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Origin and fate of dissolved organic matter in the subsoil
Patrick Wordell-Dietrich
1
∙ Leena Renita Dsilva
1
∙ Patrick Liebmann
2
∙ Robert Mikutta
3
∙ Georg Guggenberger
2
∙Karsten Kalbitz
1
1
Institute of Soil Science and Site Ecology, Technische Universität Dresden, Germany
2
Institute of Soil Science, Leibniz University Hannover, Germany
3
Soil Science and Soil Protection, Martin Luther University Halle-Wittenberg, Germany
Dissolved organic matter (DOM) is the most mobile form of organic matter in soils
and represents a major source of carbon (C) in the subsoil. However, the
transformation processes of DOM during the migration in soils remain unclear. The
preferential removal of more sportive DOM compounds (e.g. phenols) in the
topsoil and leaching of more labile once (carbohydrates) cannot explain e.g. the
depletion of
14
C in DOM in the subsoil nor the higher stability of DOM against
microbial decomposition. The cascade model (Fig. 1) proposes that DOM is
subjected to continuous sorption on mineral surface combined with microbial
processing and remobilization when migrating through the soil
[1]
. However, there is
lack of experimental evidence under field conditions, which supports such a
model. By using
13
C labeled leaf litter we hypothesize that
the contribution of fresh litter-derived C in mineral-associated organic matter
(MOM) and DOM decreases with increasing soil depth
the removal of labeled litter introduce a pulse of DO
13
C mobilized from MOM to
larger depth due to exchange processes with fresh non-labeled DOM
Labeling experiment on a Dystric Cambisol in a beech forest
1
st
Addition of
13
C enriched beech litter
(124 g C m
-2
, 1880 ‰)
2
nd
After 2 years stop of
13
C input of labeled litter and
replacement with original litter
Soil sampling 0 and 18 month after replacement (0-180 cm)
Analysis
Water extractable organic C (WEOC)
Density fractionation (fPOM, oPOM and MOM)
TOC and
13
C measurements of all fractions
UV and fluorescence spectroscopy of WEOC
13
C
Materials and Methods
Background
C/N
phenols
sugars
14
C age
DOC
Fig. 1 After Kaiser and Kalbitz 2012.
Cascade model
Results
negli
Conclusion
Fresh litter-derived C is only minor source of DOM and MOM in the subsoil
DOM in subsoil originated more from microbial processed OM
Exchange processes of fresh and altered DOM not evident in MOM, but in DOM?
First field indications of the cascade model for DOM migration in the soil
WEOC more sensitive for labeled
litter-derived C than MOM
Contribution of litter-derived C
decreases with depth and time
Small pulse of DO
13
C in 5-10 cm
after 18 month (blue arrow)
Decline of litter-derived C in
between 0-60 cm
Decreasing C/N ratio of indicate more
processed organic matter (N
min
?)
Shift in DOM composition with depth
indicated by change in SUVA and HIX
Higher proportion of plant-derived
compounds between 0-50 cm
Below increasing proportion of
microbial-derived compounds
Litter-derived C in MOM and DOM
Reference:
[1]
Kaiser, K. and Kalbitz, K.: Cycling downwards – dissolved organic matter in soils, Soil Biol. Biochem., 52, 29–32,
doi:10.1016/j.soilbio.2012.04.002, 2012.
patrick.wordell-dietrich@tu-dresden.de
www.researchgate.net/profile/Patrick_Wordell-Dietrich
precipitation sorption
desorption
dissolution
aged plant & microbial
compounds
plant
compounds
microbial
processing
This research is part of the research unit FOR 1806 (SUBSOM): The Forgotten Part of Carbon Cycling:
Organic Matter Storage and Turnover in Subsoils. Founded by the German Research Foundation (DFG)
Fig. 2 Labeled litter-derived C in (a) MOM and (b) water extractable organic C for the two
samplings. Means and standard error (n = 3).
(a)
(b)
Fig. 3 C/N ratio of MOM
Fig. 4
DOM composition characterized by
UV and fluorescence spectroscopy.
Means and standard errors (n = 6).
MOM as DOM source
Ideas for further analyses on samples with low DOC
concentrations 1 mg L
-1
and small volume (5 mL)?
DO
13
C pulse?