Lecture 4: Terrestrial Carbon Process

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Lecture 4 Terrestrial Carbon Process I. Carbon
Stocks and Fluxes in Terrestrial Ecosystems II.
Terrestrial Ecosystems A. Ecosystem Concept
B. Ecosystem Carbon Balance (GPP, NPP, NEP,
NBP) III. Missing Carbon sinks
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• Fluxes and Pools of Carbon on the Earth

Terminology
Carbon pool The reservoir containing carbon
(carbon stock) Carbon flux The rate of exchange
of carbon between pools (i.e.
reservoirs) Carbon sinks Carbon reservoirs and
conditions that take-in and store more carbon
(e.g. carbon sequestration) than they
release. Carbon sources Carbon reservoirs and
conditions that release more carbon than
take-in and store Carbon sequestration The
uptake and storage of carbon
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2). The Units of Carbon
Pg C petagrams of carbon (1 Pg C 1015g C 1
billion tons C) Gt C gitatonnes of carbon (1
Gt C 1 Pg C) Mt C megatonnes of carbon (1
Mt C 1012 g C) Tg C teragrams of carbon (1
Tg C 1 Mt C) (1 Pg C 3.7 Pg carbon dioxide)
1 C 3.7 CO2
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1. Terrestrial ecosystems are important
components in the global carbon cycle that
create many of the sources and sinks of CO2,
(methane, and nitrous oxide)

• The Ecosystem Concept
• The ecosystem including not only the
  organism-complex, but the whole
• complex of physical factors forming what we call
  the environment
• (Tansley, 1935)
• Whittaker (1975) suggested that an ecosystem is
  a functional system that includes assemblage of
  interacting organisms (plants, animals and
  microbes) and their environment, which acts on
  them and on which they act.

Ecosystem Biotic community Plant, animal,
microbial community Abiotic environment
atmosphere, soil and geological substrate
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2. The dynamics of terrestrial ecosystem
depend on interactions between a variety
of biogeochemical cycles, particularly
the carbon cycle, the cycles of nutrient and
water.
For example the interconnectedness of the
various living and nonliving components of
ecosystem that a change in any one will results
in a subsequent change in almost all others.
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3.  Carbon Balance of Terrestrial Ecosystem (4
important concepts)

• GPP (Gross Primary Production)
• The total amount of carbon fixed in the process
  of photosynthesis
• by plants in an ecosystem, such a stand of
  trees.
• Global total GPP 120 Gt C yr-1

(b) NPP (Net Primary Production) The net
production of organic matter by plants in an
ecosystem that is, GPP reduced by losses
resulting from the respiration of plants
(autotrophic respiration, Ra).   NPP GPP-
Ra  Global total NPP 60 Gt C yr-1
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(c) NEP (Net Ecosystem Production) The net
accumulation of organic matter or carbon by an
ecosystem.   NEP NPP-Rh (heterotrophic
respiration)   Rh includes losses by herbivory
and the decomposition of organic debris by soil
biota ( 50 Gt C yr-1) Global total NEP 10 Gt
C yr-1   (NEP is also called NEE- net ecosystem
exchange)
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(d) NBP (Net Biome Production) The net
production of organic matter in a region
containing a range of ecosystem (a biome). NBP
NEP non-respiratory C losses
through ecosystem disturbances
(harvest, forest fire, clearance,
insect etc.) (by Schulze, E-D and M. Heimann,
1998) Global NBP is small (about ?1 Pg C for
1989-1998) NEP and NBP are key indicators used
to describe the annual net C balance of forest
ecosystems  
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CO2
Disturbance (9 Gt C yr-1)
Plant Respiration (60Gt C yr-1)
Decomposition (50Gt C yr-1)
GPP 120 Gt C yr-1
Med.-term Carbon Storage
Long-term Carbon Storage
Short-term Carbon Uptake
CO2 assimilation
NPP 60 Gt C yr-1
NEP 10 Gt C yr-1
NBP 1 Gt C yr-1
0.5
lt5
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4.Carbon Stocks and Flows in Major Biomes of
Terrestrial Ecosystems
       -Total ecosystem area 151.2 ? 106
km2        -Vegetation carbon ?500 Gt C       
-Soil carbon ?2000 Gt C        -Total
terrestrial biomes ? 2500 Gt C
       Forests contain a large part of carbon
stored on land, in the form of biomass
(trucks, branches, foliage, roots etc.) and
in the form of soil organic carbon.      
       Grassland ecosystems store most of their
carbon in soils, where turnover is
relatively slow.
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•   Wetlands are important carbon reservoirs.
• - Undrained peatlands in high latitude - C
  sinks
• (0.2-0.5 t C ha-1 yr-1), but, sources of
  methane
• (0.03-0.3 t CH4 ha-1 yr-1)
•  
• - Peatlands that are drained for agriculture
• or for afforestation release carbon as CO2,
• No significant methane release.

In croplands, carbon stocks are primarily in the
form of below-ground plant organic matter and
soil  
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Atmospheric evidence of large carbon exchanges
by the biosphere
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3). Major carbon pools on the Earth

• The Earth contains about 1023 g of carbon
• Active surface Pools
• Atmosphere 7.5 x 1017 g ( 750 Pg )
• Land plants 6.1 x 1017 g (610 Pg)
• Soils 1.6 x 1018 g (1, 600 Pg)
• Ocean Dissolved Inorganic carbon 3.9 x 1019 g
• (surface and deep oceans) (39,000 Pg)
• Fossil Fuels and cement production 4 x 1018
  (4,000 Pg)
• - Sedimentary rocks
• organic compounds 1.56 x 1022 g
• carbonate 6.5 x 1022 g

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4). Major carbon fluxes on the Earth
NPP (net primary production) on land 60 Pg
yr.-1 - Photosynthetic Uptake 120 Pg yr.-1
- Plant Respiratory Loss - 60 Pg yr.-1 Soil
Respiration - 60 Pg yr.-1 Fossil Fuels 5.5 Pg
yr.-1 Land Use Changes 1.6 Pg yr.-1 Ocean
Uptake (physiochemical diffusion) 92 Pg yr.-1
Ocean Release (physiochemical diffusion) 90 Pg
yr.-1
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Two important factors contributing global carbon
budgets
Contemporary carbon cycle is controlled primarily
by the rate at that carbon moves in and out of
these pools, not by their size
For example Calcium carbonate in desert soil
930 Pg C (930 x1015 g C) Flux from this pool to
atmosphere 0.0023 Pg C yr 1 (85,000 years
turnover time!)
Small change in the rate of large carbon pools
can have a dramatic impact on the atmospheric
CO2
For example A 0.1 increase in the rate of
decomposition on land, release about 0.6 Pg C
/yr to the atmosphere
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2. The Missing Carbon Sinks
Atmospheric Imbalance - the Missing Carbon Sink?
Net sources of CO2 to the atmosphere (in units of
1015g) Fossil Fuels - 5.5 Pg yr.-1 Land Use
Change - 1.6 Pg yr.-1 Net sinks for CO2 from
the atmosphere Atmospheric Increase 3.2 Pg
yr.-1 Oceanic uptake (increase) 2.0 Pg yr.-1
Net carbon emissions Net carbon
sinks Imbalance 1.9 Pg yr.-1 (unknown carbon
sinks) (ref. 1980 to 1989)
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Missing Carbon Sinks The imbalance between
carbon emissions and sinks (of about 1.9 Pg C
yr-1 for 1980s) is often refereed to as the
missing carbon sinks
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Finding the Missing Carbon Sinks
Hypothesis 1 There is a large terrestrial sinks
for anthropogenic CO2 in the Northern Hemisphere
Hypothesis 2 The ocean carbon sinks will
continue to increase in response to rising
atmospheric concentrations, But, the rate of
increase will be modulated by changes in ocean
circulation, biology and chemistry
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Take home message Two Questions 1. What are
the evidences in support of the terrestrial
carbon sinks? 2. What are the major factors
contributing the terrestrial carbon sinks?
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1. What are the evidences in support of the
terrestrial carbon sinks?
A. Expected CO2 North-South Gradient The
smaller than expected north-south gradient of
atmospheric CO2, combine With data on the partial
pressure of CO2 in ocean surface waters, suggests
that There is a large terrestrial carbon sinks at
temperate latitudes in the Northern Hemisphere
(Tans et al., 1990)
Tans, P., I.Y. Fung, and T. Takahashi. 1990.
Observational constraints on the global
atmospheric CO2 budget. Science, 247 4131-1438.
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B. Isotopic Fractionation The existence of a
large terrestrial sink at northern latitude is
supported by 13C/12C ratio measurement in
atmospheric CO2 (Ciais et al., 1995) and by
measurements of the O2/N2 ratio (Keeling et al.,
1996).

• Ciais, P., P.P. Tans, M. Trolier, J.W. C. White,
  and R.J. Francey. 1995. A large north hemisphere
• terrestrial CO2 sinks indicated by the 13C/12C
  ration of atmospheric CO2. Science 269
  1098-1102.

• Keeling, R.F., S.C. Piper, and M. Heimann. 1996.
  Global and hemisphere CO2 sinks deduced from
• changes in atmospheric O2 concentration. Nature,
  381 218-221.

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C. New Technique of Eddy Covariance Flux
measurements obtained by this technique in
different ecosystems have demonstrated the
ability of some forest to act as significant net
sinks for atmospheric CO2 (Wofsy et al., 1993)

• Wofsy, S.C et al. 1993. Net exchange of CO2 in a
  mid-latitude forest. Science, 260 1314-1317.

D. Recent Forest Inventory suggested that there
has been a substantial increase in the carbon
stock in northern forest biomass, of the order of
06-0.8 Pg C y-1 (Caspersen et al., 2000 Fang et
al. 2001 Goodale et al. 2002)
Caspersen, J. P., et al. 2000 Contributions of
land-use history to carbon accumulation in U.S.
forests. Science 290, 1148-1151.7
Fang, J. A. Chen, C. Peng, X. Zhao, and L. Ci,
2001 Changes in forest biomass carbon storage in
China between 1949 and 1998. Science 292,
2320-2322.
Goodale, C.L. et al. 2002. Forest carbon sinks in
the Northern Hemisphere. Ecological. Application,
12 891-899
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E. Remote Sensing Estimates A large carbon sink,
of order 0.68 Pg C, Was found in the woody
biomass of Northern forests based on 19 years of
data from remote-sensing spacecraft and forest
inventory.
Myneni, R.B. et al. 2001. A large carbon sink in
the woody biomass of Northern forests. PNAS 98
14784-14789
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2. What are the major factors contributing the
terrestrial carbon sinks?

• Several factors contribute to carbon sinks
  including
• Direct human impact (e.g. reforestation and tree
  regrowth)
• Indirect human impacts (e.g. CO2 and nitrogen
  fertilization)
• Natural factors (e.g. climate variability, Fire
  )

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• Schimel et al. (1995) attributed the missing
  carbon sinks to
• Enhance forest growth due to CO2 fertilization
  (Beta factor)
• e.g. CO2 stimulation of plant growth (32-41)
• (2) Forest regrowth due to land-use and land
  cover change
• (3) Increase N deposition
• (4) Positive response to climate anomalies
  (climate variability)

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Land Cover Change in USA
Ref. G. C. Hurtt et al., Proc. Natl. Acad. Sci.
USA 99, 1389 (2002).