Soil Organic Matter
the unanswered questions
Dr Frances Hoyle
Department of Agriculture and Food WA, South Perth
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What is organic matter?
Total Organic Carbon or Total Carbon or Organic Matter?
• TOC - measure of carbon derived from organic material.
• TC - measure of all carbon in the soil (e.g. carbonates).
• Organic matter is different to TOC in that it contains nutrients
Living organic
matter
15%
10%
10%
Fresh residues
Living organisms
65%
Particulate organic
matter
Humus
Resistant organic matter
Dead and decaying organic matter
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Influencing soil organic matter
Soil organic carbon stocks differ between regions and seasons
• Clay content – protects organic carbon from microbes
= Potential C
• Climate – determines crop and pasture productivity
= Attainable C
• Management – decisions, economics, constraints
= Actual C
Ingram and
Fernandez
(2001)
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Role of SOC fractions: function
Soil type interacts with soil organic matter to determine its function
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Hoyle, Baldock and Murphy (2011)
1. How quickly does SOM content change?
•
Different fractions of SOM decompose at different rates
Particulate organic
carbon Fresh residues,
living organisms
Labile (POC)
SOM
1-5 y
Humus organic carbon
‘Resistant’ residues,
physically protected
Slow (HOC)
SOM
20-40 y
Particulate organic
carbon Protected
humus, charcoal
Stable (ROC)
SOM
500-1000 y
•
Mineralisation rate doubles for each 10°C increase in soil temperature
•
Large additions of organic matter required to change SOM
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1. How quickly does SOM content change?
A natural equilibrium exists for the retention and loss of organic
matter, with significant seasonal variability
0
50
Loamy sand
Organic C retained in soil
(% of input)
45
40
Clayey sand
10
Loamy sand
Sandy clay
loam
30
35
30
Loamy sand
Grace et al., 2006
(Soil Biology &
Biochemistry)
25
20
Light medium
clay
60
Loamy sand
0
5
10
15
20
25
Medium clay
30
CEC (meq/100 g)
Grace et al. (2006)
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90
CEC 0-5
CEC 5-30
1. How quickly does SOM content change?
How do I calculate changes in SOC?
Step 1 calculate amount of organic carbon in soil
•
Soil carbon stock of a soil with mass of 1200 (bulk density 1.2 g/cm3, 10
cm depth) and 1.20 % organic carbon
= (1.2/100) x 1200
= 0.012 x 1200
= 14.4 t C ha
Step 2 Calculate rate of addition
•
3 tonnes stubble = 4 tonnes organic matter including roots
•
45% carbon content
•
Coarse textured sand soil might retain 25% of what is added
= (4 x 0.45) x 0.25
= 1.8 x 0.25
= 0.45 t C ha (equivalent to 1.24% carbon)
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1. How quickly does SOM content change?
Roth-C simulation
100 years for a soil with 3% clay, bulk density 1.2 g/cm3, 60%WUE in crops, 95%
stubble retained, GSRF, wheat yield 2.5 t/ha
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1. How quickly does SOM content change?
We also need to
understand if soil
constraints are limiting
plant growth and thus
soil organic carbon build
up at some sites.
Chemical e.g. acidity
Physical e.g. compaction
Biological e.g. disease
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www.soilquality.org.au
2. Does biological activity influence SOM?
2. If I increase microbial activity by liming (or other management),
will I deplete my OM or do I create more?
–
–
–
–
Microbial activity is primarily limited by soil moisture and labile carbon
Improving soil condition will increase plant production and biomass
Increasing biomass will lead to greater organic matter inputs to soil
Increased microbial activity will result
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2. Does biological activity influence SOM?
–
–
60
Stubble
retained
50
40
30
20
Stubble
burnt
10
0
0
10
20
30
Temperature (°C)
Hoyle et al.your
(2006)success
Soil Biology & Biochemistry
Supporting
40
CO2 -C evolution (µg CO2 -C g-1 soil)
(µg CO2-C g-1 soil d-1)
CO2-C evolved
Increasing biomass will lead to greater organic matter inputs to soil
Increased microbial activity will result
25
y = 5.8389x + 0.2779
2
R = 0.9403
20
15
10
5
0
0.0
1.0
2.0
3.0
4.0
Gross N mineralisation rate (µg N g-1 soil)
2. Does biological activity influence SOM?
Example (Merredin long term trial)
Treatment
Soil organic
carbon (%)
Labile C
(mg/kg soil)
Microbial biomass
(mg/kg soil)
Straw burnt
1.2
139
142
Straw retained
1.3
182
211
10% (NS)
31%
49%
Significance
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Hoyle and Murphy (2007)
3. Will SOM influence plant available water?
As a general rule of thumb:
1% increase in soil organic carbon = 2% increase in water holding capacity
For a soil that held 200 mm of soil water = additional 4 mm of water
As 60% of soil organic matter (0-30 cm) is in the top 10 cm of the soil,
increases in water holding capacity beyond 10 cm are less likely.
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6
Extra water from a 1% increase in SOC
SOC% x BD (soil mass kg/m2) x retention factor*
5
(mm water)
Change in water holding capacity
3. Will SOM influence plant available water?
= 0.01 x (1.4 x 100) x 4
= 0.01 x 140 x 4
= 1.4 kg/m2 x 4
4
3
2
1% SOC = 14 t C ha = 5.6 mm
1
0
0
10
20
30
40
- Calculated to 10 cm depth
- Assumes SOM holds 4 x weight of water
- If change was observed to 30 cm then
= [0.01 x (1.4 x 300)] x 4 = 16.8 mm
Clay content (% of soil mass)
Change in water holding capacity for the 0-10 cm soil layer of South Australian Red-brown
earths with a one per cent increase in soil organic carbon content (Hoyle et al. 2011).
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3. Will SOM influence plant available water?
Soil texture, amount and type of organic matter influences water
repellence.
- Sands develop repellence more rapidly than finer textured clay soils
with lower amounts of organic matter.
- Associated with
alkyl carbon
MED severity (King 1981)
Nil
Very low
Low
Moderate
Severe
Very severe
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Hoyle and Murphy, unpublished
4. SOM and nutrients, how does it work?
4. I have heard that
i)
increasing SOM will lock up N, P, S, K and
ii)
that increasing SOM will supply more nutrients to my crop
Which is true?
Part 1 Nutrient lock up in stable SOM
Every tonne of organic carbon (humus) = 85 kg N, 20 kg P and 15 kg S stored in
stable organic matter pools.
• Organic residues take years to break down so nutrients not all required at once
• AND only a proportion of carbon enters the humus pool (5-15%).
This is roughly the equivalent of 25-100 kg C each year in current systems.
As nutrients enter the humus pool, other nutrients are released (SOM
decomposes at an average rate of 3%)
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4. SOM and nutrients, how does it work?
Part 2 Nutrient turnover
N release
(kg N per hectare 0-10 cm)
• Microbes use available nutrients to break down organic residues which
form part of the particulate organic carbon fraction (POC)
• Nutrients requirements dependent on the quality and quantity of residues.
140
120
100
80
60
40
20
0
-20
-40
Lupin
High C to N ratio = immobilisation*
Field pea
Oat
0
10
20
30
40
C:N ratio (plant residues)
Low C to N ratio = supply
50
*Due to the continual turnover of organic matter this immobilisation is
relatively short lived (approx. 8 weeks).
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Hoyle et al. (2011)
4. SOM and nutrients, how does it work?
Fresh residues and Particulate
organic carbon (POC)
Nutrients in stubble
Mean
Per cent
Carbon
45.0
Potassium
1.3
Nitrogen
0.5
Calcium
0.2
Magnesium
0.2
Sulphur
0.1
Phosphorous
0.1
1 tonne stubble would contain
- 13kg K, 5 kg N etc.
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14 ppm K
0.6 t/ha
27 ppm K
1.9 t/ha
Manmanning 2000, wheat
(Pluske and Bowden)
4. SOM and nutrients, how does it work?
Part 3 Nutrient supply from the humus fraction
A soil with mass of 1200 t/ha (bulk density 1.2 g/cm3, 10 cm depth)
Soil has 1.5% organic carbon (2.6% organic matter)
= 1200 x 0.015
= 18 tonnes organic carbon (31 tonnes organic matter)
Assume 3% turnover of soil organic matter
= 18 x 0.03
= 0.54 tonnes C ha/year
At a ratio of 1000 kg C: 85 kg N: 20 kg P: 15 kg S
C = 540 kg ha
N = 46 kg ha
P = 11 kg ha
S = 8 kg ha
Supplied annually from the humus pool to 10 cm
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4. Nitrogen budget in a WA soil
Continuous
Wheat
(kg N ha)
Lupin-Wheat
Rotation
(kg N ha)
Annual
Pasture
(kg N ha)
Mineralisation
100
120
282
Immobilisation
57
61
160
Net mineralisation
43
59
122
Microbial biomass
25
41
49
Murphy et al. (1998) Aust. J. Soil Res. Vol 49
•
Plant available nitrogen is the result of soil supply (mineralisation) and
demand (immobilisation)
•
Turnover of the actual microbial biomass contributes to soil N supply
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5. What rate does SOC change in our soils?
5. I’ve heard reported SOC increases of 1% in 2 years - can you
explain this?
Quite simply – not possible
Let’s look at it...
In a soil (bulk density 1.3 g/cm3) with carbon you have
1300 tonnes soil to 10 cm depth
Organic carbon (t/ha)
1300 x 1.2% SOC
15.6
1300 x 2.2% SOC
28.6
Difference
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13
5. What rate does SOC change in our soils?
• Plant matter is 45% carbon
• Soils might retain 30% of what you put in (this varies by soil type)
• Assume a root biomass is half the shoot biomass
• To get an additional 13 t/ha organic carbon in soil you would
need to add
13 tonnes ÷ 0.45 (percent carbon) =
29 ÷ 0.30 (retention factor)
=
29 tonnes organic matter
96 tonnes organic matter
(64 t/ha above-ground and 32 below ground)
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5. Broadbalk Experiment 1843 – present
Siltyloam
clay loam
Silty clay
Manure
% C in soil
3
Manure stopped
2
NPK
Control
1
0
1850
1900
Sandy loam
Sandy loam
1950
2000
% C in soil
3
No difference
between
treatments
2
1
0
1850
1900
1950
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2000
5. What rate does SOC change in our soils?
Possible reasons
Sampling error, not adjusted for bulk density/stone content, included root
material, sampling depth, redistribution of carbon, time of sampling
Questions?
Fran Hoyle [email protected]
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