Grassland Soil Carbon Stocks: Status,
Opportunities and Vulnerability
What do we know?
Richard Conant
Natural Resource Ecology Laboratory &
School of Global Environmental Sustainability
Colorado State University
Institute for Sustainable Resources
Queensland University of Technology
2007 IPCC AR4: effects of
warming evident; cost of
reducing emissions far less
than damage they will cause
CO2 concentrations are increasing:
Human activities are driving increases in atmospheric CO2
10000
8000
MMt C
6000
1995 IPCC 2nd report:
“signature of human
activities”
Fossil fuel emissions
Tropical LUC
Temperate LUC
1956 Phillips: 1st
somewhat realistic
global climate model
1897 Chamberlin: model of
global C exchange
1896 Arrhenius: 1st calculation
of anthropogenic global
warming
1938 Callendar: CO2
greenhouse global
warming is underway
4000
1859Tyndall: some
gasses absorb IR;
2000drive climate
could
change
1930s Global
warming trend since
late 19th century
reported
1988 IPCC established;
1st report 1990
1976 Deforestation
recognized as important
driver of climate change
1958 Keeling: Atm. CO2
measurements begin at
Mauna Loa
0
1860
1880
1900
1920
1940
1960
1980
2000
Perspective
Climate policy timeline
2008-2012: 1st Kyoto
compliance period
10000
Fossil fuel emissions
Tropical LUC
Temperate LUC
8000
A modest proposal?
MMt C
6000
2005: Kyoto into effect
Near-term implementation
of a global policy that affects
all parts of everyone’s lives.
4000
2001: Marrakech accords
1997: Kyoto Protocol
1992: US Energy policy
act; incl. Section 1605(b)
2000
1992: Rio Treaty –
establishes UNFCCC
0
1860
1880
1900
1920
1940
1960
1980
2000
Carbon flow in grassland ecosystems
What do we know?
CO2
Forage
consumption
Soil carbon
Stocks
1. The things we do on the
land impact carbon stocks
• Grazing
intensity/seasonality
• Species composition
• Soil fertility
2. Other factors influence
carbon stocks
• Climate
• Soil characteristics
• Past land use
3. Given information about
these things, soil carbon
stocks can be predicted
• Current stocks
• gains or losses w/
changes in
management
Carbon stocks in grassland ecosystems
What do we know? --management influences C stocks
System carbon
Sequestration potential = f(C loss)
Disturbance
Time
Management
change
Carbon flow in grassland ecosystems
What do we know?
Some numbers:
5.1Bha  730Mha converted to cropland
~415Pg C (in top 20cm)  ~at least 30 PgC lost due to conversion to cropland
Degraded soils
Cultivation
Domestic exploitation
Deforestation
Overgrazing
Industrial exploitation
(Glasod database; Oldeman 1994)
Potential C sequestration (tC/ha/yr)
-0.50 - -0.25
-0.25 - 0.00
0.00 - 0.25
0.25 - 0.55
(Conant and Paustian 2002)
Carbon stocks in grassland ecosystems
What do we know? --management influences C stocks
(Conant et al. 2001)
Carbon stocks in grassland ecosystems
What do we know? --technical potential for sequestration is large
Carbon stocks in grassland ecosystems
What do we know?
1. Grassland soil C stocks are substantial.
2. Management impacts carbon stocks – C stocks
are susceptible to loss
3. Carbon lost from grassland systems can be regained through changes in management
4. Improved management practices can increase
soil carbon stocks
5. Technical potential for sequestration in grasslands
is large
Grassland Soil Carbon Stocks: Status,
Opportunities and Vulnerability
What don’t we know?
Richard Conant
Natural Resource Ecology Laboratory &
School of Global Environmental Sustainability
Colorado State University
Institute for Sustainable Resources
Queensland University of Technology
Carbon stocks in grassland ecosystems
What do we know? --measurements on the ground are sparse
• Data are very sparse globally.
(Conant and Paustian 2002)
Carbon stocks in grassland ecosystems
What do we know? --measurements on the ground are sparse
• Data are very sparse globally.
• We measure soil C stock changes (either
before/after or neighboring fields)
Carbon stocks in grassland ecosystems
System carbon
What do we know? --management influences C stocks
Disturbance
Time
Now Management
The future
change
Carbon stocks in grassland ecosystems
What do we know? --measurements on the ground are sparse
• Data are very sparse globally.
• We measure soil C stock changes (either
before/after or neighboring fields)
• This approach is uncertain and has limitations
• One way forward: more data, better models, tighter
data-model linkages.
Carbon stocks in grassland ecosystems
What don’t we know?
1. We still have limited information on
how management influences C stocks
• We have a growing database of on-theground studies
• Data are still limited
• Little consistency between sources
• Synthesis still challenging
All Grassland Studies
25000
y = 0.994x + 93.113
R2 = 0.9557
modSOMSC
20000
15000
10000
5000
0
0
5000
10000
15000
20000
25000
m easSOC
All Grassland Studies
6000
y = 0.754x - 293.69
R2 = 0.6542
modSOMSC DIFF
4000
2000
0
-6000
-4000
-2000
0
-2000
-4000
-6000
m easSOC DIFF
2000
4000
6000
Grazing Differences
25000
y = 1.02x - 74.94
R2 = 0.98
modeled somsc
20000
15000
10000
5000
0
0
5000
10000
15000
20000
25000
measured SOC
Grazing Differences
5000
y = 0.53x - 14.01
R2 = 0.64
4000
modeled somsc change
3000
2000
1000
0
-5000
-4000
-3000
-2000
-1000
0
1000
-1000
-2000
-3000
-4000
-5000
measured SOC change
2000
3000
4000
5000
Carbon stocks in grassland ecosystems
What don’t we know?
1. Data on the ground are sparse
2. We still have limited information on how
management influences C stocks
• Few studies focus on mechanisms driving
changes in C stocks
• Current models have limited capability to
represent observed C stock changes.
• This inability to understand drivers of change
limits our ability to forecast C stock changes.
Carbon stocks in grassland ecosystems
What do we know? --management influences C stocks
System carbon
Sequestration potential = f(C loss,
loss)
propensity for C gain)
Disturbance
Time
Management
change
Carbon stocks in grassland ecosystems
What don’t we know? -- unknown capacity to change management
(IPCC AR4 CH8)
Carbon stocks in grassland ecosystems
What don’t we know?
1. Data on the ground are sparse
2. We still have limited information on how
management influences C stocks
3. We don’t know how widely practices that
sequester carbon can/will be implemented
• Technical potential differs from – and is easier to
assess than – economic potential
• Technical potential is just one piece of the
equation
• Activity and economic data are limited in the US,
but much (MUCH!) more-so world-wide
Food production is part of the C cycle
human
respiration
CO2
Forage
consumption
How does harvesting biomass
affect ecosystem carbon
stocks?
Soil carbon
Stocks
Are there ways to maximize
both harvest and ecosystem
carbon stocks?
Carbon stocks in grassland ecosystems
What don’t we know? -- limited knowledge about costs/benefits
Carbon stocks in grassland ecosystems
What don’t we know?
1. Data on the ground are sparse
2. We still have limited information on how
management influences C stocks
3. How widely practices that sequester carbon
can/will be implemented; limited information
about costs vs. benefits
4. Impacts of carbon-sequestering practices on
other greenhouse gas emissions
• Upstream – energy use, inputs
• Downstream – transport, CH4 emissions
Carbon stocks in grassland ecosystems
Some recommendations
1. Expand on-the-ground data collection,
greater focus on understanding mechanisms
2. Coordinate data collection and release to
lower barriers to meaningful synthesis
3. Take a systems perspective; understand
sequestration within the larger context of
economic costs and benefits