elucidate the mechanisms affecting BC decay. After fractionation of soil into particlesize, density, and aggregate-density pools BC was characterized by scanning electron microscopy (SEMi) and energy-dispersive X-ray spectrometry (EDX). Benzenepolycarboxylic acids (BPCA, revised method) served as specific molecular BC markers, the sources of which having been characterized using compound-specific natural 14C abundance measurements. For the assessment of the stability of BC, a long-term incubationexperiment was performed.

The SEM/EDX analysis identified BC as particles of low O/C ratio. The BC particles
itself had a round to irregular shape, with smooth to rough surfaces. This morphology
was not specific for a given soil fraction. It indicated different BC sources that were
variably distributed among the soil C pools. Black carbon in the surface soil of Bad
Lauchstädt mainly derived from coal combustion and vegetation fires. Compoundspecific
radiocarbon measurements ascertained that a significant part of black carbon
(about 60% in the surface soil of Halle) was of fossil origin. The analysis is currently extended to archived samples and particle size fractions to better trace back the fossil C input. First results indicate that major fossil C inputs originated from the last 50 years. This fossil BC, however, was not stable but incorporated into soil humus. The incubation experiment revealed that initial BC decay may be rapid and cannot
be assigned to one single, stable C pool. Investigations by REM strongly indicated
that there are chemical interactions of BC with the mineral phase, however. Aggregate
and aggregate-density fractionation suggested that BC was embedded into microaggregates. Both chemical interactions and physical entrapment contribute, therefore, to a stabilization of various soil BC forms in the long-term run.

(1) Institute of Soil Science, University of Bonn, 53115 Bonn, Germany,
sonja.brodowski@uni-bonn.de, (2) Leibniz-Labor für Altersbestimmung und
Isotopenforschung, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Str. 11-13, 24118 Kiel,Germany, (3) Institute of Soil Science and Soil Geography, University of Bayreuth, 95440 Bayreuth, Germany

Geophysical Research Abstracts, Vol. 7, 07892, 2005
SRef-ID: 1607-7962/gra/EGU05-A-07892
© European Geosciences Union 2005