Soil carbon in a carbon accountingframework

Jeff BaldockCSIRO Land and WaterAdelaide, SA

Topics to be examined

• Amount of organic carbon in soils• Significance globally and potential to alter

CO2-C concentration• Factors controlling organic carbon content

• Biologically significant fractions• Why is it important to consider fractions?

• Carbon sequestration potential• Defining the sequestration potential of a soil• Rates of soil carbon change

Significance of carbon in soils

Annual fluxes (1015 g C/yr)Emissions

Fossil fuel burning 6 Land use change 2

Responses

Atmospheric increase 3 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 15000-3 m depth £ 2300

Houghton (2005)

1330

Potential for soils to sequester C

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

Potential for soils to sequester C

0 cm

10 cm

30 cm

100 cm

• SOC pool size: 1500 Pg

• Rapid cycling SOC: 500-750 Pg

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

• CO2-C emissions: 8 Pg/yr

Issues• Permanency of increase• Native unmanaged soils• Constraints on C inputs (biophysical,

economic, social)

What determines soil organic carboncontent?

Throttles or rate determinants

Soil organic carboncontent

Inputs oforganic carbon

Losses oforganic carbon= ,f

Inputs• Net primary

productivity• Addition of off

site organicmaterial

Losses• Conversion of

organic C toCO2 bydecomposition

Years

Soil

orga

nic

carb

on

(g C

kg-

1 soi

l)

0

5

10

15

20

25

30

0 20 40 60 80 100 120 140

Influence of the balance between inputsand outputs

Inputs > Outputs

Inputs >> Outputs

Inputs < Outputs

Inputs << Outputs

Inputs = Outputs

Years

Soil

orga

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

Changes in total soil organic carbon withtime

15 43

Initiatewheat/fallow

18 y 10 y

Biologically significant fractions of soilorganic matter

• Crop residues on the soilsurface (SPR)

• Buried crop residues(>2 mm) (BPR)

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

• Humus (<0.05 mm)(HumC)

Extent ofdecompositionincreases

C/N/P ratiodecreases (becomenutrient rich)

Dominated bycharcoal with variableproperties

• Resistant organic matter(ROC)

Identification of biologically significant soilorganic fractions

Humus(HumC)

Particulate material(POC)

Charcoal(ROC)

Morphology of charcoal found in soil

CO2

Plantproduction

Photosynthesis

Death/Harvest

Plantresidues

Mineralisation

Soil animalsand microbes

Recalcitrantorganic C

(ROC)

Burning

Soil carbon cycle

Particulateorganic C

Humusorganic C

Increasingextent of

decomposition

Importance of quantifying allocation of C tosoil organic fractions

Soi

l Org

anic

Car

bon

(g C

kg-

1 soi

l)

Time

0

5

10

25

15

20

Soil 120 g SOC kg-1 soil

Soil 220 g SOC kg-1 soil

Time

0

5

10

25

15

20

Active C

Active CSoi

l Org

anic

Car

bon

(g C

kg-

1 soi

l)Inert C

10 g Char-C kg-1soil

Inert C

2.5 g Char-C kg-1soil

Years

Soil

orga

nic

carb

on

(g C

kg-

1 soi

l)

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70

TOC

Conversion topermanent

pasture

33

Importance of allocating C to soil organicfractions

15 43

Humus C

ROCPOC

Initiatewheat/fallow

18 y 10 y

~30% less humus C

~800% more POC

Vulnerability of soil carbon content tovariations in management practices

Years

Soil

orga

nic

carb

on

(g C

kg-1

soi

l)

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70

TOC Humus

ROCPOC

Conversionto

wheat/fallow

18 y

Conversionto pasture

10 y

15 4333

9 y

52

Initiatewheat/fallow

Variation in amount of C associated withsoil organic fractions

0

5

10

15

20

25

Average for Hamilton (long term pasture)

Org

anic

car

bon

in 0

-10

cm la

yer

(Mg

C/h

a)

Surface plant residues(SPR)

Buried plant residues(BPR)

Particulate organic matter(POM)

Humus

Recalcitrant(ROM - charcoal)

Variation in amount of C associated withsoil organic fractions

Pasture PastureCropped Mix Mix

0

5

10

15

20

25

301P 8P 32P

NoT

ill (M

edN

)

NoT

ill (H

ighN

)

Stra

t (M

edN

)

Stra

t (H

ighN

)

0P

125P

250P

Arbo

retu

m

Perm

Pas

ture

W2P

F

Can

ola/

whe

at

Puls

e/w

heat

Past

ure/

whe

at

Hamilton Hart Yass Urrbrae Waikerie

Org

anic

C in

0-1

0 cm

laye

r(M

g C

/ha)

SPRBPRPOCHumCROC

Minimum requirements for tracking soilorganic carbon for accounting purposes

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

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

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

For 0-10 cm soil with a bulk density of 1.0 Mg/m3 anda carbon content of 1.0%

=Mass ofCarbon

(Mg C/ha)

Depth(cm) 10 Mg C/hax

Bulkdensity(g/cm3)

xCarboncontent

(%)=

New 30 cm depth

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

Management induced compaction

Correcting soil carbon for managementinduced changes in bulk density

Original soil surface

Original 30 cm depth

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

Depth for equivalent mass (cm) 30.0 27.5 25.4 23.6

Organic C loading (Mg/ha)

1% OC, no BD correction 33 36 39 42

1% OC, with BD correction 33 33 33 33

Plant dry matter additions required to altersoil C content

0

10

20

30

40

50

60

70

80

90

0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7

Bulk density

(g/cm3)

Mass c

arb

on s

tore

d in s

oil

(Mg/h

a/ 10 c

m d

epth

layer)

1% SOC

2% SOC

3% SOC

4% SOC

5% SOC24

48

Amount of C required: 24 Mg C 50 Mg Dry Matter (DM)

Rate per year: 10 Mg DM/y (no loss) 20 Mg DM/y (50% loss)10

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 o

rgan

ic C

(M

g C

ha-1

)

POC Humus IOCTOC

Irrigated Kikuyu pasture – Waite rotation trial

Modelling measurable SOC fractions

DPM

RPM

Plant

Inputs

BIO

HUM

CO2Decomposit ion

Decomposit ion

BIO

HUM

CO2

Decomposit ion

RothC Model

IOMFire

RPM = POC

IOM = UV/photo-oxidat ion resistant

(Char C)

HUM = TOC ‒ (POC + Char C)

Predicting soil organic carbon contents

• 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

Defining soil C dynamics at Roseworthy SAunder continuous wheat production

3.88Grain yield used (80% water limited potential - Mg/ha)0.45Harvest index (Mg grain/Mg dry matter)8.62Dry matter production (Mg/ha)

4.56Water limited potential grain yield (Mg/ha)

20110

French-Schultz constantsSlopeIntercept

338Average growing season rainfall (mm)

2.88

C content of 0-10 cmlayer (%)

80.54Total

Amount of C in 0-30cmlayer (Mg C/ha)Type of C

2.20Recalcitrant55.68Humus22.66Particulate

Equilibrium conditions (model for 500 years)

Estimates of organic carbon in the 0-30 cmlayer under wheat at Roseworthy, SA

0

50

100

150

200

250

0 100 200 300 400 500

Years since start of simulation

Am

ou

nt

of

so

il o

rgan

ic c

arb

on

(Mg

C/h

a f

or

0-3

0 c

m layer)

0.5 Mg/ha

1 Mg/ha

2 Mg/ha

3 Mg/ha

4 Mg/ha

6 Mg/ha

8 Mg/ha

10 Mg/ha

Average

wheat grain

yield

58 Mg C/ha

Estimates of organic carbon in the 0-30 cmlayer under wheat at Roseworthy, SA

0

25

50

75

100

125

150

175

0 5 10 15 20

Years since start of simulation

Am

ou

nt

of

so

il o

rgan

ic c

arb

on

(Mg

C/h

a f

or

0-3

0 c

m layer)

0.5 Mg/ha

1 Mg/ha

2 Mg/ha

3 Mg/ha

4 Mg/ha

6 Mg/ha

8 Mg/ha

10 Mg/ha

Average

wheat grain

yield

Defining soil C dynamics at Yass, NSWunder permanent pasture

5.0Average pasture shoot dry matter production (Mg dm/ha)

Retained and returnedConsumed by animals

0.500.50

Fate of pasture shoot dry matter

Proportion of consumed dry matter0.67Used by animal (wool, wt gain, respiration)0.33Excreted as faeces and urine

Net proportion of shoot residues0.335Removed from the paddock0.665Returned to the paddock

RootsShoots

0.400.60

Allocation of pasture dry matter

Estimates of organic carbon in the 0-30 cmlayer under pasture at Yass, NSW

0

50

100

150

200

250

300

0 100 200 300 400 500

Years since start of simulation

Am

ou

nt

of

so

il c

arb

on

carb

on

(Mg

C/h

a f

or

0-3

0 c

m layer)

2 Mg/ha

4 Mg/ha

6 Mg/ha

8 Mg/ha

10 Mg/ha

15 Mg/ha

20 Mg/ha

Shoot dry

matter

production

42.6 Mg C/ha

Estimates of organic carbon in the 0-30 cmlayer under pasture at Yass, NSW

0

25

50

75

100

125

150

0 5 10 15 20

Years since start of simulation

Am

ou

nt

of

so

il c

arb

on

carb

on

(Mg

C/h

a f

or

0-3

0 c

m layer)

2 Mg/ha

4 Mg/ha

6 Mg/ha

8 Mg/ha

10 Mg/ha

15 Mg/ha

20 Mg/ha

Shoot dry

matter

production

Evaluating potential C sequestration in soilS

oil c

arbo

n se

ques

tratio

n si

tuat

ion

Stable soil organic carbon (e.g. t1/2 ³ 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 inputand reducelosses

Add externalsources ofcarbon

Defining inputs of organic carbon to soil – drylandconditions

• Availability of water – amount and distribution ofrainfall imposes constraints on productivity and options

Beverly, WA

0

15

30

45

60

75

90

Jan

Mar

May

Jul

Sep

Nov

Month of the year

Avera

ge m

on

thly

rain

fall (

mm

)

0

50

100

150

200

250

300

Rain (mm)

Pan Evaporation (mm)

Roseworthy, SA

0

15

30

45

60

75

90

Jan

Mar

May

Jul

Sep

Nov

Month of the year

0

50

100

150

200

250

300

Rain (mm)

Pan Evaporation (mm)

Mudgee, NSW

0

15

30

45

60

75

90

Jan

Mar

May

Jul

Sep

Nov

Month of the year

0

50

100

150

200

250

300

Avera

ge m

on

thly

pan

evap

ora

tio

n (

mm

)

Rain (mm)

Pan Evaporation (mm)

$$ for C sequestration – fact or fiction

• There is no doubt that soils could hold more carbon

• Challenge – increase soil C while maintainingeconomic viability

• Options• Perennial vegetation in regions of negative returns• Reduce stocking, rotational grazing, green manure• Optimise farm management to achieve 100% of water limited

potential yield

• 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

Summary

• Soils represent a significant global pool of carbon

• The organic carbon content achieved is determined by thebalance between inputs and losses

• Defining the composition of soil organic matter allowsenhanced understanding of C dynamics

• Soils have a finite ability to build organic carbon

• Optimising plant productivity will maximise inputs and soilorganic carbon content

• External sources of organic matter can enhance soil organiccarbon – but continued addition is required

Thank you

CSIRO Land and WaterJeff BaldockResearch ScientistPhone: +61 8 8303 8537Email: jeff.baldock@csiro.auWeb: http://www.clw.csiro.au/staff/BaldockJ/

AcknowledgementsJan Skjemstad, Kris Broos, Evelyn KrullSteve Szarvas, Leonie Spouncer, Athina MassisJanine McGowan

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