25th International Meeting on Organic Geochemistry IMOG 2011

P-385
Source determination and depth translocation of PAH in Chinese
paddy and non-paddy soils
Cornelia Mueller-Niggemann 1 , Philipp Roth 2 , Eva Lehndorff 2 , Lorenz Schwark 1
1 Institute of Geosciences, Christian-Albrechts-University of Kiel, 24118 Kiel, Germany, 2 Institute of Crop
Science and Resource Conservation, Soil Science and Soil Ecology, University of Bonn, 53115 Bonn,
Germany (corresponding author:cmn@gpi.uni-kiel.de)
The incomplete combustion of organic residues leads
to the formation of polycyclic aromatic hydrocarbons
(PAH) and a macromolecular, highly condensed
complex residue termed black carbon (BC).
Widespread fertilization practices are burning of crop
residues in the field or adding ashes from domestic
burning to the field, which introduces large quantities
of condensed organic matter (OM) into soils.
We present results from a chronosequence study of
paddy and non-paddy soils with different and well
known starting dates of cultivation in the Zhejiang
province (Yangtze River delta) by land reclamation
through the building of protective dikes over the past
2000 years. The end member of natural sediments
subjected to land reclamation, a marine tidal mudflat
in the Yangtze delta, represent the substrate on which
the soil evolution started. In the study area a
traditional method of rice straw disposal is open field
burning, which leads to the incorporation of specific
PAH and BC into paddy soils.
As expected, the quantitatively dominant fraction of
the condensed, highly aromatic soil OM is
macromolecular BC with a range of 1.1 to 2.1 mg/g
dw in paddy topsoils (TS) and 0.7 to 1.1 mg/g dw in
non-paddy TS. Preferentially adsorbed to the BC or to
clay minerals are monomolecular PAH that occur in
trace amounts only. The sum of PAH detected in
paddy TS is higher than in non-paddy TS, with a
range from 247 to 978 ng/g dw in paddies and from
54 to 167 ng/g dw in non-paddies. Additionally, in
paddy TS an increasing trend with cultivation time
was observed.
The lowest BC (0.3-0.8 mg/g dw) and PAH (10-30
ng/g dw) concentrations were found in regular
subsoils, with no significant difference between paddy
and upland use. Extremely high concentrations of BC
were detected in dark buried horizons (at the 700 and
1000 year old site), which were either assumed to be
former A-horizons or sedimentary layers due to
inundations of soils by marine or limnic flooding.
The higher compositional variability of PAH, despite of
their low concentration, were used for differentiation
of combustion effects in substrates, topsoils and
subsoils (local combustion and soil erosion of the
hinterland).
Fig.1. Relative distribution of 5-ring PAH in substrate
end-members and soils.
The marine tidal flat substrate contains predominantly
OM of terrigenic origin, derived from soil and
sediment erosion in the catchment area of the
Yangtze River. This is shown by enrichment with
perylene and depletion in benzofluoranthenes and
benzopyrenes, also observed in other studies [1].
Regular subsoils show a PAH pattern identical to that
of the marine substrate, whereas TS and buried
horizons yield a distinctive rice ash derived PAH
pattern (Fig.1). Subsoils adjacent to the buried Ahorizons
reveal a PAH pattern similar to that of the
latter indicating postdepositional translocation of BC
and PAH from buried TS into over- and underlying
strata. A major proportion of the SOM in paddy rice
soils is derived from in-field rice straw combustion.
This management practice is expected to phase out
over the next decade due to environmental legislation.
A proposed diminished input of combustion residues
may lead to significant changes and yet unpredicted
consequences in stabilization of SOM in paddy soils.
[1] Liu et al. (2008) Environ. Pollution 154, 298-
305.
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