Soil Carbon

Richard Eckard

• Recent media focus on soil carbon

– Need more science at the forefront

• Carbon Farming Initiative

– Crediting mechanism

• Land sector abatement and sinks

– Including soil carbon

Introduction

Soil Carbon Scheme Could Offset all

of Australia’s Greenhouse Gas

Emissions

It is feasible and practical to stop

global warming right now - The SOIL

CARBON SOLUTION

Desert soils: < 1% Agric soils: 1-5% Forest soils: 1-10%

Organic soils:

up to 100%

In top 15 cm SOM typically ranges:

• Carbon forms in soil

– Inorganic forms

• carbonates, graphite, CO2, HCO3

– Organic

• living, dead; labile, non-labile

What is Soil Carbon?

• Soil Organic Matter (SOM)

– The sum total of all organic carbon-containing

substances in soils:

– Living biomass, decomposed residues and humus

• Soil Organic Carbon (SOC)

– Carbon component of the SOM

• Total Organic Carbon (TOC)

– SOC

What is Soil Carbon?

• Crop residues

– Shoot and root residues less than 2 mm found in the soil

and on the soil surface

– Energy to soil microbes

• Particulate Organic Carbon (POC)

– Individual pieces of plant debris that are smaller than 2

mm but larger than 0.053 mm

– Slower decomposition than residues

– Provides energy and nutrients for microbes

What is Soil Carbon?

400 µm400 µm400 µm

Source: Jeff Baldock

• Humus

– Decomposed materials less than 0.053 mm that are

dominated by molecules stuck to soil minerals

– Energy and source of N

• Recalcitrant or resistant organic carbon (ROC)

– Biologically stable; typically in the form of charcoal.

What is Soil Carbon?

10 µm10 µm10 µm

20 µm20 µm Source: Jeff Baldock

Why is it important?

- Biochemical energy

- Reservoir of nutrients

- Increased resilience

- Biodiversity

Biological

roles

- Structural stability

- Water retention

- Thermal properties

- Erosion

Physical

roles

Chemical

roles

- Cation exchange

- pH buffering

- Complexes cations

Roles of organic carbon (and associated elements) in

defining soil productivity

Climate change – Soils can store carbon

Source: Jeff Baldock

Tropical forests

Temperate forests

Boreal forests

Tropical savannas

Temperate grass & shrublands

Deserts & Semi-deserts

Tundra

Croplands

Plants Soils Area

2 115 5.6

Global Carbon Stock (Pg C) Mill km2

57 338 13.7

139 153 10.4

340 213 17.5

79 247 27.6

23 176 15.0

10 159 27.7

4 165 13.5

Total 654 1567• Most terrestrial C is in soil

• 6,000 to 32,400 B tonnes (Gt) CO2e stored in soils worldwide

Saugier et al (2001)

How does soil carbon compare to

other sinks globally?

• A big, slow-changing input : output equation

– Inputs: Plant residues & fire residues

– Outputs: Decomposition & mineralisation

• Limited by

– Climate (temperature), soil type (clay),

management & nutrients

– Water and temperature

• Good seasons = more soil C

• Drought = less soil C

What determines soil organic

carbon content?

Source: Jeff Baldock

How fractions differ between soils

Soil

1

Soil

2

Soil

3

Soil

4

Soil

5

Soil

6

Soil

7

Soil

org

an

ic c

arb

on

sto

ck

(Mg

C/h

a)

10

20

30

40

50

Particulate organic carbon

Humus organic carbon

Resistant organic carbon

0

Understanding composition provides information on the vulnerability

of soil organic carbon to change

Source: Jeff Baldock

• Kyoto sinks

– Reforestation

– Afforestation

• Kyoto sources

– Enteric methane

– Nitrous oxide

• Non-Kyoto sinks

– Soil C sequestration

– Managed forests

– Non-forest revegetation

What are the Policy Drivers for Soil

Carbon sequestration?

The Carbon Farming Initiative

• Non-Kyoto Carbon Fund • $250 million program over 6 years

• Purchase credits for non-Kyoto land-based offsets

– More certainty in market

• Non-Kyoto price lower than $23/t CO2e

– CFI will credit projects for 7 years or more

• But need to quantify changes

• And guarantee permanance >100 years

The Policy Drivers for Soil Carbon

Grace per comm

• Likely changes in Victorian cropping systems

– Good rainfall + good clay + min tillage (6-7 t DM/y)

– 330 kg C/ha/yr = 0.3% in 10 years

– 1.2 t CO2e/y = $12 – $18/ha/y

• Rothamstead expt

– Arable to pasture

– >300 years

– 1.2% to 2.7% in 110 years

– Max 0.4% in 25 years

Can we quantify changes?

• Will not be able measure in short-term

• CFI will allow a deeming method

– i.e. modelling

– Various industry models can be used

• If peer reviewed and validated.

– Add measured points as validation

Can we quantify changes?

0

10

20

30

40

50

60

0 5 10 15 20Soil

org

an

ic c

arb

on

(Mg

C/h

a)

Time (years) Source: Jeff Baldock

• Studies show that a range of models

– Can produce similar results (eg. Ranatunga et al.)

• If the biophysical assumptions are similar

• If driven by correct climatic and edaphic parameters

• For CFI methods

– Top down must align with bottom up accounting

• Industry models and inventory must align

– Models must be validated and peer reviewed

• Demonstrated skill in predicted soil C changes

How prepared are our models?

• Carbon is not carbon

– Soils differ in their fractions

– Fractions decompose at different rates

• Soil C will be developed as CFI offset method

– Soil C changes can be modelled

– Models must be validated & peer reviewed

– But must be

• Capable of long term (100 year) simulation

In Summary

• Soil carbon sequestration

– Building soil carbon is good practice!

– Trading soil C is a separate discussion

• Non-Kyoto offsets will be lower priced

– Plus returns per ha & per year will be very modest

• Measureable changes may be in decades

– Obligations will be >100 years

• Rainfall & temperature

– Biggest determinant of input vs losses of soil carbon

• Price and Permanence

– The big sleepers in soil C trading!

In Summary