building soil carbon: benefits, possibilities, and modeling

Building soil organic carbon: benefits, possibilities and modeling

Jeff BaldockCSIRO Land and WaterAdelaide, SA

J.A. Baldock, CSIRO: Orange soil carbon workshop, 19 November 2008

Take home messages

• Carbon exists in soils in different forms - influences the vulnerability of soil carbon to change

• Storing more carbon in soils has benefits beyond carbon trading

• Altering current management systems will be required to store additional carbon

• Models and calculators can be used to predict outcomes of management on soil carbon contents

• Australian soils do have the potential to store more carbon


Composition of soil organic carbon

Extent of decomposition increases

Vulnerability to change decreases

C/N/P ratio decreases (become nutrient rich)

Crop residues on the soil surface (SPR)

Buried crop residues (>2 mm) (BPR)

Particulate organic matter (2 mm – 0.05 mm) (POC)

Humus (<0.05 mm) (HumC)

Dominated by charcoal with variable properties

Resistant organic matter (ROC)


Variation in amount of C associated with soil organic fractions

0

5

10

15

20

25

301P 8P

32P

NoT

ill (

Med

N)

NoT

ill (

Hig

h N

)

Str

at (

Med

N)

Str

at (

Hig

h N

)

0P 11P

22P

Arb

oret

um

Per

m P

astu

re

W2P

F

Can

ola/

whe

at

Pul

se/w

heat

Pas

ture

/whe

at

HamiltonPasture

HartCropping

YassPasture

UrrbraeVarious

WaikerieVarious

Org

anic

C in

0-1

0 c

m la

yer

(t C

/ha)

SPR

BPR

POC

HumC

ROC


Years

So

il or

ga

nic

carb

on

(g C

kg-1

soi

l)

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70

Total soil organic C

Conversion topermanent

pasture

33

Importance of allocating C to soil organic fractions

15 43

Humus

ROCPOC

~30% less humus

~800% more POC18 y 10 y


Vulnerability of soil carbon content to variations in management practices

Years

So

il or

ga

nic

carb

on

(g C

kg

-1 s

oil)

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70

TOC Humus

ROCPOC

Conversion to intensive cultivation

18 y

Conversion to pasture

10 y

15 4333

9 y

52


Predicting allocation of soil carbon to fractions using mid-infrared spectroscopy

1

2

3

4

5000 4500 4000 3500 3000 2500 2000 1500 1000 500

Inte

nsi

ty

Frequency (cm-1)

Fourier Transform Infrared Spectrum• Estimates of the

amount of each type of carbon in a sample and other soil properties


n = 177Range: 0.8 – 62.0 g C/kgR2 = 0.94

n = 141Range: 0.2 – 16.8 g C/kgR2 = 0.71

n = 121Range: 0.0 – 11.3 g C/kgR2 = 0.86

Predicting the amount of each form of soil carbon using MIR

Total organic carbon

(mg C/g soil)

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70

Measured

MIR

pre

dict

ed

Particulate organic carbon

(mg C/g soil)

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20

Measured

MIR

pre

dict

ed

Recalcitrant organic carbon

(mg C/g soil)

0

2

4

6

8

10

12

0 2 4 6 8 10 12

Measured

MIR

pre

dict

ed


Spatial variation in total oragnic carbon and charcoal carbon (0-10 cm layer)

0.00

0.40

0.80

1.20

1.60

2.00

2.40

0 25 50 75 100

Western Boundary (m)

TOC

0

20

40

60

80

100

120

140

160

180

200

Nor

ther

n B

ound

ary

(m)

0 1 1 1

2 2 2

3 3 3

4 4 4

5 5 5

6 6 6

7 7 7

8 8 8

9 9 9

10 10 10

11 11 11

12 12 12

13 13 13

14 14 14

15 15 15

16 16 16

17 17 17

19 19 19

20 20 20

21 21 21

22 22 22

23 23 23

24 24 24

25 25 25

26 26 26

27 27 27

29 29 29

30 30 30

31 31 31

32 32 32

33 33 33

34 34 34

35 35 35

18 18 18

35 3534333231302928272625242322212019181716151413121110987654321 0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0 25 50 75 100

Western Boundary (m)

Charcoal C

0

20

40

60

80

100

120

140

160

180

200

Nor

ther

n B

ound

ary

(m)

0 1 1 1

2 2 2

3 3 3

4 4 4

5 5 5

6 6 6

7 7 7

8 8 8

9 9 9

10 10 10

11 11 11

12 12 12

13 13 13

14 14 14

15 15 15

16 16 16

17 17 17

19 19 19

20 20 20

21 21 21

22 22 22

23 23 23

24 24 24

25 25 25

26 26 26

27 27 27

29 29 29

30 30 30

31 31 31

32 32 32

33 33 33

34 34 34

35 35 35

18 18 18

35 W FW FP P F WP P F WP P F WP P F WPerm. Past.Contour bankW O O(g) FW O O(g) FW O O(g) FW O O(g) FB Pe WB Pe WB Pe WW P P W P P W P P W WW W P P P P PW W P P P P PW W P P P P PW W P P P P PW W P P P P PW W P P P P PW O FW O FW O FW O(g) FW O(g) FW O(g) FW PeW PePerm. PastPerm. Past


Functions of organic matter in soil

Biological functions- energy for biological processes

- reservoir of nutrients

- contributes to resilience

- cation exchange capacity

- buffers changes in pH

- complexes cations

Chemical functionsPhysical functions

- improves structural stability

- influences water retention

- alters soil thermal properties

Functions of SOM


Plant-available water holding capacity

• How much plant-available water can a soil hold

• Upper limit (wetter soil) - soil water content after drainage

• Lower limit (drier soil) - soil water content at which plants can no longer extract water

Analogy of a sponge removed from a bucket of water

Stops dripping

Upperlimit

Squeeze out as much water

as possible

Lowerlimit

Remove sponge from bucket


Plant available soil water

Water unavailable to plants

Changes in plant available soil water with clay content

Am

ou

nt o

f wa

ter

(mm

wa

ter/

cm s

oil

dep

th)

0

1

2

3

4

Sand SandyLoam

Loam SiltLoam

ClayLoam

Clay

UpperLimit

Lowerlimit

Increasing clay content


Change in water holding capacity with a 1% increase in soil organic carbon content

For 0-10 cm layer of South Australian Red-brown earths

3 mm extra stored rainfall for 10 rainfall events equates to 30 mm total or 600 kg of grain

Issue: harder to build up soil carbon on a sandy soil than a clay

0

1

2

3

4

5

6

0 10 20 30 40

Clay content (% of soil mass)

Ch

an

ge

in w

ate

r h

old

ing

ca

pac

ity

(mm

wat

er)


Influence of soil organic C composition on nitrogen supply

• N supply is governed by the rate of decomposition and the C/N ratio

Soil organic matter

C/N=10

10 unitsof C

1 unitof N

WheatResidueC/N=100

100 unitsof C

1 unitof N

70 units of C to carbon dioxide

30 units of C

1 unit of N

N required= 30/10= 3

2 units of N required


Influence of soil organic C composition on nitrogen supply

SOMC/N=10

Medic ResidueC/N= 20

70 30

5N required= 3

+2

SOMC/N=10

Wheat ResidueC/N=100

70 30

1N required= 3

-2

SOMC/N=10

Soil HumusC/N= 10

70 30

10N required= 3

+7


Variation in C/N ratio of different fractions of soil organic matter

Min Max

SPR 18.7 104.7

BPR 14.1 60.4

POC 12.8 19.6

Humus 6.0 10.1

0

20

40

60

80

100

120

SPR BPR POC HumusType of organic matter

C/N

ra

tio

(we

igh

t b

asi

s)

Maximum values

Minimum values

29 soils from southern Australia with total organic carbon contents ranging from 0.8% to 5.7%


Amount of nitrogen associated with soil organic matter

Assumption: C/N ratio = 10

0

2000

4000

6000

8000

10000

0.8 1 1.2 1.4 1.6 1.8 2

Soil bulk density(Mg soil/m3)

Nitr

oge

n in

the

0-1

0 cm

laye

r(k

g N

/ha)

SOC=1%

SOC=2%

SOC=3%

SOC=4%

SOC=5%

4200 kg N/ha

1400 kg N/haDecrease from 3% to 1% SOC releases 2800

kg N/ha


What determines the amount of carbon present in a soil?

• Soil properties (clay content, mineralogy, depth)

• Balance between inputs and losses

Inputs

• Carbon captured by plants and added to soil

• Addition of waste organic materials

Losses• Conversion of

organic C to CO2

• Erosion


Evaluating potential C sequestration in soilS

oil c

arbo

n se

ques

trat

ion

situ

atio

n

Stable soil organic carbon (e.g. t1/2 10 years)

Attainablesequestration

SOCattainable

RainfallTemperatureLight

Limitingfactors

Potential sequestration

SOCpotential

Reactive surfacesDepthBulk density

Definingfactors

Actualsequestration

SOCactual

Soil managementPlant species/crop selectionResidue managementSoil and nutrient lossesInefficient water and nutrient useDisrupted biology/disease

Reducingfactors

Optimise input and reduce losses

Add external sources of carbon


Increasing the capture of carbon in soils

1. Maintain current production system

• Maximise resource use efficiency (e.g. carbon capture per mm water or per kg nutrient)

• For dryland systems – starts with water use efficiency

• Maximise stubble retention (carbon return)

2. Shift to alternative production systems

• Introduction of perennial vegetation where appropriate

• Alternative crops - lower harvest index

• Alternative pasture species – increased below ground allocation

• Increased use of green manures

Options


Options need to be tailored to site conditions

Beverley, WA

0

50

100

150

200

250

300Ja

n

Ma

r

Ma

y

Jul

Se

p

No

v

Month of the year

Av

era

ge

mo

nth

ly r

ain

fall

or

pa

n

ev

ap

ora

tio

n (

mm

)

Rain (mm)

Pan Evaporation (mm)

Mudgee, NSW

0

50

100

150

200

250

300

Jan

Ma

r

Ma

y

Jul

Se

p

No

v

Hamilton, Vic

0

50

100

150

200

250

300

Jan

Ma

r

Ma

y

Jul

Se

p

No

v


Soil carbon models and calculators

• Range of different forms• Carbon balance calculations• Spreadsheet based calculators (usually empirical)• System simulation models (more mechanistic)

• Provide estimates of what may be possible under a defined set of conditions

• Climatic conditions, soil properties, crop/pasture production

• Only as good as the data used to perform calculations or validate the model


Modelling soil organic carbon – RothC model

DPM

RPM

PlantInputs

BIO

HUM

CO2Decomposition

Decomposition

BIO

HUM

CO2

DecompositionIOMFire

RPM = POCIOM = ROCHUM = TOC – (POC + Char C)


Model calibration and verification

0 350

Kilometres

700

Verification Sites

Brigalow

Tarlee

Calibration Sites


Calibration of RothC to Australian conditions

• Clearing of Brigalow bushland

0

10

20

30

40

50

60

70

1982 1987 1992 1997

Year

C (

t/h

a)

RPM

HUM

IOM

TOC

TOC

HUM

CHAR

POC

Measured fractions

Modelled fractions

0

10

20

30

40

50

60

70

1982 1987 1992 1997

Year

C (

t/h

a)

RPM

HUM

IOM

TOC

RPM RPM

HUM HUM

IOM IOM

TOCTOC

TOC

HUM

CHAR

POC

TOCTOC

HUMHUM

CHARCHAR

POCPOC

Measured fractions

Modelled fractions


Tamworth – wheat/fallow

0

10

20

30

40

50

1970 1980 1990 2000

Year

So

il C

(t/

ha)

Wagga – wheat/pasture

0

20

40

60

1988 1990 1992 1994 1996 1998Year

So

il C

(t/

ha)

Salmon Gums – wheat/wheat

01020304050

1979 1983 1987 1991

Year

So

il C

(t/

ha)

Salmon Gums - wheat/ 3 pasture

Year

So

il C

(t/

ha)

01020304050

1979 1983 1987 1991

DPM

RPM

HUM

IOM

BIO

Soil

Modeled

POC

HUM

CHAR

TOC

Measured

Model Verification: (sites with archived soil samples)


Model verification: (paired sites)

• Is this result due poor model performance or poor pairing of the sites?

• Did the sites start off similar or are there significant shifts in soil/plant/environmental properties between paired individuals?

Kindon - pasture 15 y

0

10

20

30

40

50

Year

So

il C

(t/

ha)

1986 1991 1996 2001

Dunkerry South - crop

0

10

20

30

1967 1977 1987 1997

Year

So

il C

(t/

ha)

DPM

RPM

HUM

IOM

BIO

Soil

ModeledPOC

HUM

CHAR

TOC

Measured


Influence of altering the water use efficiency of wheat at Gawler, SA

20 year change in carbon

WUE tC/ha

0.50 0

0.75 12.1

1.00 24.1

0

20

40

60

80

100

120

0 10 20 30 40 50

Years since start of simulation

Am

ou

nt

of

so

il o

rga

nic

ca

rbo

n(t

C/h

a f

or

0-3

0 c

m la

ye

r)

WUE=0.50

WUE=0.75

WUE=1.00


Influence of altering the harvest index of wheat at Gawler, SA

0102030405060708090

100

0 10 20 30 40 50


Am

ou

nt

of

so

il o

rga

nic

ca

rbo

n(t

C/h

a f

or

0-3

0 c

m la

ye

r)

HI = 0.25

HI = 0.35

HI = 0.45


Harvest index

tC/ha

0.25 12.6

0.35 0

0.45 -7.0


Influence of altering the root shoot ratio of wheat at Gawler, SA


R/S ratio

tC/ha

0.50 0

0.75 5.2

1.00 10.50

1020

3040

50

6070

8090

100

0 10 20 30 40 50


Am

ou

nt

of

so

il o

rga

nic

ca

rbo

n(t

C/h

a f

or

0-3

0 c

m la

ye

r)

R/S ratio = 0.50

R/S ratio = 0.75

R/S ratio = 1.00


Frequency of addition of 5 t compost C/ha

0

20

40

60

80

100

120

140

160

180

200

0 100 200 300 400 500


To

tal o

rga

nic

ca

rbo

n in

0-3

0 c

m s

oil

lay

er

(t C

/ha

)

Annualaddition

1 year in 2

1 year in 3

1 year in 5

1 year in 10

1 year in 20

Fequency of adding 5 t C/ha


Take home messages

• Carbon exists in soils in different forms• composition influences the vulnerability of soil carbon

to change






Take home messages

• Decision to enter a carbon trading scheme will required consideration of the following issues:

• production system options, • economics (profitability), • food security, • implications into the future (liability and flexibility)

Thank you

CSIRO Land and WaterJeff BaldockResearch ScientistPhone: +61 8 8303 8537Email: [email protected]: http://www.clw.csiro.au/staff/BaldockJ/

AcknowledgementsJan Skjemstad, Kris Broos, Evelyn KrullSteve Szarvas, Leonie Spouncer, Athina Massis

Contact UsPhone: 1300 363 400 or +61 3 9545 2176

Email: [email protected] Web: www.csiro.au

http://www.clw.csiro.au/staff/BaldockJ/


Dynamic nature of SOC and its fractions

0

8

16

24

32

1/6/98 6/2/99 14/10/99 20/6/00 25/2/01

Date of sample collection

Am

ount

of

orga

nic

C

(Mg

C h

a-1in

0-1

0 cm

) POC Humus ROCTOC

Irrigated Kikuyu pasture – Waite rotation trial


Soil bulk density (Mg/m3) 1.1 1.2 1.3 1.4

Management induced compaction

Correcting soil carbon for management induced changes in bulk density

Original soil surface

Mass Soil 0-30 cm (Mg/ha) 3300 3600 3900 4200

Depth for equivalent mass (cm) 30.0 27.5 25.4 23.6

Original 30 cm depth

New 30 cm depth

Organic C loading (Mg/ha)

1% OC, no BD correction 33 36 39 42

1% OC, with BD correction 33 33 33 33


Influence of tillage on changes in soil carbon with depth

If red region > blue region = sequestration

For 0-10 cm layerred region > blue region (sequestration)

For 0-30 cm layerred region = blue region (no sequestation)

Cultivated to 10 cm

Uncultivated

Organic carbon content (% soil mass)

So

il de

pth

(cm

)

0.0 0.5 1.0 1.5 2.0 2.50

10

20

30

40

50

60

70

80


The carbon cycle in agricultural systems: where do options exist for sequestration

CO2

Plant carbon

Photosynthesis

Decomposition

Soil carbon

Death and addition of residues to soil

Agricultural products

Harvest

Long lived products(biochar)

Short lived products

(grains, meat)


Potential for soils to sequester C

0 cm

10 cm

30 cm

Potential for sequestration of C in soil• Global SOC pool size: 1500 Pg

• Rapid cycling SOC: 500-750 Pg

• 1% increase in stored SOC/yr: 5 - 7.5 Pg/yr

• Anthropogenic CO2-C emissions: 8 Pg/yr

Issues• Native unmanaged soils

• Variations in soil properties

• Permanency of increase

• Constraints on C inputs to soil (biophysical, economic, social)


Take home messages




Issues to consider• Production system options• Economics (profitability)• Food security• Implications into the future (liability and flexibility)



Quantifying SOC allocation of SOC to fractions

Recalcitrant Charcoal C

Humus + recalcitrant

HF treatment, UV-PO, & NMR

<53 µm fraction>53 µm fraction

Na saturate, disperse, sieve <53 µm

Total soil organic carbon

Density fractionation

Buried plant residue carbon

Soil sieved to <2mmSoil sieved to >2mm

Surface plant residue carbon

Quadrat collection

Particulateorganic carbon

Density fractionation

Humus = <53µm - Recalcitrant


Take home messages

• Atmospheric carbon can be captured by increasing the size of long lived forms of terrestrial carbon

• Carbon capture by forests

– without harvesting and storage in long lived products, carbon capture can only be counted once

– potential exists to create continuous capture and storage

• Soils do have a potential to capture and store carbon; however, issues exist that may limit carbon storage opportunities

– biophysical constraints on production (rainfall, nutrient, etc)

– economic situation of the farm business

– food security


Composition of soil organic carbon

Extent of decomposition increases

Rate of decomposition decreases

C/N/P ratio decreases (become nutrient rich)

Crop residues on the soil surface (SPR)

Buried crop residues (>2 mm) (BPR)

Particulate organic matter (2 mm – 0.05 mm) (POC)

Humus (<0.05 mm) (HumC)

Dominated by charcoal with variable properties

Resistant organic matter (ROC)


Years

So

il o

rgan

ic c

arb

on

(g

C k

g-1 s

oil

)

0

5

10

15

20

25

30

0 20 40 60 80 100 120 140

Balance between inputs and outputs

Inputs x 2

Inputs x 3

Inputs / 2

Inputs / 3

Inputs = Outputs


Minimum requirements for tracking soil organic carbon for accounting purposes

1. Collection of a representative soil sample to a minimum depth of 30 cm

2. An accurate estimate of the bulk density of the sample

3. An accurate measure of the organic carbon content of a soil sample

For 0-30 cm soil with a bulk density of 1.0 Mg/m3 and a carbon content of 1.0%

=Mass ofCarbon

(Mg C/ha)

Depth(cm)

30 Mg C/haxBulk

density(g/cm3)

xCarboncontent

(%)=


Importance of defining composition of organic N on mineralisation

Amount of N present (kg N/ha)

Fraction

(C/N ratio)

Residues/Particulate

(50)

Humus

(10)

Inert/char

(50)Total

Soil 1 300 2100 200 2600

Soil 2 500 1300 800 2600

Portion that decomposes

0.3 0.1 0.001

Amount of N mineralised (kg N/ha)

Residues/Particulate Humus Inert/char Total

Soil 1 - 45 147 0 102

Soil 2 - 75 91 0 16


0

100

200

300

400

500

600

1.0 1.2 1.4 1.6

Bulk density (g cm-3)A

mo

un

t of P

(kg

/ha

)

1.0% SOC 2.0% SOC

3.0% SOC 4.0% SOC

C/P=120

Requirements to increase soil carbon:the nutrient perspective

0

1000

2000

3000

4000

5000

6000

7000

1.0 1.2 1.4 1.6

Bulk density (g cm-3)

Am

ou

nt o

f N (

kg/h

a)

1.0% SOC 2.0% SOC

3.0% SOC 4.0% SOC

2400 kg N/ha

4800 kg N/ha

200 kg P/ha

400 kg P/ha

C/N=10


Options for increasing soil carbon content

• Principal: increase inputs of carbon to the soil• Maximise capture of CO2 by photosynthesis and addition of

carbon to soil

• Options• Maximise water use efficiency (kg total dry matter/mm water)• Maximise stubble retention• Introduction of perennial vegetation where appropriate

(afforestation, pastures, native vegetation)• Alternative crops - lower harvest index• Alternative pasture species – increased below ground allocation• Green manure crops – legume based for N supply • Addition of offsite organic materials – diversion of waste streams


Options need to be tailored to site conditions: the amount and distribution of rainfall

Beverley, WA

0

10

20

30

40

50

60

70

80

90

100Ja

n

Ma

r

Ma

y

Jul

Se

p

No

v

Month of the year

Av

era

ge

mo

nth

ly r

ain

fall

(mm

)

Mudgee, NSW

0

10

20

30

40

50

60

70

80

90

100

Jan

Ma

r

Ma

y

Jul

Se

p

No

v

Hamilton, Vic

0

10

20

30

40

50

60

70

80

90

100

Jan

Ma

r

Ma

y

Jul

Se

p

No

v


$$ for C sequestration – fact or fiction

• There is no doubt that soils could hold more carbon

• Challenge – increase soil C while maintaining economic viability

• Options do exist but they must be tailored to soil and climatic conditions

• Under current C trading prices• Difficult to justify managing for soil C on the basis of C

trading alone• Do it for all the other benefits enhanced soil carbon gives

• Careful consideration of liabilities and possible future restrictions in management options is required


0

10

20

30

40

50

60

70

Soil type

Ca

rbo

n i

n 0

- 3

0c

m s

oil

la

ye

r(t

C/h

a)

Influence of tillage and stubble on soil carbon

Kandosol(n=106)

Sodosol(n=63)

Vertosol(n=226)

Reduced Tillage (Stubble burnt, baled or retained)

Chromosol(n=119)

Traditional Tillage (Stubble burn or removed)

Traditional Tillage (Stubble retained or burnt late) Direct Drill (Stubble retained)

Pasture/Native


Influence of tillage systems on soil carbon contained in the 0-30 cm layer

0

10

20

30

40

50

60

Chromosol Kandosol Vertisol Sodosol

Soil Type

Am

ou

nt

of

C i

n 0

-30

cm s

oil

lay

er

(t C

/ha)

Tilled - stubble

Tilled + stubble or late burn

Reduced tillage

Direct drill


The carbon cycle: adding compost to soil

CO2

Plant production

Photosynthesis Respiration

Soil animals

and microbes

Death

Residues

Particulate organic C

Humus organic C

Harvested products

Harvest

Respiration

Death

Green wastes, manures and composts


Rate of annual compost addition

0

50

100

150

200

250

300

350

0 100 200 300 400 500


To

tal o

rgan

ic c

arb

on

in 0

-30

cm s

oil

laye

r (t

C/h

a)

0.0

0.5

1.0

2.0

3.0

5.0

10.0

Rate of compost C addition (t C/ha/y)


Frequency of addition of 5 t compost C/ha

0

20

40

60

80

100

120

140

160

180

200

0 100 200 300 400 500


To

tal o

rga

nic

ca

rbo

n in

0-3

0 c

m s

oil

lay

er

(t C

/ha

)

Annualaddition

1 year in 2

1 year in 3

1 year in 5

1 year in 10

1 year in 20

Fequency of adding 5 t C/ha


Simulation modelling: Using RothC to predict changes to soil carbon

• Clearing of Brigalow bushland

0

10

20

30

40

50

60

70

1982 1987 1992 1997

Year

C (

t/h

a)

RPM

HUM

IOM

TOC

TOC

HUM

CHAR

POC

Measured fractions

Modelled fractions

0

10

20

30

40

50

60

70

1982 1987 1992 1997

Year

C (

t/h

a)

RPM

HUM

IOM

TOC

RPM RPM

HUM HUM

IOM IOM

TOCTOC

TOC

HUM

CHAR

POC

TOCTOC

HUMHUM

CHARCHAR

POCPOC

Measured fractions

Modelled fractions


Influence of pasture production on soil carbon at Bairnsdale

Pasture lost = 50%Root/shoot ratio = 1Pasture grows from March to November

0

20

40

60

80

100

120

140

160

180

200

0 200 400 600 800 1000

Duration of simulation (years)

Am

ou

nt

of

ca

rbo

n in

th

e 0

-30

cm

s

oil

lay

er

(t C

/ha

) 2 t/ha

4 t/ha

6 t/ha

8 t/ha

10 t/ha

12 t/ha

16 t/ha

Increase pasture growth from 6 to 8 t dm/ha gives an additional 9.4 t C/ha in 25 years(10t dm/ha gives 19 t C/ha)


Changes in soil C for different levels of average grain yield (Roseworthy, SA)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0 5 10 15 20


So

il o

rgan

ic C

(0-

10 c

m l

ayer

) (

% o

f to

tal

soil

mas

s) 0.5 T/ha

1 T/ha

2 T/ha

3 T/ha

4 T/ha

6 T/ha

8 T/ha

10 T/ha

Shift yield from 4 to 8 T grain/ha = 1.0 %C increase over 20 yearsShift yield from 4 to 6 T grain/ha = 0.4 %C increase over 20 years


Significance of carbon in soils

Annual fluxes (1015 g C/yr)

Emissions

• Fossil fuel burning6• Land use change 2

Responses

• Atmospheric increase3

• Oceanic uptake 2• Other 3

World wide C pools (1015 g C)

• Atmosphere (CO2 C) 780

• Living Biomass (plants, animals) 550

• Soil

0-1 m depth 1500

0-3 m depth 2300Houghton (2005)

1330


Distribution and turnover of organic carbon in soil

0 cm

10 cm

30 cm

100 cm

SOCcontent

High

Low

Verylow

Proportion ofprofile SOC

30-50%

20-30%

10-30%

Relativeresponse time

Rapid

Intermediateto slow

Slow


0

200

400

600

800

1000

0.00 0.05 0.10 0.15 0.20

So

il de

pth

(m

m)

Volumetric Water Content

(cm3 cm-3)Euston

Impact of subsoil constraints

Plant-available water (no constraints) = 97 mm

Plant-available water (with constraints) = 59 mm

UpperLimit

LowerLimit

Lower limitwith subsoilconstraints

building soil carbon: benefits, possibilities, and modeling

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