Biogeosciences
www.biogeosciences.net
1726-4170
1726-4189
4
3
2007
10.5194/bg-4-425-2007
http://www.biogeosciences.net/4/425/2007/
http://www.biogeosciences.net/4/425/2007/bg-4-425-2007.html
http://www.biogeosciences.net/4/425/2007/bg-4-425-2007.pdf
425
432
2007-06-27
Mobility of black carbon in drained peatland soils
J. Leifeld
jens.leifeld@art.admin.ch
S. Fenner
M. Müller
Air Pollution/Climate Group, Agroscope Reckenholz-Tänikon Research Station ART, Zurich, Switzerland
Bern University of Applied Sciences, Swiss Collage of Agriculture, Zollikofen, Switzerland
Amount, stability, and distribution of black carbon (BC) were studied at
four sites of a large peatland ("Witzwil") formerly used as a disposal for
combustion residues from households to derive BC displacement rates in the
profile. Possible artefacts from thermal oxidation with Differential
Scanning Calorimetry (DSC) on BC quantification of C-rich deposits were
inferred by choosing three sites from a second peatland with no historical
record of waste disposal as a reference ("Seebodenalp"). All sites were
under grassland at time of sampling, but were partially cropped in the past
at Witzwil. Mean BC contents in topsoils of Witzwil ranged from 10.7 to 91.5
(0–30 cm) and from 0.44 to 51.3 (30–140 cm) mg BC g<sup>−1</sup> soil,
corresponding to BC/OC ratios of 0.04 to 0.3 (topsoil) and 0.02 to 0.18
(deeper soil). At three sites of Seebodenalp, BC was below the detection
limit of 0.4 mg g<sup>−1</sup> organic soil, indicating negligible formation of BC
during thermal oxidation of peat. <sup>13</sup>C NMR spectra corroborated the high
BC contents at Witzwil. The data support a considerable vertical transport
of BC given that soils were ploughed not deeper than 30 cm since abandonment
of waste application about 50 years ago. The total amount of BC in the
Witzwil profiles ranged from 3.2 to 7.5 kg BC m<sup>−2</sup>, with 21 to 69
percent of it stemming from below the former ploughing depth. Under the
premise of negligible rates of BC consumption since abandonment of waste
application, minimum BC transport rates in these peats are 0.6 to 1.2 cm a<sup>−1</sup>. The high mobility of BC might be explained by high macro-pore
volumes in combination with occasional water saturation. By means of DSC
peak temperatures, different types of BC could be distinguished, with deeper
horizons containing BC of higher thermal stability. Application of
combustion residues likely involved a mixture of various BC types, of which
thermally more stable ones, most likely soots, were preferentially
transported downwards.
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