1. Soil Carbon – What does it mean?
Farming Ahead 2010
Soil Carbon – What does it mean?
Daniel Murphy, University of Western Australia

2. Outline of talk Opportunities
Climate change research program – soil carbon
Challenges
The carbon balance in soil – what defines storage capacity
Practical options for farmers and grower groups
Cropping example from the Liebe Group
Perennial pasture example from the Esperance sand plains

3. Climate change research program
Australia’s farming future
Sustainable management of soil, in particular soil carbon is essential for the continued viability of the 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.

4. Climate change research program
Australia’s farming future
Soil carbon – What does it mean?
Soil organic carbon comes from the decay of organic matter from plants, animals and microorganisms.
It is the total amount of organic carbon in soil (less than 2 mm) which ranges between 5 to 250 tonnes of carbon per hectare (0-30 cm soil layer).
Soil organic carbon is not the same as soil organic matter which includes the weight of other elements such as N, S, P, O, H.
Science for Decision Makers March 2010 “Soil Carbon Management and Carbon Trading” or Department of Agriculture, Fisheries and Forestry.
Hoyle F.C., Baldock J.A. and Murphy D.V. (2010). Soil organic carbon – Role in rainfed farming systems with particular reference to Australian conditions In : Rainfed Farming Systems (Tow Ed.), Springer International, in press.

5. Climate change research program
Australia’s farming future
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

6. Soil organic carbon (%)
Potential soil organic carbon content:
Clay protects organic matter from microbial decay
Upper value
Lower value
Clay content (%)
Soil organic carbon (%)
Average

7. Attainable soil organic carbon content:
Climate defines net primary productivity
Plant biomass (t C per ha per year) squares with dashed line
Soil organic carbon (t C per ha) circles with upper and lower values
Net primary productivity (t C per ha per year) solid line
Western Australia – Grain growing regions

8. Some rainfall not converted to plant biomass
Attainable soil organic carbon content:
Climate defines net primary productivity
Plant biomass (t C per ha per year) squares with dashed line
Soil organic carbon (t C per ha) circles with upper and lower values
Net primary productivity (t C per ha per year) solid line
Some rainfall not converted to plant biomass
Western Australia – Grain growing regions

9. Lower plant biomass means less C inputs
Attainable soil organic carbon content:
Climate defines net primary productivity
Plant biomass (t C per ha per year) squares with dashed line
Soil organic carbon (t C per ha) circles with upper and lower values
Net primary productivity (t C per ha per year) solid line
Lower plant biomass means less C inputs
Western Australia – Grain growing regions

10. Actual soil organic carbon content:
Practical options: Cropping example from Liebe group
Time
Organic C
Actual levels
Attainable upper limit
Lower limit
Management can move soil C closer to or further from attainable levels
L = lupin, brown manured
W = Wheat (grain offtake)
Removal of surface acidity and sub-soil compaction improved soil quality and increased yield towards potential grain yield. Thus more plant carbon return to soil.
Set-up phase. Soil constraints removed

11. Actual soil organic carbon content:
Practical options: Cropping example from Liebe group
Time
Organic C
Actual levels
Attainable upper limit
Lower limit
Management can move soil C closer to or further from attainable levels
Set-up phase. Soil constraints removed
Difference in soil carbon due to management = 3.5 t C per ha in top-soil.

12. Actual soil organic carbon content:
Practical options: 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. 12

13. Actual soil organic carbon content:
Practical options: Perennial 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.

14. Actual soil organic carbon content:
Practical options: Perennial pasture example
Actual = measured carbon (dots)
Attainable = modelled carbon: likely and optimal scenarios
These preliminary model runs highlight that some paddocks are storing as much carbon as we would expect while others have possibly further capacity if plant inputs can be increased through optimal soil management and agronomy.

15. Actual soil organic carbon content:
Practical options: 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

16. Management Climate practices PLANT GROWTH Soil organic carbon PHYSICAL
BIOLOGICAL
Disease
Microbial biomass
Soil N supply
CHEMICAL pH
Salinity
CEC
Water repellence
PHYSICAL
Soil texture
Bulk density
Compaction
Climate
Management
practices
PLANT GROWTH
Soil organic carbon
The soil quality monitoring is being extended nationally and will link to some of the Climate Change Research Program sites and engage in soil quality extension with Grower Groups.

17. Acknowledgements
This project is funded by the Australian Government’s Climate Change Research Program and the Grains Research and Development Corporation.
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