Transcript Slide 1

Biogeochemical Investigation at Prairie Ridge, NC
Prairie Ridge Soil Profile
Amy Keyworth
Jovi Saquing
November 2006
Prairie Ridge Soil Profile
Outline
• What we expect to see… and why?
• What we do see… and how come?
• What can we conclude?
Soil Profile Description
Litter (undecomposed)
Organic layer, fermented
Organic layer, humified
Mineral layer with organic
carbon and leached
minerals
Mineral layer with precipitation
of oxides/hydroxides and/or
carbon
Unaltered parent substrate
Source: Gleixner, G. 2005. Stable isotope composition
of soil organic matter. In Stable isotopes and
biosphere-atmosphere interactions. ed. Flanagan,
L.B., E.J. Ehleringer and D.E. Patake.
Prairie Ridge Soil Profile
What we expect to see..
•
•
•
•
•
13C – increase with depth
C/N – decrease with depth
% C – decrease with depth
% N – increase/decrease with depth
Carboxylic and aromatic groups –
present in organic layers, increasing
aromaticity with depth
Prairie Ridge Soil Profile
Organic Compounds
Cellulose
Monosaccharide
(e.g. glucose)
Lipid
Lignin
Lignin
monomers
Intermediates
Protein
Amino acid
Ammonium
(e.g. acetic acid)
CO2
Source: Gleixner, G. 2005. Stable isotope composition of
soil organic matter. In Stable isotopes and biosphereatmosphere interactions. ed. Flanagan, L.B., E.J. Ehleringer
and D.E. Patake.
Humic
Substances
Nitrites/Nitrates
N2, N2O
What we expect to see - 13C
Carbon isotopic composition profiles.
Undisturbed site
Disturbed (agricultural) site
(Fig 2 middle, J.G. Wynn, et al., 2006)
What we expect to see – [C]
Carbon concentration profiles.
Undisturbed site
Disturbed (agricultural) site
“Kink” in the Cz curve reflects root depth or productivity zone
(Fig 2. Top, J.G. Wynn, et al., 2006)
What we expect to see – C/N
Source: C/N of soil organic matter from different depth intervals
(Gleixner, 2005)
Why do we expect to see it ?
1.
2.
3.
4.
Suess effect
Soil carbon mixing
Preferential microbial decomposition
Kinetic fractionation
Why we expect to see it ?
1.
2.
3.
4.
Suess effect
Soil carbon mixing
Preferential microbial decomposition
Kinetic fractionation
Why we expect to see it
•
Suess effect –
– Older, deeper SOM originated when
atmospheric 13C was more positive (CO2
was heavier)
– From 1744 to 1993, difference in 13C app 1.3 ‰
– Typical soil profile differences = 3 ‰
1. Suess effect
Mixing of SOC derived from the modern atmosphere versus
that derived from a pre-Industrial Revolution
atmosphere. (Fig. 1A, J.G. Wynn, et al., 2006)
Why we expect to see it ?
1.
2.
3.
4.
Suess effect
Soil carbon mixing
Preferential microbial decomposition
Kinetic fractionation
2a. Soil carbon mixing
- Surface litter (depleted) vs. root derived (enriched) SOM
Mixing of leaf litter-derived SOC and root-derived SOC.
(Fig. 1B, J.G. Wynn, et al., 2006)
2b. Soil carbon mixing
- Variable biomass inputs (C3 vs. C4 plants)
Mixing of SOC formed under two different vegetation communities,
e.g. C3 vs C4. Slope could vary from positive to negative depending
on direction of shift. (Fig. 1C, J.G. Wynn, et al., 2006)
2. Soil carbon mixing
c. Some of the carbon incorporated into SOM
by these critters has an atmospheric or soil
gas, not SOM, source.
d. Atmospheric C is heavier. Atmospheric CO2
in the soil is 4.4 ‰ heavier than CO2
metabolized by decomposition (Wedin,
1995)
Why we expect to see it ?
1.
2.
3.
4.
Suess effect
Soil carbon mixing
Preferential microbial decomposition
Kinetic fractionation
3. Preferential microbial decomposition
– Lipids, lignin, cellulose - 13C depleted with
respect to whole plant
– Sugars, amino acids, hemi-cellulose, pectin
- 13C enriched
– Lipids and lignin are preferentially
accumulated in early decomposition
– Works against soil depth enrichment
– More C than N are lost from soil as SOM
decomposes due to internal recycling of N.
Why we expect to see it ?
1.
2.
3.
4.
Suess effect
Soil carbon mixing
Preferential microbial decomposition
Kinetic fractionation
4. Kinetic fractionation
– Microbes choose lighter C
– Microbial respiration of CO2 – 12C
preferentially respired
– Frequently use Rayleigh distillation analyses
(Wynn 2006)
– No direct evidence for this (Ehleringer 2000)
– Preferential preservation of 13C enriched
decomposition products of microbial
transformation
4. Kinetic fractionation
13C
distillation during decomposing SOM. The gray lines
show the model with varying fractionation factors from 0.997
to 0.999. (Fig. 1D, J.G. Wynn, et al., 2006)
4. Kinetic fractionation
Rayleigh distillation

  13Cf



1
 1000

1

F 
 13Ci    1  e t  1  

 1  
1 
 1000
   e  1t  1  t  
•
•
•
•
•
•
fraction of remaining soil organic matter (SOC) – approximated by the
calculated value of fSOC
13Cf isotopic composition of SOC when sampled
13Ci isotopic composition of input from biomass
α
fractionation factor between SOC and respired CO2
e
efficiency of microbial assimilation
t
fraction of assimilated carbon retained by a stabilized pool of SOM
F
Assumptions by Wynn etal
•
Open system
– All components decompose
– Contribute to soil-respired CO2 at same rate with depth
•
FSOC  fSOC
Anthropogenic mixing (agriculture)
Various reasons that disturbed land
might not conform to nice regression curve in fig 1D (Wynn fig 9 )
A – natural
B – introduce C4 plants, enriched
in 13C
C – Cropping – removes new, low
13C material, leading to surface
enrichment
D – Erosion – removes upper layer,
moving the whole curve up
E – Reintroduce soil organic carbon
(better management practices) –
reverses the trends in C, D, and E
What we do see - results
δ13C
%C
%N
Mean
C:N
Mole
O- horizon
PRS-15 Bulk
-19.11
1.49
0.12
14.38
A- horizon (0-6 cm)
PRS-16 Bulk
-18.95
2.01
0.18
13.36
AP horizon (6-11 cm)
PRS-17 Bulk
-15.92
0.81
0.05
17.28
B horizon (11+ cm)
PRS-18 Bulk
-22.84
0.73
0.05
15.99
O- horizon
PRS-15 Plant Fragment
-21.27
36.77
1.37
31.43
A- horizon (0-6 cm)
PRS-16 Plant Fragment
-29.63
39.13
1.93
23.68
AP horizon (6-11 cm)
PRS-17 Plant Fragment
-27.01
18.71
0.64
34.07
B horizon (11+ cm)
PRS-18 Plant Fragment
O- horizon
PRS-15 Heavy Fraction
-19.00
1.50
0.11
15.42
A- horizon (0-6 cm)
PRS-16 Heavy Fraction
-18.71
1.19
0.10
14.66
AP horizon (6-11 cm)
PRS-17 Heavy Fraction
-15.60
0.71
0.05
17.66
B horizon (11+ cm)
PRS-18 Heavy Fraction
What we do see - results
• 13C – increase 3 ‰ to 8 cm (PRS 18 =
anomaly)
• C/N – increases to 8 cm, then decreases
• % C – decrease with depth (PRS 15 =
anomaly)
• % N – decrease with depth (PRS 15 =
anomaly)
What we do see - 13C
Depth vs delta 13C
delta 13C
-25
-20
-15
0
2
Depth (cm)
4
6
8
10
12
14
16
Increase of 3 ‰ to 8 cm (PRS 18 = anomaly)
What we do see - C/N
Depth vs C/N
C/N
0
5
10
15
20
0
2
Depth (cm)
4
6
8
10
12
14
16
Increases to 8 cm, then decreases
What we do see - % C
Depth vs %C
%C
0
2
4
0
2
Depth (cm)
4
6
8
10
12
14
16
Decrease with depth (PRS 15 = anomaly)
What we do see - % N
Depth vs %N
%N
0
0.2
0.4
0
2
Depth (cm)
4
6
8
10
12
14
16
Decrease with depth (PRS 15 = anomaly)
3000
2500
2000
1500 - Normalized
1000
500
Soil FTIR
wave number
0
Soil FTIR (normalized)
1
15
16
17
18
7
0.6
0.4
0.2
0
4000
3500
3000
2500
2000
1500
1000
500
0
Wave number (cm-1)
15
16
17
18
7
• PRS 7 and PRS 15, both surface soils, have similar absorbencies
• All soils have peak at wavelength 1032
• All 5 spectra have similar peaks, though not necessarily similar
absorbencies
• In our bulk and heavy samples, are the mineral spectra masking the
organics, as in Poirier’s M-SOM?
Absorbance
0.8
Wavenumbr
Description
Possible functional groups
3700
sharp peak
O-H stretching region (3800-3400 for clay mineral)
3622
sharp peak
O-H stretching region (3800-3400 for clay mineral)
Bands due to Si-O-O-OH vibration.
3464
broad, strong intensity
O-H , N-H
Since it's broad and strong intensity,
this is due to O-H bond rather
than N-H bond.
2935
tiny broad
C-H (3150-2850)
The peak is below 3000, so it is an
aliphatic C-H vibration.
Medium intensity absortions at
1450 and 1375 cm-1 will
indicate -CH3 bend.
strectching.
1655
medium intensity
C=C (1680-1600 for aromatic and alkenes); C=O
vibrations (1680-1630 for amide), C=N (16901630) and also of N-H bend (1650-1475)
Some soil literature assigned this to
C=O vibratios of carboxylates
and aromatic. Vibrations
involving most polar bonds,
such as C=O and O-H have
the most intense IR
absorptions. This peak has
medium intensity and most
likely due to N-H bending.
1450 & 1400
weak
C-H, alkanes, -CH3 (bend, 1450 and 1375), -CH2
(bend, at 1465),
Most likely CH3 bending.
1099-1034
sharp & strongest peak
Si-O vibration of clay minerals
Consistent with FTIR spectra of soil
in the literarture
800
medium intensity, saw tooth
NH2 wagging and twisting, =C-H bend, alkenes
696
medium intensity, sharp
540
medium intensity, sharp
N-C=O bend for secondary amides
472
strong intensity, sharp
C-C=O bend for secondary amides, SiO3
Intense absorption at 460-475
-2
corresponds to SiO3
vibration. In the literarture,
bands at 800,780,650,590,530
and 470 are attributed to
inorganic materials, such as
clay and quartz minerals.
cm
Comments
-1
-2
Problems with Methods
– Random protocol on soil sampling at the site
(i.e. depth interval, mass of soil)
– Inconsistent sample preparation procedure
(i.e. different mass, subjective sorting)
– Poor implementation of IRMS protocols (i.e.
sample size, standard calibration)
– Insufficient samples for statistical accuracy