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

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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.