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Building Soil Carbon: Benefits, Possibilities, and Modeling

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Dr Jeff Baldock, from CSIRO Land & Water, is a central figure in soil carbon science in Australia. His views count because they indicate the centre of gravity in official thinking, such is his …

Dr Jeff Baldock, from CSIRO Land & Water, is a central figure in soil carbon science in Australia. His views count because they indicate the centre of gravity in official thinking, such is his influence. Jeff is a mentor and a friend of the soil carbon movement.

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  • 1. Building soil organic carbon: benefits, possibilities and modeling Jeff Baldock CSIRO Land and Water Adelaide, SA
  • 2. 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
  • 3. 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)
  • 4. 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
  • 5. 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
  • 6. 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
  • 7. 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
  • 8. 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
  • 9. 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
  • 10. 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
  • 11. 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
  • 12. 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
  • 13. 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)
  • 14. 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
  • 15. 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
  • 16. 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%
  • 17. 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
  • 18. 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
  • 19. 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
  • 20. 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
  • 21. Options need to be tailored to site conditions
  • 22. 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
  • 23. 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
  • 24. Model calibration and verification 0 350 Kilometres 700 Verification Sites Brigalow Tarlee Calibration Sites
  • 25. 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
  • 26. 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
  • 27. 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
  • 28. 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
  • 29. 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
  • 30. 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
  • 31. Frequency of addition of 5 t compost C/ha
  • 32. Take home messages
    • Carbon exists in soils in different forms
      • composition 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
  • 33. 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)
  • 34. 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
  • 35.  
  • 36. 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
  • 37. 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
  • 38. 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
  • 39. 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)
  • 40. 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
  • 41. Take home messages
    • Australian soils do have the potential to store more carbon
    • Storing more carbon in soils has benefits beyond carbon trading
    • Altering current management systems will be required to store additional carbon
      • Issues to consider
      • Production system options
      • Economics (profitability)
      • Food security
      • Implications into the future (liability and flexibility)
    • Models and calculators can be used to predict outcomes of management on soil carbon contents
  • 42.  
  • 43. 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
  • 44. 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
  • 45. 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)
  • 46. 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
  • 47. 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 (%) =
  • 48. 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
  • 49. 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
  • 50. 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
  • 51. Options need to be tailored to site conditions: the amount and distribution of rainfall
  • 52. $$ 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
  • 53. 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
  • 54. 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
  • 55. 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
  • 56. 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)
  • 57. Frequency of addition of 5 t compost C/ha
  • 58. Simulation modelling: Using RothC to predict changes to soil carbon
    • Clearing of Brigalow bushland
  • 59. 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)
  • 60. 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
  • 61. 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
  • 62. 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
  • 63. 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