Chapter 5: The carbon cycle of terrestrial ecosystems

1
Chapter 5 The carbon cycle of terrestrial
ecosystems
2
Photosynthesis review
Photosynthesis conversion of CO2, H2O, and
nutrients into organic matter. Chlorophyll,
contains Mg Photons oxidize some chlorophyll
molecules, passing the electron to a series of
electron transfer proteins ultimately leading to
the reduction of nicotinamide adenine
dinucleotide phosphate (NADP) to NADPH The
chlorophyll molecules then regain an electron
from a water molecule which is split by an enzyme
containing Mn, Ca, and Cl. This reaction is the
origin of O2 in the atmosphere 2H2O ? 4H 4e-
O2
3
Photosynthesis review

• Protons produced above build up within a
  chloroplasts or cell membranes, creating
  potential energy used to build adenosine
  triphosphate (ATP) from adenosine diphosphate
  (ADP).
• NADPH and ATP are then used along with enzymes to
  reduce CO2 and build carbohydrates.
• This reaction involves rubulose bisphosphate
  carboxylase (Rubisco), sometimes referred to as
  the CO2-capturing enzyme, which adds CO2 to the
  basic carbohydrate unit
• CO2 H2O ? CH2O O2

4
Photosynthesis review
This is a two-stage process 1. Light energy
allows water molecules to be split and high
energy molecules to form 2. CO2 is reduced to
carbohydrate
5
(No Transcript)
6
Photosynthesis review

• CO2 diffuses through stomata
• Stomatal conductance is a major factor and is
  strongly affected by
• The boundary layer (layer of still air next to
  leaf
• Stomatal aperture and morphology
• Water leaves plants via stomata, and plants
  close stomata when under water stress
• Thus, plants do not carry on photosynthesis when
  under water stress.

7
Photosynthesis review

• Water Loss and Photosynthesis
• Book quotes evapotranspiration (ET) as a
  percentage of total precipitation for Hubbard
  Brook Experimental Forest
• In actuality, ET is more or less a constant
  amount over time rather than a percentage, and
  thus the percentage of ET varies with precip
  amount.
• The same is true of Walker Branch, Tennessee
• See next slide

8
(No Transcript)
9
(No Transcript)
10

• Water use efficiency
• WUE mmoles CO2 fixed/moles H2O lost
• Typically ranges from 0.86 to 1.50 mmol/mol
• WUE is higher at lower stomatal conductance
• Rising CO2 allows greater WUE, also because the
  concentration of CO2 in the stomatal chamber (Ci)
  increases.
• Some evidence that number of stomata per leaf
  area has decreased as CO2 levels in the
  atmosphere have risen.

11

• Water use efficiency
• ?13C can be used to gain an index of long-term
  water use efficiency.
• 12CO2 is favored over 13CO2 because it is lighter
• Atmospheric CO2 ?13C -8.0 o/oo most plants
  show a ?13C of approximately 28 o/oo
• But when stomata are closed, basically all CO2
  with stomatal cavity is taken up, and the signal
  is closer to atmospheric.
• See Fig. 5.2
• The ?13C signal of plants has shown greater WUE
  over time, possibly because of greater
  atmospheric CO2 levels.

12
(No Transcript)
13

• Nutrient use efficiency (NUE)
• Rate of photosynthesis is largely correlated with
  leaf N concentration on a mass basis.
• This is because N is the most commonly limiting
  plant nutrient.
• Most leaf N is contained in enzymes rubisco
  accounts for 20-30 alone.
• Sometimes P concentration is related to
  photosynthesis
• Book states that Mg and Mn are seldom limiting,
  but Mg deficiencies in forests of central Europe
  are not unusual.

14

• Nutrient use efficiency (NUE)
• Nitrogen use efficienty (NUE) rate of
  photosynthesis per unit leaf N (Figure 5.3)
• Seems to be rather constant among plants
  differences subtle.

15
(No Transcript)
16

• Nutrient use efficiency (NUE) and fertilization
• Book notes that NUE declines with fertilization
• But does this reflect low efficiency or an error
  in semantics?
• Is it not more efficient for a plant to
  increase its N concentration in times of plenty,
  store it for later?
• This is one of many reasons several scientists do
  not like the term nutrient use efficiency.
• It should be more precisely defines, for example,
  as PS/N, or photosynthesis per unit N.
• Nutrient use efficiency is further confused
  because it often refers to biomass production per
  unit N uptake, which can be interpreted as the
  inverse of simple concentration.

17

• Respiration
• Plants respire just like we do release CO2.
• Thus, measured photosynthesis is actually net
  photosynthesis, the difference between
  photosynthesis and respiration.
• Gross photosynthesis is about twice net
  photosynthesis
• But is a function of temp, so plants can burn
  themselves out
• Respiration is correlated with N content (p.
  134)
• But he really means concentration, given what is
  on Figures 5.3 and 5.4

18
(No Transcript)
19
(No Transcript)
20

• Net primary production (NPP)
• Net photosynthesis integrated over longer times
  than measured in the lab
• Gross Primary Production (GPP) Plant
  respiration (Rp) Net Primary Production (NPP)
• NPP includes losses due to herbivores, grazers,
  litterfall it is greater than standing crop
• Net annual accumulation of C per unit land (p.
  135) should be amended to specify that is refers
  to plants only, I think, not including C loss
  from soils.
• Plants are 45 50 C (I usually get 47), so we
  can take this biomass and express it as C Units
  g m-2 or kg ha-1, or Mg ha-1.

21

• Net primary production (NPP)
• Measurement is often not easy, especially
  belowground (root) production. In forests, it
  equals litterfall increment root turnover
  (smaller than standing crop). Increment is
  estimated from biomass regressions based on dbh.
• In grasslands, it is not equal to standing crop
  and is difficult to measure, especially when
  grazers are involved.
• Allocation to roots, foliage, and stems varies
  greatly with species.
• Boreal forests allocate more to wood than
  tropical forests, maybe because tropical forests
  must spend a greater amount of their capital on
  respiration.

22
Net primary production (NPP) Good
relationship between leaf biomass and aboveground
NPP (ANPP), seen in Figure 5.5 ANPP is often all
that is reported because roots are so difficult
to measure.
23
(No Transcript)
24

• Net primary production (NPP)
• When it is measured, root growth is often found
  to contribute a lot to NPP
• e.g., more than half in a deciduous forest in
  Eastern Tennessee Edwards and Harris, 1977, and
  also in a subalpine fir forest in Washington
  Cascades Vogt et al, 1982 Table 5.1
• However, there are serious problems in
  measurement. Sequential coring may miss some
  peaks and troughs, and greatly underestimate root
  NPP

25
(No Transcript)
26
Net primary production (NPP) New method of
estimating NPP Eddy flux. Measure CO2 fluxes
into canopy during day and out at night using
towers, gas samplers at different heights, and
sensitive anemometers. However, there have been
some recent problems reconciling eddy flux
measurements with on the ground measurements
(sampling over time). Eddy flux measurements are
sometimes unaccountably high
27
Net primary production (NPP) Eddy flux
websites Generic for Ameriflux http//public.or
nl.gov/ameriflux/Participants/Sites/Map/index.cfm
Duke Example http//www.env.duke.edu/other/AMERIF
LUX/amerflux.html
28
Remote sensing of Primary Production and
biomass Based on absorption and reflectance
of light by chlorophyll Chlorophyll absorbs red
and blue, reflects green and absorbs little
infrared (Fig 5.6). Selective measurements of
reflectance give an index of leaf area index
(LAI m2 m-2)
29
(No Transcript)
30
Remote sensing of Primary Production and
biomass There is a good correlation between
LAI and production (Fig 5.8) From advanced very
high resolution radiometer (AVHRR) from NOAA
satellite, Goward et al (1985) developed
normalized difference vegetation index
(NDVI) NDVI (NIR VIS)/(NIR VIS). This
simply maximizes the difference between visible
and NIR
31
(No Transcript)
32
(No Transcript)
33

• Global estimates of NPP and Biomass
• So far, these are based on compilations of data
  from harvest measurements. Noteworthy sources are
  Whittaker and Likens (1973), Olson et al (1983)
• Values range from 45 to 65 x 1015 g C yr-1
• Olson et al (1983) estimate land biomass is 560 x
  1015 g C yr-1
• This gives a mean residence time of 9 yr, but
  ranges from 4 in deserts to gt20 in some forests.
• Recall that these are weighted averages, so in
  forests, for example, wood turns over in decades
  to centuries whereas foliage every year or
  several years.

34

• Global estimates of NPP and Biomass
• Production is greatest in tropics, least in shrub
  tundra.
• Leith (1975) correlated NPP in 52 sites with MAT
  and MAP, suggesting that soils played a lesser
  role. Figs 5.10 and 5.11.
• Melillo et al (1993) suggests
• NPP NPPmax x PAR x LAI x T x CO2 x H2O x NA
• Where
• PAR photosynthetically active radiation
• LAI leaf area index
• T temp
• NA index of nutrient availability (fudge
  factor)

35
(No Transcript)
36
(No Transcript)
37

• NPP and Global Change
• Direct harvest of plants for food, fuel, and
  shelter accounts for about 3.3 x 1015 g C yr-1,
  or 3 of NPP worldwide.
• Humans may reduce NPP by 25 to 40 by inadvertent
  activities like fire, pollution.
• Largest diversion of NPP to support a single
  species in the history of the planet, and thought
  provoking given the rate of human population
  growth.

38
Now for my bias on all this The 25 to 40
value seems quite high to me but I dont know
enough to question it seriously Also note that
humans can inadvertently increase NPP by N
pollution, and this is certainly the case for
forests in Europe at this time (handout). Environm
entalists often focus on the problems, the gloom
and doom, and often do not offer positive
solutions (such as increasing growth), only to
stop this and that. While we must face the
facts, whatever they may be, to throw up our
hands and give up is not a good option.
39
Now for my bias on all this We must manage
this NPP, not make some completely futile
attempts to get it set back to pre-human
levels. Whether we like it or not, humans are
here, and to simply decry their presence and
continuously attempt to prove that they are
destructive and somehow ecologically immoral
accomplishes nothing.
40
Global C P. 147 Most of the increase in
atmospheric carbon dioxide is due to the burning
of fossil fuels is this really so? Take a
look at the global carbon cycle how about fire,
how about a slight imbalance in detrital input
and decomposition? Who is to say that these are
in balance? On the other hand, the isotopic
signature seems to suggest that fossil fuels are
the major cause of the increases, I am told.
41
The Global Carbon Cycle
Atmosphere 748 Annual Increase 3.9
Fossil Fuels 6
100-115
Fire 5.6
Respiration 60
Detritus 60
Decomposition 60
Terrestrial Biota 500-800
Oceans 1000
Soil and Litter 1500-1600
42
The Global Carbon Cycle The rate of biomass
regrowth does not equal the rate of
harvest Simply put, the carbon storage in
agricultural crops or forest regrowth is less
than the carbon contained in the original forest
biomass. This is true but what about the wood
products? They may last longer than wood in the
forest might have.
43
The Global Carbon Cycle The book quotes
Houghton et al (1983) as to losses of biomass by
110 x 1015 g C yr-1 Or by about 13 of
preindustrial biomass. Includes the release from
soils. We checked on the release from soils in
literature reviews and, and it is flawed in a
major way (Johnson and Curtis, 2001)
44
(No Transcript)
45
Effects of forest harvesting on soil C and N -
literature review
Johnson, D.W., and P.S. Curtis. 2001. Effects of
forest management on soil carbon and nitrogen
storage Meta Analysis. For. Ecol. Managem. 140
227-238.
46
Johnson, D.W., and P.S. Curtis. 2001. Effects of
forest management on soil carbon and nitrogen
storage Meta Analysis. For. Ecol. Managem. 140
227-238.
47
Reforestation of the eastern US There has
been a net reforestation in the eastern US,
especially the southeast. Globally, this might
amount to 0.7 x 1015 g C yr-1
48

• Elevated CO2
• Increasing CO2 might help sequester more carbon
  how possible is this?
• Large increases in growth have been found with
  elevated CO2 in lab studies, open top chamber
  studies
• This is because CO2 facilitates greater WUE and
  NUE
• There is less need for rubisco, so foliar N
  declines
• Greater production per unit N uptake
• Therefore, a growth response is possible even
  with water and N limitations.

49

• Elevated CO2 Example results from lab and open
  top chamber studies
• The majority of studies show lower foliar N
  concentration with elevated CO2
• Various studies have shown
• Constant N uptake but greater growth
• Lower N uptake with either greater growth or no
  growth increase
• Greater N uptake and greater growth (growth
  increase offsets N concentration effect).
• Where does the extra N come from?
• Soil mining?
• Free-living N fixation?
• Reduced N toxicity with elevated CO2 in ponderosa
  pine (our studies)

50
Examples of tree seedling response to elevated CO2
Examples from our studies
51
Examples from our studies
Johnson, D.W., R.B. Thomas, K.L. Griffin, D.T.
Tissue, J.T. Ball, B.R. Strain, and R.F. Walker.
1998. Effects of CO2 and N on growth and N
uptake in ponderosa and loblolly pine. J.
Environ. Qual. 27 414-425.
52
Examples from our studies
Examples from our studies
Johnson, D.W., R.B. Thomas, K.L. Griffin, D.T.
Tissue, J.T. Ball, B.R. Strain, and R.F. Walker.
1998. Effects of CO2 and N on growth and N
uptake in ponderosa and loblolly pine. J.
Environ. Qual. 27 414-425.
53
Examples from our studies
Examples from our studies
Johnson, D.W., R.B. Thomas, K.L. Griffin, D.T.
Tissue, J.T. Ball, B.R. Strain, and R.F. Walker.
1998. Effects of CO2 and N on growth and N
uptake in ponderosa and loblolly pine. J.
Environ. Qual. 27 414-425.
54
Examples from our studies
Examples from our studies
Johnson, D.W., R.B. Thomas, K.L. Griffin, D.T.
Tissue, J.T. Ball, B.R. Strain, and R.F. Walker.
1998. Effects of CO2 and N on growth and N
uptake in ponderosa and loblolly pine. J.
Environ. Qual. 27 414-425.
55
Examples from our studies
Examples from our studies
Johnson, D.W., R.B. Thomas, K.L. Griffin, D.T.
Tissue, J.T. Ball, B.R. Strain, and R.F. Walker.
1998. Effects of CO2 and N on growth and N
uptake in ponderosa and loblolly pine. J.
Environ. Qual. 27 414-425.
56
Examples from our studies
Johnson, D.W., R.B. Thomas, K.L. Griffin, D.T.
Tissue, J.T. Ball, B.R. Strain, and R.F. Walker.
1998. Effects of CO2 and N on growth and N
uptake in ponderosa and loblolly pine. J.
Environ. Qual. 27 414-425.
57

• Elevated CO2
• How relevant are lab and open top chamber
  studies to the real world?
• Most forests are in a closed-canopy, closed root
  system steady-state, not the early stage of
  foliage/root buildup
• Can elevated CO2 allow greater soil exploration
  in older stands where roots have been exploring
  for decades?
• How about feedbacks lower litterfall N
  concentration --gt reduced decomposition --gt less
  N mineralization --gt less N available for uptake?

58

• Elevated CO2
• Free Air CO2 studies started about a decade ago
  first one at Duke, and Schlesinger is now in
  charge of the follow-up study. Others in progress
  all over the world
• In general, results so far have shown
• Growth increase, but less than with smaller
  plants.
• Consistently lower foliar N
• Often greater N uptake
• Higher soil respiration
• More results to come soon.

59
Free Air CO2 (FACE) Studies
http//cdiac.esd.ornl.gov/programs/FACE/face.html
60
The fate of net primary production Biomass
reaches steady-state after a time, and though NPP
is positive, there is no true increment any more.
Fig 5.13 shows a Calluna shrubland after
fire Other examples from forest development from
ERS 497/697 Only in young ecosystems is there
really any net biomass increment In older
ecosystems, the biomass is produced and delivered
to the forest floor and soil to decompose. The
litter layer can continue to accumulate long
after biomass reaches steady-state in cold
systems, but in warm systems it, too, stabilizes
and much more quickly.
61
(No Transcript)
62

• The fate of net primary production
• Other examples from forest development from ERS
  497/697
• Only in young ecosystems is there really any net
  biomass increment
• In older ecosystems, the biomass is produced and
  delivered to the forest floor and soil to
  decompose.
• The litter layer can continue to accumulate long
  after biomass reaches steady-state in cold
  systems, but in warm systems it, too, stabilizes
  and much more quickly.

63
Forest stand development
64

• The fate of net primary production
• Odum suggests that increasing fractions of gross
  primary productivity (GPP, defined as NPP plant
  respiration) is lost to plant respiration and
  decomposition over time (Fig 5.14).
• He defines net ecosystem production (NEP) as
• NEP NPP (Rh Rd)
• Where
• Rh respiration of herbivores
• Rd respiration of decomposers
• Since NPP GPP Rp, we can say that
• NEP GPP Rt, where
• Rt total ecosystem respiration

65
(No Transcript)
66
Fire Fire is equivalent to instantaneous
decomposition, converting biomass to CO2. By
some estimates, fire delivers nearly as much CO2
to the atmosphere ( 5 x 1015 g yr-1) as fossil
fuel combustion ( 6 x 1015 g yr-1), and is on
the increase. Questions What does fire
contribute to global warming? Will there be
positive feedbacks between warming and fire?
67
The Global Carbon Cycle
Atmosphere 748 Annual Increase 3.9
Fossil Fuels 6
100-115
Fire 5.6
Respiration 60
Detritus 60
Decomposition 60
Terrestrial Biota 500-800
Oceans 1000
Soil and Litter 1500-1600
68

• Detritus production and decomposition
• Schlesinger presents the simple decay model
• I will give more detail and draw upon a classic
  paper by Olson (Ecology, 1963) which provides
  some very useful models for decomposition

69
A simple model of decomposition (Olson, 1963)
70
A simple model of decomposition (Olson, 1963)
71
A simple model of decomposition (Olson, 1963)
72
A simple model of decomposition (Olson, 1963)
73
A simple model of decomposition (Olson, 1963)
74
A simple model of decomposition (Olson, 1963)
Decomposition
Mass
Primary Production

Forest Floor Mass
Mean Annual Temperature
Fi
gur
e
2
.

S
che
m
a
ti
c

d
i
ag
r
a
m
o
f

t
he

change
s

i
n
li
t
te
r
f
a
ll
,

deco
m
po
s
i
ti
on

r
a
t
e
, and

f
o
r
es
t
fl
oor

accu
m
u
l
a
ti
on

w
i
th
i
nc
r
eas
i
ng
m
ean

annua
l

te
mp
e
ra
t
u
r
e

(A
ft
e
r
O
ls
on,

1963)
.
75
A simple model of decomposition (Olson, 1963)
76

• Soil Organic matter and humus formation
• The final products of decomposition are CO2 and
  nutrients
• However, the next to final product is humus, and
  it is a vital factor in soil fertility for many
  reasons (CEC, water holding capacity, N and other
  nutrient storage)

77
Soil Organic matter and humus formation Humus
is formed from partially decomposed organic
matter, is very complex, and is full of aromatic
compounds leading to its stability. N is often
a link between these compounds, and N is a major
factor in humus formation (not noted in the
book). Schlesinger gives standard methods for
fractionating humus and some background on bomb
14C work read this, I will not go through it.
I will here present some material on the
importance of CN ratio to decomposition which
should be in the book and also some newer results
of decomposition studies which had not been done
yet when the book was written
78

• The role of N in humus
• formation
• Abiotic incorporation
• of N with organic matter
• Paul and Clark, 1989

79
CN ratio a major factor controlling
decomposition rate and N release to plants and
nitrifiers Material C/N ratio Soil
Microbes Bacteria 61 Actinomycetes
61 Fungi 121 Litter
Types Alfalfa 131 Clover 201 Straw 8
01 Deciduous litter 401 to
801 Coniferous litter 601 to 1301 Woody
litter 2501 to 6001 Soil
Organic Matter 121 to 501
80

• CN ratio
• In order for soil microbes to decompose most
  litter types, they must initially incorporate N
  from the soil.
• Thus, inputs of high C/N ratio litter can cause N
  deficiency to plants unless accompanied by
  fertilization.
• As C is lost at CO2 gas, the C/N ratio of the
  litter decreases to a value ranging from 201 to
  301, at which point N is released from
  decomposing litter
• More on this later when we get to chapter 12

81

• LigninN ratio
• Cromack (1973) and later Aber and Melillo (1978)
  found that ligninN ratio was a good predictor of
  decomposition rate
• Higher lignin ---gt slower decomposition
• Lignin immobilizes N chemically
• Tannins will also do this but have not been
  studied much

82
Soil Organic matter and humus formation Björn
Berg has shown that the amount of final humus
formation, the asymptote of litter decay, is
greater in litter with higher initial N
concentration. This is despite the fact that
initial rates of litter decay are higher in
litter with higher N concentration (lower CN
ratio, as discussed above).
83
(No Transcript)
84
C Contents of O horizon and soil The book
says O horizon soil C is usually greater than
vegetation This is often not the case in forests,
especially older forests with high biomass and
southern forests with low O horizon and soil C
pools
85
The Integrated Forest Study (Johnson and
Lindberg, 1992)
Legend CPPinus strobus , Coweeta, NC DLPinus
taeda, Duke, NC GS Pinus taeda, B.F. Grant
Forest, GA LP Pinus taeda, Oak Ridge, TN FS
Pinus eliottii , Bradford Forest, FL
DFPseudotsuga menziesii , Thompson, WA
RAAlnus rubra, Thompson, WA NSPicea abies,
Nordmoen, Norway HFnorthern hardwood,
Huntington Forest, NY MSPicea rubens ,
Howland, ME WF Picea rubens, Whiteface, NY
ST Picea rubens, Clingmans Dome, NC LVPinus
contorta/P. jeffreyii , Little Valley, NV.
86

• C fluxes into and out of soil
• Measuring C into soil from litterfall is easy
• Measuring C into soil from root turnover is
  difficult
• Measuring CO2 flux from soil is possible, but
  tricky
• Using CO2 flux from soil to get soil balance is
  near impossible because there are two sources
• Heterotrophs decomposing organic C from plants
• Root and mycorrhizal respiration
• Figure 5.18 shows that measured soil respiration
  exceeds litterfall input by an average 2.5 x

87
(No Transcript)
88
Photosynthate
Organic Matter
CO2
Microbes
Roots
89
C fluxes into and out of soil Raich and
Nadelhoffer (1989) suggest that we can assume
soil and O horizon pools are in steady-state, and
then subtract this from total respiration and
estimate root respiration Rt Rh Rr Rt
total respiration Rh heterotroph respiration
(from dead OM) Rr root respiration (from
recently fixed C by plant) What are the
potential problems with this method?
90
Soil C and global change Soils constitute a
large C pool, and slight changes in it could
contribute greatly to atmospheric CO2 Conversion
of wildland soils to agriculture usually results
in a net C loss from soils of 20 to
40 (exceptions may occur in arid soils put to
irrigation) This no doubt contributed greatly to
the atmospheric CO2 increase during the last
century Example from Fig. 5.19 This C can
reaccumulate when former ag lands are reverted to
nature Table 5.5
91
(No Transcript)
92
(No Transcript)
93

• Soil C and global change
• Schlesinger has held that soil C accumulation
  rates are too low to matter for the global C
  budget, even though soil C pools are so large
• He comes up with an average value of 0.04 x 1015g
  yr-1 (only 0.003 of soil C per year)
• Rattan Lal at Ohio State University contends that
  soils could be an important sink for CO2

94

• Soil C and global change
• How accurate are the numbers?
• What kind of error bounds might they have?
• What would be the global implication of say, a 5
  error in the balance between detrital input (60 x
  1015g yr-1) and heterotrophic respiration (60 x
  1015g yr-1)?
• How well do we know the global C budget?
• Would you be willing to base multi-billion dollar
  policy decisions on the data at hand?

95

• Soil C and global change potential effects of
  warming
• Warming will increase decomposition, lower O
  horizon contents
• Soil C pools may tend to be lower however, many
  other factors strongly affect soil C
• Soil texture
• Fe, Al hydrous oxides (very important)
• N availability
• Primary production
• We know that O horizon pools decrease with
  temperature despite greater litterfall
• Things are not so clear with soil C pools

96

• Soil C and global change potential effects of
  warming
• Also keep in mind that aboveground C pools will
  be generally greater with warming, and in forests
  this will be a large effect
• Bottom line effects of warming are not
  completely clear, especially when combined with
  the effects of elevated CO2 and increased N
  pollution, which together could cause large
  aboveground C increases