Soil carbon under perennial pastures in SE NSW

Susan OrgillResearch Officer – Soil Carbon

Wagga Wagga Agricultural InstitutePhD candidate - CSU

CSU and FFI CRCSupervisors:Dr Jason Condon (CSU)Dr Mark Conyers (DPI)Dr Brian Murphy (OE&H)Dr Richard Greene (ANU)

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?

Monaro and Boorowa regions, SE NSW

Monaro basalt

Monaro (and Boorowa) deep granite

Monaro shallow granite

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

Soil C profiles: TC and LC g/100g

Total C (g/100g)

Dep

th (

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 GraniteNativeIntroduced

Labile C (g/100g)

Dep

th (

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 GraniteNativeIntroduced

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

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)

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<0.05Monaro region 76.5 (11 se) Boorowa region 51.8 (3 se)

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

Ave

rage

rai

nfal

l (m

m)

−505

1015202530

Tem

pera

ture

(°C

)

Mean monthly °C max and min● Monaro▲ Boorowa

Mean monthly mmDark MonaroLight Boorowa

Similar potential to grow biomassDifferent 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

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

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…

2009

Basalt derived soil

104 Mg C/ha 0.30m 2012 131 Mg C/ha

+8.8 t/C/ha/yr from 09-12

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 + errorClass = parent material and vegetation typeError = multivariate normal with mean zero

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)

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

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

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

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

Susan Orgill | Research Officer - Soil CarbonNSW Department of Primary IndustriesWagga Wagga Agricultural Institute M: 0428 424 566 E: susan.orgill@dpi.nsw.gov.au

• Parent material, soil depth and climate significantly influencedsoil 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