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?