Compound-specific stable-isotopes

to investigate soil organic carbon source/sink

relocations between areas of land degradation

and deposition areas in a mountainous

watershed in Northwest Vietnam

C. Brandta, F. Raschea, H. Fröhlicha, N.T. Lamb, T. D. Vienb and G. Cadischa

a Hohenheim University, Department of Plant Production and Agroecology in the Tropics and

Subtropics, Stuttgart, Germany b Center for Agricultural Research and Ecological Studies (CARES), Hanoi, Vietnam

FAO/IAEA International Symposium on Managing Soils for

Food Security and Climate Change Adaptation and Mitigation

23 – 27 July 2012, Vienna

1. Problem definition

2. Concept of CSSI

3. Objectives

4. Study area

5. Preliminary results

6. Conclusion

2/21

Contents

Land degradation in

South East Asia

Population pressure

Agricultural commercialization

Deforestation

Expansive maize production

From traditional shifting

cultivation systems to

continuous monocropping

Problem definition

3/21

Problem definition

Sediment deposition and

siltation of reservoirs

Land use intensification in upland areas

Soil degradation on slopes

Soil relocation

Movement of SOC

and Nutrients

4/21

Problem definition

Maize Teak

Cassava

Bamboo

Mix of different

sources

Which are the hot spots of

critical land degradation?

5/21

Identification of hot spots of critical land degradation

for a cost-effective implementation of precision

conservation measures

How to apportion the source soils of sediments?

Compound specific stable isotope (δ13C)

approach and CSSI based mixing models

Main goal

6/21

Problem definition

Concept of CSSI

Marker compounds

Land-use fingerprint

Plants produce a range of organic compounds that leach

from leaves or stems or exude from roots

Different photosynthetic fixation pathways for CO2 (C3,C4)

Genetic and environmental factors influencing stomata

conductance as well as nutrient status

Same organic compounds of different plants have slightly

different δ13C isotopic signature (‰)

7/21

Long-chain fatty acids (C12:0 – C24:0)

Abundant in environment

Highly polar (acids)

Fatty acid - clay mineral associations in upper soil

Concept of CSSI

Marker compounds of choice

Palmitic acid; C16:0

Stearic acid; C18:0

Lignoceric acid; C22:0

8/21

Significant differences of δ13C values of fatty acids in

source-soils of various land uses can be utilized to

identify the provenances of sediments.

Therefore it is possible to trace soil organic carbon

source/sink relationships in a small agricultural

catchment on a spatial level.

Concept of CSSI

Hypothesis

9/21

Sediment

sample

Land use

4

Land use

3

Land use

2

Land use

1

Sediment source ascription

δ13C value

3

δ13C value

2

δ13C value

1

δ13C value

4

Reference database of

source soils

Concept of CSSI

Gas Chromatography Combustion Isotope Ratio Mass Spectrometry (GC/C/IRMS)

Mixture of source soils of unknown proportions

Soil preparation (max 50°C/<2mm)

Fatty acid extraction and fractionation

Methylization FA FAME

10/21

Significant differences between isotopic ratio of δ13C

of individual fatty acids in source soils

Proportional contributions of source soils to colluvial

sediment can then be estimated by mixing models

(Isosource, SIAR)

Concept of CSSI

Sediment source ascription

11/21

Are CSSI and CSSI-based mixing models applicable to

identify and trace SOC source/sink relationships in a

small tropical catchment?

Development of a standard procedure combining an

optimized fatty acid extraction and measurement

strategy and the implementation of statistical

analyses and mixing models

Implementation of the CSSI-approach at catchment

level

Objectives

12/21

Introduction – Study Area Study area

13/23

Chieng Khoi

catchment

Son La Province

14/21

Introduction – Study Area Study area

0 250 500 Meters

Upland land use sites

in Chieng Khoi [M] Maize

[C] Cassava

[SBF] Sec. Forest and Bamboo

[PNF] Protected natural forest

[MFP] Mixed fruit plantation

[TW] Timber plantation

Soil sampling plot [10 - 20m²]

Sediment sampling site [4 - 10m²]

14/23

Introduction – Study Area Study area

Maize

Cassava

Teak

Bamboo

0 50 100 Meters

Preliminary results

Cassava field

Teak plantation

Maize field

16/23

A

B

C

Fatty acids (δ13

C ‰) Source soils (δ13

C ‰; SEM)

Maize Cassava Teak

C16:0 Methyl palmitate -23,5 ±0,36 -28,9 ±0,48 -29,7 ±0,33

C16:1 Methyl palmitoleate -22,5 ±0,35 -27,6 ±0,46 -27,9 ±0,61

C18:0 Methyl stearate -21,6 ±0,71 -27,2 ±0,50 -29,0 ±0,10

C18:1 Methyl oleate -21,8 ±0,74 -27,5 ±0,54 -27,8 ±0,51

C18:2 Methyl linoleate -23,4 ±0,36 -30,9 ±2.64 -28,3 ±0,44

C18:3 Methyl linolenate -33,1 ±0,58 -28,3 ±3,37 -39,5 ±3,35

17/22

Maize

Cassava

Teak

Preliminary results

Identification of fatty acids which are present in

all investigated soil and sediment samples

Maize

Cassava

Teak

Preliminary results

C M T

24

26

28

30

C M T

25

30

35

40

45

Cassava Maize Teak Cassava Maize Teak

δ1

3C

(‰

)

C16:0 C18:3

Selection of suites of δ13C - FAME marker with

significant differences - Tukey HSD (Honest

Significant Difference)

Preliminary results

Proportional contributions of potential sources

estimated by SIAR

0.0

0.2

0.4

0.6

0.8

1.0

Proportions by group: 1

Source

Pro

po

rtio

n

M T C

0.0

0.2

0.4

0.6

0.8

1.0

Proportions by group: 1

Source

Pro

po

rtio

n

M T C

0.0

0.2

0.4

0.6

0.8

1.0

Proportions by group: 1

Source

Pro

po

rtio

n

M T CCassava Maize Teak

Sediment A

Cassava Maize Teak Cassava Maize Teak

Sediment C Sediment B

Pro

portion

19/21

1. Different land uses show contrasting δ13C signatures for

identical soil fatty acids

2. At least one suite of marker with sufficient discriminatory

power exists to describe distinct isotopic fingerprint

profiles

promising for the use of the CSSI-FAME

approach in a small tropical watershed

20/21

Conclusions

The Uplands Program SFB 564

Acknowledgements

Thank you for your attention