The Importance of Soil Carbon

Amanda Schapel amanda.schapel@sa.gov.au

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

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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

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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

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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

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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

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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

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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

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3

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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

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OC

%

Clay content (%)

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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

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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

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sto

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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

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1990 1995 2000 2005 2010 2015 2020

Pe

rce

nta

ge

MENZIES Organic Carbon

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DUNCAN Organic Carbon

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HAINES Organic Carbon

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1990 1995 2000 2005 2010 2015 2020

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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

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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

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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

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Flats2014

Flats2018

Slope2014

Slope2018

North Coast Sandy

pH C

aCl2

OC

(%

)

OC (%) pH (CaCl2)

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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