Optimising the management of organic
carbon: potential role in plant nutrition and
drought resilience
Jeff Baldock
CSIRO AGRICULTURE
Outline
1. Why are we interested in soil carbon?
2. What is soil organic carbon/organic matter composed of?
3. Role of organic carbon/organic matter in plant nutrition and
drought resilience.
4. Summary
Soil organic matter/organic carbon: why are we
interested in it?
Positive contribution to productivity
of soils
Sequestration of CO2-C to reduce net
GHG emissions
CO2
Addition
from
offsite
Product
removal
Residue
removal
Soil
organic
carbon
C additions
C loses
What is soil organic carbon/organic
matter composed of?
Soil organic
carbon
Composition of soil organic carbon
Crop residues on the soil surface (SPR)
High CH2O
(energy rich)
Buried crop residues (>2 mm) (BPR)
Recalcitrance
increases
Particulate organic carbon (2 mm – 0.05 mm) (POC)
Humus organic carbon (<0.05 mm) (HOC)
Decreasing C/N/P
(nutrient rich)
Resistant organic carbon (ROC): dominated by charcoal
Particulate carbon
(2mm – 0.05 mm)
Humus carbon
(<0.05mm)
Resistant
(charcoal <2mm)
Fractionation of soil organic carbon
≤2 mm soil
TC, IC and OC
Disperse soil
Sieve to 50 µm
Coarse fraction
(2000-50 µm)
2000-50 µm fraction
Fine fraction
(≤50 µm)
≤ 50 µm fraction
Why are we interested in soil carbon composition?
90
Carbon content (mg C/g soil)
80
70
60
48
32
50
20
11
40
30
36
48
0
HOC
31
60
20
10
POC
59
9
49
17
20
29
NSW000073
NSW000045
NSW000066
ROC
59
20
21
33
NSW000077
NSW000101
NSW000079
Location and soil
Why are we interested in soil carbon composition?
90
POC
Vulnerability
=
to change
HOC + ROC
Carbon content (mg C/g soil)
80
70
60
48
32
50
20
11
40
30
36
48
0
HOC
31
60
20
10
POC
59
9
49
17
20
29
NSW000073
NSW000045
NSW000066
ROC
59
20
21
33
NSW000077
NSW000101
NSW000079
Location and soil
Allocation of soil carbon to its component fractions
(312 soils from across Australia’s agricultural zone)
Mean
Minimum
Maximum
19.2
0.6
59.9
Mean
Minimum
Maximum
56.1
19.1
115.0
Mean
Minimum
Maximum
26.2
6.6
74.2
Fundamentals of organic
carbon/organic matter
management
Identifying soils/regions with a potential to increase soil
organic carbon
Identify areas (soils) where carbon capture per unit of available
resource is not maximised
Define whether or not resource use efficiency can be enhanced by
management (consider local climate and soil specificity)
Maintain current production
system
• Maximise resource use efficiency (e.g.
carbon capture per mm water or per
kg nutrient)
• Maximise carbon retention and return
to the soil
• Examples – liming, fertilisation,
rotational grazing
Shift to alternative production
systems
• Introduction of perennial vegetation
where appropriate
• Cover crops - legumes
• Organic amendments or green
manures
Permanence of changes in soil carbon stocks
Soil C changes take place
over long time periods
Soil organic carbon (Mg C/ha)
Soil C storage capacity is finite for a
defined rate
100of input and the largest
changes happen early
80
Management changes that build
soil C must be maintained to
maintain soil C
60
40
20
0
Control (no additions)
Manure addition then stopped
Manure addition maintained
1860
Petersen et al. (2005) Soil Biol Biochem 37: 359
1900
1940
1980
Plant nutrition and drought
resistance (more efficient use of
available water)
Soil N supply: Impact of C/N ratio of organic matter
70 kg ha-1 of C to
carbon dioxide
Wheat
Residue
C/N=100
100 kg ha-1
of C
1 kg ha-1
of N
30 kg ha-1 of C
1 kg ha-1 of N
2 kg ha-1 of N
required
N required
= 30/10
=3
Soil
organic
matter
C/N=10
10 kg ha-1
of C
1 kg ha-1
of N
Soil N supply: Impact of C/N ratio of organic matter
70
Wheat Residue
C/N=100
30
1
N required
=3
SOM
C/N=10
-2
70 kg ha-1 of C to
carbon dioxide
Pasture
legume
C/N=20
100 kg ha-1
of C
5 kg ha-1
of N
30 kg ha-1 of C
5 kg ha-1 of N
2 kg ha-1 of N
released
N required
= 30/10
= 3 kg ha-1
Soil
organic
matter
C/N=10
10 kg ha-1
of C
1 kg ha-1
of N
Soil N supply: Impact of C/N ratio of organic matter
70
Wheat Residue
C/N=100
30
1
N required
=3
SOM
C/N=10
N required
=3
SOM
C/N=10
-2
70
Medic Residue
C/N= 20
30
1
+2
70 kg ha-1 of C to
carbon dioxide
Soil Humus
C/N=10
100 kg ha-1
of C
10 kg ha-1
of N
30 kg ha-1 of C
10 kg ha-1 of N
+7 kg ha-1 of
N released
N required
= 30/10
= 3 kg ha-1
Soil
organic
matter
C/N=10
10 kg ha-1
of C
1 kg ha-1
of N
Accounting for variations in composition of soil organic
carbon – C to nutrient content ratios
Plant residues Soil organic carbon
C:N ratios for Australian soils (GRDC project
and SCaRP)
Avg
Range
SCaRP
(333 soils)
Avg
Range
SPR
51
20-105
BPR
31
16-60
POC
15
13-19
21
10-100
HOC
10
8-12
10
5-40
ROC
50
50-100
50
HOC
• high variability in plant residues
• extent of variability decreases with
increasing extent of decomposition
Relative Frequency
Fraction
GRDC
(29 soils)
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
HOC
POC
0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60
C/N Ratio
Importance of defining composition of organic N on
mineralisation
Amount of N present (kg N/ha)
Fraction
(C/N ratio)
POC
(50)
HOC
(10)
ROC
(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
Impact of organic carbon/matter on soil water status
Transpiration
Enhancing water use efficiency
• Minimise E, maximise T
• Use of surface residues to reduce E
Rainfall
Irrigation
Evaporation
Infiltration rate
IR (mm/h) = a + b(x) + c(SOC or fraction)
Runoff
Infiltration
Soil water
holding
capacity
Drainage
Root
zone
Subsoil
Soil water holding capacity
WHC (mm) = a + b(x) + c(SOC or fraction)
Considerations
• Uniformity across different soils
• Forms of carbon
• Interactions with environment
Influence of organic matter on water holding capacity
(no subsoil constraints)
Predictive functions have been derived
to define the upper and lower limits
θv = aψ mb





 −4.15 + 0.68  lnClay  + 0.42  lnOC  




a = exp 



 +0.27  lnBD 





b=
−0.54 + 0.11 lnClay  + 0.02 lnOC 



+0.10  lnBD 


da Silva and Kay 1997 Soil Science Society of
America Journal 61 877-883

Change in plant available water holding capacity with a 1%
increase in soil organic carbon content
Change in water holding capacity
(mm water)
For 0-10 cm layer of South Australian Red-brown earths
(7-31% clay & 0.7-2.8% SOC)
3 mm extra stored
6
rainfall for 10 rainfall
events equates to 30
5
y = -0.1229x + 5.5029
mm total or 600 kg of
2
R = 0.8212
grain
4
3
Issue: harder to build
up soil carbon on a
sandy soil than a clay
2
1
0
0
10
20
30
Clay content (% of soil mass)
40
Summary
Summary
1. Organic carbon/matter is composed of a variety of materials.
• Measurement techniques now exist to quantify composition
2. Knowing composition as well as content is informative
• Vulnerability of soil carbon to change
• Provision of nutrients
3. Predictive equations have been derived describing the impact of
organic matter on availability of soil water
• Typically site/soil type specific
• Improvements are expected now that we can measure soil
carbon fractions
Thank you
Jeff Baldock
PMB 2, Glen Osmond, SA 5064
Email: [email protected]
Phone: (08) 8303 8537
CSIRO LAND AND WATER/ SUSTAINABLE AGRICULTURE FLAGSHIP
Presentation title | Presenter name | Page 24
Sampling locations and soil samples collected and analysed
Samples collected and analysed by SCaRP
- 17,721 samples
- 3,836 sites
Additional samples analysed
- 2774 samples
- 690 sites
Total
- 20,495 samples
- 4,526 sites
>92% from farmer paddocks
Provision of nutrients from soil organic matter – the
concept of elemental stoichiometry
Recognises that for any living plant, microbe or animal, the mixture
of component biomolecules must exist within defined boundaries.
Outside of these boundaries – organisms do not function correctly
or die
Consequence
– elements (C, N, P, S and others) must
also be present within defined
boundaries.
– soil organic matter has a defined
C/N/P/S ratio