Genesis of the use of RothC to model soil organic carbon

1
Genesis of the use of RothC to model soil organic
carbon
2
Outline

• Composition of soil organic carbon isolating
  biologically important fractions
• Methodology for quantifying C allocation to
  fractions
• Why attempt to understand allocation to
  fractions?
• Modelling soil carbon with RothC
• Substitution of conceptual with measureable C
  pools in RothC
• MIR prediction of soil carbon fractions

3
Composition of soil organic matter
4
Biologically significant soil organic fractions
Humus (HumC)
Particulate material (POC)
Charcoal (ROC)
5
Quantifying SOC allocation of SOC to fractions
Total soil organic carbon
Humus lt53µm - Recalcitrant
6
Variation in amount of C associated with soil
organic fractions
Surface plant residue C (SPR)
Buried plant residue C (BPR)
(Mg C/ha)
Organic carbon in 0-10 cm layer
Particulate organic carbon (POC)
Humus C (HumC)
Recalcitrant C (ROC - charcoal)
Average for Hamilton (long term pasture)
7
Variation in amount of C associated with soil
organic fractions
30
SPR
25
BPR
POC
20
HumC
ROC
15
Organic C in 0-10 cm layer
(Mg C/ha)
10
5
0
0P
1P
8P
32P
11P
22P
W2PF
Arboretum
Pulse/wheat
Strat (MedN)
Strat (HighN)
NoTill (MedN)
Perm Pasture
Canola/wheat
NoTill (HighN)
Pasture/wheat
Hamilton
Hart
Yass
Urrbrae
Waikerie
Pasture
Pasture
Cropped
Mix
Mix
8
Changes in total soil organic carbon with time
Initiate wheat/fallow
9
Importance of allocating C to soil organic
fractions
10
Vulnerability of soil carbon content to
variations in management practices
30
25
Conversion to pasture
20
Soil organic carbon (g C kg-1 soil)
15
10
5
0
10
20
30
50
0
40
70
60
43
Years
15
33
11
Importance of quantifying allocation of C to soil
organic fractions
Soil 2 20 g SOC kg-1 soil
Soil 1 20 g SOC kg-1 soil
25
25
20
20
15
Soil Organic Carbon (g C kg-1 soil)
Soil Organic Carbon (g C kg-1 soil)
15
10
10
5
5
0
0
Time
Time
12
Summary SOC fractions
Total soil organic carbon
Humus lt53µm - Recalcitrant
13
RothC Model (Version 26.3)
Plant Inputs
Original configuration monthly time step
14
Roth C data requirements

• Monthly climate data rainfall (mm), open pan
  evaporation (mm), average monthly air temperature
  (C)
• Soil clay content ( soil OD mass)
• Soil cover (vegetated or bare)
• Monthly plant residue additions (t C ha-1)
• Decomposability of plant residue additions
• Monthly manure additions (t C ha-1)
• Soil depth (cm)
• Initial amount of C contained in each pool

15
RothC model structure partitioning residue
inputs into decomposable and resistant material

• All plant material entering the soil is
  partitioned into DPM and RPM via DPM/RPM ratio

Management DPM/RPM
Grassland and most agricultural crops 1.44
Unimproved grassland and scrub (savannas) 0.67
Deciduous and tropical woodlands 0.25
16
RothC model structure amount of each type of
carbon decomposed

• The amount of carbon associated with each pool
  that decomposes follows an exponential decay

a the rate modifying factor for temperature b
the plant retainment rate modifying factor c
the rate modifying factor for soil water k the
annual decomposition rate constant for a type of
carbon t 0.0833, since k is based on a yearly
decomposition rate.
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RothC model structure calculation of rate
constant modifying factors

• Temperature modifying factor (a)

tm average monthly temperature
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RothC model structure calculation of rate
constant modifying factors

• Soil water modifying factor calculated based on
  top soil moisture deficit (TSMD)

19
RothC model structure calculation of rate
constant modifying factors

• Calculation of maximum TSMD

20
RothC model structure calculation of rate
constant modifying factors

• Calculation of the rate modifying factor (c)

if TSMDacc lt 0.444 MaxTSMD then c1.0 otherwise,
1.0
c
0.2
MaxTSMD
0.444 MaxTSMD
21
RothC model structure amount of each type of
carbon decomposed

• The amount of carbon associated with each pool
  that decomposes follows an exponential decay

a the rate modifying factor for temperature b
the plant retainment rate modifying factor c
the rate modifying factor for soil water k the
annual decomposition rate constant for a type of
carbon t 0.0833, since k is based on a yearly
decomposition rate.
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RothC Model (Version 26.3)
Plant Inputs
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RothC model structure partitioning of
decomposition products

• Fraction decomposing organic matter that goes to
  CO2, humus and biomass
• Partitioning to CO2 is defined by clay content

Biomass Humus partitioning 46 Bio 54 Hum
24
RothC output under constant inputs and climate
to define equilibrium SOC
25
Modelling the measurable
Plant Inputs
RPM POC IOM ROC (Charcoal C) HUM TOC (POC
ROC)
26
Requirements for calibration
Soil samples Representative composite soil samples collected at the beginning and end of a period gt10 years to a soil depth of 30 cm.
Bulk density Measured at time of sampling using soil core weight/volume.
Crop yields Yield of grain and pasture over each year to be modelled and estimates of harvest index and root/shoot ratios
Management Details of individual crops, rotations, fallow periods, stubble burning and incorporation. If grazing occurred, estimates of consumption and return from animals.
Climate Details of average monthly air temperature, rainfall and pan evaporation
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Model calibration and verification sites
28
Brigalow calibration site influence of modifying
RPM decomposition constant (k)
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Model Verification (sites with archived soil
samples)

Wagga wheat/pasture
Tamworth wheat/fallow
30
Model verification (paired sites)

• Is this result due poor model performance or poor
  pairing of the sites?
• Did the sites start off similar or were there
  significant initial differences in
  soil/plant/environmental properties?

31
Quantifying SOC allocation of SOC to fractions
Total soil organic carbon
Humus lt53µm - Recalcitrant
32
Predicting total organic carbon and its
allocation to SOC fractions using MIR

• Dependence on soil chemical properties
• Prediction of allocation of carbon to fractions
  via calibration and PLS

33
Prediction of total organic carbon (TOC)
177 Australian soils (all states) from varying
depths within the 0-50 cm layer
n 177 Range 0.8 62.0 g C/kg R2 0.94
MIR predicted TOC (g C/kg soil)
Measured TOC (g C/kg soil)
Janik et al. 2007 Aust J Soil Res 45 73-81
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Tasmanian soils project
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MIR prediction of particulate organic carbon
141 Australian soils (all states) from varying
depths within the 0-50 cm layer
n 141 Range 0.2 16.8 g C/kg R2 0.71
MIR predicted POC (g C/kg soil)
Variability in crop residue type exits
Measured POC (g C/kg soil)
Janik et al. 2007 Aust J Soil Res 45 73-81
36
MIR prediction of charcoal C
121 Australian soils (all states) from varying
depths within the 0-50 cm layer
n 121 Range 0.0 11.3 g C/kg R2 0.86
MIR predicted Char C (g/kg)
Measured Char C (g/kg)
Janik et al. 2007 Aust J Soil Res 45 73-81
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Summary

• Methodologies exist to quantify biologically
  significant pools of carbon
• Understanding the dynamics of the pools allows
  accurate interpretation of potential changes
• Substitution of measureable fractions for
  conceptual pools in models is possible
• Rapid methods for predicting soil carbon
  allocation to pools exist

38
Thank you
CSIRO Land and Water Jeff Baldock Research
Scientist Phone [phone] Email
jeff.baldock_at_csiro.au Web http//www.clw.csiro.au
/staff/BaldockJ/ Acknowledgements Jan Skjemstad,
Kris Broos, Evelyn Krull, Ryan Farquharson, Steve
Szarvas, Leonie Spouncer, Athina Massis
Contact UsPhone 1300 363 400 or 61 3 9545
2176Email Enquiries_at_csiro.au Web www.csiro.au
39
Model Calibration
Brigalow South ws64 (RPM 0.15)
40
Defining soil C dynamics at Roseworthy, SA under
continuous wheat production
Average growing season (Apr-Oct) rainfall (mm) 338
Water limited potential grain yield (Mg/ha) 4.56
Grain yield used (Mg/ha) (85 water use efficiency) 3.88
Harvest index (Mg grain/Mg dry matter) 0.45
Total shoot dry matter production (Mg/ha) 8.62
Equilibrium conditions (model for 500 years)
Soil clay content () Amount of C in 0-30cm layer (Mg C/ha) C content of 0-10 cm layer ()
5 65 2.32
15 78 2.79
30 93 3.32

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Changes in soil C for different levels of average
grain yield
42
Changes in soil C for different levels of average
grain yield
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
43
Composition of methodologically defined SOC
fractions

• Particulate organic carbon (POC)
• Fragments of plant residues gt53 µm (living and
  dead)
• Molecules sorbed to mineral particles gt53 µm
• Large pieces of charcoal
• Humus (HUM-C)
• Fragments lt53 µm
• Molecules sorbed to particles lt53 µm
• Recalcitrant (ROC)
• Materials lt53 µm that survive photo-oxidation
• Dominated by material with a charcoal-like
  chemical structure
• NMR to quantify char-C

44
Spatial variation in soil charcoal and carbon
contents (0-10 cm layer)
45
Predicting soil organic carbon contents

• Clearing of Brigalow bushland

46
Options for increasing soil carbon content

• Principal increase inputs of carbon to the soil
• Maximise capture of CO2 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
• Alternative crops - lower harvest index
• Alternative pasture species increased below
  ground allocation
• Addition of offsite organic materials diversion
  of waste streams
• Green manure crops legume based for N supply

47
Options for increasing soil carbon content

• Constraints
• Soil type protection and storage of carbon
• Local environmental conditions
• Dryland conditions amount and distribution of
  rainfall
• Irrigation maximise water use efficiency
• Economic considerations alterations to existing
  systems must remain profitable
• Social
• Options need to be tailored to local conditions
  and farm business situation

48
Defining inputs of organic carbon to soil
dryland conditions

• Availability of water amount and distribution
  of rainfall imposes constraints on productivity
  and options

49
Evaluating potential C sequestration in soil
Optimise input and reduce losses
Add external sources of carbon
50
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
• External sources of carbon
• 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

51
Incorporation into a decision support framework
MIR Analysis
52
Options for sequestering carbon
CO2
Carbon sequestration options
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What determines soil organic carbon content?
54
Balance between inputs and outputs
30
25
20
Soil organic carbon (g C kg-1 soil)
15
Inputs Outputs
10
5
0
20
40
60
100
0
80
140
120
Years
55
Understanding the residue input requirements to
change soil carbon content
Amount of C required 24 Mg C 50 Mg Dry
Matter (DM) Rate per year (no losses) 10 Mg
DM/y 50 allocation below ground equates to
5 Mg shoot DM/y Rate per year (with 50
loss) 20 Mg DM/y (50 loss) 50 allocation
below ground 10 Mg shoot DM/y
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Nutrients associated with soil carbon
Assumptions C/N 10 and C/P120)
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(No Transcript)
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Variation in C/N ratio of different fractions of
soil organic matter
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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

60
Dynamic nature of SOC and its fractions
Irrigated Kikuyu pasture Waite rotation trial
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Dynamic nature of SOC and its fractions
Dryland Pasture/Wheat/Wheat Waite rotation trial
Amount of organic C
(Mg C ha-1 in 0-10 cm)
Date of sample collection
62
Correcting soil carbon for management induced
changes in bulk density
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
63
Predicted equilibrium soil organic C contents for
3 regions in SA with different climate type
Clare Roseworthy Waikerie
Growing season rain (mm) 491 338 170
Water limited potential grain yield (T/ha) 6.2 4.6 1.8
Grain yield (T/ha) (85 WUE) 5.3 3.9 1.5
Total shoot dry matter (T/ha) 11.7 8.6 3.4

Equilibrium soil carbon content
Modelled amount of C in 0-30 cm (t C/ha) 98 78 41
Estimated C in 0-10 cm soil layer 3.5 2.8 1.5
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Take home messages

• Organic matter (carbon other elements) is
  composed of a variety of materials and improves
  soil productivity
• Different soils can hold different amounts of
  carbon
• Nature of soil minerals, depth and bulk density
• Balance between inputs and losses goal is to
  maximise production per mm available water
• Measuring changes in soil carbon requires careful
  consideration
• Options to increase carbon must be tailored to
  the local conditions and economic considerations
  of the farmer
• Computer models exist to predict the impact of
  management on soil carbon

65
Tasmanian soils project

• Objective Prediction of total organic carbon
• Samples
• 154 soils collected from 0-10 cm layer of a
  diverse set of soil x management combinations
• 30 measured values used to derive the calibration
• All other samples predicted from this calibration
• Range of Walkley-black C contents
• 3.7 99.9 g C/kg soil

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Tasmanian soils project
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Functions of organic matter in soil
Functions of SOM
68
Distribution and turnover of organic carbon in
soil
69
Variation in soil organic carbon with depth for
different soils
70
Significance of carbon in soils
Annual fluxes (1015 g C/yr)

• Emissions
• Fossil fuel burning 6
• Land use change 2

• Responses
• Atmospheric increase 3
• Oceanic uptake 2
• Other 3

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Potential for soils to sequester C

• Potential does exist to sequester C in soil
• SOC pool size 1500 Pg
• Rapid cycling SOC 500-750 Pg
• 1 increase in stored SOC/yr 5 - 7.5 Pg/yr
• CO2-C emissions 8 Pg/yr

• Issues
• Permanency of increase
• Native unmanaged soils
• Constraints on C inputs (biophysical, economic,
  social)

72
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

73
Understanding the residue input requirements to
change soil carbon content
Amount of C required 14 Mg C 28 Mg Dry
Matter (DM) Rate per year (no losses) 5.6
Mg DM/y 50 allocation below ground 2.8 Mg
shoot DM/y Rate per year (with 50 loss)
11.2 Mg DM/y (50 loss) 50 allocation below
ground 5.6 Mg shoot DM/y
74
Soil organic carbon content influence of
management

• 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 millennia
• 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
• example soil carbon model RothC

75
Changes in soil C for different climates at a
constant wheat grain yield
76
Nutrients associated with soil carbon
Assumptions C/N 10 and C/P120)
77
Significance of carbon in soils

• Annual fluxes (1015 g C/yr)

• Emissions
• Fossil fuel burning 6
• Land use change 2

• Responses
• Atmospheric increase 3
• Oceanic uptake 2
• Other 3

78
Chemical function Cation exchange capacity
79
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?

80
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)
0.1 increase in soil organic C 1.5
81
Adding charcoal to soil the Terra Preta
phenomenon
Terra Preta
Oxisol

• High soil organic carbon significant charcoal
• High P contents 200400 mg P/kg
• Higher cation exchange capacity
• Higher pH and base saturation