Soils and Climate Change: Greenhouse gas emissions implications and research requirementsJeff Baldock, Ichansi Wheeler, NeilMcKenzie and Alex McBratneyCCRSPI 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
Generation of greenhouse gases by soil CO2 N fertiliser & N2O CH4 Animal waste Soil surface Denitrification Respiration NH4 NO3 Methanogenic Nitrification organisms MineralisationAssimilation Decompositionand mineral protection Organic Organic carbon nitrogen Aerobic Anaerobic soil soil Soil organic matter conditions conditions including decomposer organisms
Consumption of greenhouse gases in soil CO2Photosynthesis 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 & NO3Organic Organic Aerobiccarbon nitrogen soil Immobilisation conditions Soil organic matterincluding decomposer organisms
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 mediumfuture emissions profile relative to 1980-1999
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 productivityProductharvest Other constraints (water, fertility, disease) may reduce efficiencies and lead to Actual NPP < Potential NPP Product removal – harvest index issue
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 NoProductharvest Implement Consider management alternative changes and production capture systems that may additional be better suited carbon to constraints
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 carbonProduct that is being decomposedharvest Issues - residue placement – surface residues vs roots Soil organic - reduced incorporation carbon
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.
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 IrrigatedIncreased in tropics and subtropics Increased in all regionsDecreased in cooler temperate regions
Nitrous oxide: mitigation strategiesKey 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
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 strategiesKey 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
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) On training data On training dataAverage absolute error 0.1 Average absolute error 0.09 R2 0.59 R2 0.55 On test data On test dataAverage absolute error 0. 14 Average absolute error 0. 11 R2 0.45 R2 0.38 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.