How do we estimate soil carbon levels

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How do we estimate soil carbon levels?

Making best use of existing science and knowledge

Brian Murphy

Cowra

November 2008

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Soil science knowledge that can help measuring and estimating soil carbon

Use known or developed relationships for soil properties to predict soil carbon levels

- Relationships between soil properties

- Relationships between soil properties and land management activities / practices

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Gaunaut report

“A comprehensive mitigation strategy will also require government intervention to promote abatement activity in sectors not covered by the emissions trading scheme. ………………………………………………………………..

The most significant opportunities may be in the area of improved carbon sequestration through better management of soil carbon.”

Measurement / estimate of soil carbon levels is Measurement / estimate of soil carbon levels is required for this to become effectiverequired for this to become effective

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1. Percentage soil carbon (C%)

- g carbon/100g soil

No volume taken into account

cannot be used for carbon accounting alone

2. Carbon density (CD)

- CD = C% x BD x soil depth

- t/ha

can be used for carbon accounting

requires measure of bulk density

requires depth to be specified

30 cm is the standard Kyoto depth.

Measuring soil carbon

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Changes in soil carbon density and changes in soil carbon content

0

5

10

15

20

25

30

35

40

45

0 1 2 3 4 5

Change in soil carbon content g/100g

Cha

nge

in s

oil c

arbo

n de

nsity

t/h

a

Soil carbondensity to 5cm

Soil carbondensity to 10cm

Poly. (Soilcarbondensity to 10cm)Poly. (Soilcarbondensity to 5cm)

On one paddock going from traditional tillage to long term pasture carbon % for 0 to 10 cm went from 1.20% to 1.92% over 15 years (10 cm), about 600 mm rainfall

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Bulk Density

Mass of soil solids per unit volume of soil- Usually taken as oven dry weight

Does not include the mass of water or air in the density calculation

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Soil surface

10 cm depth

Wheat root

Bulk density, mass of soil solids per unit volume

Soil pores can be filled by air or water

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Soil surface

Soil pores can be filled by air or water

Bulk density, mass of soil solids per unit volume

10 cm depth

Wild oats seed root

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Measuring Soil Carbon

Where to sample in the field

_what do we know?

What to measure in the field, how deep?_

_what do we know?

What to measure in the laboratory_

_what do we know?

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Where to sample in the field_Measuring Soil Carbon for Different Purposes

Scientific References Sites- Monitoring soil condition, testing or calibrating soil

carbon models

- 25 m grid or quadrat has many advantages from a scientific viewpoint

Estimating the soil carbon density of a paddock- Needs a different approach

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Soil analysed for soil carbon

1500 t soil/ha

10 kg soil collected?

0.1 to 0.5 kg sent to lab?

0.001 kg tested

Needs to be representative

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Selecting a representative sample in the field (based on fertiliser handbook)

Check target area for notable features that can influence soil type

– slope, drainage, soil colour, management history etc

Draw a sketch map and identify “individual areas” to be sampled. “Individual areas” should be uniform based on the above features. Satellite imagery and aerial photos can be very helpful

Avoid sampling across soil types and when soils are very wet

Take a number of cores and make into a composite sample for each “individual area”. Numbers of cores for a composite sample and the number of composite samples to characterise a paddock need to be determined.

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What to measure in the field, how deep? _ what do we know?

30 cm is the standard depth for carbon density.

Need to sample soils with a standard core so bulk density can be calculated

Care needs to be taken so that organic matter does not contaminate the soil samples below the surface

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What to measure in the laboratory _ what do we know?

Measure carbon content – treat with acid if carbonate present- LECO furnace – standard

- MIR spectrophotometer – quick and cheaper???

Measure soil moisture and weight to get bulk density.

Calculate carbon density (C% x bulk density) – a slight buffer to storing carbon / unit depth?

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Using Knowledge of Soil Relationships

What accuracy for soil carbon density, soil carbon % is acceptable ?

- ± 10t/ha, ± 1t/ha, ± 0.1 t/ha, ± 0.01 t/ha _ costs tend to increase exponentially with increased accuracy.

Use known or developed relationships for soil properties to predict soil carbon levels

- Relationships between soil properties

- Relationships between soil properties and land management activities

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Relationships between soil properties

Bulk density v soil texture, soil carbon and soil type

Soil carbon density to 10 cm and soil carbon density to 30 cm- Using soil carbon depth functions

Other????- MIR in the field

- Soil colour

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Wagga_LECO carbon levels g/100g

0

5

10

15

20

25

30

35

40

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00

LECO carbon levels g/100g

Dep

th (c

m) DD stubble retain

TT stubble retain

DD stubble burn

TT stubble burn

650 mm rainfall, red earths

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Changes in soil carbon density with clearing

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10

20

30

40

50

60

70

80

90

100

0.00 0.50 1.00 1.50 2.00 2.50 3.00

Soil carbon density (t/ha/cm of soil)

So

il d

epth

(cm

)

Uncleared

4 years

3 years

90 years

29 years

34 years

10 years

9 years

Rainfall 450 to 550 mm, red earths,Bimble Box

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Caution – likely to be soil type and climate specific!!!

Predicting soil carbon density to 30 cm using soil carbon density to 10 cm - based on NSW

Paired sites (Murphy et al. 2003)

y = 1.6546x + 7.3616

R20.929 =

0

10

20

30

40

50

60

70

0 5 10 15 20 25 30 35

Soil carbon density to 10 cm t/ha

So

il c

arb

on

den

sity

to

30

cm

t/h

a

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Relationships between soil carbon levels and land management practices Many known relationships between land

management practices and soil carbon levels. Problem of continual developing and

improving land management practices Available data is scattered and does not

include all permutations and combinations However, modelling can be used to fill in the

gaps, with real data providing the benchmarks to work around

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t / ha / 30 cm

20

30

40

S* N* R* T*E*

annual average

evaporation

moderate

annual average rainfall

annual average

temperature 0C

1500 mm 25

long term nutrient levels

650 mm

Soil Carbon Potential

Soil texture, clay content

fine sandy loam (15%)

Land Management Practice

50

60

70

80

0

10

25

t / ha / 30 cmfine sandy

loam (15%)

Land Management Practice

50

60

70

long term nutrient levels

650 mm

Soil Carbon Potential

Soil texture, clay content

Long term perennial pasture

annual average

evaporation

moderate

annual average rainfall

annual average

temperature 0C

1500 mm 25

S* N* R* T*E*

80

0

10

20

30

40

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t / ha / 30 cm

20

30

40

S* N* R* T*E*

annual average

evaporation

moderate

annual average rainfall

annual average

temperature 0C

1500 mm 25

long term nutrient levels

650 mm

Soil Carbon Potential

Soil texture, clay content

fine sandy loam (15%)

Land Management Practice

50

60

70

Long term perennial pasture

Scalds

80

0

10

27

t / ha / 30 cm

Land Management Practice

50

60

70

0

10

Direct drilling with stubble burning (hot)

long term nutrient levels

650 mm

Soil Carbon Potential

Soil texture, clay content

annual average

evaporation

moderate

annual average rainfall

annual average

temperature 0C

1500 mm 25

N* R* T*E*

20

30

40

S*

fine sandy loam (15%)

Long term perennial pasture

Scalds

80

28

t / ha / 30 cm

80

S*

fine sandy loam (15%)

Long term perennial pasture

Scalds

N* R* T*E*

annual average

evaporation

moderate

annual average rainfall

annual average

temperature 0C

1500 mm 25

Direct drilling with stubble burning (hot)

No till with stubble retention and

controlled traffic

long term nutrient levels

650 mm

Soil Carbon Potential

Soil texture, clay content

Land Management Practice

50

60

70

0

10

20

30

40

29

t / ha / 30 cm

Land Management Practice

50

60

70

0

10

20

30

40

Direct drilling with stubble burning (hot)

No till with stubble retention and

controlled traffic

Annual pasture, volunteer pasture

long term nutrient levels

650 mm

Native vegetation - minimal disturbance

Soil Carbon Potential

Soil texture, clay content

annual average

evaporation

moderate

annual average rainfall

annual average

temperature 0C

Traditional tillage with multiple

cultivations and stubble burning

1500 mm 25

N* R* T*E*

80

S*

fine sandy loam (15%)

Long term perennial pasture

Scalds

30

Soil texture, clay content

annual average rainfall annual average

temperature 0C

Direct drilling with stubble burning (hot)

Scalds

1500 mm 25

60

Long term perennial pasture

No till with stubble retention and

controlled traffic

Direct drilling with stubble retention

long term nutrient levels

650 mm

Traditional tillage with multiple

cultivations and stubble burning

Soil Carbon Potential t/ha/30 cm

10

0

Annual pasture, improved pasture

Annual pasture, volunteer pasture

50

40

30

20

80

70

High nutrients . Low disturbance

S* N* R* T*E*

annual average evaporation

Native vegetation - minimal disturbance

fine sandy loam (15%) moderate

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Extra Information about land management practices to predict soil carbon levels.

Biomass and yields – use of fertilisers?

Stubble management – amount of stubble retained – not burnt (early hot v late cold burn), grazing

Soil disturbance, amount and type of tillage – threshold of tillage to start reducing soil carbon

Grazing intensity and timing of resting pasture – impact on biomass and plant growth

Species, grasses v herbaceous dicots

Perennial v annual – some knowledge yet to be gained.

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Conclusions 1

Methodologies to measure soil carbon are available - cost is the issue

Need to apply ALL existing soils knowledge and soil science to soil carbon issues

Exploring and investigating relationships between soil properties can bring down costs of measuring soil carbon

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Conclusions 2

Investigating the relationships between land management activities and soil carbon can be developed in a two step process

- Putting existing soil carbon measurements into a framework of climate x soil type x land management activities to give a soil carbon potential

- Using soil carbon models to fill in the gaps where there is no measured data.

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Chart Titley = -0.159x + 1.6046

R2 = 0.6666

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

0 0.5 1 1.5 2 2.5 3 3.5

BD

Linear (BD)

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Department of Environment & Climate Change NSW

First level dot point

- Second level dot point

Third level dot point

First level dot point

Soil carbon and available water - fine sandy loam mm/10 cm

0

5

10

15

20

25

0 1 2 3 4 5 6

Soil carbon g/100 g

Av

ail

ab

le w

ate

r m

m/1

0 c

m o

f s

oil

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Changes in soil carbon density and changes in soil carbon content

0

5

10

15

20

25

30

35

40

45

0 1 2 3 4 5

Soil carbon content g/100g

Cha

nge

in s

oil c

arbo

n de

nsity

t/h

a

Soil carbondensity to 5cm

Soil carbondensity to 10cm

Poly. (Soilcarbondensity to 10cm)Poly. (Soilcarbondensity to 5cm)

On one paddock going from traditional tillage to long term pasture carbon % went from 1.20% to 1.92% over 15 years (10 cm)