# Presentation: 276671

## Stable Isotopes in a C3-C4 Tallgrass Prairie

- By
- Chun-Ta Lai1, Andy Schauer1, Clenton Owensby2, Jay Ham2 and Jim Ehleringer1
- 1 Department of Biology, University of Utah, Utah
- 2 Department of Agronomy, Kansas State University, Kansas

## By

Chun-Ta Lai1, Andy Schauer1, Clenton Owensby2, Jay Ham2 and Jim Ehleringer1

1 Department of Biology, University of Utah, Utah
2 Department of Agronomy, Kansas State University, Kansas

- Chun-Ta Lai1, Andy Schauer1, Clenton Owensby2, Jay Ham2 and Jim Ehleringer1
- 1 Department of Biology, University of Utah, Utah
- 2 Department of Agronomy, Kansas State University, Kansas

- CO2

- Stable Isotopes in a C3-C4 Tallgrass Prairie

## By

Chun-Ta Lai1, Andy Schauer1, Clenton Owensby2, Jay Ham2 and Jim Ehleringer1

1 Department of Biology, University of Utah, Utah
2 Department of Agronomy, Kansas State University, Kansas

- Chun-Ta Lai1, Andy Schauer1, Clenton Owensby2, Jay Ham2 and Jim Ehleringer1
- 1 Department of Biology, University of Utah, Utah
- 2 Department of Agronomy, Kansas State University, Kansas

- 13COO

- 12COO

- C18OO

- C17OO

- C16OO

- Stable Isotopes in a C3-C4 Tallgrass Prairie

## The Role of Isotopes in Carbon Studies

- Isotopes of CO2 are influenced by gas exchanges between terrestrial ecosystem and the atmosphere
- The global isotopic budget should constrain estimates of gross carbon fluxes from land biosphere versus marine sources/sinks

## C4

- C3

- Mesophyll Cell

- Bundle Sheath Cell

- There are two photosynthetic pathways

## Slide 6

## Rannells Flint Hills Prairie, Kansas

## Study Period:

- LAI=1.8

- LAI=2.1

-  = 0.14

-  = 0.41

- July, 2002

- dry

- wet

## There are detectable diurnal variations in 

- July 17, 2002

- There are detectable diurnal variations in
- • CO2 concentration
- • 13C of CO2
- • 18O of CO2
- ...detectable even under **extreme drought** conditions.

******13****C (‰)**

******18****O (‰)**

**CO****2**** (********mol mol****-1****)**

## Measured NEE values during dry and wet periods, but what proportions of NEE come                        from C3 versus C4 species?

## Photosynthesis-stomatal conductance models for C3 and C4

- Maximum Rubisco Capacity

-   Farquhar et al. 1980

## In this grassland, Rubisco capacity is a strong function of soil moisture content

- Redrawn from Collelo et al. (1998)

## A comparison between modeled and measured diurnal NEE patterns

## Carbon isotope analyses provide an independent test to verify the C3-C4 proportions that are photosynthetically active

- 13C of organic matter:    -27.9 ‰      for    C3
-                                          -12.3 ‰      for    C4

- 13C of ecosystem respiration
- (integrated estimate)

- Keeling Plot

## Rannells Flint Hills Prairie, Kansas

## Keeling plot, dry period

- Y = -15.6 + 2715X; R2=0.97

- 13C (‰)

- 1/CO2 (ppm-1)

- 13CR = -15.6

## Carbon isotope analyses provide an independent test to verify C3-C4 proportions that are photosynthetically active

- Isotopic approach

- Independently confirms the model results !

- 100

- 0

- 50

- %

- C4 contribution

## C3 forbs

## ... but over time we see that this ecosystem “appears” 

- 13CR (‰)

- DOY

- Rannells Prairie, Kansas

- ... but over time we see that this ecosystem “appears”
- not to stay as a C4 dominated ecosystem

## Our C4 prairie is

- surrounded by
- C3 sources

- (C3)

- (C3)

- (C4)

- (C3)

## Wind speed (m s-1)

******13****C****R**** (****‰)**

- Wind speed (m s-1)

- C3

- C4

## Our C4 prairie is

- surrounded by
- C3 sources

- (C3)

- (C3)

- (C4)

- (C3)

## Significant variations of 13CR are also observed in forest ecosystems

- 13CR (‰)

- DOY

- Harvard Forest, 2001-2002

- Significant variations of 13CR are also observed in forest ecosystems

## Acknowledgements

-  Isotopes can help partition the contributions of C3 and C4 plants to the net ecosystem exchange fluxes
-  Isotopes can be used to assess the fetch of air sampling and identify non-biological sources

- In summary: