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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: [email protected]: http://www.clw.csiro.au/staff/BaldockJ/
AcknowledgementsJan Skjemstad, Kris Broos, Evelyn KrullSteve Szarvas, Leonie Spouncer, Athina MassisJanine McGowan
Contact UsPhone: 1300 363 400 or +61 3 9545 2176
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