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Partitioning of Net Ecosystem Exchange
from Agroecosystems into
Photosynthesis and Respiration
Nithya Rajan and Sumit Sharma
Dept. Soil and Crop Sciences, Texas A&M University
Stephen Maas
Dept. Plant and Soil Science, Texas Tech University
2015 ASA-CSSA-SSSA Annual Meeting)
Measuring Net Ecosystem CO2 Flux
Why NEE is important?
 We can partition NEE
into its two gross
components: canopy
photosynthesis and
ecosystem respiration
 We can also study
how and why these
fluxes vary with time
and environmental
conditions.
GPP (Gross Photosynthesis)
Reco (Respiration)
Sonic
Anemometer
• Ecosystem-scale flux
measurements of
atmospheric
CO2 were made
above the crop
canopy using fast
response instruments
• Measurements were
made at 10 Hz
Infrared Gas Analyzer
(IRGA)
Eddy Covariance Method
CO2 FLUX= Average of the covariance of concentration of
CO2 with the deviation in vertical component of the wind.
𝐶𝑂2 𝐹𝑙𝑢𝑥 = 𝐶𝑂2 ′ × 𝑤′
Eddy Covariance Method
CO2 Flux is Net Ecosystem CO2 Exchange (NEE) = GPP - Reco
NEE and GPP are negative when the CO2 flux is directed
to the surface. NEE and Reco are positive when the CO2
flux is directed toward the atmosphere.
GPP (Gross Photosynthesis)
WIND
Reco (Respiration)
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Methods for flux partitioning
• Several popular methods
Night time NEE represents ecosystem respiration.
 An empirical function describing the relationship
between ecosystem respiration and temperature
is derived using nighttime NEE measurements
 Daytime canopy photosynthesis is then estimated
by subtracting empirical estimates of daytime
respiration from daytime measurements of NEE
(Reichstein et al., 2005)
Issues related to current method
for flux partitioning
• How well estimates of daytime ecosystem
respiration can be represented as a function of
temperature?
• Daytime ecosystem respiration, at a given
temperature, may be different than nighttime
respiration at the same temperature.
(Baldocchi et al., 2015)
Soil Respiration – a main component
of ecosystem respiration
 Root respiration (30-50%)
 Rhizo-microbial
respiration
 Microbial respiration of
dead plant residues
 Soil organic matter
derived CO2
Study Sites
Forage Sorghum
Objectives
 Investigate the carbon flux
dynamics and investigate
the seasonal pattern in
NEE, GPP and Reco
measured using eddy
covariance flux towers in
the Southern High Plains
Southern
High
Plains
 Investigate the diurnal and
seasonal changes in soil
respiration component of
ecosystem respiration
Soil CO2 Flux Measurements
Perennial grassland
LI-8100A Automated Soil Gas Flux System
Irrigated Cotton
Dryland Cotton
Soil temperature and soil moisture (CS-655 sensors)
at 5 cm depth
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Results: Chamber 1 vs Chamber 2
Precipitation/Irrigation and Soil CO2 Flux
Slope = 1.46
Intercept = -0.11
R2 = 0.72 (n = 487)
High Soil CO2 Flux after Irrigation/Rain
Diurnal Respiration and Net Ecosystem
CO2 Exchange (DOY 158-162)
Cultivation
Diurnal Respiration and NEE (DOY 225228 & 254-257)
Respiration – Effect of crop growth stage
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Respiration-Photosynthesis (DOY 158-162)
Respiration-Photosynthesis (DOY 254-257)
Added a 6 hr lag
Respiration-Photosynthesis (DOY 254-257)
Soil Respiration – Temperature (No crop)
 The temperature
sensitivity of soil
respiration is
commonly called Q10
 With no-crop (JanMay 2015)
Q10 = 2.07
Soil Respiration: Mid to late-season
Seasonal pattern of daily integrated NEE,
GPP and Reco
Q10 = 1.53
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Conclusions
Acknowledgement
 Flux partitioning separates NEE into its two main
components; Gross Primary Production (GPP) and
Ecosystem Respiration (Reco).
 Current method of flux partitioning based on night time
Reco and temperature relationships may not capture
precipitation or irrigation pulse events and can
underestimate Reco
 Need to investigate more about the control of soil
moisture and photosynthesis on soil respiration to
improve flux partitioning
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