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.

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

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

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

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)

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

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

21. Options need to be tailored to site conditions

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

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

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

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

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)

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

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

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

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

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

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

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