Land Carbon Sink and Nitrogen Regulation

under Elevated CO2: Central Tendency

Yiqi Luo

University of Oklahoma

NCEAS Working group: William Currie, Jeffrey Dukes, Christopher Field, ,Adrien Finzi, Ueli Hartwig, Bruce Hungate, Yiqi Luo, Ross McMurtrie, Ram Oren, William Parton, Diane Pataki, Rebecca Shaw, Bo Su, Donald Zak

Other collaborators: Dafeng Hui and Deqiang Zhang

Probing mechanism toward predictive understanding

Meta-analysis to reveal central tendency

Meta analysis 104 published papers, 940 lines

Category variables:

Response variables (18):1. Biomass in shoot, root, and whole plant; 2. C pools in shoot, root, whole plant, litter, and soil3. N pools in shoot, root, whole plant, litter, and soil; 4. Ratios of C and N in shoot, root, litter, and soil pools; 5. Root/shoot ratio.

• sources of data• experimental facilities• ecosystem types, • field sites,

• exposure times, • nitrogen treatments• CO2 concentrations of

treatments

• 22-32% increases in averaged C contents (~30 g C m-2 yr-1)

c

t

XX

RR ln

• 21% increase in litter C

• 5.6% increase in soil C

• Ecosystem C increases by ~100 g m-2 yr-1

• Large variation among studies

Response Ratio

-0.6 -0.3 0.0 0.3 0.6 0.9 1.2 1.5

Freq

uenc

y

0

10

20

30

Freq

uenc

y

0

5

10

15

20

25

Freq

uenc

y

0

5

10

15

20

25

30

35

e: whole plant

c: Root

a: Shoot

Mean = 0.207Se = 0.02319n = 189P < 0.001

Mean = 0.275Se = 0.0286n = 168P < 0.001

Mean = 0.202Se = 0.0173n = 186P < 0.001

Response Ratio

-0.1 0.1 0.3 0.5 0.7

Soybean

Swiss 3 yrs

Florida

Sorghum

Duke 6 yrs

Duke 3 yrs

Swiss 2 yrs

Swiss 3 yrs

P. nigra

Ca grassland

Swiss 1 yr

Oak Ridge

P. alba

P. x euram

Response Ratio

-0.3 -0.1 0.1 0.3 0.5

Freq

uenc

y

0

2

4

6

8

10

12

14

e

Littercarbon

Mean = 0.054Se = 0.0117n = 40P < 0.001

Mean = 0.187Se = 0.0376n = 14P < 0.001

d

Soil carbon

Luo et al. 2006 Ecology

[ Mauna Loa Data from Keeling and Whorf (1994) ]

Year1960 1970 1980 1990 2000

CO

2 C

once

ntra

tion

(ppm

v)

310

320

330

340

350

360

370

380 As atm CO2 is rising, productivity usually increases

How does nitrogen regulates ecosystem responses to rising CO2? NHNH44

++NONO33

--

COCO22

NCEAS Working group

Progressive N limitation in plant and ecosystem responses to elevated CO2

NPP

N sequestered inbiomass & litter

C input to soil N sequestered

in SOM

labile soil N

N uptake N availability

C:N

CO2

Progressive Nitrogen Limitation

Luo et al. 2004 BioScineces

Two Approaches to Study C and N Coupling in Land Ecosystems

1. Global assessment

2. Meta-analysis of site-specific data from CO2 experiments

Hungate et al.2003 Science

Ecosystem models with N cycling processes incorporated predict carbon sinks more realistically that models without N cycling.

Freq

uenc

y

0

5

10

15

20

25

Freq

uenc

y

0

5

10

15

20

25

30

35

Response Ratio

-0.6 -0.3 0.0 0.3 0.6 0.9 1.2 1.5

Freq

uenc

y

0

10

20

30

Freq

uenc

y

0

4

8

12

16

20

24

28

32

Freq

uenc

y

0

5

10

15

20

25

Mean = 0.202Se = 0.0173n = 186P < 0.001

Mean = 0.275Se = 0.0286n = 168P < 0.001

Mean = 0.207Se = 0.02319n = 189P < 0.001

Mean = 0.045Se = 0.021n = 113P = 0.0342

Mean = 0.096Se = 0.0261n = 84P < 0.001

Response Ratio

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

Freq

uenc

y0

4

8

12

16Mean = 0.098Se = 0.0265n = 53P < 0.001

a: Shoot

Carbon in plant pools

b: Shoot

Nitrogen in plant pools

c: Root d: Root

e: whole plant

f: whole plant

• 22-32% increases in averaged C contents (~30 g C m-2 yr-1)

• 4-10% increases in averaged N contents (~0.44 g N m-2 yr-1)

Results of meta-analysis

Luo et al. Ecology In press

-0.2 0.0 0.2 0.4 0.6

SoybeanFlorida

Duke 6 yrsDuke 3 yrs

SorghumOak Ridge

Swiss 6 yrs

Soybean

Swiss 3 yrs

Florida

Sorghum

Duke 6 yrs

Duke 3 yrs

Swiss 2 yrs

Swiss 3 yrs

P. nigra

Ca grassland

Swiss 1 yr

Oak Ridge

P. alba

P. x euram

Freq

uenc

y

0

2

4

6

8

10

12

14

Response Ratio

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5

Freq

uenc

y

0

2

4

6

8

10

12

14

c: Carbon

d: Nitrogen

Nitrogen

carbon

Response Ratio

Mean = 0.054Se = 0.0117n = 40P < 0.001

Mean = 0.106Se = 0.0322n = 36P = 0.002

Mean = 0.187Se = 0.0376n = 14P < 0.001

Mean = 0.227Se = 0.0666n = 7P = 0.011

a

b

Litter pools Soil pools

Luo et al. Ecology In press

• 21% increase in litter C

• 25% increase in litter N

• 5.6% increase in soil C

• 11.2% increase in soil N

• Ecosystem C increases by ~100 g m-2 yr-1

• Ecosystem N increases by ~1 g m-2 yr-1

1. Complete downregulation of CO2 stimulation of

ecosystem C processes is unlikely to be pervasive across ecosystems.

2. Net N accumulation likely support, at least partially, long-term ecosystem C sequestration in response to rising atmospheric CO2.

Implications

Freq

uenc

y

0

4

8

12

16

20

Response Ratio

-0.4 -0.2 0.0 0.2 0.4 0.6

Freq

uenc

y

0

4

8

12

16

-0.4 -0.2 0.0 0.2 0.4 0.6

Freq

uenc

y

0

4

8

12

C/N ratio in plant pools

Mean = 0.110Se = 0.0209n = 57P <0.001

Mean = 0.103Se = 0.024n = 39P <0.001

Mean = 0.028Se = 0.011n = 36P =0.015

a: Shoot

b: Root d: Soil

Freq

uenc

y

0

1

2

3 c: Litter Mean = 0.026Se = 0.0355n = 8P =0.490

C/N ratio in litter and soil pools

Response Ratio

Stoichiometrical Flexibility

C/N increases by

• 11.6% in shoot

• 10.8% in root

• N.S. in litter

• 2.9% in soil

Luo et al. Ecology In press

Flexible C/N can support short-term CO2 stimulation of plant growth and C sequestration

1. Coupling of C and N in ecosystems is poorly understood, hindering model development.

2. Ecosystem models that incorporate N processes can better predict C sequestration.

3. Ecosystems do have mechanisms to increase N stocks to support long-term land C sequestration in response to rising atmospheric CO2.

4. Stochastic modeling may be the only viable approach to account for diverse C and N responses to elevated CO2.

Concluding Remarks

Acknowledgement

The Terrestrial Carbon Program, the Office of Science (BER), U.S. Department of Energy, Grant No. DE-FG03-99ER62800

The National Center for Ecological Analysis and Synthesis, a center funded by the National Science Foundation (DEB-94-21535), the University of California at Santa Barbara, and the State of California.

The National Science Foundation, Grant Nos. DEB 0092642 and DEB 0444518.

Variable FACE OTC GCShoot C 11.59* 13.87* 16.22*Root C 47.23* 1.33 36.47*

Plant C 4.57* 7.94* 21.22*Soil C 5.75* 6.62*Shoot N 21.11* 12.58* 4.35Root N 27.73* 19.41* 12.27*Plant N 26.25* 12.80* 14.66*Soil N 3.52 11.52*

CO2 Facility

Little systematic biases caused by facility

Luo et al. Ecology In press

Variable cropland forest grassland desert wetlandShoot C 14.21* 21.50* 9.80* 9.66 3.43Root C 22.54* 48.76* 40.49* 11.4 -12.97*Plant C 15.72* 26.72* 0.54 24.60* -8.51Soil C 2.81 5.56* 10.49* -0.73Shoot N -1.6 31.28* 20.46* 2.9 -10.5*Root N 24.49* 26.76* -3.64 -0.60Plant N 15.08* 25.67* -0.91 8.98*Soil N 18.29* 5.71* -8.52

Ecosystem Type

Desert, wetland and cropland have different responses, largely due to small sample sizes

Luo et al. Ecology In press

PNLoccurs

PNL may not occur

PNL may not develop

CO2

If NPP is stimulated?

N demand

Can N supplymeet demand?

Yes

Yes

No

No

Nevada DesertAlaska Tundra

Kansas prairieDuke ForestOak Ridge

Texas grasslandFlorida woodland

Examples

Types

Variable control + NShoot C 2.98 22.42*Root C 38.98* 51.96*Plant C 12.26* 28.35*Soil C -4.28 13.35*Shoot N 20.45* 31.02*Root N 14.07* 30.73*Plant N 24.90* 27.71*Soil N -9.18* 13.35*

N addition stimulates more C and N accumulation

Nitrogen Treatment

Luo et al. Ecology In press