Soil organic carbon value for sequestration
versus production
Frances Hoyle, Natalie Hogg, Justin Laycock, Liam Ryan (Department of Agriculture
and Food Western Australia) and Mingenew-Irwin Group staff
Key messages
 No short term change in soil organic carbon (SOC) stocks associated with any of
the treatments imposed at this site.
 There was no financial gain from increasing seeding rates.
 Less microorganisms were found in the green manure and chemical fallow
treatments in comparison to the control, high seeding rate and abandoned
treatments.
 Highest economic returns were associated with the control treatment (standard
practice).
 Increasing stubble retention, minimising soil disturbance and optimising plant
productivity would be considered the best strategy in this case to support a very
slow (decades) incremental change in SOC.
 Soil pH currently constitutes a limitation to plant growth and may limit any
agronomic or soil health benefits at this site by restricting root growth and late
access to water.
 50% of soil organic carbon (0-30cm) was located in the topsoil.
Aim
To determine the effects of various farm practices to increase the storage of soil
organic carbon (SOC). The project aimed to determine any increase in soil organic
carbon.
Background
Growers are constantly assessing the long term profitability and sustainability of their
farming systems. Often growers look to target an optimum gross margin rather than
highest yield. This demonstration trial was established in 2013 and carried on into
2014 to determine whether measureable changes in soil organic carbon (SOC) and
productivity could be associated with differing farm practices.
In this instance the influence of abandonment, chemical fallow, green manure and
increased seeding rate. The impact of different inputs was assessed by considering
any changes in SOC storage, yield and/or profitability.
Supporting your success
Trial details
(List of abbreviations available at end of document)
Detail
Description
Property
Plot size and replications
Soil type
Paddock rotation
Irwin River Station, Mingenew
100m x 18m x 3 replicates
Red loamy earth (Red Kandosol)
2009-2013: continuous wheat
#
Treatment (2013)
Treatment (2014)
1
Abandonment
Abandonment
2
Chemical fallow
Canola (3kg/ha)
3
Control (wheat 70kg/ha)
Control (canola 3kg/ha)
4
Green manure (wheat 70kg/ha)
Green manure (canola 3kg/ha)
5
High seeding rate (wheat 140kg/ha)
High seeding rate (canola 6kg/ha)
Activity
2013
Crop type
Sowing date
Wheat: Wyalkatchem
Canola: Snapper
3 June
2 May
Ausplow DBS 18.3m, 12 DBS, 12’’ knife points, press
inch (300 mm) spacing
wheels
258 mm
233 mm
70 kg/ha
3kg/ha
140 kg/ha
6kg/ha
45 kg/ha MAP
Nil
45 kg/ha flexi-N
Nil
5kg/ha MAP, 45L/ha UAN
Nil
100kg/ha NS41
1.2lt roundup attack
Nil
1.5lt Treflan
0.7lt velocity
1.1kg/ha Atrazine
0.44lt MCPA LVE
1kg/ha Propyzamide
10gm Logran
Nil
1.15kg/ha Atrazine
500ml/ha Select
0.15lt Lorsban
200ml/ha Alpha-cypermethrin
100ml/ha Chlorpyrifos
Seeding machinery
Growing season rainfall
Seeding rate - normal
Seeding rate – high
Fertiliser – pre seeding
Fertiliser – at seeding
Fertiliser – post emergent
Herbicide - knockdown
Herbicide – pre emergent
Herbicide – post emergent
Insecticide
2014
Results
2013
Soil
Baseline testing at the site prior to imposing treatments in 2013 indicated that soil pH
(CaCl2) levels were below target in the topsoil and marginal in subsoil at a depth
greater than 10cm (Table 1). Inorganic nitrogen was also low, with other nutrients
including phosphorous (P), potassium (K) and inorganic nitrogen (ammonium and
nitrate) at levels sufficient for crop growth (Table 1). No water repellence was
measured at this site.
Soil organic carbon (SOC) in the surface was below what would be considered
optimal for a sandy soil (0.8-1.0%, Table 1). The stock (or amount) of soil carbon in
tonnes C/ha is calculated using the weight of soil for a given area (bulk density) from
0 to 30cm depth, measured 24.3 t C/ha. Carbon concentration was highest at the
surface with 46% of SOC located in the top 10cm.
Table 1 Soil test results for 03MIG13 prior to imposing treatments in 2013 for 3 soil
depths (0-10, 10-20, 20-30cm) on a red loamy earth near Mingenew
0-10
Bulk
density
(g/cm3)
1.6
4.2
5
34
216
5
4.4
Organic
carbon
(%)
0.72
10-20
1.6
3.6
6
12
168
12
4.5
0.50
20-30
1.6
3.0
2
6
139
22
4.9
0.32
LSD
(<0.05)
ns
0.8
1
4
15
5
0.3
0.04
Depth
(cm)
Ammonium Nitrate
(mg/kg)
(mg/kg)
P
(mg/kg)
K
(mg/kg)
S
(mg/kg)
pH
(CaCl2)
Grain yield
Grain yield did not change with increasing seeding rate at this site despite large
differences in plant numbers measured 4 weeks after sowing. Grain protein and
resultant nitrogen use (N uptake) were also unchanged. Screenings were slightly
higher at the lower seeding rate (Table 2).
Table 2 Wheat (cv. Wyalkatchem) yield and quality at two seeding rates on a red
loam at Irwin River in 2013. Data is the average of all treatments imposed in 2013.
Plant
Protein yield
Protein
number
(N uptake; kg
(%)
(plants/m2)
N/ha)
Seeding rate
Yield
(t/ha)
70 kg/ha
1.36
95
9.4
21.3
81
3.3
350
140 kg/ha
1.52
178
9.5
24.1
82
2.6
390
ns
61 (p<0.1)
ns
ns
0.5
LSD (p<0.05)
HLW Screenings
(kg/hl)
(%)
*Price Notes: As at 18th December 2013 for AGP1 and assuming yield change is real.
Returns*
($/ha)
2014
Soil
Similar to 2013 (with the exception of sulphur) soil nutrients declined with depth
(Table 3). The soil is typical of a low fertility, sandy soil with low water holding
capacity (average 26%) and poor cation exchange capacity (CEC) with a low
potential to store and hold onto nutrients (Table 3). Soil pH (CaCl2) showed a slight
variation in the surface from 2013 and averaged 4.7 at the site (Table 3). SOC
distribution in the profile did not change from 2013, with half the SOC present in the
soil surface (Table 3). Carbon stocks adjusted for bulk density were 22.2 t/ha (0-30
cm) and showed no real measurable change from 2013. Based on the turnover of
soil organic matter and an average C:N ratio of 11:1, we might anticipate
approximately 30 kg N/ha would be available through the season, a portion of which
will be plant available.
Table 3 Soil test results measured at three depths (0-10, 10-20, 20-30 cm) on a red
loamy earth near Mingenew in 2014 in March. Data is the average of all treatments
Soil
depth
(cm)
CEC
Ammonium Nitrate
P
K
S
pH
(meq/
(mg/kg)
(mg/kg) (mg/kg) (mg/kg) (mg/kg) (CaCl2)
100g)
Organic
carbon*
(SOC%)
C
stock
(t/ha)
0-10
5.0
12.1
35
241
6
4.7
2.7
0.68
10.5
10-20
1.9
4.4
18
180
7
4.6
2.6
0.51
8.7
20-30
0.2
2.3
5
142
15
4.8
2.6
0.33
5.3
LSD
<0.05
0.9
2.1
7
22
4
0.1
ns
0.04
0.7
* Combustion analysis by Elementar.
Data has not been presented for variables where no change was measured in 2014
as a result of imposed treatments. This included SOC stocks.
There were only small changes in soil attributes measured as a result of treatments
in 2014 (Table 4). The chemical fallow treatment showed lower available K, higher
nitrate and a smaller microbial biomass (mass of soil microorganisms; MBC) than the
control (standard farming practice). In this instance a lower MBC was associated
with a lower potential for the biological supply of nitrogen (PMN). These measures of
soil biological health were also significantly decreased in the green manure (95
kg/ha) treatments (Table 4). Cropped treatments often demonstrated greater
potentially mineralisable nitrogen associated with a larger microbial biomass (Table
4). Soil bulk density (BDfe; <2mm fine earth) was variable but did in this instance
suggest treatment effects, with a lower BD in control and green manure treatments
(Table 4). These differences were primarily limited to the surface (0-10cm) layer
(data not presented).
Table 4 Soil attributes under imposed treatments on a red loamy earth near
Mingenew in 2014. Data is the average of all depths to 30cm (0-10, 10-20, 20-30cm)
Abandonment Abandonment
8
206
11
CEC
(meq/
100g)
3.1
Chemical
Fallow
Canola
(3kg/ha)
11
162
10
Control
(Wheat,
70kg/ha)
Control
(Canola,
3kg/ha)
3
190
Green
manure
(Wheat,
70kg/ha)
Green
manure
(Canola,
3kg/ha)
7
High seeding
rate (Wheat,
140kg/ha)
High seeding
rate (Canola,
6kg/ha)
LSD (p<0.05) LSD (p<0.05)
Treatment
(2013)
Treatment
(2014)
Nitrate
K
S
(mg/kg) (mg/kg) (mg/kg)
MBC
(t/ha)
PMN
BDfe
(kg/ha) (g/cm3)
114
3
1.71
2.4
84
3
1.67
10
2.5
129
10
1.50
198
8
2.6
95
4
1.50
3
181
7
2.5
121
9
1.63
3
28
2
0.5
17
3
0.07
Grain yield
High seeding rate did not alter canola yield significantly despite having established
significantly higher plant numbers (Table 5). This could reflect lower growing season
rainfall. The canola post fallow was significantly higher yielding in 2014 (Table 5)
despite few significant changes in soil chemical condition and had the same oil levels
as other treatments (average 45% oil). This suggests soil water conservation and
higher nitrogen availability as a result of the fallow treatment in 2013 contributed to
grain yield in 2014.
Table 5 Plant growth responses to treatments imposed in 2013 and 2014 on a red
loamy earth near Mingenew
Yield
(t/ha)
0
Plant number
(m2)
0
Canola (3kg/ha)
0.97
86
0.12
Control (wheat 70kg/ha)
Control (canola 3kg/ha)
0.53
51
0.13
Green manure
(wheat 70kg/ha)
Green manure
(canola 3kg/ha)
0
78
0.15
High seeding rate
(wheat 140kg/ha)
High seeding rate
(canola 6kg/ha)
0.44
123
0.13
LSD (p<0.05)
LSD (p<0.05)
0.11
26
ns
Treatment (2013)
Treatment (2014)
Abandonment
Abandonment
Chemical Fallow
NDVI
0.09
Economic analysis
Returns can vary widely depending on management inputs and the cost of seed
(particularly if sourced from on-farm). Economic benefits should consider any
variation to costs as outlined on a case by case basis (Table 6). For this trial
conducted in 2013 and 2014, the highest production returns were associated with
‘standard practice’. Given no change in SOC stocks was associated with alternate
treatments, this would indicate no current benefit (as determined by the measures
presented here) from altering management in the context of a carbon farming
environment. Increasing stubble retention, minimising soil disturbance and optimising
plant productivity would be considered the best strategy in this case to support a
very slow (decades) incremental change in soil carbon.
Table 6 Returns to treatments imposed in 2013 and 2014 on a red loamy earth near
Mingenew
Treatment (2013)
Treatment (2014)
Abandonment
Abandonment
Chemical Fallow
Canola (3kg/ha)
Control
Control
(wheat 70kg/ha)
(canola 3kg/ha)
Green manure
Green manure a
(wheat 70kg/ha)
(canola 3kg/ha)
High seeding rate
High seeding rate
(wheat 140kg/ha)
(canola 6kg/ha)
2013
returns
($/ha)
2014
returns
($/ha)
Current
balance
$0
$0
$0
-$20
$270
$250
$298
$42
$340
-$147
-$233
-$380ª
$203
-$80
$123
Note prices and costs used can vary widely and should be calculated for individual circumstances.
*Price Notes: Assuming $518/t received for canola in 2014 and including deduction of costs
for seed at $25/t and management costs as outlined above in paper. Assuming a cost of
green manure is the same as the control but with no income. In 2013 assumed a price of
seed at $48/ha at normal density and $96/ha at high density and management costs of
$99/ha. Machinery costs/transport not included in these calculations.
ª In 2014 the treatment that was a green manure in 2013 should have been harvested and
does not necessarily represent a typical return from what would normally be suggested is
used as a 1 in 4-5 year rotation.
Comments
This is a low rainfall environment in which crops are often constrained by moisture
deficits late in the season. This is highlighted by the crop response after fallow in
2014 where soil moisture was conserved. Based on growing season rainfall this site
should have yielded significantly higher (3.0-4.5 t/ha based on producing 12-20 kg of
grain for each mm of water available). Soil tests taken in March suggest soil acidity is
likely to be a constraint to plant growth and management strategies should focus in
this area. Nutrients (phosphorous, potassium, sulphur) did not appear to be limiting
crop growth.
Due to a grain crop not following the initial green manure phase it is difficult to
establish yield response for this treatment. Trials conducted throughout Western
Australia (n=45 trials) suggest a typical yield response would have been
approximately 10-20% above the control treatment (Hoyle, unpublished).
Background turnover of soil organic matter (assumed at 3% per year) could have
been expected to supply about 30 kg N/ha, of which a proportion will be unavailable
to plants.
Acknowledgements
Thanks to the Mingenew-Irwin group and their growers for continued interest and
support for this project and management of the trial site. A special thanks to Hugh
and Brendon Rowe for access to the trial site and assistance for the duration of the
trial. Also thanks to Justin Laycock (DAFWA) and Richard Bowles (UWA) for trial site
sampling and analyses.
This project is led by the Department of Agriculture & Food WA in collaboration with
the Mingenew-Irwin Group and is supported by funding from the Australian
Government Department of Agriculture - Action on the Ground program and the
GRDC.
Contact: Frances Hoyle frances.hoyle@agric.wa.gov.au
Abbreviation Meaning
g/ha
kg/ha
t/ha
L/ha
kg/hl
mg/kg
g/cm3
mm
Kg
C/ha
N/ha
TSP
LSD
CEC
N
C
NH4
NO3;
P
K
S
OC
CaCl2
BD
DBS
MAP
UAN
NS41
MCPA LVE
HLW
MBC
PMN
BDfe
NDVI
C/N ratio
N/ha
Gram per hectare
Kilogram per hectare
Tonne per hectare
Litre per hectare
Kilogram per hectolitre
Milligram per kilogram
Gram per cubic centimetre
Millimetre
Kilogram
Carbon per hectare
Nitrogen per hectare
Trisodium phosphate
Least significant difference
Cation exchange capacity
Nitrogen
Carbon
Ammonium
Nitrate
Phosphorus
Potassium
Sulphur
Organic carbon
Calcium chloride
Bulk density
Deep blade system
Mono ammonium phosphate
Urea and ammonium nitrate (in solution)
Nitrogen 35% and Sulphur 8.9% fertiliser
MCPA herbicide (present as the ethylhexyl ester) low volatile ester
Hectolitre weight
Microbial biomass-carbon
Potentially mineralisable nitrogen
Bulk density fine earth
Normalised differenced vegetation index
Carbon nitrogen ratio
Nitrogen per hectare