biogeochemical investigation at prairie ridge, nc prairie ridge soil profile amy keyworth jovi...
TRANSCRIPT
Biogeochemical Investigation at Prairie Ridge, NC
Prairie Ridge Soil Profile
Amy Keyworth
Jovi Saquing
November 2006
Outline
• What we expect to see… and why?
• What we do see… and how come?
• What can we conclude?
Prairie Ridge Soil Profile
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.
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 CompoundsCellulose
Monosaccharide (e.g. glucose)
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
Intermediates
(e.g. acetic acid)
CO2
Amino acid
Ammonium
Nitrites/Nitrates
N2, N2O
Lignin monomers
Humic Substances
ProteinLigninLipid
Carbon isotopic composition profiles. Undisturbed site Disturbed (agricultural) site (Fig 2 middle, J.G. Wynn, et al., 2006)
What we expect to see - 13C
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]
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. Suess effect
2. Soil carbon mixing
3. Preferential microbial decomposition
4. Kinetic fractionation
Why we expect to see it ?
1. Suess effect
2. Soil carbon mixing
3. Preferential microbial decomposition
4. 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. Suess effect
2. Soil carbon mixing
3. Preferential microbial decomposition
4. 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. Suess effect
2. Soil carbon mixing
3. Preferential microbial decomposition
4. 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. Suess effect
2. Soil carbon mixing
3. Preferential microbial decomposition
4. 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 fractionationRayleigh distillation
Assumptions by Wynn etal• Open system
– All components decompose– Contribute to soil-respired CO2 at same rate with depth
• FSOC fSOC
11111
1
1100013
1100013
tteteCi
Cf
F
• F 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
Anthropogenic mixing (agriculture)Various reasons that disturbed land might not conform to nice regres-sion 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
δ13C % C % N C:N
Mean Mole
O- horizon PRS-15 Bulk-19.11
1.49 0.1214.3
8
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
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
Increase of 3 ‰ to 8 cm (PRS 18 = anomaly)
Depth vs delta 13C
0
2
4
6
8
10
12
14
16
-25 -20 -15
delta 13CD
epth
(cm
)
What we do see - C/N
Increases to 8 cm, then decreases
Depth vs C/N
0
2
4
6
8
10
12
14
16
0 5 10 15 20
C/ND
epth
(cm
)
What we do see - % C
Decrease with depth (PRS 15 = anomaly)
Depth vs %C
0
2
4
6
8
10
12
14
16
0 2 4
%CD
epth
(cm
)
What we do see - % N
Decrease with depth (PRS 15 = anomaly)
Depth vs %N
0
2
4
6
8
10
12
14
16
0 0.2 0.4
%ND
epth
(cm
)
Soil FTIR (normalized)
Wave number (cm-1)
• 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?
Abs
orb
ance
Wavenumbr Description Possible functional groups Comments
cm-1
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 (1690-1630) 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 Intense absorption at 460-475 corresponds to SiO3
-2
vibration. In the literarture, bands at 800,780,650,590,530 and 470 are attributed to inorganic materials, such as clay and quartz minerals.
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, SiO3-2
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