Origin, Function, and Reactivity of Black Carbon in the Arable Soil Environment.
Sonja Brodowski, Institute of Crop Science and Resource Conservation - Division of Soil Science, Univ of Bonn, Nussallee 13, 53115 Bonn, Germany
Chernozems contain stable, black humus components which originate from charred or coal-derived particles. However, little is known about the origin, fate, and contribution of such black carbon (BC) compounds to stable humus. The main aims of this study were (i) to quantify BC and identify the origin of BC in soils of three degraded Chernozems and a Luvisol in Germany, (ii) to elucidate the fate of BC in these soils by localizing BC in different pools, (iii) to investigate the factors affecting BC decay using a long-term incubation experiment. BC was quantified using benzene polycarboxylic acids (BPCAs) as specific markers, the sources of which were characterized using compound-specific C-14 dating. BC was characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDX). BC made up to 2.7% to 13% of the organic C in the surface soils. The BPCA pattern and compound-specific radiocarbon dating of the Chernozems suggested that about 1/3 of BC derived from vegetation fires while the remaining 2/3 were of fossil origin. The analyses of archived samples from Halle revealed that fossil BC was mainly deposited until 1967/70, a result of fossil fuel combustion and coal mining. This was confirmed by SEM-EDX. The surface of BC particles was partially oxidized, and hence is partially reactive. Analysis of physical soil fractions showed that BC is partly associated with the mineral phase and likely, physically stabilized by them. Fractionating soil into aggregate-size and -density classes confirmed that BC was preferentially embedded within microaggregates. From these results we deduce that oxidized BC may actively be involved in soil aggregation. Hence, chemical interactions as well as physical entrapment likely contribute to BC stabilization in soils. In the incubation experiment, BC degradation occurred biologically and was promoted by the addition of an easily available C-source (glucose).