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: jeff.baldock@csiro.au

Contact UsPhone: 1300 363 400 or +61 3 9545 2176Email: Enquiries@csiro.au 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