CZO National Meeting, Arizona, May 2011
Distribution and Speciation
of Soil Organic Carbon
along
l
Hillslope
Hill l
Transects:
T
t
Importance of Fe-Redox Cycling
Chunmei Chen, Peter Leinweber,
Anthony K.
K Aufdenkampe,
Aufdenkampe Donald L
L. Sparks
Introduction
Organic
O
i matter-mineral
tt
i
l interaction:
i t
ti
¾ One of major mechanisms for C stabilization
¾ Impacted by coupled geomorphological, hydrological & microbial processes
bedrocck
Saproolite
Soiil
Precipitation
Well-drained
Poorly-drained
Mineralogical
transformation
(Fe oxides)
Slow C
decomposition,
eroded C
accumulation
Enhancing
DOC release
Lignin
Preservation
C fractionation
Objectives
¾ Investigate
g
soil organic
g
matter molecular
composition at varying landscape topographic
positions
¾ Assess interactions of C with specific soil minerals at
the molecular scale
¾ Determine Fe speciation of soils along redox
gradients
Field Site
2
34 1
6
5
Base
Top
Advanced Characterization
Pyrolysis-Field Ionization Mass Spectrometry (Py-FIMS)
• Soft ionization enables detection of
non-fragmented
g
molecules which can
be assigned to compound classes and
reflect composition of original sample.
• Temperature-resolved pyrolysis
indicates thermal stability of
compounds, reflects biological stability.
Near-edge X-ray Absorption Fine Structure (NEXAFS)
• Characterize C functional groups of soil organic matter without any
pretreatment.
Scanning Transmission X-ray Microscopy (STXM) -NEXAFS
• M
Map di
distribution
t ib ti off C andd C forms
f
andd the
th major
j elements
l
t (K,
(K Ca,
C Fe,
F Al,
Al Si)
in soils at nanometer scale.
• Assess the interactive mechanisms of C with specific soil minerals.
Soil Properties
p
Depth
(cm)
Amorphous
p
Fe
oxides (g kg-1)
Crystalline
y
Fe
oxides (g kg-1)
Cla content (%)
Clay
Top
Base
Top
Base
Top
Base
0-15
3.91
3.00
5.47
3.84
16%
22%
15-25
4.14
3.92
7.20
3.43
22%
25%
Reduction of Fe oxides occurred at the base of the hillslope.
Carbon and Soil Mineral Surface Area
Depth
(cm)
Mineral surface
area (m2 g-1)
%C
Fraction
C loading (C/mineral
surface area) (mg m-2)
Top
Base
Top
Base
Top
Base
Bulk soil
3.06
2.76
14.52
11.52
2.11
2.46
clay
7.26
6.87
70.79
53.22
1.02
1.29
Bulk soil
1 75
1.75
0 52
0.52
19 41
19.41
10 44
10.44
0 90
0.90
0 50
0.50
clay
4.91
1.33
72.23
50.40
0.68
0.26
0-15cm
15-25 cm
Elevation
Lower C concentration at the base of the hillslope is likely due to
enhanced DOC leaching as a result of reduction of Fe oxides.
Soil
Increased Fe
reduction
Soil
Downhill distance
Reduced mineral
surface area
and %C
R
Relative
abund
dance of
organic compo
o
ouns as %
OM compound classes from Py-FIMS
S il depth:
Soil
d th 00-15cm
15
12
10
Top
Base
Bulk soil
Clay
8
6
4
2
0
Relativ
ve abundance of
organic compouns a
as %
Soil depth: 15-25cm
25
20
Bulk soil
Clay
15
10
5
0
s
s
s
s
rs
rs
r a t e p ound D i m e Lipi d n om e om a t ic
d
y
o
r
bo h N c om Lign in
n M AlkylA
i
n
Ca r
ig
l s +L
o
n
P he
s
s
s
s
rs
rs
r a t e poun d D im e Lipi d nom e om a t i c
d
y
Mo
lAr
boh N c o m Li gnin
nin Alky
Ca r
g
i
ls + L
o
n
P he
Volatilization Temperatures by Py-FIMS
Soil depth:
p 0-15cm
Bulk soil
Soil depth: 15
15-25cm
25cm
clay
Bulk soil
clay
Intensity (%TIII)
0.30
Top
Base
0 25
0.25
0.20
Peptides
0.15
0.10
0.05
0 00
0.00
Intensity
y (%TII)
0
200
400
600
Temperature (οC)
800 0
200
400
600
Temperature (οC)
800 0
200
400
600
Temperature (οC)
800 0
200
400
600
Temperature (οC)
800
0.6
C b h d t
Carbohydrates
0.4
0.2
0.0
0
200
400
600
T
Temperature
t
(οC)
8000
200
400
600
T
Temperature
t
(οC)
8000
200
400
600
T
Temperature
t
(οC)
800 0
200
400
600
800
T
Temperature
t
(οC)
Inte
ensity (%TII)
0.6
0.5
0.4
0.3
N compounds
02
0.2
0.1
0.0
0
200
400
600
Temperature (οC)
8000
200
400
600
Temperature (οC)
8000
200
400
600
Temperature (οC)
800 0
200
400
600
800
Temperature (οC)
Volatilization Temperatures by Py-FIMS
Soil depth: 0-15cm
Bulk soil
Soil depth: 15-25cm
clayy
Bulk soil
clayy
Intens
sity (%TII)
0.6
Top
Base
0.5
0.4
Lipids
p
0.3
0.2
0.1
0.0
Intensity (%
%TII)
0
200
400
600
T
Temperature
t
(οC)
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
800 0
200
400
600
T
Temperature
t
(οC)
800 0
200
400
600
T
Temperature
t
(οC)
800 0
200
400
600
T
Temperature
t
(οC)
800
Top
Base
0
200
400
600
Temperature (οC)
Phenols
and Lignin
Monomers
800 0
200
400
600
Temperature (οC)
800 0
200
400
600
Temperature (οC)
800 0
200
400
600
Temperature (οC)
800
C NEXAFS spectra of clay fractions
S il depth:
Soil
d th 00-15
15 cm
Aromatic-C
Aliphatic-C
Hill base
b
soil:
il enriched
i h d in
i aromatic
ti C
depleted in aliphatic C
Conclusions
¾ C concentration and speciation along the pasture
hillslope transect are linked to redox cycling of Fe
oxides.
¾ The stability of soil organic carbon is related to
carbon content and/or the normalized carbon content
by mineral surface area.
Acknowledgements
• Kyungsoo Yoo at the
University of Minnesota
• Beamline scientists at the
g Sources: Jian
Canadian Light
Wang, J.J.Dynes, and Tom
Regier
• Environmental Soil Chemistry
Group at the University of
Delaware
• National Science Foundation
(NSF)