Building Soil Carbon: Benefits, Possibilities, and Modeling

Building soil organic carbon: benefits, possibilities and modeling Jeff Baldock CSIRO Land and Water Adelaide, SA

Take home messages

Carbon exists in soils in different forms - influences the vulnerability of soil carbon to change

Storing more carbon in soils has benefits beyond carbon trading

Altering current management systems will be required to store additional carbon

Models and calculators can be used to predict outcomes of management on soil carbon contents

Australian soils do have the potential to store more carbon

Composition of soil organic carbon Crop residues on the soil surface (SPR) Buried crop residues (>2 mm) (BPR) Particulate organic matter (2 mm – 0.05 mm) (POC) Humus (<0.05 mm) (HumC) Extent of decomposition increases Vulnerability to change decreases C/N/P ratio decreases (become nutrient rich) Dominated by charcoal with variable properties Resistant organic matter (ROC)

Variation in amount of C associated with soil organic fractions 0 5 10 15 20 25 30 1P 8P 32P NoTill (Med N) NoTill (High N) Strat (Med N) Strat (High N) 0P 11P 22P Arboretum Perm Pasture W2PF Canola/wheat Pulse/wheat Pasture/wheat Hamilton Pasture Hart Cropping Yass Pasture Urrbrae Various Waikerie Various Organic C in 0-10 cm layer (t C/ha) SPR BPR POC HumC ROC

Importance of allocating C to soil organic fractions Years Soil organic carbon (g C kg -1 soil) 0 5 10 15 20 25 30 0 10 20 30 40 50 60 70 Total soil organic C Conversion to permanent pasture 33 15 43 Humus ROC POC ~30% less humus ~800% more POC 18 y 10 y

Vulnerability of soil carbon content to variations in management practices Years Soil organic carbon (g C kg -1 soil) 0 5 10 15 20 25 30 0 10 20 30 40 50 60 70 Conversion to pasture 15 43 33 TOC Humus ROC POC Conversion to intensive cultivation 18 y 10 y 9 y 52

Predicting allocation of soil carbon to fractions using mid-infrared spectroscopy

Estimates of the amount of each type of carbon in a sample and other soil properties

1 2 3 4 5000 4500 4000 3500 3000 2500 2000 1500 1000 500 Intensity Frequency (cm -1 ) Fourier Transform Infrared Spectrum

Predicting the amount of each form of soil carbon using MIR n = 177 Range: 0.8 – 62.0 g C/kg R 2 = 0.94 n = 141 Range: 0.2 – 16.8 g C/kg R 2 = 0.71 n = 121 Range: 0.0 – 11.3 g C/kg R 2 = 0.86 Total organic carbon (mg C/g soil) 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 Measured MIR predicted Particulate organic carbon (mg C/g soil) 0 2 4 6 8 10 12 14 16 18 20 0 5 10 15 20 Measured MIR predicted Recalcitrant organic carbon (mg C/g soil) 0 2 4 6 8 10 12 0 2 4 6 8 10 12 Measured MIR predicted

Spatial variation in total oragnic carbon and charcoal carbon (0-10 cm layer) 0.00 0.40 0.80 1.20 1.60 2.00 2.40 0 25 50 75 100 Western Boundary (m) TOC 0 20 40 60 80 100 120 140 160 180 200 N o r t h e r n B o u n d a r y ( m ) 0 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 7 7 7 8 8 8 9 9 9 10 10 10 11 11 11 12 12 12 13 13 13 14 14 14 15 15 15 16 16 16 17 17 17 19 19 19 20 20 20 21 21 21 22 22 22 23 23 23 24 24 24 25 25 25 26 26 26 27 27 27 29 29 29 30 30 30 31 31 31 32 32 32 33 33 33 34 34 34 35 35 35 18 18 18 35 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0 25 50 75 100 Western Boundary (m) Charcoal C 0 20 40 60 80 100 120 140 160 180 200 N o r t h e r n B o u n d a r y ( m ) 0 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 7 7 7 8 8 8 9 9 9 10 10 10 11 11 11 12 12 12 13 13 13 14 14 14 15 15 15 16 16 16 17 17 17 19 19 19 20 20 20 21 21 21 22 22 22 23 23 23 24 24 24 25 25 25 26 26 26 27 27 27 29 29 29 30 30 30 31 31 31 32 32 32 33 33 33 34 34 34 35 35 35 18 18 18 35 W F W F P P F W P P F W P P F W P P F W Perm. Past. Contour bank W O O(g) F W O O(g) F W O O(g) F W O O(g) F B Pe W B Pe W B Pe W W P P W P P W P P W W W W P P P P P W W P P P P P W W P P P P P W W P P P P P W W P P P P P W W P P P P P W O F W O F W O F W O(g) F W O(g) F W O(g) F W Pe W Pe Perm. Past Perm. Past

Functions of organic matter in soil Functions of SOM Biological functions - energy for biological processes - reservoir of nutrients - contributes to resilience - cation exchange capacity - buffers changes in pH - complexes cations Chemical functions Physical functions - improves structural stability - influences water retention - alters soil thermal properties

Plant-available water holding capacity

How much plant-available water can a soil hold

Upper limit (wetter soil) - soil water content after drainage

Lower limit (drier soil) - soil water content at which plants can no longer extract water

Analogy of a sponge removed from a bucket of water Stops dripping Upper limit Squeeze out as much water as possible Lower limit Remove sponge from bucket

Changes in plant available soil water with clay content Amount of water (mm water/cm soil depth) 0 1 2 3 4 Sand Sandy Loam Loam Silt Loam Clay Loam Clay Increasing clay content Plant available soil water Water unavailable to plants Upper Limit Lower limit

Change in water holding capacity with a 1% increase in soil organic carbon content For 0-10 cm layer of South Australian Red-brown earths 3 mm extra stored rainfall for 10 rainfall events equates to 30 mm total or 600 kg of grain Issue: harder to build up soil carbon on a sandy soil than a clay 0 1 2 3 4 5 6 0 10 20 30 40 Clay content (% of soil mass) Change in water holding capacity (mm water)

Influence of soil organic C composition on nitrogen supply

N supply is governed by the rate of decomposition and the C/N ratio

N required = 30/10 = 3 Soil organic matter C/N=10 10 units of C 1 unit of N Wheat Residue C/N=100 100 units of C 1 unit of N 70 units of C to carbon dioxide 30 units of C 1 unit of N 2 units of N required

Influence of soil organic C composition on nitrogen supply SOM C/N=10 Medic Residue C/N= 20 70 30 5 N required = 3 +2 SOM C/N=10 Wheat Residue C/N=100 70 30 1 N required = 3 -2 SOM C/N=10 Soil Humus C/N= 10 70 30 10 N required = 3 +7

Variation in C/N ratio of different fractions of soil organic matter Min Max SPR 18.7 104.7 BPR 14.1 60.4 POC 12.8 19.6 Humus 6.0 10.1 0 20 40 60 80 100 120 SPR BPR POC Humus Type of organic matter C/N ratio (weight basis) Maximum values Minimum values 29 soils from southern Australia with total organic carbon contents ranging from 0.8% to 5.7%

Amount of nitrogen associated with soil organic matter Decrease from 3% to 1% SOC releases 2800 kg N/ha Assumption: C/N ratio = 10 0 2000 4000 6000 8000 10000 0.8 1 1.2 1.4 1.6 1.8 2 Soil bulk density (Mg soil/m 3 ) Nitrogen in the 0-10 cm layer (kg N/ha) SOC=1% SOC=2% SOC=3% SOC=4% SOC=5% 4200 kg N/ha 1400 kg N/ha

What determines the amount of carbon present in a soil?

Soil properties (clay content, mineralogy, depth)

Balance between inputs and losses

Inputs

Carbon captured by plants and added to soil

Addition of waste organic materials

Losses

Conversion of organic C to CO 2

Erosion

Evaluating potential C sequestration in soil Optimise input and reduce losses Add external sources of carbon Soil carbon sequestration situation S table soil organic carbon (e.g. t 1/2  10 years ) Attainable sequestration SOC attainable Rainfall Temperature Light Limiting factors Potential sequestration SOC potential Reactive surfaces Depth Bulk density Defining factors Actual sequestration SOC actual Soil management Plant species/crop selection Residue management Soil and nutrient losses Inefficient water and nutrient use Disrupted biology/disease Reducing factors

Increasing the capture of carbon in soils

Maintain current production system

Maximise resource use efficiency (e.g. carbon capture per mm water or per kg nutrient)

For dryland systems – starts with water use efficiency

Maximise stubble retention (carbon return)

Shift to alternative production systems

Introduction of perennial vegetation where appropriate

Alternative crops - lower harvest index

Alternative pasture species – increased below ground allocati on

Increased use of green manures

Options

Options need to be tailored to site conditions

Soil carbon models and calculators

Range of different forms

Carbon balance calculations

Spreadsheet based calculators (usually empirical)

System simulation models (more mechanistic)

Provide estimates of what may be possible under a defined set of conditions

Climatic conditions, soil properties, crop/pasture production

Only as good as the data used to perform calculations or validate the model

Modelling soil organic carbon – RothC model RPM = POC IOM = ROC HUM = TOC – (POC + Char C) DPM RPM Plant Inputs BIO HUM CO 2 Decomposition Decomposition BIO HUM CO 2 Decomposition IOM Fire

Model calibration and verification 0 350 Kilometres 700 Verification Sites Brigalow Tarlee Calibration Sites

Calibration of RothC to Australian conditions

Clearing of Brigalow bushland

0 10 20 30 40 50 60 70 1982 1987 1992 1997 Year C (t/ha) RPM HUM IOM TOC TOC HUM CHAR POC Measured fractions Modelled fractions 0 10 20 30 40 50 60 70 1982 1987 1992 1997 Year C (t/ha) RPM HUM IOM TOC RPM RPM HUM HUM IOM IOM TOC TOC TOC HUM CHAR POC TOC TOC HUM HUM CHAR CHAR POC POC Measured fractions Modelled fractions

Model Verification: (sites with archived soil samples) Tamworth – wheat/fallow Wagga – wheat/pasture 0 10 20 30 40 50 1970 1980 1990 2000 Year Soil C (t/ha) 0 20 40 60 1988 1990 1992 1994 1996 1998 Year Soil C (t/ha) Salmon Gums – wheat/wheat 0 10 20 30 40 50 1979 1983 1987 1991 Year Soil C (t/ha) Salmon Gums - wheat/ 3 pasture Year Soil C (t/ha) 0 10 20 30 40 50 1979 1983 1987 1991 DPM RPM HUM IOM BIO Soil Modeled POC HUM CHAR TOC Measured

Model verification: (paired sites)

Is this result due poor model performance or poor pairing of the sites?

Did the sites start off similar or are there significant shifts in soil/plant/environmental properties between paired individuals?

Kindon - pasture 15 y 0 10 20 30 40 50 Year Soil C (t/ha) 1986 1991 1996 2001 Dunkerry South - crop 0 10 20 30 1967 1977 1987 1997 Year Soil C (t/ha) DPM RPM HUM IOM BIO Soil Modeled POC HUM CHAR TOC Measured

Influence of altering the water use efficiency of wheat at Gawler, SA 20 year change in carbon WUE tC/ha 0.50 0 0.75 12.1 1.00 24.1

Influence of altering the harvest index of wheat at Gawler, SA 20 year change in carbon Harvest index tC/ha 0.25 12.6 0.35 0 0.45 -7.0

Influence of altering the root shoot ratio of wheat at Gawler, SA 20 year change in carbon R/S ratio tC/ha 0.50 0 0.75 5.2 1.00 10.5

Frequency of addition of 5 t compost C/ha

Take home messages

Carbon exists in soils in different forms

composition influences the vulnerability of soil carbon to change

Take home messages

Decision to enter a carbon trading scheme will required consideration of the following issues:

production system options,

economics (profitability),

food security,

implications into the future (liability and flexibility)

Thank you CSIRO Land and Water Jeff Baldock Research Scientist Phone: +61 8 8303 8537 Email: jeff.baldock@csiro.au Web: http://www.clw.csiro.au/staff/BaldockJ/ Acknowledgements Jan Skjemstad, Kris Broos, Evelyn Krull Steve Szarvas, Leonie Spouncer, Athina Massis Contact Us Phone: 1300 363 400 or +61 3 9545 2176 Email: Enquiries@csiro.au Web: www.csiro.au

Dynamic nature of SOC and its fractions Irrigated Kikuyu pasture – Waite rotation trial 0 8 16 24 32 1/6/98 6/2/99 14/10/99 20/6/00 25/2/01 Date of sample collection Amount of organic C (Mg C ha -1 in 0-10 cm) POC Humus ROC TOC

Correcting soil carbon for management induced changes in bulk density Original soil surface Mass Soil 0-30 cm (Mg/ha) 3300 3600 3900 4200 Depth for equivalent mass (cm) 30.0 27.5 25.4 23.6 Organic C loading (Mg/ha) 1% OC, no BD correction 33 36 39 42 1% OC, with BD correction 33 33 33 33 Soil bulk density (Mg/m 3 ) 1.1 1.2 1.3 1.4 Management induced compaction Original 30 cm depth New 30 cm depth

Influence of tillage on changes in soil carbon with depth If red region > blue region = sequestration For 0-10 cm layer red region > blue region (sequestration) For 0-30 cm layer red region = blue region (no sequestation) Cultivated to 10 cm Uncultivated Organic carbon content (% soil mass) Soil depth (cm) 0.0 0.5 1.0 1.5 2.0 2.5 0 10 20 30 40 50 60 70 80

The carbon cycle in agricultural systems: where do options exist for sequestration CO 2 Plant carbon Photosynthesis Decomposition Soil carbon Death and addition of residues to soil Agricultural products Harvest Long lived products (biochar) Short lived products (grains, meat)

Potential for soils to sequester C

Potential for sequestration of C in soil

Global SOC pool size: 1500 Pg

Rapid cycling SOC: 500-750 Pg

1% increase in stored SOC/yr: 5 - 7.5 Pg/yr

Anthropogenic CO 2 -C emissions: 8 Pg/yr

Issues

Native unmanaged soils

Variations in soil properties

Permanency of increase

Constraints on C inputs to soil (biophysical, economic, social)

0 cm 10 cm 30 cm

Take home messages

Issues to consider

Production system options

Economics (profitability)

Food security

Implications into the future (liability and flexibility)

Quantifying SOC allocation of SOC to fractions Total soil organic carbon Humus = <53µm - Recalcitrant Recalcitrant Charcoal C Humus + recalcitrant HF treatment, UV-PO, & NMR <53 µm fraction >53 µm fraction Na saturate, disperse, sieve <53 µm Density fractionation Buried plant residue carbon Soil sieved to <2mm Soil sieved to >2mm Surface plant residue carbon Quadrat collection Particulate organic carbon Density fractionation

Take home messages

Atmospheric carbon can be captured by increasing the size of long lived forms of terrestrial carbon

Carbon capture by forests

without harvesting and storage in long lived products, carbon capture can only be counted once

potential exists to create continuous capture and storage

Soils do have a potential to capture and store carbon; however, issues exist that may limit carbon storage opportunities

biophysical constraints on production (rainfall, nutrient, etc)

economic situation of the farm business

food security

Composition of soil organic carbon Crop residues on the soil surface (SPR) Buried crop residues (>2 mm) (BPR) Particulate organic matter (2 mm – 0.05 mm) (POC) Humus (<0.05 mm) (HumC) Extent of decomposition increases Rate of decomposition decreases C/N/P ratio decreases (become nutrient rich) Dominated by charcoal with variable properties Resistant organic matter (ROC)

Balance between inputs and outputs Years Soil organic carbon (g C kg -1 soil) 0 5 10 15 20 25 30 0 20 40 60 80 100 120 140 Inputs = Outputs Inputs x 2 Inputs x 3 Inputs / 2 Inputs / 3

Minimum requirements for tracking soil organic carbon for accounting purposes

Collection of a representative soil sample to a minimum depth of 30 cm

An accurate estimate of the bulk density of the sample

An accurate measure of the organic carbon content of a soil sample

For 0-30 cm soil with a bulk density of 1.0 Mg/m 3 and a carbon content of 1.0% = Mass of Carbon (Mg C/ha) Depth (cm) 30 Mg C/ha x Bulk density (g/cm 3 ) x Carbon content (%) =

Importance of defining composition of organic N on mineralisation Amount of N present (kg N/ha) Fraction (C/N ratio) Residues/Particulate (50) Humus (10) Inert/char (50) Total Soil 1 300 2100 200 2600 Soil 2 500 1300 800 2600 Portion that decomposes 0.3 0.1 0.001 Amount of N mineralised (kg N/ha) Residues/Particulate Humus Inert/char Total Soil 1 - 45 147 0 102 Soil 2 - 75 91 0 16

Requirements to increase soil carbon: the nutrient perspective C/N=10 C/P=120 2400 kg N/ha 4800 kg N/ha 200 kg P/ha 400 kg P/ha

Options for increasing soil carbon content

Principal: increase inputs of carbon to the soil

Maximise capture of CO 2 by photosynthesis and addition of carbon to soil

Options

Maximise water use efficiency (kg total dry matter/mm water)

Maximise stubble retention

Introduction of perennial vegetation where appropriate (afforestation, pastures, native vegetation)

Alternative crops - lower harvest index

Alternative pasture species – increased below ground allocation

Green manure crops – legume based for N supply

Addition of offsite organic materials – diversion of waste streams

Options need to be tailored to site conditions: the amount and distribution of rainfall

$$ for C sequestration – fact or fiction

There is no doubt that soils could hold more carbon

Challenge – increase soil C while maintaining economic viability

Options do exist but they must be tailored to soil and climatic conditions

Under current C trading prices

Difficult to justify managing for soil C on the basis of C trading alone

Do it for all the other benefits enhanced soil carbon gives

Careful consideration of liabilities and possible future restrictions in management options is required

Influence of tillage and stubble on soil carbon 0 10 20 30 40 50 60 70 Soil type Carbon in 0 - 30cm soil layer (t C/ha) Kandosol (n=106) Sodosol (n=63) Vertosol (n=226) Reduced Tillage (Stubble burnt, baled or retained) Chromosol (n=119) Traditional Tillage (Stubble burn or removed) Traditional Tillage (Stubble retained or burnt late) Direct Drill (Stubble retained) Pasture/Native

Influence of tillage systems on soil carbon contained in the 0-30 cm layer 0 10 20 30 40 50 60 Chromosol Kandosol Vertisol Sodosol Soil Type Amount of C in 0-30 cm soil layer (t C/ha) Tilled - stubble Tilled + stubble or late burn Reduced tillage Direct drill

The carbon cycle: adding compost to soil CO 2 Plant production Photosynthesis Respiration Soil animals and microbes Death Residues Particulate organic C Humus organic C Harvested products Harvest Respiration Death Green wastes, manures and composts

Rate of annual compost addition 0 50 100 150 200 250 300 350 0 100 200 300 400 500 Years since start of simulation Total organic carbon in 0-30 cm soil layer (t C/ha) 0.0 0.5 1.0 2.0 3.0 5.0 10.0 Rate of compost C addition (t C/ha/y)

Frequency of addition of 5 t compost C/ha

Simulation modelling: Using RothC to predict changes to soil carbon

Clearing of Brigalow bushland

Influence of pasture production on soil carbon at Bairnsdale Pasture lost = 50% Root/shoot ratio = 1 Pasture grows from March to November Increase pasture growth from 6 to 8 t dm/ha gives an additional 9.4 t C/ha in 25 years (10t dm/ha gives 19 t C/ha)

Changes in soil C for different levels of average grain yield (Roseworthy, SA) Shift yield from 4 to 8 T grain/ha = 1.0 %C increase over 20 years Shift yield from 4 to 6 T grain/ha = 0.4 %C increase over 20 years

Significance of carbon in soils Annual fluxes (10 15 g C/yr)

Emissions

Fossil fuel burning 6

Land use change 2

Responses

Atmospheric increase 3

Oceanic uptake 2

Other 3

World wide C pools (10 15 g C)

Atmosphere (CO 2 C) 780

Living Biomass (plants, animals) 550

Soil

0-1 m depth 1500

0-3 m depth  2300

Houghton (2005)

1330

Distribution and turnover of organic carbon in soil 0 cm 10 cm 30 cm 100 cm SOC content High Low Very low Proportion of profile SOC 30-50% 20-30% 10-30% Relative response time Rapid Intermediate to slow Slow

Impact of subsoil constraints 0 200 400 600 800 1000 0.00 0.05 0.10 0.15 0.20 Soil depth (mm) Volumetric Water Content (cm 3 cm -3 ) Euston Plant-available water (no constraints) = 97 mm Plant-available water (with constraints) = 59 mm Upper Limit Lower Limit Lower limit with subsoil constraints