Sequestration of carbon in soil: myth or reality Jeff Baldock - PowerPoint PPT Presentation

Sequestration of carbon in soil: myth or reality Jeff Baldock CSIRO Land and Water Adelaide, SA

Outline • How much and where is organic carbon located in a soil? • What is soil organic matter composed of and why is it important to define its composition? • Why would we want to increase the amount of carbon stored in soils? • How do we change soil carbon content through management practices? • What do we need to consider when measuring changes in soil organic carbon? • Issues associated with measurement • Ground truthing results • Economics and whole of system issues CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Where is carbon located in a soil and how much can soils hold? Soil organic carbon content (% by weight) 0 1 0 1 2 0 1 2 3 0 2 4 6 0 Soil Depth (cm) 50 100 150 200 Mallee Red Black Krasnozems sands earths earths • Pattern with depth is consistent with root distributions • Surface soil carbon cycles faster • Different soils hold different amounts of carbon CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

What is the soil organic carbon composed of? Crop residues on the soil surface (SPR) Extent of decomposition increases Buried crop residues (>2 mm) (BPR) Rate of decomposition decreases Particulate organic matter Soil organic carbon (2 mm – 0.05 mm) (POC) C/N/P ratio decreases (become nutrient rich) Humus (<0.05 mm) (HumC) Resistant organic matter Dominated by charcoal (ROC) with variable properties CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Types of soil organic carbon Particulate material Humus Resistant (Char) (POC) (HumC) (ROC) CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Variation in amount of C associated with soil organic fractions 30 Organic C in 0-10 cm layer SPR 25 BPR POC 20 HumC (Mg C/ha) ROC 15 10 5 0 Hamilton, Hart, Yass, Urrbrae, Waikerie, Vic SA NSW SA SA Mixed Mixed Pasture Cropped Pasture rotation rotation CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Why is it important to define the composition of soil organic carbon? 30 Total - TOC Humus Conversion to 25 Particulate - POC permanent Soil organic carbon Resistant - ROC pasture (g C kg -1 soil) 20 15 ~30% less humus 10 5 18 y 10 y ~800% more POC 0 10 20 30 50 0 40 70 60 15 33 43 Years CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Vulnerability of soil carbon content to variations in management practices 30 Conversion TOC to intensive Humus 25 cultivation Soil organic carbon POC Conversion ROC (g C kg -1 soil) to pasture 20 15 10 5 18 y 10 y 9 y 0 10 20 30 50 0 40 70 60 43 52 15 33 Years CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Why would we want to increase the amount of carbon stored in soils? Beneficial contributions that organic carbon makes to soil Biological Physical Chemical roles of soil carbon roles of soil carbon roles of soil carbon - biochemical energy - structural stability - cation exchange - reservoir of nutrients - water retention - pH buffering - increased resilience - thermal properties - complexes cations The contributions that organic carbon makes to each function varies with: • Composition (POC, Humus and ROC) • Soil type (texture and mineralogy) CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Water holding capacity (water retention) • How much plant-available water can a soil hold • Upper limit (wetter soil) - soil water content after drainage • Lower limit (drier soil) - soil water content at which plants can no longer extract water Analogy of a sponge removed from a bucket of water Remove sponge Squeeze out from water as much water Stops as possible dripping Upper Lower limit limit CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Change in water holding capacity with a 1% increase in soil organic carbon content For 0-10 cm layer of South Australian Red-brown earths Change in water holding capacity 6 y = -0.1229x + 5.5029 3 mm extra stored R 2 = 0.82 rainfall for 10 rainfall 5 events equates to 30 (mm water) Average mm total 4 ~3 mm 3 Sandy soils Issue: harder to ~5 mm 2 build up soil carbon on a sandy 1 Clay loams soil than a clay ~2 mm 0 0 10 20 30 40 Clay content (% of soil mass) CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

The carbon cycle: options for sequestering carbon Decomposition CO 2 Photosynthesis Harvested Decomposition products Death Harvest Plant Death production Burning or Residues Biochar production Soil Resistant Increasing Particulate animals organic C extent of organic C and decomposition microbes Humus Carbon sequestration options organic C 1) increase C stored in long lived plants 2) increase C stored in long lived products 3) increase C stored in one or all soil components 4) mover more carbon into resistant forms (e.g. charcoal) CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

What determines soil organic carbon content? Soil organic carbon Inputs of Losses of f = , content organic carbon organic carbon Inputs Losses • Conversion of • Net primary productivity organic C to CO 2 (return of • Protection offered residues) by soil minerals • Addition of waste • Extent and organic materials frequency of cultivation CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Balance between inputs and outputs 30 25 Soil organic carbon Inputs x 3 (g C kg -1 soil) 20 Inputs x 2 15 Inputs = Outputs Inputs / 2 10 Inputs / 3 5 0 20 40 60 100 0 80 140 120 Years CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Influence of variations in climate and management 30 25 Soil organic carbon (g C kg -1 soil) 20 15 10 5 0 20 40 60 100 0 80 140 120 Years CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Minimum requirements for tracking soil organic carbon for accounting purposes 1. Collection of a representative soil sample to a minimum depth of 30 cm 2. An accurate estimate of the bulk density of the sample 3. An accurate measure of the organic carbon content of a soil sample For 0-30 cm soil with a bulk density of 1.0 Mg/m 3 and a carbon content of 1.0% Bulk Mass of Carbon Depth 30 Mg C/ha Carbon = x density x content = (cm) (g/cm 3 ) (Mg C/ha) (%) CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Dynamic nature of SOC and its fractions Irrigated Kikuyu pasture – Waite rotation trial 32 Amount of organic C TOC POC Humus ROC (Mg C ha -1 in 0-10 cm) 24 16 8 0 1/6/98 6/2/99 14/10/99 20/6/00 25/2/01 Date of sample collection CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Correcting soil carbon for management induced changes in bulk density Management induced compaction Original soil surface Original 30 cm depth New 30 cm depth Soil bulk density (Mg/m 3 ) 1.1 1.2 1.3 1.4 Mass Soil 0-30 cm (Mg/ha) 3300 3600 3900 4200 Depth for equivalent mass (cm) 30.0 27.5 25.4 23.6 Organic C loading (Mg/ha) 1% OC, no BD correction 33 36 39 42 1% OC, with BD correction 33 33 33 33 CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Influence of tillage on changes in soil carbon with depth Organic carbon content (% soil mass) 0.0 0.5 1.0 1.5 2.0 2.5 0 Cultivated 10 to 10 cm Soil depth (cm) 20 Uncultivated 30 40 If red region > blue region = sequestration 50 For 0-10 cm layer 60 red region > blue region (sequestration) 70 For 0-30 cm layer 80 red region = blue region (no sequestation) CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Ground truthing claims: the carbon perspective Changing soil carbon from 2% Amount of carbon in the 0-10 cm layer to 4% over 5 years 90 Amount of C required: 80 24 Mg C 70 50 Mg Dry Matter (DM) 1% SOC (Mg C/ ha) 60 2% SOC 50 48 Rate per year (no losses): 3% SOC 10 Mg DM/y 40 4% SOC 50% allocation below ground 30 24 equates to 5 Mg shoot DM/y 20 10 Rate per year (with 50% loss) 0 20 Mg DM/y (50% loss) 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 50% allocation below ground Bulk density 10 Mg shoot DM/y (g/cm3) CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Ground truthing claims: the nutrient perspective 7000 600 C/N=10 C/P=120 6000 Amount of N (kg/ha) Amount of P (kg/ha) 500 4800 5000 400 400 4000 300 3000 2400 200 200 2000 100 1000 0 0 1.0 1.2 1.4 1.6 1.0 1.2 1.4 1.6 Bulk density (g cm -3 ) Bulk density (g cm -3 ) 1.0% SOC 2.0% SOC 1.0% SOC 2.0% SOC 3.0% SOC 4.0% SOC 3.0% SOC 4.0% SOC CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Modelling changes in soil carbon content Measured 70 POC HUM 60 0-30 cm Soil C (t/ha) CHAR 50 TOC 40 Modeled DPM 30 RPM 20 HUM IOM 10 BIO 0 Soil 1982 1987 1992 1997 Year CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009

Modelling changes in soil organic carbon under permanent pasture at Nimmitabel 180 Model conditions Rain 150 • Location: Nimmitabel, NSW Pan Evap mm Water 120 • Annual rain 695mm, • Apr-Oct rain 380mm 90 60 • Dry Matter production 30 • 6 t shoot dry matter • 50% of shoot dry matter ingested 0 Mar May Nov Jan Sep Jul • 67% of ingested carbon lost • root/shoot ratio = 1 Month of the year • Soil properties • 0-30 cm soil layer • bulk density = 1.4 Mg/m 3 • clay content = 15% CSIRO. Monaro Farming Systems – Carbon Forum 6/07/2009