Prairie Ridge Soil Profile

Amy Keyworth

Jovi Saquing

November 2006

Outline

• What we expect to see

• Why

• What we do see

What we expect to see

13C – increase with depth

• C/N – decrease with depth

• % C – decrease with depth

• % N – decrease with depth

Fig 2 middle. Carbon isotopic composition profiles. Undisturbed site Disturbed (agricultural) site (J.G. Wynn, et al., 2006)

What we expect to see

Fig 2. Carbon concentration profiles. Undisturbed site Disturbed (agricultural) site “Kink” in the Cz curve reflects root depth or productivity zone (J.G. Wynn, et al., 2006)

What we expect to see

Why we expect to see it

• 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 ‰

Suess effect

Fig. 1A. Mixing of SOC derived from the modern atmosphere versus that derived from a pre-Industrial Revolution

atmosphere. (J.G. Wynn, et al., 2006)

Why we expect to see it

• Soil carbon mixing– Surface litter (depleted) vs. root derived

(enriched) SOM– Variable biomass inputs (C3 vs. C4 plants)– Some of the carbon incorporated into SOM

by these critters has an atmospheric, not SOM source.

– Atmospheric C is heavier. Atmospheric CO2 in the soil is 4.4 ‰ heavier than CO2 metabolized by decomposition (Wedin, 1995)

Soil carbon mixing- Surface litter (depleted) vs. root derived (enriched) SOM

Fig. 1B. Mixing of leaf litter-derived SOC and root-derived SOC. (J.G. Wynn, et al., 2006)

Soil carbon mixing- Variable biomass inputs (C3 vs. C4 plants)

Fig. 1C. 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). (J.G. Wynn, et al., 2006)

Why we expect to see it

• Preferential microbial decomposition– Lipids, lignin, cellulose - 13C depleted with

respect to whole plant– Sugars, amino acids, hemicellulose, pectin -

13C enriched– Lipids and lignin are preferentially

accumulated in early decomposition– Works against soil depth enrichment

Why we expect to see it

• 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

Kinetic fractionation

Fig. 1D. 13C distillation during decomposing SOM. The gray lines show the model with varying fractionation factors from

0.997 to 0.999. (J.G. Wynn, et al., 2006)

Kinetic fractionationRayleigh distillation

• 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

Assumptions by Wynn etal• Open system

– All components decompose– Contribute to soil-respired CO2 at same rate with depth

• FSOC fSOC

111

11

1

11000

13

11000

13

tte

teCi

Cf

F

Anthropogenic mixing (agriculture)Wynn fig 9 – various reasons that disturbed land might not conform to nice regression curve in fig 1D

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

Order of decay of compounds(Melillo 1989)

1. Loss of C fractions depleted in 13C (p 192)– tannins– non-polar components– water-soluble compounds– lignin, also depleted in 13C is conserved

2. Cellulose – C fraction enriched in 13C3. Lignin

– Recalcitrant– Can be enhanced by addition of simple sugars– N may slow lignin decay (Fenn etal 1981, Keyser etal

1978) – not proven

Controls on decay

Melillo, et al, 1989

• Temperature

• Moisture

• Soil texture

• Availability of labile C and N

What we do see - results

13C – increase 0.4 ‰ with 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 0.4 ‰ with 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)

Abs

orba

nce

FTIR results

• 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?

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

•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?