Soil Carbon
Richard Eckard
Introduction
• 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
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
What is Soil Carbon?
• Carbon forms in soil
– Inorganic forms
• carbonates, graphite, CO2, HCO3
– Organic
• living, dead; labile, non-labile
In top 15 cm SOM typically ranges:
Desert soils: < 1%
Agric soils: 1-5%
Forest soils: 1-10%
Organic soils:
up to 100%
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
400 µm
Source: Jeff Baldock
What is Soil Carbon?
• Humus
– Decomposed materials less than 0.053 mm that are
dominated by molecules stuck to soil minerals
– Energy and source of N
10 µm
• Recalcitrant or resistant organic carbon (ROC)
– Biologically stable; typically in the form of charcoal.
20 µm
Source: Jeff Baldock
Why is it important?
Roles of organic carbon (and associated elements) in
defining soil productivity
Biological
roles
Physical
roles
Chemical
roles
- Biochemical energy
- Structural stability
- Cation exchange
- Reservoir of nutrients
- Water retention
- pH buffering
- Increased resilience
- Thermal properties
- Complexes cations
- Biodiversity
- Erosion
Climate change – Soils can store carbon
Source: Jeff Baldock
How does soil carbon compare to
other sinks globally?
Global Carbon Stock (Pg C) Mill km2
Plants
Tundra
2
Soils Area
115
5.6
Boreal forests
57
338
13.7
Temperate forests
139
153
10.4
Tropical forests
340
213
17.5
Tropical savannas
79
247
27.6
Temperate grass & shrublands
23
176
15.0
Deserts & Semi-deserts
10
159
27.7
Croplands
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)
What determines soil organic
carbon content?
• 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
Source: Jeff Baldock
How fractions differ between soils
Particulate organic carbon
Humus organic carbon
Soil organic carbon stock
(Mg C/ha)
50
Resistant organic carbon
40
30
20
10
0
Soil
1
Soil
2
Soil
3
Soil
4
Soil
5
Soil
6
Soil
7
Understanding composition provides information on the vulnerability
of soil organic carbon to change
Source: Jeff Baldock
What are the Policy Drivers for Soil
Carbon sequestration?
The Carbon Farming Initiative
• Kyoto sinks
• Non-Kyoto sinks
– Reforestation
– Afforestation
• Kyoto sources
– Soil C sequestration
– Managed forests
– Non-forest revegetation
– Enteric methane
– Nitrous oxide
The Policy Drivers for Soil Carbon
• 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
Grace per comm
Can we quantify changes?
• 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.
Soil organic carbon
(Mg C/ha)
– Add measured points as validation
60
50
40
30
20
10
0
0
5
10
15
Time (years)
20
Source: Jeff Baldock
How prepared are our models?
• 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
In Summary
• 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!