Soils and Climate Change: Greenhouse gas emissions implications and research requirements
Jeff Baldock, Ichansi Wheeler, Neil McKenzie and Alex McBratney
CCRSPI Conference, Melbourne15-17 February, 2011
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
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
Aerobic soil
conditions
Anaerobic soil
conditions
Soil surface
CH4
Methanogenicorganisms
N2O
Mineralisation
Denitrification
NH4 NO3
Nitrification
Assimilation and mineral protection
CO2
Decomposition
Respiration
Organic carbon
Soil organic matter including decomposer
organisms
Organic nitrogen
Generation of greenhouse gases by soil
N fertiliser &Animal waste
Aerobic soil
conditions
Soil surface
N2O
Biological transformationsassociated with
N cycling
Inorganic N NH4 & NO3
CH4
Methanotrophicorganisms
CO2
Organic carbon
Soil organic matter including decomposer
organisms
Organic nitrogen
Root dry matter
Pla
nt d
ry
mat
ter
Shoot dry matter
Residuedeposition
Photosynthesis
Immobilisation
Uptake
Consumption of greenhouse gases in soil
Projected changes to Australia’s climate
Australian agricultural regions• warmer and drier • altered seasonality• greater extremes
Such changes will undoubtedly influence rates of net greenhouse gas emissions
Magnitude of change will be defined by the sum of the climate change influence on all processes
Change in annual rainfall (%)-40 -20 -10 -5 -2 2 5 10 20 40
Change in annual potential evapotranspiration (%)-4 -2 2 4 8 12 16
Change in average annual temperature (°C)
2030 2050 2070
0.3 0.6 1.0 1.5 2.0 2.5 3.0 4.0 5.0
Source: http://climatechangeinaustralia.com.au - 50th percentile of projected changes under the medium future emissions profile relative to 1980-1999
CO2 / Soil carbon: inputs of carbon
1) The amount of PAR2) Fraction of PAR used 3) Efficiency of carbon capture,4) Proportion lost to respiration5) Proportion removed in products.
Factors 1-4 define potential net primary productivity
Other constraints (water, fertility, disease) may reduce efficiencies and lead to Actual NPP < Potential NPP
Product removal – harvest index issue
Controls on potential carbon input
CO2
Product harvest
Photosyntheticallyactive radiation (PAR)
CO2 / Soil carbon: inputs of carbon
Identify systems that are not achieving 100% resource use efficiency (water and nutrients)
Identify constraints and define whether or not they can be managed
Yes
Implement management changes and
capture additional
carbon
Where can carbon inputs be increased?
No
Consider alternative production
systems that may be better suited to constraints
CO2
Product harvest
Photosyntheticallyactive radiation (PAR)
CO2 / Soil carbon: fate of carbon inputs
Issues - residue placement – surface
residues vs roots- reduced incorporation
The remainder resists decomposition and replaces the soil organic carbon that is being decomposed
CO2
Product harvest
Photosyntheticallyactive radiation (PAR)
Soil organic carbon
What happens to the carbon inputs?
The majority is decomposed and returned to the atmosphere as CO2
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
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
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.
• Strong influence of temperature and water availability
• Net change will depend on the relative responses
Nitrous oxide: climate change impacts
40 60 80 100
0.2
0.0
0.4
0.6
Water filled pore space (%)
Soil water content
Rel
ativ
e N
2O e
mis
sion
Temperature
Tota
l N2O
em
issi
on(µ
g N
kg-1
)
Incubation Temperature (°C)Chen et al 2010 SBB 42 660 Dalal et al 2003 AJSR 41 165
DrylandIncreased in tropics and subtropicsDecreased in cooler temperate regions
IrrigatedIncreased in all regions
Nitrous oxide: mitigation strategies
• 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
Key requirement – minimise the concentration of inorganic N
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
Methane: mitigation strategies
• 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
Key requirement – maintain soil in an oxidative state
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.
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.
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
Regional soil carbon estimation (Wheeler et al.2011a)
0.45R2
0. 14Average absolute error
On test data
0.59R2
0.1Average absolute error
On training data
0.38R2
0. 11Average absolute error
On test data
0.55R2
0.09Average absolute error
On training data
0 – 10 cm 0 – 30 cm
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.
Thank you
Jeff BaldockSustainable Agriculture Flagship
Phone: (08) 8303 8537Email: [email protected]
Contact UsPhone: 1300 363 400 or +61 3 9545 2176Email: [email protected] Web: www.csiro.au
CO2 / Soil carbon: composition
Particulate organic carbon (2 mm – 0.05 mm) (POC)
Humus (<0.05 mm) (HumC)
Resistance to decomposition
increases
Resistant organic carbon (ROC): dominated by charcoal
Humus carbon(<0.05mm)
10 m10 m10 m
Resistant(charcoal <2mm)
20 m20 m
Particulate carbon(2mm – 0.05 mm)
400 m400 m400 m