Photosynthesis and
Net Primary Production
OCN 401 - Biogeochemical Systems 4 September 2012
Reading: Schlesinger, Chapter 5
1. Photosynthesis • Chemical Mechanism
• Water-Use Efficiency
• Nutrient-Use Efficiency
• Net Primary Production and Respiration
• Land-Air Fluxes - The Eddy Correlation Technique
2. Global Estimates of Net Primary Production and
Biomass
3. Global Simulation Models of Net Primary Production
Outline
Photosynthesis CO2 + H2O CH2O + O2
• Process by which carbon is reduced from CO2 to
organic carbon
• Provides the energy for the biosphere (except for
chemoautotrophy at hydrothermal vents and other
redox discontinuities)
• Affects composition of atmosphere
• Affects development of soils
• Responsible for Earth’s distinctive biogeochemistry
• Rates of net plant growth vary widely from ~0 to
>1000 g C m-2 yr-1
Chemical Mechanism
1. Capture of light energy
• Chlorophyll molecule oxidized by light
• NADP reduced to NADPH
• Water split: 2H2O 4H+ + 4e- + O2
• ATP generated from proton gradient
2. Carbon reduction
• NADPH and ATP
used to reduce
CO2 to form
organic matter
• Enzyme = ribulose
biphosphate
carboxylase
(Rubisco)
Carbon dioxide enters leaves via stomates on lower side of
leaf
Stomatal conductance (cm/sec) controlled by water
availability
Under optimal water conditions, Rubisco amount and
activity controls photosynthetic rate
Water-Use Efficiency
Stomatal conductance controls CO2, O2 and H2O transfer
H2O loss is transpiration, major mechanism of water
transfer to atmosphere (e.g., 25% of precipitation in NH forest
lost by transpiration)
Water Use Efficiency (WUE) = mmoles CO2 fixed/ moles
H2O lost
Typical values = 0.86 - 1.50. Lower stomatal conductance,
higher WUE
Higher atmos [CO2] allows lower conductance, higher WUE
Thus, number of leaf stomates may be getting smaller as
atmospheric CO2 increases
C isotope fractionation can be used to determine WUE
1.1% of atm CO2 is 13C, which diffuses and reacts slower than 12C
Plant tissue contains ~ 2% fewer 13C than atmospheric CO2
[ 13Cplant-tissue = -20‰ (per mil)]
13C 13C/12C samp- 13C/12C std x 1000
13C/12C std
13Catmosphere = -8‰
Thus, for most land plants, 13Cplant-tissue -8‰ + -20‰ = -28‰
When conductance is
low, less isotopic
discrimination, as
more of total CO2
inside leaf used
When stomatal
conductance is high,
more isotopic
discrimination, 13C is
more negative
13C in preserved plant material show WUE has increased in plants since
last glacial max as atmospheric CO2 has risen, particularly since the
Industrial Revolution
Nutrient-Use Efficiency (NUE)
In many species, nitrogen-limitation affects Rubisco
content affects photosynthetic rate
P may be limiting for some species
Mg and Mn are limiting in rare cases
N-limitation is most
common
Rate of net
photosynthesis / N in
leaf = NUE = slope
Similar for most species
NUE inversely related to
WUE
Net Primary Production and Respiration
Photosynthesis measured is usually “Net” (i.e., net amount of
CO2 taken up or O2 released by total plant metabolism)
Net Primary Production (NPP) = Gross Primary Production
(GPP) - Plant Respiration (Rp)
GPP does not all go to plant growth, herbivores, litterfall etc.
Plant respiration 50% of photosynthesis. Thus, gross
photosynthesis 2x net photosynthesis
Plant biomass 50% C by weight
NPP 1% of intercepted light energy
NPP measured by harvesting at peak growth or by seasonal
change in mass of tissue -- correct for in-season consumption
and loss
NPP separated into above-ground and below-ground
Above-ground split between leaves and stem (woody) growth
Forests: ~25% of above-ground NPP in leaves
Shrublands: 35 - 60% of above-ground NPP in leaves
Old forests: smaller % in leaves than in young forests
Boreal forests have a higher proportion of woody growth than
in tropics -- more respiration at higher temperatures
Above-ground annual NPP correlates with leaf biomass:
• Root growth difficult
to measure but can be
high fraction (>50%) of
total NPP
• In forests, higher %
for root growth in less-
fertile soils
Land-Air Fluxes - The Eddy Correlation (Eddy
Covariance) technique
Used to measure flux of CO2 into/out-of a terrestrial
ecosystem
Downward air flows have higher CO2 levels than upward
air flow (due to CO2 consumption by plants)
Requires very fast measurements of [CO2] and 3-D wind
velocity
sofia.usgs.gov/projects/evapotrans/photogallery.html www.st.hirosaki-u.ac.jp/.../turbulence.html
Example of data -- Massachusetts deciduous forest:
GPP = 1070-1210 g C m-2 yr-1 (overall CO2 uptake during
day)
Plant and soil respiration = 810 -1140 g C m-2 yr-1 (CO2
release at night)
Net accumulation rate of C (or wood growth) by the
ecosystem (true increment) = 140 - 280 g C m-2 yr-1
NPP is greater than the increment since NPP includes leaves
and other short-lived tissues
Global Estimates of NPP and Biomass
1. Harvest measurements: Need detailed regional coverage
for global models (very expensive)
2. Remote sensing:
• Thematic Mapper: Use ratio of surface reflectance in two
wavebands to estimate chlorophyll inventory (TM4 / TM3):
Ground truth satellite data: TM4 /
TM3 reflectance ratio Leaf area
index (LAI) (m2leaf/m
2ground)
Leaf area Index NPP Thus, TM4/TM3-ratio NPP
• Normalized difference vegetation index (NDVI):
Ratio light reflectance:
NDVI = (TM4 – TM3) / (TM4 + TM3)
NDVI is measure of density of green vegetation (“greeness”)
Total biomass 560 x 1015 g C
Pixel size and regional validity of calibration are still big issues Woody tissues hard to estimate by color, but water-filled tissue
can be estimated via reflected microwaves using SAR:
SAR bands C (5.9 cm/5.3 GHz), L (24
cm/1.25 GHz), and P (65 cm/440 MHz)
and their scattering mechanisms for
different vegetation types.
www.gecoz.com/SARApplications/whatisSAR.htm
Global NPP
Mean global residence time for C in plant tissue =
biomass / NPP = 9 yr
Varies from ~4 yr in deserts to >20 yr in forests
Values average
• Short-turnover materials such as leaves (<1 yr)
• Long-turnover tree wood (decades to centuries)
Global estimates still based on harvest methods -- are
biased by selection of representative regions!
Current values of biomass may be too high, as ecologists
tend to study mature, well-developed sites
NPP values indicate latitude effect:
Tropical forests > boreal forests > shrub tundra
Latitude effect decreases with decreasing precipitation:
Forests > grasslands > deserts
Hig
hly
pro
du
ctive
. W
e’ll
dis
cu
ss in
de
tail
late
r.
Global distribution of NPP reflects NPP vs. T relationship:
…and also NPP vs. precipitation relationship:
NPP generally decreases with elevation -- a temperature
effect
However, NPP can increase with elevation if precipitation
increases with elevation
Use temp and precipitation relationships to develop global
maps of NPP
Observed good agreement with satellite data implies T and
precipitation are main NPP limiting factors.
Global annual NPP 45 - 65 x 1015 g C yr-1
Global Simulation Models of Net Primary Production
NPP = NPP(max) • PAR • LAI • T • [CO2]atm • H2Osoil • NA
PAR = photosynthetically active radiation
LAI = leaf area
T = temp
H2Osoil = soil moisture
NA = index of nutrient availability
Calculated global NPP = 53 x 1015 g C yr-1 (sum of calculations
from 56,000 pixels of data)
Models can be used to predict changes in NPP as CO2 and
precipitation vary
http://earthobservatory.nasa.gov/Newsroom/NPP/npp.html
Visualization of Global NPP Data
Combine to calculate NPP:
• Space-based measurements of a
range of plant properties collected
by the Moderate Resolution Imaging
Spectroradiometer (MODIS)
• Suite of other satellite and surface-
based measurements
When you average the productivity
rates over the whole world, the
ocean is roughly equal to the land
Movie showing two years of data reveals strong seasonal
cycles of plant growth (NPP), especially at high latitudes
across North America, Europe, and Asia:
http://earthobservatory.nasa.gov/GlobalMaps/view.php?d1
=MOD17A2_M_PSN#
The movie also reveals the almost immediate response of
land plants to changing daily weather patterns.