Keith PaustianDept. of Soil and Crop Sciences
and Natural Resource Ecology Lab
Colorado State University
Fort Collins, CO
Soil carbon sequestration: What’s the
potential and why do we care?
Soil Health Institute – July 13-14, 2017
Outline
Why do we care?
Basic principles
What are the practices?
‘Conventional’ best management practices
Frontier technologies
‘What’s the potential ?
3
[Intended Nationally Determined Contributions]
* Kevin Anderson and Glen Peters, “The Trouble with Negative Emissions”, Science, 14 Oct 2016
(for 2oC)
Negative
Emissions
The 2oC goal requires cumulatively-substantial CO2 removal this
century, starting ~2030 and reaching > 15 GtCO2/yr by 2100.
Biological negative emissions (BNE)
Agricultural soils – best mgmt. practices & frontier
technologies
Afforestation or reforestation
Biomass energy with CO2 capture and storage (BECCS)
Chemically-based negative emission
Direct air CO2 capture and storage (DACCS)
Accelerated weathering/mineralization of CO2 (land or ocean)
Ocean iron fertilization
4
Most negative emissions technologies are technologically
immature, and risks with deployment at scale not well known.
Soil organicmatter
CO2
The soil C balance
Harvest
Soil organicmatter
CO2
The soil C balance
Harvest
Increasing C inputs
Soil organicmatter
CO2
The soil C balance
HarvestDecreasing C losses
Soil C sequestration- key principles
• Duration of C sequestration is finite – SOC
balance tends towards equilibrium.
• Total amount of storage is limited – SOC
stabilization is subject to saturation.
• Soils depleted in SOC have the greatest
capacity to gain C, but often the least
propensity to do so.
• Gains are reversible. To maintain the C
gain, the practices must remain.
Conventional technologies - soil C BMPs
No till, cover crops, intensified rotations
Meta-analyses of no-till adoption
0.2-0.5 tonne C/ha/y
Meta-analysis of cover crops
0.35±0.08 tonne C/ha/y
Franzluebbers (2005) found NT + cover
crops 2X C storage of NT alone
Set-aside, grassland restoration, conversion
to pasture
CRP land – South Dakota
Restored tallgrass prairie – Wisconsin
System ΔSOC
tC/ha/y
Source
Cropland to
pasture (global)
0.87 Conant et al. 2017
Restored prairie 0.77 Tillman et al. 2006
Cropland to
pasture (SE USA)
0.84 Franzluebbers 2010
Improved pastures and grazing systems
• Moderate stocking rates
• Pasture improvement
(nutrient mgmt., legume over-seeding)
Adaptive Multi-Paddock (AMP) grazing
0.3-1.3 tC/ha/y Morgan et al. 2010
0.3-0.7 tC/ha/y Conant et al. 2016
TAMU
Arkansas pasture – USDA/NRCS
• Short, intensive grazing periods
• Extended ‘rest’ between grazings
Teague et al. reported ~3 tC/ha/y
increase w/ conversion to AMP
(relatively few field studies)
Other soil management practices
Compost (other organic) amendments
Restoration of drained, cultivated organic
(peat) soils (‘rewetting’)
Non-CO2 GHG emission reductions
N management options for N2O
CH4 abatement from flooded (e.g. rice) soil
Frontier technologies
Biochar addition to soils
• Coproduct of pyrolysis for biofuel • High porosity, resistant to decomposition
Current constraints to scale-up: High cost, feedstock supply
Understanding soil C and GHG impacts of biochar use requires a more complex life-cycle accounting
1.8 Gt CO2/y mitigation potential, global, estimated by Woolf et al. (2010)
• Added C persists for 100s years• > Plant growth in many soils• < N2O emissions
‘High C input’ crop development
Perennial cereals?Annuals with more and deeper roots?
For US ag land:
• 0.4-1.4 t C ha-1 yr-1
• up to 0.8 Gt CO2e yr-1
Paustian et al. 2016 – ARPA-E report
Current constraints to
scale-up: Breeding, yields,
economics
What is the aggregate potential?
Paustian et al. 2016 Nature
Global GHG mitigation potential for aggregated ag practices
Total technical potential ~4-8 Pg CO2e yr-1
Study/Citation Estimate
Gt CO2eq/y
Scope
Paustian et al. 1998 1.5-3.3 Improved cropland management, set-aside, restoration
of degraded land
Lal & Bruce 1999 1.7-2.2 Improved cropland management, restoration of
degraded land
IPCC 2000 3 Improved cropland & grassland management, setaside,
agroforestry, restored peat soils
Lal 2004 1.5-4.4 Improved cropland & grassland management, setaside,
agroforestry, restored degraded lands
Smith et al. 2008 5-5.4 Improved cropland & grassland management, setaside,
agroforestry, restored degraded lands, restored peat
soils§
Sommer & Bossio
20142.5-5.1 Improved cropland & grassland management, setaside,
agroforestry, restored degraded lands
Paustian et al. 2016a 2-5 Improved cropland & grassland management, set-aside,
agroforestry, restored degraded lands, restored peat
soils
Paustian et al. 2016a 4-8 Improved cropland & grassland management, set-aside,
agroforestry, restored degraded lands, restored peat
soils, + biochar, +high root C crop phenotypes
Table 3. Published estimates of global soil carbon sequestration potential.
Closing thoughts
Soil carbon sequestration can play a crucial role
in global GHG mitigation
The ‘technical potential’ of ‘conventional
practices’ is relatively well-characterized
The biggest unknown is adoption rates – what
policies and incentives can achieve the highest
and lowest cost rates?
Implementing SCS: Two-stage strategy: 1)
rapidly ramp-up existing BMPs now, and 2) R&D
to allow scale-up of ‘frontier technologies’ before
2050
Thanks for your attention!
Questions?
