Does N limit C sequestration in terrestrial ecosystems?

If so, how?

Yiqi LuoYiqi LuoDepartment of Botany and Department of Botany and

MicrobiologyMicrobiologyUniversity of OklahomaUniversity of Oklahoma

USAUSA

Key points

1. Mineral N regulates plant growth and its responses to global change

2. N capital in organic form determines long-term carbon sequestration

Working hypotheses

1. CO2 stimulation of carbon sequestration will be down-regulated by limited N supply over time.

2. Climate warming stimulates N mineralization and increases N availability, which will enhance C sequestration

3. N deposition increases mineral N availability, stimulate plant growth, and thus will enhance C sequestration

Carbon cycle

Soil Mineral N

Nitrogen cycle

Plantassimilation

Warming

N deposition

Elevated CO2

Effects of nitrogen on plant growth, overall and grouped by biome

LeBauer and Treseder 2008

Plant

Carbon cycle

Soil Mineral N

Nitrogen cycle

assimilation

Atm CO2

Litter / CWD

Soil Organic Matter

N deposition

N fixation

denitrification

N leaching

respiration

Internalfast

Externalslow

mineralization

photosynthesis

litterfall & mortality

decomposition

(i)

Thornton et al. 2009

Effects of N addition on C and N cycles

Meta-analysis of data from 206 papers

Lu et al. 2011 New Phytologist (N cycle)Lu et al. 2011 Agricultural Ecosystems &

Environment (C cycle)

Lea

chin

g 46

1%

N2O

134

%

Den

84%

N addition

NH4+

47%

N uptake NO3-

429%

Abovegroundplant N 44%

Belowgroundplant N 53%

N-M

in 2

5%

Lit

ter/

OH

dec

omp

osit

ion

Litter N24%

Organic HorizonN 6.1%

Soil N pool 6.2%

Microbialbiomass N5.8%

DON 21%

Nit. 154%

Extremely leaking system

Lu et al. 2011a

O’Sullivan et al. 2011 GCB

Once N fertilization stops, mineral N gradually reset to the control level

N additions

Aboveground plant C 35.7%

Ps

Belowground plant C 23%

Lit

ter/

OH

dec

omp

osit

ion

Litter C 20.9%

Organic Horizon C 1.8%

Deep layer SOM

DOC 11%

R:S 14.5

Rs

4.3

%

Soil organic C 2.2%

Microbe C 6.4%

1. Reduce C input into soil systems2. Little contributions of aboveground

biomass and litter production to soil C3. Increased C loss via decomposition and respiration4. Increased C loss via DOC

Lu et al. 2011b

Mack et al. 2004 Nature

Mineral N does not set the level of soil N capital over time

“dummy” heater Infrared heater

clipunclip

unclip

clip

Long-term (12 years)warming and clipping

C and N interactions under experimental warming

Plant communityC4/C3 species

Leaf Ps

Phenology

Growing season

Plant growth

Microbial communityFungi/bacteria

Plant N uptake

Plant & soil C

Available N

Quality of bulk litter

Respiration

NUE

Litter Decomposition

Luo, 2007. Ann. Rev. Ecol. Evol. System

Plant community C4/C3 species

Leaf Ps

Phenology

Growing season

Plant growth

Microbial community Fungi/bacteria

Plant N uptake

Plant & soil C

Available N

Quality of bulk litter

Respiration

NUE

Litter Decomposition

Sherry et al. 2007, PNAS Zhou et al. 2007a,

JIPBZhou et al. 2006, GBC; 2007b, GCBLuo et al. 2001, Nature

Zhang et al. 2005 GCBZhou et al. In review

An et al. 2005, GCB

Cheng et al. 2010 Agric Ecosystems

Wan et al. 2005, GBC

Luo et al. 2009, GCB-E

Sherry et al. 2008, GCB

Niu et al. 2010, Ecology

clipped

2000 2002 2004 2006 2008

Incr

emen

t of

pla

nt C

con

tent

(gC

m-2

)

-40

-20

0

20

40

60

unclipped

-60

-40

-20

0

20

40

60

80

Total CC due to NC due to NUE

unclipped

Incr

emen

t of

pla

nt C

con

tent

(gC

m-2

)

-5

0

5

10

15

20

clipped

C C-N C-NUE0

5

10

15

20

a

c

b

d

Niu et al. 2010 Ecology

NUE is the main mechanism underlying warming-induced increases in plant C storage

Lu et al. In preparation

Warming effects on carbon processes

Lu et al. In preparation

NPP

N sequestered in

biomass & 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

NPP

N sequestered inbiomass & litter

C input to soil N sequestered

in SOM

labile soil N

N uptake N availability

C:N

CO2

Luo et al. 2004 BioScineces

PNL may not occur if

N fixation

N loss

-0.2 0.0 0.2 0.4 0.6

Soybean

Florida

Duke 6 yrs

Duke 3 yrs

Sorghum

Oak 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

Fre

quen

cy0

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

Fre

quen

cy

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. 2006 Ecology

• 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

No complete down-regulation

Working hypotheses

N capital increased by ~1 g N m-1 yr-1 to alleviate N limitation

Increased N mineralization enhances biomass growth but not soil C sequestration.

Yes for plant pools, not for soil pools

CO2 stimulation of carbon sequestration will be down-regulated by limited N supply over time.

Climate warming stimulates N mineralization and increases N availability, which will enhance C sequestration

N deposition increases mineral N availability, stimulate plant growth, and thus will enhance C sequestration

Soil mineral N availability regulates plant growth but does not determine long-term C sequestration

Which N processes determine long-term C sequestration?

Rastteter et al. 1997

N capital in organic form Long-term C sequestration

Redistribution of N among pools intermediate C sequestration

Adjustment in C/N ratio short-term C sequestration

Rastteter et al. 1997

Binkley et al. 2000 Ecosystems

N capital

Adding inorganic N Lu et al. 2011, New Phytologist

Fire Wan et al. 2001, Ecological Appl

Plant invasion Liao et al. 2008, New Phytologist

Forest succession Yang et al. 2011, New Phytologist

Forest plantation Liao et al. 2010, PloS One

Elevated CO2 Luo et al. 2006, Ecology

Experimental warming Lu et al. In preparation

Net change in organic N capital(the key variable to determine long-term C sequestration)

A database of 124 published papers from the literature

Carbon and nitrogen coupling during forest succession

Yang et al. 2011 New Phytologists

Yang et al. 2011 New Phytologists

The rates of C pool changes declined with forest age and approached an equilibrium state

Yang et al. 2011 New Phytologists

The rate of relative N change was positively associated with the rate of relative C change with different slopes among various ecosystem components

Yang et al. 2011 New Phytologists

The rate of absolute N change increased linearly with that of C pool change

Yang et al. Unpublished

Yang et al. 2011 New Phytologists

The relative change in C: N ratio was larger than 1.0 in both aboveground plant and woody tissues, but close to 1.0 in other ecosystem components

Conclusions

1. Mineral N limits plant growth but does not regulate long-term carbon sequestration

2. Organic N capital determines long-term carbon sequestration

http://ecolab.ou.edu

AcknowledgementFinancial support:

U.S. National Science Foundation US Department of Energy

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

Meta analysis collaborators: Dafeng Hui, Chengzhang Liao, Meng Lu, Shuli Niu, Shiqiang Wan, Yuanhe Yang, Deqiang Zhang, Xuhui Zhou