The Importance of Soil Carbon
Amanda Schapel [email protected]
DEW, Natural Resources KI
4 February 2019
Soil organic carbon (OC)
• Soil OC is important for
– plant productivity
– soil health
– offsetting greenhouse gases
• High soil health = carbon sequestration
• Soil OC of 8% are achievable in SA
• Soil OC increases under no till
• OC in soil comes from photosynthesis
• Plants roots contribute more to soil OC than shoots
Burning issues?
Questions?
Things that need to be covered?
IMPORTANCE OF SOIL CARBON
Photo credit: Mary-Anne Young, Amanda Schapel PIRSA
Why is soil important?
• The world is facing multiple challenges
– food security
– environmental sustainability
– soil protection
– climate change
• Soil is fundamental to human life and environmental systems
• The decisions we make and actions we take affect not just the land
but also fresh water, oceans, the atmosphere and the life they support
Why is soil organic carbon important?
• Plant productivity
• Resilience
• Offsetting greenhouse gases
• Soil health / function
What is soil carbon?
Soil carbon is present in both inorganic (IC) and organic (OC) forms
– IC is mineral based and not strongly influenced by land management practices
– Is the OC measured in soil organic matter
• makes up ~ 40-60% of the mass of soil organic matter
• decomposing organic compounds of plants, animal and microbial origin
• STABILISED with clay minerals and aggregates through biological, physical and chemical
processes
• influenced by land management practices
Role of organic matter
Physical Chemical Biologicalbetter structural stability
(aggregation)
lower bulk density
rapid infiltration of water
better drainage
better root growth
less erosion
improved water holding capacity
improved cation exchange
source of nutrients
continual release of nutrients
sorption and deactivation of
contaminants
increased biological activity
increased diversity
improved suppression of soil borne
pathogens
OC pools
OC turnover
DOC = minutes to hours
POC = years
HOC = decades
ROC = centuries
Microbes are critical to turnover OC
OC pools and influence on soil properties
Source: Krull et al. 2004
What determines soil OC?
OC Input is driven by
• Residue above ground
• Roots below ground
• Exudates from roots
• Decomposing soil biology
Influenced by
• Soil factors
• Rainfall
• Crop type
• Management
OC input
OC
loss
OC in soil
OC loss is driven by
• Mineralisation
• Erosion
• Leaching
Influenced by
• Soil texture
• Soil disturbance
• Temperature
• Form of OM residue
Factors that influence soil OC
Source: Ingram and Fernandes 2001
OC in soil
Soil has a finite capacity to protect
OM from biological attack
= capacity to bond OM
OC in soil
Free OccludedBound
Decomposition
risk = high
Decomposition
risk = low
Decomposition
risk = low
Chemical Physical
STABILISATION
SOIL CARBON SEQUESTRATION
OC Sequestration
Sequestration = Stabilisation
– of OC in soil after interactions with soil microbes, mineral and aggregates
• Turnover of soil OC is dynamic
• Human impact can turn soil OC in a net source or sink
OC increase is not linear
0
20
40
60
80
100
120
0 10 20 30 40 50 60 70 80 90 100
OC
sto
ck (
t/ha)
Time (yrs)
OC increase is not linear – equilibrium will rule
0
10
20
30
40
50
60
0 20 40 60 80 100
OC
sto
ck (
t/ha)
Time (yrs)
Microbes
equalises
OC increase
Timeframe for equilibrium to occur?
OC Sequestration
We know that soil
• 3 x more OC than above ground vegetation
• 2 x more OC than the atmosphere
• soil OC varies with depth and time
What we aren’t sure of
• the permanence of soil OC
• long term impact of change in management practices
Terrestrial Carbon Cycle
Source: Trivedi et al. 2018 Soil carbon:
introduction, importance, status threat,
and mitigation. In Soil Carbon Storage
Green are natural fluxes
Red are human contributions
Gt of C per year
1Gt = 1 billion or 109
1 t C = 3.67 t CO2 equivalents
OC Sequestration – OC stock
• Stock is the unit used in soil carbon accounting
• Soil OC stock is reported as
– t C / ha (same as Mg C / ha)
– or CO2 equivalents 1 t C / ha = 3.67 t CO2e
– in the top 30 cm of soil
To get stock need the soil bulk density (mass of soil / volume of soil)
OC stock (t/ha) = OC (%) x bulk density (g/cm3) x depth (cm) x (100-gravel %)
OC Sequestration – OC stock
• Number of samples
• Laboratory Analysis
– Total vs OC analysis – Leco vs Walkley Black
– Calcareous soils and impact on OC
• Labs report OC as % (measure of mg of C per g of soil)
0.7
0.8
0.9
1.0
1.1
1 4 7 10 13 16 19
OC
(%
)
Ave No. x samples
Can changed
agricultural
practices sequester
large amounts of
soil OC ?
How much OM would be needed to increase OC from 2 to 4%?
Assumptions:
BD = 1.2 g/cm3, OC of OM = 45%, 50% decomposition of OM additions / year
If increase OC stock from 24 to 48 t/ha over 5 years
Will be difficult to grow that much dry matter – we grow 5-10 t/ha per year
Add 50 t/ha
Yr 1
Add 20 t/ha Yr 1-5
150t/ha
by Yr 5
Factors that influence soil OC
Source: Ingram and Fernandes 2001
Effect of soil type on soil OC
Clay content of South Australia agricultural soils SSLIF and effect on OC content
0
2
4
6
8
10
3-5 5-10 10-20 20-25 25-35 35-40 40-45 45-55 50-60
Soil
OC
co
nte
t (%
)
Clay content (%)
Ave Min Max
Effect of climate on soil OC
0
1
2
3
4
5
6
250
350
450
550
750
900
250
350
450
550
750
900
250
350
450
550
750
900
250
350
450
550
750
900
250
350
450
550
750
900
250
350
450
550
750
900
250
350
450
550
750
900
0-5 5-10 10-15 15-20 20-25 25-30 30-40
OC
WB
con
tent
(%
)
Rainfall (mm) and clay concentration (%)
Rainfall – sharp increase in OC between 500-550 mm for all clay contents
Effect of soil type on soil OC
Critical OC content by clay content of South Australia agricultural soils
Texture Low Moderate High
Sand 0.5 0.5 - 1.0 1.0
Sandy loam 0.7 0.7 - 1.4 1.4
Loam 0.9 0.9 - 1.8 1.8
Clay loam/clay 1.2 1.2 - 2.0 2.0
Effect of soil type on soil OC
Soil properties – chemicals in the soil that physically bind OC (Al, Fe)
0
1
2
3
4
5
6
250 300 350 400 450 500 550 650 750 850 900
OC
WB
(%
)
Rainfall (mm)
Clay concentration 10-15 %All Ironstone
Effect of soil type on soil OC
Soil properties – pH, salinity etc that affect microbial activity
0
1
2
3
4
5
6
4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0
OC
% 0
-10
cm
pH CaCl2
Effect of soil depth on soil OC
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 10 20 30 40
OC
%
Clay content (%)
0-10 '10-20 20-30 30-50
0
10
20
30
40
50
60
70
80
0 0.2 0.4 0.6 0.8
Dep
th (
cm)
OC WB %
Effect of management on OC
From Sanderman et al. 2010 report:
• Largest soil OC gains occurs in first 5-10 years diminishing by 40 years
• Theorised best management actions;
– Large addition of organic materials (manure, green waste, compost etc)
– Maximising pasture phases in mixed cropping systems
– Shifting from annual to perennial species in permanent pastures
Sanderman J. Farquharson R. and Baldock J. (2010). Soil Carbon Sequestration Potential: A review for Australian agriculture. A report prepared for Department of Climate Change and Energy Efficiency CSIRO. http://www.csiro.au/files/files/pwiv.pdf.
Management activity SOC
Benefits aConfidence b
Production /
Profit Benefits aJustification for SOC benefits Production/profit outcome
EXISTING CROPPING / MIXED SYSTEM
Maximise water-use efficiencies through
conserving soil moisture, timely sowing, disease
management and adequate nutrition
0 / + L +++Yield and efficiency increases do not necessarily
translate to increased C return to soil
Increased production and in most cases also
profitability
Maximising nutrient-use efficiencies 0 / + L ++Yield and efficiency increases do not necessarily
translate to increased C return to soilIncreased productivity and profitability
Increased productivity through irrigation 0 / + L ++/+++Potential trade-off between increased C return to
soil and increased decomposition rates
Increased frequency of crops and pastures results
in more C returned to the soil BUT presence of
soil moisture during warmer periods increases
biological activity and hence decomposition of C
Increased productivity through fertiliser
application 0 / + L
May have slow increases in soil carbon, but
potential trade-off between increased C return to
soil and increased decomposition rates
If harvest index (the ratio of harvested grain to
total shoot dry matter) is maintained profit and
organic matter should increase
Eliminate burning/grazing stubble + M -/+Greater C return to the soil should increase SOC
stocks
Influence on production and profit may be
positive or negative. Outcome will depend on
circumstances, if high weed burden profit should
increase. Burning results in lost fertility which
could decrease yields.
Retain crop and pasture residues+ M
0/+Higher nutrient cycling capacity and biological
fertility, with greater inputs and lower erosion risk,
C should slowly increase.
Long term production increase due to greater
nutrient recycling.
Reduce tillage 0 M 0/+Reduced till has shown little SOC benefit.
Improved crop establishment on poorly
structured soils and reduced soil erosion on sandy
soil
Implement direct drilling/no-till 0 / + M +
Direct drill reduces erosion and destruction of soil
structure thus slowing decomposition rates;
however, surface residues decompose with only
minor contribution to SOC pool.
Increased moisture retention, seed placement
and more timely sowing can improve yield
potential and profit.
Change rotation to eliminate fallow and replace
with cover crop + M +
Soil carbon losses continue during fallow without
any new carbon input, implementation of cover
crops mitigate this loss
May be a short term economic cost but medium
to long term profit outcomes
Source: Sanderman 2010, Hoyle 2011 Reseigh et al 2014
Management activity SOC
Benefits aConfidence b
Production /
Profit Benefits aJustification for SOC benefits Production/profit outcome
EXISTING CROPPING / MIXED SYSTEM
Increase proportion of pasture to crops+ / ++ H
-/+Pastures generally return more C to soil than
crops due to greater root mass and/or deeper
roots
Can be dependent on stock/meat/wool prices. Positive
outcomes where good pasture establishment, weed and
erosion risks are managed.
Implement pasture cropping ++ M +Pasture cropping increases carbon return with
the benefits of perennial grasses but research is
lacking
Dependent on opportunity costs, the cost of operation
and subsequent seasonal conditions.
Addition of organic matter and other
offsite additions++ / +++ H -/++
Direct input of C, often in a more stable form,
into the soil; additional stimulation of plant
productivity
Economic outcomes likely to be constrained by
application and transport costs. Agronomic responses
vary widely, seek expert advice and undertake soil and
additive analysis.
EXISTING PASTURE SYSTEM
Increased productivity through
implementation of irrigation0 / + L ++
Potential trade-off between increased C return
to soil and increased decomposition rates
Higher biomass production can have a large on-farm
impact but costs can be high.
Increased productivity through
implementation of fertilisation0 / + L ++
Potential trade-off between increased C return
to soil and increased decomposition rates
Production of greater pasture biomass, particularly
introduced species.
Implementation of rotational grazing + L +Increased productivity, including root turnover
and incorporation of residues by trampling but
research is lacking.
Any production/profit outcomes will be dependent
implementation costs, type of enterprise and the prices
of commodities such as stock, meat and wool.
Shift to perennial species ++ M +Plants can utilise water throughout year,
increased below ground allocation but few
studies to date.
Any production/profit outcomes will be dependent on
establishment costs, enterprise type and the prices of
commodities such as stock, meat and wool.
CHANGE TO A DIFFERENT SYSTEM
Conventional to organic farming system 0 / + / ++ L -/+Likely highly variable depending on the specifics
of the organic system (i.e. manuring, cover
crops, etc…)
Viability dependent on opportunity costs, cost of
operation and subsequent seasonal conditions
Cropping to pasture system + / ++ M -/+Generally greater C return to soil in pasture
systems; will likely depend greatly upon the
specifics of the switch
Any production/profit outcomes will be dependent
enterprise type and the prices of commodities such as
stock, meat and wool.
Retirement of land and restoration of
degraded land++ / +++ H -/0
Annual production, minus natural loss, is now
returned to soil; active management to replant
native species often results in large C gains
Opportunity costs should be considered over the long-
term. Economic viability dependent on foregone
opportunity costs, carbon price and market
opportunities.
South Australian examples
SA Pasture Management Rotational Grazing Unviable cropping
land
Remediating
degraded land
Introduction /
Increase perennials
INCREASE SOC
Reduce soil erosion + ++ + ?/+
Reduce soil aggregate breakdown ++ ++
Increased duration of OM ++ ++ +++ ++
Greater above ground biomass
(productivity)? ? +++ +/++
Greater below ground biomass
(productivity)++ ++ ++
Greater below ground biomass (deeper
roots)+++ +++ +++ +++
Increased surface cover ++ ++ +++ ++
Utilise summer rains (WUE) ++ ++ +++ ++
Increased return OM to soil +
Decrease C photo-degradation +++ +
DECREASE SOC
Increased decomposition by microbes as
result of increased OM during warmer
periods with increased soil moisture
++ ++ ++ ++
Qualitative assessment of the SOC benefits and production/profit benefits of a given management practice (0 = nil, + = low, ++ = moderate, +++ = high, ? = unknownReseigh J, Wurst M, Schapel A. 2015. Perennial pasture management systems for soil carbon stocks in cereal zones. AOTGR1-0044 report
Expected changes to SOC with a change in management
SOC of SA’s Agricultural SoilsIdentified soils with the greatest potential
for increasing carbon stocks state wide
• G – sand over clay, H - deep sands
• F – deep loamy texture contrast with brown or
dark subsoil
• J – Ironstone, K2 - Shallow to moderately deep
acidic soils on rock, C – gradational soils with
highly calcareous lower subsoil
Soil Organic Carbon in South Australia ’s
Agricultural Soils
A review of the current status of and opportunities to
increase stocks of soil organic carbon in the State ’s
agricultural lands
Mary-Anne Young, David Davenport, Amanda Schapel, Brian
Hughes.
Prepared for Department of Environment, Water and Natural
Resources
March 2017 (updated from June 2015)
DEWNR Technical report
G H FOver 1100 characterisation sites from the
State Land and Soil Information
Framework were analysed
Soil photos extracted from Hall et al. 2009
The Soils of Southern South Australia
Soil OC stock of SA’s Agricultural Soils
DRAFT soil OC stock map
Created from data in previous report
DEW / Goyder project
Joint funding through DEW and
Goyder Institute
• Collated and analysed OC
from existing clay-modified
and unmodified sandy soil –
49 sites
• Correlation of climatic,
chemical and soil parameters
• Clay modification increased
OC compared to unmodified
sandy soil
Soil carbon in clay modified soils – Goyder/DEW
• Addition of subsoil clay to sands increased OC stock 4.9 tha-1
(range -1.0 to 8.2 tha-1)
350-400
mm
400-450
mm
450-500
mm> 500 mm
Clay concentration
Water storage
Nutrition
Rainfall
• Highest OC stock at > 500 mm but
unmodified also high
Greatest OC opportunity rainfall < 500 mm
0
5
10
15
20
25
30
35
350-400 400-450 450-500 >500
OC
sto
ck (
tha-
1)
Rainfall (mm)
unmodified clay-modified
Depth to subsoil
• Larger increase in OC stock with greater depth to subsoil clay
For greatest OC opportunity subsoil clay should be > 30 cm
0
5
10
15
20
25
30
35
20 30 40 60
OC
sto
ck (
t/ha)
Depth to subsoil clay (cm)
unmodified clay-modified
Kangaroo Island examples
KI - long term OC trends
OC summary from Kangaroo Island Soil Health Report 2016 show
• 1990 to 2015 average OC between 2-4%
– 97% samples > 1.5% OC
– 89% samples > 2% OC
• Trend lines demonstrate OC is increasing over time
– Result of long history of grazing with limited continuous and conventional cropping
– Current cropping practices include minimum tillage
– Hundreds with lowest OC
• Menzies, Dudley and Haines generally have lower rainfall and sandy soils
PIRSA and AGKI. Kangaroo Island Soil Health Report 2016.
KI - long term OC trends
0
1
2
3
4
5
6
7
8
1990 1995 2000 2005 2010 2015 2020
Pe
rce
nta
ge
MENZIES Organic Carbon
0
1
2
3
4
5
6
7
8
1990 1995 2000 2005 2010 2015 2020
Pe
rce
nta
ge
DUNCAN Organic Carbon
0
1
2
3
4
5
6
7
8
1990 1995 2000 2005 2010 2015 2020
Pe
rce
nta
ge
HAINES Organic Carbon
0
1
2
3
4
5
6
7
8
1990 1995 2000 2005 2010 2015 2020
Pe
rce
nta
ge
MACGILLIVRAY Organic Carbon
Menzies and
Haines are
sandier and
generally lower
rainfall
All have similar
lower OC but
there is a
difference in
upper OC
levels
KI - OC trends in kikuyu
Little change in soil OC
after 14 yrs under kikuyu
• Likely due to high
starting OC from
existing perennial
grass
• fresh OM 7x more
decomposable than
stable OM
In WA where starting OC
~ 15 t C/ha after
introduction kike OC
levels nearly doubled
after 12-15 yrs
Sanderman 2010 Can Kikuyu improve soil organic carbon levels on Kangaroo Island? KI Agricultural Trials 2010 Results
AgKI Potential Project 2013-2018
Aim to
– increase productivity and profitability
– maintain ground cover and improve soil health
• Perennial pastures
– to support a rotational grazing system, apply nitrogenous fertilisers to drive stock feed to minimise supplementary feeding
– boosted production on a range of soil types (sandy, ironstone, clay loam)
• Summer and cover crops in continuous cropping
– increase diversity and improve soil health
– lower the water table and reduce winter water logging
– success varied with soil type (sandy, clay loam)
Flanagan, G. 2019. AGKI Potential Project Finding the Production limits of Grazing and Cropping on KI 2013-2018 Final Report - DRAFT
AgKI Potential Project 2013-2018
Flanagan, G. 2019. AGKI Potential Project Finding the Production limits of Grazing and Cropping on KI 2013-2018 Final Report - DRAFT
2.59
2.03
2.59
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Control 2014 Bare 2018 Kike 2018
South Coast Sandy Topview
pH C
aCl2
OC
(%
)
OC (%) pH (CaCl2)
3.09
3.86
4.88
3.81
0.0
1.0
2.0
3.0
4.0
5.0
6.0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Cell 12014
Cell 12018
Cell 52018
Cell 5 Kike2018
Plateau Ironstone
pH C
aCl2
OC
(%
)
OC (%) pH (CaCl2)
Perennial pastures
AgKI Potential Project 2013-2018
Flanagan, G. 2019. AGKI Potential Project Finding the Production limits of Grazing and Cropping on KI 2013-2018 Final Report - DRAFT
Continuous cropping + summer crops
3.36
2.35
1.68
2.68
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Flats2014
Flats2018
Slope2014
Slope2018
North Coast Sandy
pH C
aCl2
OC
(%
)
OC (%) pH (CaCl2)
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Crop 12014
Crop 12018
Crop 22014
Crop 22018
Cygnet River Clay loampH
CaC
l2
OC
(%
)
OC (%) pH (CaCl2)
How can we build
soil OC?
How to build soil OC in the long term
• Minimise erosion
– Cultivation practices, grazing management
• Keep cover on your soil as long as possible
– Pasture – annual vs perennial species
• Maximise OM inputs to the soil
– Grow as much biomass above and below ground
– Grow roots deeper
– Address soil constraints to production – pH, compaction etc
– Nutrition targeted to production
• Ensure practices are profitable and sustainable
• Learn and adapt as new information comes along
How to build
soil health
Stirling G, Hayden H, Pattison T and Stirling M (2016). Soil
health, soil biology, soilborne diseases and sustainable
agriculture : a guide. CSIRO Publishing, Clayton VIC
Things
to
consider
Importance of Soil OC
Soil OC is important for
• food for microbes
• soil health
• plant productivity
• offsetting greenhouse gases
Is your goal high OC or high soil function?
• Are they mutually exclusive?
Profitability and sustainability
To better understand your OC results
Sample collection
• Number of samples, depth
Laboratory analysis
• How accurate can you measure
• Total vs OC analysis – Leco vs Walkley Black
• What else should be measured with OC?
– pH, salinity, other properties that may limit plant growth, microbial function
• What is included in a soil sample?
– Fine roots, stubble
Things to consider
OC % vs stock
• Bulk density, equivalent soil mass for comparison over time
How long before a stabilised change can be expected?
• 5 to 50 years
• OC change is not linear
Soil OC is a stock• paid to maintain or increase stock over time
• if you build OC and trade it you HAVE to maintain that level
• CH4 and N2O are gases - once emissions stop will get rewarded
Top Related