Soil carbon in a carbon accounting framework: concepts and calculators

Slide 1: Soil carbon in a carbon accounting framework: concepts and calculators Jeff Baldock CSIRO Land and Water Adelaide, SA

Slide 2: Take home messages • Soil organic matter provides many benefits to soil • Different soils can hold different amounts of carbon • Soil carbon represents the balance between additions and losses • Soil carbon is composed of a variety of materials • Understanding soil carbon composition allows more accurate assessment of management impacts • Measuring changes in soil carbon requires careful consideration • Computer models exist to predict the impact of management on soil carbon • Options to improve soil carbon and productivity need to be tailored to local conditions

Slide 3: Functions of organic matter in soil Biological functions - energy for biological processes - reservoir of nutrients - contributes to resilience Functions of SOM Physical functions Chemical functions - improves structural stability - cation exchange capacity - influences water retention - buffers changes in pH - alters soil thermal properties - complexes cations

Slide 4: Distribution and turnover of organic carbon in soil SOC Proportion of Relative content profile SOC response time 0 cm High 30-50% Rapid 10 cm Low 20-30% Intermediate to slow 30 cm Very 10-30% Slow low 100 cm

Slide 5: Variation in soil organic carbon with depth for different soils Soil organic carbon content (% by weight) 01 01 01 0123 0 2 4 6 0 50 Soil Depth (cm) 100 150 200 Red Grey Red Black brown Krazonzems clays earths earths earths

Slide 6: What determines soil organic carbon content? Soil organic carbon Inputs of Losses of f = , content organic carbon organic carbon Inputs Throttles or • Net primary rate determinants productivity • Addition of off Losses site organic • Conversion of material organic C to CO2 by decomposition

Slide 7: Influence of the balance between inputs and outputs 30 25 Soil organic carbon Inputs >> Outputs (g C kg-1 soil) 20 Inputs > Outputs 15 Inputs = Outputs 10 Inputs < Outputs 5 Inputs << Outputs 0 20 40 60 100 0 80 140 120 Years

Slide 8: Changes in total soil organic carbon with time Initiate wheat/fallow 30 Total soil organic C Conversion to 25 Soil organic carbon permanent pasture (g C kg-1 soil) 20 15 10 5 10 y 18 y 0 10 20 30 50 0 40 70 60 15 33 43 Years

Slide 9: Biologically significant fractions of soil organic matter • Crop residues on the soil surface (SPR) Extent of decomposition • Buried crop residues increases (>2 mm) (BPR) C/N/P ratio decreases • Particulate organic matter (become nutrient rich) (2 mm – 0.05 mm) (POC) • Humus (<0.05 mm) (HumC) Dominated by charcoal • Resistant organic matter with variable properties (ROC)

Slide 10: Identification of biologically significant soil organic fractions Particulate material Humus Charcoal (POC) (HumC) (ROC)

Slide 11: Soil carbon cycle Photosynthesis CO2 Mineralisation Death/Harvest Plant production Burning Plant residues Recalcitrant Particulate Soil animals Increasing organic C organic C and microbes extent of (ROC) decomposition Humus organic C Carbon sequestration options 1) move more carbon into the recalcitrant pool 2) increase C stored in plants – e.g. grow a forest 3) increase C stored in one or all soil components

Slide 12: Importance of quantifying allocation of C to soil organic fractions Soil 2 Soil 1 20 g SOC kg-1 soil 20 g SOC kg-1 soil 10 g Char-C kg-1soil 2.5 g Char-C kg-1soil 25 25 Soil Organic Carbon Soil Organic Carbon 20 20 (g C kg-1 soil) (g C kg-1 soil) 15 15 Active C 10 10 Active C 5 5 Inert C Inert C 0 0 Time Time

Slide 13: Importance of allocating C to soil organic fractions Initiate wheat/fallow 30 TOC Humus C Conversion to 25 POC Soil organic carbon permanent ROC pasture (g C kg-1 soil) 20 15 ~30% less humus C 10 5 10 y 18 y ~800% more POC 0 10 20 30 50 0 40 70 60 15 33 43 Years

Slide 14: Vulnerability of soil carbon content to variations in management practices Initiate wheat/fallow 30 TOC Conversion to Humus 25 wheat/fallo POC Soil organic carbon Conversion w ROC (g C kg-1 soil) to pasture 20 15 10 5 10 y 18 y 9y 0 10 20 30 50 0 40 70 60 43 52 15 33 Years

Slide 15: Variation in amount of C associated with soil organic fractions 25 Organic carbon in 0-10 cm layer Surface plant residues 20 (SPR) (Mg C/ha) Buried plant residues 15 (BPR) Particulate organic matter 10 (POM) Humus 5 Recalcitrant (ROM - charcoal) 0 Average for Hamilton (long term pasture)

Slide 16: Organic C in 0-10 cm layer (Mg C/ha) 0 5 10 15 20 25 30 1P 8P Pasture Hamilton 32P NoTill (MedN) organic fractions NoTill (HighN) Hart Strat (MedN) Cropped Strat (HighN) 0P 125P Yass Pasture 250P Arboretum Perm Pasture Mix Urrbrae W2PF Canola/wheat Variation in amount of C associated with soil Pulse/wheat Mix BPR SPR POC ROC HumC Waikerie Pasture/wheat

Slide 17: Minimum requirements for tracking soil organic carbon for accounting purposes 1. Collection of a representative soil sample to a minimum depth of 30 cm 2. An accurate estimate of the bulk density of the sample 3. An accurate measure of the organic carbon content of a soil sample For 0-10 cm soil with a bulk density of 1.0 Mg/m3 and a carbon content of 1.0% Bulk Mass of Carbon Depth x density x content = 10 Mg C/ha Carbon = (cm) (g/cm3) (Mg C/ha) (%)

Slide 18: Understanding the requirements for changing soil carbon content Amount of carbon in the 0-10 cm layer 90 80 70 Amount of C required: 1% SOC (Mg C/ ha) 60 24 Mg C 2% SOC 48 50 50 Mg Dry Matter (DM) 3% SOC 40 4% SOC 30 Rate per year: 5% SOC 24 10 Mg DM/y (no loss) 20 20 Mg DM/y (50% loss) 10 10 0 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Bulk density (g/cm3)

Slide 19: Understanding the requirements for changing soil carbon content Amount of C required: 14 Mg C Amount of carbon in the 0-10 cm layer 28 Mg Dry Matter (DM) 90 80 Rate per year: 70 5.6 Mg DM/y (no loss) 1% SOC 60 11.2 Mg DM/y (50% loss) 2% SOC (Mg C/ha) 50 3% SOC 40 4% SOC 28 30 5% SOC 20 14 10 0 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Bulk density (g/cm3)

Slide 20: Correcting soil carbon for management induced changes in bulk density Management induced compaction Original soil surface Original 30 cm depth New 30 cm depth Soil bulk density (Mg/m3) 1.1 1.2 1.3 1.4 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

Slide 21: Dynamic nature of SOC and its fractions Irrigated Kikuyu pasture – Waite rotation trial 32 TOC POC Humus IOC Amount of organic C 24 (Mg C ha-1) 16 8 0 1/6/98 6/2/99 14/10/99 20/6/00 25/2/01 Date of sample collection

Slide 22: Modelling soil organic carbon content • Defining the influence of management practices on soil organic carbon is difficult • Different types of organic C respond at different rates • POC - years to decades • Humus – decades to centuries • Charcoal –centuries to millenia • Other factors may be more influential in some years than management (e.g. rainfall) • Spatial variability and within year temporal variability • Use of computer simulation models offers a way to estimate likely outcomes quickly • How valid are the results?

Slide 23: Soil organic carbon models Complex simulation models • FullCAM – Australian greenhouse office model • APSIM – plant production simulator with soil C • Century – organic C and N cycling (USA) • RothC – organic C (UK) • Soil Carbon Manager – organic C (Australia) Simple calculators • Typically based on output from the more complex simulation models • Provides a more simplified output • More limited in adaptability

Slide 24: Modelling measurable SOC fractions RothC Model DPM Plant CO2 Decomposition Inputs RPM BIO CO2 Decomposition HUM BIO Decomposition IOM Fire HUM RPM = POC IOM = (Char C) HUM = TOC – (POC + Char C)

Slide 25: Predicting soil organic carbon contents • Clearing of Brigalow bushland Measured fractions 70 TOC POC 60 HUM 50 C (t/ha) CHAR 40 Modelled fractions 30 TOC 20 RPM 10 HUM 0 IOM 1982 1987 1992 1997 Year

Slide 26: Model Verification: (sites with archived soil samples) Wagga – wheat/pasture Tamworth – wheat/fallow 60 50 Soil C (t/ha) Soil C (t/ha) 40 Measured 40 30 POC 20 20 HUM 10 CHAR 0 0 1988 1990 1992 1994 1996 1998 1970 1980 1990 2000 TOC Year Year Modeled DPM Salmon Gums - wheat/ 3 pasture Salmon Gums – wheat/wheat RPM 50 50 Soil C (t/ha) Soil C (t/ha) 40 40 HUM 30 30 IOM 20 20 BIO 10 10 0 0 Soil 1979 1983 1987 1991 1979 1983 1987 1991 Year Year

Slide 27: Model verification: (paired sites) Kindon - pasture 15 y Dunkerry South - crop 50 30 Soil C (t/ha) 40 Soil C (t/ha) 20 30 20 10 10 0 0 1986 1991 1996 2001 1967 1977 1987 1997 Year Year Measured Modeled POC CHAR DPM HUM BIO HUM RPM IOM Soil TOC • 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?

Slide 28: Defining soil C dynamics at Cowra, NSW under continuous wheat production Average growing season (Apr-Oct) rainfall (mm) 358 Average summer rainfall (mm) 255 Percentage of summer rain 42% Water limited potential grain yield (Mg/ha) 3.58 Grain yield used (Mg/ha) 3.3 Harvest index (Mg grain/Mg dry matter) 0.45 Dry matter production (Mg/ha) 7.33 Equilibrium conditions (model for 500 years) Amount of C in 0-30cm C content of 0-10 cm Type of C layer (Mg C/ha) layer (%) Particulate 17.9 Humus 44.3 Recalcitrant 2.20 Total 64.4 2.30

Slide 29: Estimates of organic carbon in the 0-30 cm layer under wheat at Cowra, NSW 200 Average 180 wheat grain Amount of soil organic carbon yield (Mg C/ha for 0-30 cm layer) 160 0.5 Mg/ha 140 1 Mg/ha 2 Mg/ha 120 3 Mg/ha 100 4 Mg/ha 80 6 Mg/ha 8 Mg/ha 60 10 Mg/ha 40 20 0 0 100 200 300 400 500 Years since start of simulation

Slide 30: Estimates of organic carbon in the 0-30 cm layer under wheat at Cowra, NSW 175 Shift yield from 3 to 6 Mg grain/ha = 22 Mg C/ha over 25 yearsAverage Shift yield from 3 to 4 Mg grain/ha = 7 Mg C/ha over 25 yearswheat grain Amount of soil organic carbon 150 yield (Mg C/ha for 0-30 cm layer) 0.5 Mg/ha 125 1 Mg/ha 2 Mg/ha 100 3 Mg/ha 4 Mg/ha 75 6 Mg/ha 8 Mg/ha 50 10 Mg/ha 25 0 0 5 10 15 20 Years since start of simulation

Slide 31: Defining soil C dynamics at Yass, NSW under permanent pasture Average pasture shoot dry matter production (Mg dm/ha) 5.0 Allocation of pasture dry matter Shoots 0.60 Roots 0.40 Fate of pasture shoot dry matter Consumed by animals 0.50 Retained and returned 0.50 Proportion of consumed dry matter Used by animal (wool, wt gain, respiration) 0.67 Excreted as faeces and urine 0.33 Net proportion of shoot residues Removed from the paddock 0.335 Returned to the paddock 0.665

Slide 32: Estimates of organic carbon in the 0-30 cm layer under pasture at Yass, NSW 300 Shoot dry matter Amount of soil carbon carbon 250 production (Mg C/ha for 0-30 cm layer) 2 Mg/ha 200 4 Mg/ha 6 Mg/ha 150 8 Mg/ha 10 Mg/ha 15 Mg/ha 100 20 Mg/ha 50 0 0 100 200 300 400 500 Years since start of simulation

Slide 33: Estimates of organic carbon in the 0-30 cm layer under pasture at Yass, NSW 150 Shoot dry matter Amount of soil carbon carbon 125 production (Mg C/ha for 0-30 cm layer) 2 Mg/ha 100 4 Mg/ha 6 Mg/ha 75 8 Mg/ha 10 Mg/ha 15 Mg/ha 50 20 Mg/ha 25 0 0 5 10 15 20 Years since start of simulation

Slide 34: Defining inputs of organic carbon to soil – dryland conditions • Availability of water – amount and distribution of rainfall imposes constraints on productivity and options Beverly, WA Roseworthy, SA Mudgee, NSW 90 300 90 300 90 300 Average monthly pan evaporation (mm) 75 250 75 250 75 250 Average monthly rainfall (mm) 60 200 60 200 60 200 45 150 45 150 45 150 30 100 30 100 30 100 15 50 15 50 15 50 0 0 0 0 0 0 May May Mar Nov May Mar Mar Nov Nov Jan Jul Sep Jan Sep Jan Jul Sep Jul Month of(mm) year Rain the Month of(mm) year Rain the Month of(mm) year Rain the Pan Evaporation (mm) Pan Evaporation (mm) Pan Evaporation (mm)

Slide 35: $$ 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 • Perennial vegetation • Regions with summer rainfall • Portions of paddocks that give negative returns • Reduce stocking, rotational grazing, green manure • Optimise farm management to achieve 100% of water limited potential yield • 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

Slide 36: Evaluating potential C sequestration in soil Reactive surfaces Defining Potential Depth factors sequestration Bulk density Soil carbon sequestration situation Rainfall Attainable Limiting Temperature sequestration factors Light Soil management Plant species/crop selection Actual Reducing Residue management Soil and nutrient losses sequestration factors Inefficient water and nutrient use Disrupted biology/disease Optimise input and reduce losses Add external sources of carbon SOCactual SOCpotential SOCattainable Stable soil organic carbon (e.g. t1/2  10 years)

Slide 37: Take home messages • Soil organic matter provides many benefits to soil • Different soils can hold different amounts of carbon • Soil carbon represents the balance between additions and losses • Soil carbon is composed of a variety of materials • Understanding soil carbon composition allows more accurate assessment of management impacts • Measuring changes in soil carbon requires careful consideration • Computer models exist to predict the impact of management on soil carbon • Options to improve soil carbon and productivity need to be tailored to local conditions

Slide 38: 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 Thank you Contact Us Phone: 1300 363 400 or +61 3 9545 2176 Email: Enquiries@csiro.au Web: www.csiro.au

Slide 40: Morphology of charcoal found in soil

Slide 41: Summary • Soils represent a significant global pool of carbon • The organic carbon content achieved is determined by the balance between inputs and losses • Defining the composition of soil organic matter allows enhanced understanding of C and N dynamics (P?) • Soils have a finite ability to build organic carbon • Optimising plant productivity will maximise inputs and soil organic carbon content • External sources of organic matter can enhance soil organic carbon – but continued addition is required

Slide 42: Defining inputs of organic carbon to soil – dryland conditions • Availability of water – amount and distribution of rainfall imposes constraints on productivity and options Beverly, WA Roseworthy, SA Mudgee, NSW 90 30 90 30 90 30 Average monthly temperature (°C) 75 25 75 25 75 25 Average monthly rainfall (mm) 60 20 60 20 60 20 45 15 45 15 45 15 30 10 30 10 30 10 15 5 15 5 15 5 0 0 0 0 0 0 May May May Mar Mar Nov Nov Mar Nov Jan Jul Sep Jan Sep Sep Jan Jul Jul Month of the(mm) Rain year Month of the(mm) Rain year Month ofRain (mm) the year Temperature Temperature Temperature

Slide 43: Rates of

Slide 44: Soil Carbon Calculators FullCAM - model built for prediction of greenhouse gas emissions APSIM - agricultural production simulator with C and N cycling model Century – carbon and nitrogen cycling model RothC – Carbon cycling model

Slide 45: Minimum requirements for tracking soil organic carbon for accounting purposes 1. Collection of a representative soil sample to a minimum depth of 30 cm 2. An accurate estimate of the bulk density of the sample 3. An accurate measure of the organic carbon content of a soil sample

Slide 46: Defining the amount of carbon present in a soil Volume Soil of Soil = Depth x 100 100 m (m3/ha) (cm) 100 m depth 1 hectare Mass Volume Bulk of soil block of soil of Soil = of soil x Density layer (Mg/ha) (m3/ha) (Mg/m3) Mass of Mass Carbon Carbon = of soil x Content x 0.01 (Mg C/ha) (Mg/ha) (%) For 0-10 cm soil with a bulk density of 1.0 Mg/m3 and a carbon content of 1.0% Volume Carbon content Mass of Carbon = (10 *100) x (1.0) x (1.0 x 0.01) = 10 Mg C/ha (Mg C/ha) Bulk density

Slide 47: Dynamic nature of SOC and its fractions Dryland Pasture/Wheat/Wheat – Waite rotation trial 36 TOC POC Humus IOC 32 Amount of organic C 28 24 (Mg C ha-1) 20 16 12 8 4 0 1/6/98 6/2/99 14/10/99 20/6/00 25/2/01 Date of sample collection

Slide 48: Nutrients associated with soil carbon Assumptions: C/N =10 and C/P=120) 1800 140 BD = 1.0 BD = 1.0 1600 120 BD = 1.2 BD = 1.2 Amount of P (kg/ha) 1400 Amount of N (kg/ha) BD = 1.4 BD = 1.4 100 BD = 1.6 BD = 1.6 1200 80 1000 800 60 600 40 400 20 200 0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Change in soil carbon Change in soil carbon (% of soil mass) (% of soil mass)

Slide 49: Variation in C/N ratio of different fractions of soil organic matter 120 Upper boundry 100 Lower boundry (weight basis) 80 C/N ratio 60 40 20 0 SPR BPR POM Humus Type of organic matter

Slide 50: Impact of climatic and tillage Average Average rainfall (mm) Temperature °C <661.5 661.5 <12 12 <17 17 117 567 <567 DD TT RT TD TT <655 655 43 57 <459 459 28 TD 79 RT 40 30 DD 34 DD 41 42 103 TT RT TD 35 46

Slide 51: Chemical function: Cation exchange capacity 600 Cation exchange capacity 500 (meq/100g C) 400 300 200 POM Humus 100 Recalitrant 0 4 5 6 7 8 9 Soil pH

Slide 52: Questions remaining – from an organic matter perspective • What is the capacity of soils to store organic matter (carbon and nutrients)? • How much of the carbon and nutrients stored in soil organic matter can be made available to microbes and plants? • What are the potential effects of alternative and new management options on organic matter levels? • Further quantification of the role of soil organic fractions is required to extend the range of soil types and environments examined. • What is the role of external sources of organic matter and do their influences persist?

Slide 53: Potential mineralisation of C from soil organic fractions 70 200 Whole soil 60 POM Cummulative CO2-C emission Cummulative CO 2-C emission ROM + ROM Humus 150 50 (mg CO2-C/g C) (mg CO2-C/g C) 40 100 30 20 50 Humus + ROM ROM 10 Humus HF ROM + + ROM + HF 0 0 0 20 40 60 80 0 20 40 60 80 Incubation time (days) Incubation time (days)

Slide 54: Effect of time after clearing 55 50 c) cleared 1910 45 d) cleared 1950 e d 40 e) cleared 1970 35 c C (t/ha) 30 All with reduced 25 stubble burning 20 & 15 increasing yield 10 5 0 1900 1920 1940 1960 1980 2000 2020 2040 2060 2080 2100 Years

Slide 55: Significance of carbon in soils •World wide C pools (1015 g C) • Soil 1500 • Atmosphere (CO2) 720 • Living Biomass (plants, animals) 560 Soil in Australia 30 World fluxes (1015 g C/year) Fossil Vegetation Missing Atmospheric Ocean Fuel + Destruction = + Uptake + Sink Increase 5 1.8 2.2 3 1.6 0.1% increase in soil organic C = 1.5

Slide 56: Organic matter fractionation Organic matter on and in soil Surface Soil sample plant residues >2mm soil <2mm soil (SPR) Buried Particulate organic <53 µm plant matter >53 µm (POM) residues HF treated (BPR) <53 µm Humus & ROM ROM

Slide 57: Variation in amount of N associated with soil organic fractions 3.5 Organic N in 0-10 cm layer SPR 3.0 BPR 2.5 POM (Mg N/ha) Humus 2.0 ROM 1.5 1.0 0.5 0 0P 32P 1P 8P 11P 22P W2PF Perm Pasture NoTill (HighN) Arboretum NoTill (MedN) Strat (MedN) Strat (HighN) Pulse/wheat Canola/wheat Pasture/wheat Hamilton Hart Yass Urrbrae Waikerie Pasture Cropped Pasture Mixed Mixed

Slide 58: Variation in C/N ratios across soils and management practices 120 Upper boundry 100 Lower boundry (weight basis) 80 C/N ratio 60 40 20 0 SPR BPR POM Humus Type of organic matter

Slide 59: Influence of C/N ratio on the mineralisation of N 70 units of C to carbon dioxide Soil 100 units 10 units 30 units of C Wheat N required organic of C of C Residue = 30/10 matter 1 unit of N C/N=100 =3 C/N=10 1 unit 1 unit of N of N 2 units of N required 70 units of C to carbon dioxide Soil 100 units 10 units 30 units of C Pasture N required organic of C of C legume = 30/10 matter 5 units of N C/N=20 =3 C/N=10 5 units 1 unit of N of N 2 units of N released

Slide 60: N transfer from residue decomposition Assumptions: C:N ratio of the soil is 12 and 70% of the stubble C is converted to carbon dioxide Amount of plant- Amount of Previous Crop C:N ratio of available N Stubble (kg/ha) stubble released (kg/ha) (kg N/ha) Wheat 6000 90 -33 Faba beans 4000 40 0 Medic pasture 3000 25 18 Represents 20% Previous research by Ladd et al. found that of medic residue 11 to 25% of legume residue N was found N returned to soil in the next cereal crop

Slide 61: Importance of defining composition of organic N on mineralisation Amount of N present (kg N/ha) Fraction Residues/Particulate Humus Inert/char Total (C/N ratio) (50) (10) (50) Soil 1 300 2100 200 2600 Soil 2 500 1300 800 2600 Portion that 0.3 0.1 0.001 decomposes Amount of N mineralised (kg N/ha) Residues/Particulate Humus Inert/char Total Soil 1 - 45 147 0 102 - 75 91 0 16 Soil 2

Slide 62: Significance of carbon in soils •World wide C pools (1015 g C) • Atmosphere (CO2-C) 780 1330 • Living Biomass (plants, animals) 550 • Soil 0-1 m depth 1500 2300 0-3 m depth Houghton (2005) •Annual fluxes (1015 g C/yr) •Emissions •Responses • Fossil fuel burning • Atmospheric increase 3 6 • Land use change • Oceanic uptake 2 2 • Other 3

Slide 63: Potential for soils to sequester C • SOC pool size: 1500 Pg 0 cm • Rapid cycling SOC: 500-750 Pg 10 cm • 1% increase in stored SOC: 5 - 7.5 Pg 30 cm • CO2-C emissions: 8 Pg/yr Issues • Permanency of increase • Native unmanaged soils • Constraints on C inputs (biophysical, economic, social) 100 cm

Slide 64: Adding charcoal to soil : the Terra Preta phenomenon Terra Oxisol Preta • High soil organic carbon – significant charcoal • High P contents – 200–400 mg P/kg • Higher cation exchange capacity • Higher pH and base saturation