1. Soils and Climate Change: Greenhouse gas emissions
implications and research requirements
Jeff Baldock, Ichansi Wheeler, Neil
McKenzie and Alex McBratney
CCRSPI Conference, Melbourne
15-17 February, 2011

2. Outline
• Introduction
• Summary of the processes that generate and consume
greenhouse gases in soil
• Climate change projections
• For each greenhouse gas (CO2, N2O, and CH4) examine:
• Potential impacts of climate change
• Mitigation options and and mitigation options
• Future research requirements
• Summary

3. Introduction
• Soils contain significant stores of carbon and nitrogen (1500 Pg
organic C and 190 Pg total N)
• These stores are continuously exposed to decomposition and
other biochemical processes that generate or consume CO2,
N2O and CH4.
• Using soil and atmospheric carbon stocks of 1500 and 720 Pg
and an atmospheric CO2 concentration of 390 ppm, a 1%
change in soil carbon = 8 ppm change in CO2 concentration
(assuming no feedbacks)
• Concern exists over the potential positive feedback that
increased temperature may have on soil carbon loss and CO2
concentration

4. Generation of greenhouse gases by soil
CO2 N fertiliser & N2O CH4
Animal waste
Soil surface
Denitrification
Respiration NH4 NO3 Methanogenic
Nitrification organisms
Mineralisation
Assimilation Decomposition
and mineral
protection
Organic Organic
carbon nitrogen Aerobic Anaerobic
soil soil
Soil organic matter conditions conditions
including decomposer
organisms

5. Consumption of greenhouse gases in soil
CO2
Photosynthesis
N2O CH4
Shoot dry Plant dry
matter matter
Soil surface
Root dry
matter Biological
transformations Methanotrophic
associated with organisms
Residue N cycling
deposition Uptake
Inorganic N
NH4 & NO3
Organic Organic Aerobic
carbon nitrogen soil
Immobilisation conditions
Soil organic matter
including decomposer
organisms

6. Projected changes to Australia’s climate
2030 2050 2070
Australian agricultural regions
• warmer and drier
• altered seasonality
• greater extremes
0.3 0.6 1.0 1.5 2.0 2.5 3.0 4.0 5.0
Change in average annual temperature ( C)
Such changes will undoubtedly
influence rates of net
greenhouse gas emissions
Magnitude of change will be
-40 -20 -10 -5 -2 2 5 10 20 40
defined by the sum of the
Change in annual rainfall (%) climate change influence on all
processes
-4 -2 2 4 8 12 16
Change in annual potential evapotranspiration (%)
Source: http://climatechangeinaustralia.com.au - 50th percentile of projected changes under the medium
future emissions profile relative to 1980-1999

7. CO2 / Soil carbon: inputs of carbon
Controls on potential carbon input
Photosynthetically 1) The amount of PAR
active radiation (PAR) 2) Fraction of PAR used
3) Efficiency of carbon capture,
4) Proportion lost to respiration
CO2 5) Proportion removed in products.
Factors 1-4 define potential net
primary productivity
Product
harvest
Other constraints
(water, fertility, disease) may reduce
efficiencies and lead to Actual NPP <
Potential NPP
Product removal – harvest index
issue

8. CO2 / Soil carbon: inputs of carbon
Where can carbon inputs be
increased?
Photosynthetically
active radiation (PAR) Identify systems that are not achieving
100% resource use efficiency (water
and nutrients)
CO2
Identify constraints and define
whether or not they can be managed
Yes No
Product
harvest
Implement Consider
management alternative
changes and production
capture systems that may
additional be better suited
carbon to constraints

9. CO2 / Soil carbon: fate of carbon inputs
Photosynthetically What happens to the carbon
active radiation (PAR) inputs?
The majority is decomposed and
CO2 returned to the atmosphere as CO2
The remainder resists decomposition
and replaces the soil organic carbon
Product that is being decomposed
harvest
Issues
- residue placement – surface
residues vs roots
Soil organic - reduced incorporation
carbon

10. CO2 / Soil carbon: controls on stability of SOC
• Most of these factors vary
spatially
• Different soils have different
capacities to stabilise SOC
• Practical implication –
management outcomes on
SOC will vary with soil type

11. CO2 / Soil carbon: climate change impacts
• Dryland agriculture
• Inputs
• Reduced potential plant growth and the inputs of carbon to soil is
likely where water is the main constraint.
• Losses
• Drier conditions are likely to reduce decomposition
• Evidence is mounting to suggest enhanced decomposition with
increasing temperature (larger relative impact on stable forms)
• Extension of cropping systems into current cold/wet environments
may occur – possible threat to existing carbon stocks
• Irrigated agriculture
• Increases inputs and rates of decomposition are likely.
• Net effect will depend on extent of alterations of inputs and losses

12. CO2 / Soil carbon: mitigation/sequestration
• The guiding principal - maximising the capture carbon given the
resources available at any particular location will maximise
SOC
• Enhanced water use efficiency (kg dm/mm water)
• Greater tolerance to subsoil constraints where possible
• Greater root: shoot ratios
• Altered composition of plant residues – increased lignin
• CO2 fertilisation may help offset reductions
• Positive impacts of building SOC on soil productivity – water
holding capacity, nutrient cycling, etc.

13. Nitrous oxide: climate change impacts
• Strong influence of temperature and water availability
Relative N2O emission
Temperature Soil water content
Total N2O emission
0.6
(µg N kg-1)
0.4
0.2
0.0
40 60 80 100
Incubation Temperature (°C) Water filled pore space (%)
Chen et al 2010 SBB 42 660 Dalal et al 2003 AJSR 41 165
• Net change will depend on the relative responses
Dryland Irrigated
Increased in tropics and subtropics Increased in all regions
Decreased in cooler temperate regions

14. Nitrous oxide: mitigation strategies
Key requirement – minimise the concentration of inorganic N
• Better matching of fertiliser N application to crop demand as
dictated by the season – develop flexible N strategies
• Increased reliance on biological N fixation to enhance soil N
status – processes controlling N mineralisation also control
plant growth
• Alteration of animal diets to avoid an intake of excess N and
excretion of high N content urine and faeces
• Application of inhibitors to reduce rates of formation and
transformation of soil ammonium – urease and nitrification
inhibitors

15. Methane: climate change impacts
• Soils can be a source or a sink for methane depending on their
oxidative condition
• Significant methane production occurs at redox potentials more
negative than -100 mV (rates increase
• Dependence on redox potential means that properties controlling
rates of oxygen diffusion and consumption exert strong control
• Where methane production conditions are met a strong response to
temperature exists (Q10 = 4 with an optimum near 35°C)
Flood irrigation Drip/sprinkler irrigation Dryland
Potential for methane emission will increase
Potential for methane consumption will increase

16. Methane: mitigation strategies
Key requirement – maintain soil in an oxidative state
• Adequate water management strategies:
• Flood irrigation - create temporary oxic conditions
(oxidises reduced species – e.g. Fe2+ to Fe3+)
• Sprinkler/drip irrigation – avoid prolonged saturation to
reduce emission, judicious control of soil water content can
optimise methane consumption
• Avoid incorporation of large amounts of degradable residues
just prior to or when soils are saturated
• Addition of SO42- - gypsum

17. Future research directions
• All gases
• Quantification of uncertainties associated with estimates
• Should build systems to define the cumulative probability of
outcomes
• N2O and CH4 from soils
• National evaluation of N2O and CH4 emissions reductions
will rely on modelling and/or emission factors
• Continued measurement of fluxes (e.g. NORP) will be
essential
• How do we best to deal with the diversity of agricultural
practice, soil type and climatic condition?
• How do we deal with climate change? Will calibration
against current conditions be good enough?
• Definition of the relative responses to temperature and soil
water content and potential interactions.

18. Future research directions
• Soil carbon
• A combination of measurement and modelling will be required
• Measurement – establish initial conditions, verify model
predictions, and allow recalibration
• Models – predict the likely outcomes of alterations to
management to help guide management
• Derivation of an appropriate statistical approach to assess the
potential of innovative practices
• Rapid and cost effective soil sampling -
• Smarter sampling of soils at different scales – use of available
spatial datasets to help direct sampling.

19. Regional soil carbon estimation (Wheeler et al.2011a)
Regional soil carbon prediction
• 3 biogeographic regions
– Brigalow (NSW portion)
– NSW South Western
Slopes
– South Eastern Highlands
• ~170 000 km2
– 65% grazing
– 18% cropping
– 11% forestry
– 6% other

20. Regional soil carbon estimation (Wheeler et al.2011a)
On training data On training data
Average absolute error 0.1 Average absolute error 0.09
R2 0.59 R2 0.55
On test data On test data
Average absolute error 0. 14 Average absolute error 0. 11
R2 0.45 R2 0.38
0 – 10 cm 0 – 30 cm

21. Summary
• Development of a robust modelling capability will be required to
• construct regional and national emission assessments and
• define the potential outcomes of on farm management decisions
and policy decisions.
• This model development will require comprehensive field data
sets to calibrate models and validate outputs.
• Improved spatial layers of model input variables collected on a
regular basis will be required to optimise accounting at regional
through to national scales.
• A diversity of agricultural practices exist in Australia. A
continual matching of practice to soil and climate and economic
assessment to optimise outcomes.

22. Jeff Baldock
Sustainable Agriculture Flagship
Phone: (08) 8303 8537
Email: jeff.baldock@csiro.au
Thank you
Contact Us
Phone: 1300 363 400 or +61 3 9545 2176
Email: Enquiries@csiro.au Web: www.csiro.au

23. CO2 / Soil carbon: composition
Particulate organic carbon (2 mm – 0.05 mm) (POC) Resistance to
decomposition
Humus (<0.05 mm) (HumC) increases
Resistant organic carbon (ROC): dominated by charcoal
Particulate carbon Humus carbon Resistant
(2mm – 0.05 mm) (<0.05mm) (charcoal <2mm)
400 m 10 m 20 m