Presentation by Dr Susan Orgill to the Riverina branch of the Australian Society of Soil Science at a meeting on 14 March 2014 at Charles Sturt University, Wagga

1. Soil carbon under perennial pastures in SE NSW
Susan Orgill
Research Officer – Soil Carbon
Wagga Wagga Agricultural Institute
PhD candidate - CSU
CSU and FFI CRC
Supervisors:
Dr Jason Condon (CSU)
Dr Mark Conyers (DPI)
Dr Brian Murphy (OE&H)
Dr Richard Greene (ANU)

2. Research aim and context
• Investigate the role of perennial
pastures in maximising organic carbon
in soil
– Does soil under introduced perennial
pastures accumulate more carbon than
native pastures?
– Does parent material influence the
capacity of soil to accumulation
carbon?
– What is the role of management in
accumulating carbon in soil under
perennial pastures?

3. Monaro and Boorowa regions,
SE NSW
Monaro basalt
Monaro (and Boorowa)
deep granite
Monaro shallow granite

4. Sampling, analysis and calculations
• Sites sampled to 0.70 m
• Sampled according to national
protocols
• Samples analysed for chemical
and physical properties
• Carbon units:
Total Carbon (LECO); g/100g
C stock (Mg C/ha) =
C conc (g/100g) x BD (g/cm3) x
depth (cm) x gravel corr factor

5. Soil C profiles: TC and LC g/100g
Total C (g/100g)
Depth(m)
0
0.2
0.4
0.6
Monaro Basalt
0
0.2
0.4
0.6
Monaro Shallow Granite
0
0.2
0.4
0.6
Monaro Deep Granite
0
0.2
0.4
0.6
0 2 4 6
Boorowa Deep Granite
Native
Introduced
Labile C (g/100g)
Depth(m)
0
0.2
0.4
0.6
Monaro Basalt
0
0.2
0.4
0.6
Monaro Shallow Granite
0
0.2
0.4
0.6
Monaro Deep Granite
0
0.2
0.4
0.6
0.0 0.5 1.0 1.5
Boorowa Deep Granite
Native
Introduced

6. Carbon density
(Mg C/ha)Region Parent material Vegetation
0-0.70 (m) s.e.
Monaro Basalt Introduced 160.38 10.8
Basalt Native 156.86 10.1
Basalt Remnant 116.29 unreplicated
Deep granite Introduced 77.99 11.4
Deep granite Native 75.00 11.1
Deep granite Remnant 44.58 unreplicated
Shallow granite Native 43.29 3.4
Boorowa Deep granite Introduced 52.35 2.8
Deep granite Native 51.25 2.8
Deep granite Remnant 49.24 unreplicated
Soil C stocks (Mg C/ha to 0.70 m)
n.s.
n.s.
n.s.
Pasture comparison: Introduced vs Native
Pastures NO difference in these regions

7. Carbon density
(Mg C/ha)Region Parent material Vegetation
0-0.70 (m) s.e.
Monaro Basalt Introduced 160.38 10.8
Basalt Native 156.86 10.1
Basalt Remnant 116.29 unreplicated
Deep granite Introduced 77.99 11.4
Deep granite Native 75.00 11.1
Deep granite Remnant 44.58 unreplicated
Shallow granite Native 43.29 3.4
Boorowa Deep granite Introduced 52.35 2.8
Deep granite Native 51.25 2.8
Deep granite Remnant 49.24 unreplicated
Soil C stocks (Mg C/ha to 0.70 m)
Parent material comparison:
Significant difference within region
P<0.05
Basalt soils 159 (11 se)
Deep granite soils 77 (11
se)
Shallow granite soils 43 (3
se)

8. Carbon density
(Mg C/ha)Region Parent material Vegetation
0-0.70 (m) s.e.
Monaro Basalt Introduced 160.38 10.8
Basalt Native 156.86 10.1
Basalt Remnant 116.29 unreplicated
Deep granite Introduced 77.99 11.4
Deep granite Native 75.00 11.1
Deep granite Remnant 44.58 unreplicated
Shallow granite Native 43.29 3.4
Boorowa Deep granite Introduced 52.35 2.8
Deep granite Native 51.25 2.8
Deep granite Remnant 49.24 unreplicated
Soil C stocks (Mg C/ha to 0.70 m)
Climate comparison: Significant difference
with region/climate
P<0.05
Monaro region 76.5 (11
se)
Boorowa region 51.8
(3 se)

9. What is driving the difference with
region? Climate and decomposition.
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
0
10
20
30
40
50
60
Averagerainfall(mm)
−5
0
5
10
15
20
25
30
Temperature(°C)
Mean monthly °C max and min
● Monaro
▲ Boorowa
Mean monthly mm
Dark Monaro
Light Boorowa
Similar potential to grow biomass
Different decomposition potential
In Monaro region:
1. Strongly summer dominant rainfall
2. Colder and for longer
In Boorowa region:
Soil moisture is less variable
seasonally and therefore more
available for OM decomposition
Evidence: Higher labile C
concentrations in deep granite-
derived soil from the Monaro region

10. Key message 1: Parent material, soil
depth and climate significantly influence
soil C stock
• Influence supply and decomposition of OM
• Parent material (texture, structure and soil depth) influences:
– Soil water and nutrient holding capacity = Ability to supply OM
– Clay content and nutrients = Ability to physically and chemically
protect OM
• Climate (rainfall and temperature) influences:
– Biomass (OM) supply
– Rate of OM decomposition
• Implications
– These factors cannot be changed and therefore define the
sequestration potential of a soil type-region

11. 2009
Granite derived soil
40 Mg C/ha 0.30m 2012 47 Mg C/ha
+2.4 t/C/ha/yr from 09-12
And in some cases … just add water…

12. 2009
Basalt derived soil
104 Mg C/ha 0.30m 2012 131 Mg C/ha
+8.8 t/C/ha/yr from 09-12

13. Explaining the variability within a
parent material group
• Assess correlations with C using a multivariate linear model
– 52 paddocks
– 7 soil chemical traits (TC, TN, labile C, available Colwell
P, extractable S, CEC and pH CaCl2)
– Average surface (0 to 0.20 m) data
• Correlation = class + region + error
Class = parent material and vegetation type
Error = multivariate normal with mean zero

14. Correlations between carbon and
nutrients under pastures
• C is significantly and positively correlated with:
– Total N, Labile C, Extractable S, CEC
10.21-0.29*0.31*-0.080.29*-0.06pH
10.220.49**0.260.210.51**CEC
1-0.090.60**0.31*0.52**Extr S
1-0.060.020.19Col P
10.74**0.85**Labile C
10.80**Total N
1Total C
pH
(CaCl2)
CEC
(cmolc/kg)
Extr S
(mg/kg)
Col P
(mg/kg)
LC
(g/100g)
TN
(g/100g)
TC
(g/100g)

15. Key message 2: Pasture nutrition
may increase soil C
• C sequestration is strongly related to soil fertility
• We can influence available S and P to maximise biomass
production and therefore OM supply to soil
• Nitrogen primarily from legumes
• Phosphorus and sulfur are commonly applied as mineral
fertilisers to increase legume production, function and
nodulation (thereby N fixation)
• Implications
– Maintaining adequate pasture nutrition may substantially
increase soil carbon stocks but this may come at a cost

16. Influence of grazing management,
Boorowa region
• Similar to Chan et al. 2010; n.s.d with
grazing management to 0.30m
• Higher C stock under CG 0.70m
• Annual stocking rate (DSE) n.s.
• All RG sites low-input (i.e. not fertiliser
for >10yrs), therefore may be more
related to nutrient management
• Importance of subsoil C sequestration
and sampling to depth
No difference 0-0.30m; signif diff 0-0.70m
49.4 (3.5)4.8 (1.3)7.5 (1.3)7.0 (1.3)14.6 (1.3)Native - RG
53.6 (3.9)6.1 (1.4)7.9 (1.4)7.1 (1.4)13.4 (1.4)Native - CG
48.0 (3.5)4.9 (1.3)7.1 (1.3)7.0 (1.3)13.0 (1.3)Introduced - RG
57.8 (3.9)6.7 (1.4)8.7 (1.4)7.7 (1.4)15.7 (1.4)Introduced - CG
0-0.700.20-0.300.10-0.200.05-0.100-0.05
Depth (m)
Vegetation

17. Of the total C stock to 0.70 m, in the 0.30 - 0.70 m soil layer…
• Monaro region
– basalt-derived soil 43%
– deep and shallow granite-derived soil 28%
• Boorowa region 0.30 - 0.70 m
– deep granite-derived soil 33%
• Implications
– Restricting to the surface 0.30 m may result in erroneous
conclusions
– Opportunities may exist for subsoil C sequestration
– Carbon in subsoil may be more permanent
Key message 3: Subsoil C is
important

18. Where to now (for me)….
Making soil sampling and analysis cheaper…

19. Susan Orgill | Research Officer - Soil Carbon
NSW Department of Primary Industries
Wagga Wagga Agricultural Institute
M: 0428 424 566 E: susan.orgill@dpi.nsw.gov.au
• Parent material, soil depth and climate significantly influenced
soil C stock under perennial pastures
• Pasture nutrition may increase soil C (but this may come at a
cost)
• Pasture type and grazing management did not increase soil C
(in the surface 0.30m)
• Subsoil C is important and should be considered when
comparing the influence of land management on soil C
Key messages