1. Carbon accumulation potential in WA soils
Daniel Murphy
The University of Western Australia – Institute of Agriculture
28th Feb 2012; Perth WA.

2. A team effort
UWA:
Daniel Murphy, Andrew Wherrett, Xiaodi Li, Richard Bowles, Georgina...
DAFWA:
Fran Hoyle, Karen Holmes, Tim Overheu, David Hall, Ted, Noel, Paul......
CSIRO:
Jeff Baldock, Elizabeth Schmidt (National coordinator)

3. Fundamentals of Soil Carbon
for a PDF copy

4. Climate change program Australia’s farming future
Sustainable management of soil, in particular soil carbon, is essential for the continued viability of Australian agriculture.
Within key areas across Australia this program will compare different land uses and management practices to identify those with the potential to raise soil carbon levels and improve production in a changing climate.

5. Soil carbon sampling Methods: Soil x land use (n=25 sites)
Site is 25m x 25m (10 points/site)
Depth is 0-30 cm
Soil sieved 2 mm
Total soil C and fractions measured
Adjusted for bulk density, gravel etc
Reported as tonnes carbon per ha
7 areas sampled (1000 sites)

6. Soil carbon terminology
The amount of soil carbon that can be stored changes from region to region and season to season across Australia. This is due to:
Clay content – physically protects organic carbon from microbes = Potential C
Climate – determines the productivity of crop and pasture = Attainable C
Management – your decisions, economics, constraints to growth = Actual C

7. The Carbon balance in soil
See book chapter by Hoyle, Baldock and Murphy for an explanation of this figure.
Source: Ingram and Fernandes (2001)

8. Soil organic carbon (%)
Potential soil organic carbon content:
Clay protects organic matter from microbes
Upper value
Lower value
Clay content (%)
Soil organic carbon (%)
Average
The 220 samples were collected from one paddock.
Each sample is under same climate and management.
Soil carbon difference due to clay content (storage).
Clay range
(No. samples)

9. Net primary productivity (t C per ha per year) = solid line
Attainable soil organic carbon content:
Influence of climate
Net primary productivity (t C per ha per year) = solid line
Above-ground plant biomass (t C per ha per year) = squares with dashed line
Soil organic carbon (t C per ha) = open circles. Lines indicate upper and lower values
Hoyle, Baldock, Murphy (2011)

10. Net primary productivity (t C per ha per year) = solid line
Attainable soil organic carbon content:
Influence of climate
Net primary productivity (t C per ha per year) = solid line
Above-ground plant biomass (t C per ha per year) = squares with dashed line
Soil organic carbon (t C per ha) = open circles. Lines indicate upper and lower values
Hoyle, Baldock, Murphy (2011)

11. Net primary productivity (t C per ha per year) = solid line
Attainable soil organic carbon content:
Influence of climate
Net primary productivity (t C per ha per year) = solid line
Above-ground plant biomass (t C per ha per year) = squares with dashed line
Soil organic carbon (t C per ha) = open circles. Lines indicate upper and lower values
Hoyle, Baldock, Murphy (2011)

12. (2004-now) Actual soil organic carbon content:
Liebe Group - Organic matter trial
Plots 80m x 10.5m x 3 replicates
Main Treatments:
No Till (Control)
Burnt
Till
Till + load up organic matter (3 x 20 t chaff/ha applied)
(2004-now)

13. Actual soil organic carbon content:
Liebe Group - Organic matter trial 2011 data (8 years)
Control (No Till) has 2.5 tonnes Carbon per hectare more than Burnt or Till
Adding organic matter (OM) has 8 tonnes Carbon per hectare more than Till

14. Soil carbon budget over 8 years
Why did adding 60 tonnes ha chaff = 8 tonnes ha soil carbon (C)
Microbes breakdown soil carbon
A large proportion is released as CO2
Retention in soil is approx. 40%
Plant is approx. 45% C
60 tonnes chaff = 27 tonnes plant C
27 tonnes plant C x 40/100 = 10.8 tonnes carbon per ha
So capturing 8 t C per ha from a maximum of 10.8 t C per ha is good.
This equals 1.3 tonnes soil carbon per year accumulation IF very large inputs of plant residue are returned to soil. So soil C change under typical management will be lower.

15. Attainable soil organic carbon content:
Organic matter trial – Roth-C model limit
Modeled Attainable C assuming: Cropping, 46% WUE, 50% stubble retained
Modeled – Annual pasture (100% WUE, No grazing, 7 months growth) = 72 t C/ha
Modeled – Perrennial pasture (100% WUE, No grazing, 12 months growth) = 73 t C/ha

16. Actual soil organic carbon content:
Perennial pasture example
Some beef producers have been moving towards perennial (kikuyu based) pasture systems in an effort to minimise the impact of the ‘autumn feed gap’.
Given the longer survival and increased biomass of kikuyu during summer months (compared to annual pastures of rye grass and clover) there is significant interest in the carbon storage capability of this grass species. 16

17. Actual soil organic carbon content:
Perennial versus annual pasture example
Paddock histories were collected to determine the management history and age of the Kikuyu (perennial pasture) which varied from 4 to 34 years.
From the 100 sites sampled there was no difference in soil carbon storage under annual and perennial (kikuyu) pasture systems. This was attributed to the low clay content (lack of physical protection of the soil organic matter).

18. Attainable soil organic carbon content:
Model: Perennial pasture example
Actual = measured carbon (dots)
Attainable and Realistic lines are modeled carbon scenarios.
These model runs highlight that some paddocks are storing as much carbon as we would expect. Others have possibly further capacity for carbon sequestration - if plant inputs can be increased through optimal soil management and agronomy.

19. Attainable soil organic carbon content:
Model: Perennial pasture example
Model predicts an increase in soil carbon under perennial pasture of 15 t C/ha over 40 years.
This represents an annual increase of 0.4 t C/ha per year.
A sampling strategy over decades will be needed to detect this change.

20. Actual soil organic carbon content:
Model: Pasture example
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
Fact sheet

21. Fact sheets on data sets
This is the first of 8

22. Key messages
Clay content physically protects soil carbon from decomposition.
Soil carbon level is the balance between inputs (plant residues, manures) and losses (microbial decomposition and erosion).
Soil carbon changes on a decadal time frame.
Be realistic of likely soil carbon gains (< 1 t C/ha each per year until the attainable limit is reached).

23. Acknowledgements
This project is funded by the Australian Government’s Climate Change Research Program and the Grains Research and Development Corporation.
The WA component is a joint collaboration between The University of Western Australia and the Department of Agriculture and Food.
National coordination is provided by CSIRO.

24. daniel.murphy@uwa.edu.au www.soilquality.org.au
Questions?