Does N limit C sequestration in
terrestrial ecosystems?
If so, how?
Yiqi Luo
Department of Botany and Microbiology
University of Oklahoma
USA
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
Nitrogen cycle
Carbon cycle
Plant
assimilation
N deposition
Soil Mineral
N
Warming
Elevated CO2
Effects of nitrogen on plant growth, overall and grouped by biome
LeBauer and Treseder 2008
Nitrogen cycle
Carbon cycle
Atm CO2
Internal
fast
photosynthesis
External
slow
denitrification
N deposition
Plant
assimilation
respiration
litterfall & mortality
Litter / CWD
(i)
Soil Mineral
N
N fixation
decomposition
mineralization
Soil Organic Matter
N leaching
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)
Litter N
24%
NH4+
47%
Microbial
biomass N
5.8%
Nit.
154%
NO3429%
Leaching 461%
Litter/OH decomposition
Belowground
plant N 53%
N uptake
Den 84%
Aboveground
plant N 44%
N-Min 25%
N addition
N2O 134%
Extremely leaking system
Soil N pool
6.2%
Organic Horizon
N 6.1%
DON 21%
Lu et al.
2011a
Once N fertilization stops, mineral N
gradually reset to the control level
O’Sullivan et al. 2011 GCB
Aboveground plant C
Litter/OH
decomposition
35.7%
Litter C
20.9%
Organic Horizon C
N additions
R:S 14.5 Belowground plant C
23%
Rs 4.3%
Ps
Microbe C
6.4%
Soil organic C
2.2%
1.8%
DOC 11%
1. Reduce C input into soil systems
2. Little contributions of aboveground
Deep layer SOM
biomass and litter production to soil C
3. Increased C loss via decomposition and respiration
Lu et al.
4. Increased C loss via DOC
2011b
Mack et al. 2004 Nature
Mineral N does not set the level of soil N capital over time
Long-term (12 years)
warming and clipping
“dummy” heater
Infrared heater
clip
unclip
unclip
clip
C and N interactions under experimental warming
Phenology
Growing
season
Leaf Ps
Respiration
Plant
growth
Plant community
C4/C3 species
Plant N
uptake
Available N
Plant &
soil C
NUE
Microbial community
Fungi/bacteria
Quality of
bulk litter
Litter
Decomposition
Luo, 2007. Ann. Rev. Ecol. Evol. System
Sherry et al. 2007, PNAS
Zhou et al. 2007a, JIPB
Phenology
Growing Leaf Ps
season
Sherry et al.
2008, GCB
Plant
growth
Luo et al.
2009, GCB-E
Zhou et al. 2006, GBC;
2007b, GCB
Luo et al. 2001, Nature
Respiration
Plant &
soil C
Microbial community
NUE
Fungi/bacteria
Quality of
bulk litter
Plant N
Zhang et al. 2005 GCB
uptake
Zhou et al. In review
Litter
Decomposition
Available N
Plant community
C4/C3 species
An et al.
2005, GCB
Wan et al.
2005, GBC
Niu et al.
2010, Ecology
Cheng et al. 2010
Agric Ecosystems
20
80
60
a
unclipped
unclipped
b
15
10
20
0
5
-20
0
-40
-60
Total C
C due to N
C due to NUE
60
c
-5
clipped
d
20
40
15
20
10
0
-20
Increment of plant C content (gC m -2)
Increment of plant C content (gC m -2)
40
NUE is the
main
mechanism
underlying
warminginduced
increases in
plant C
storage
5
clipped
-40
2000
2002
2004
2006
2008
C
C-N
C-NUE
0
Niu et al. 2010 Ecology
Lu et al. In
preparation
Warming effects on carbon processes
Lu et al. In
preparation
Progressive Nitrogen Limitation
N sequestered
in
biomass & litter
CO2
C:N
NPP
C input
to soil
N uptake
labile soil N
N sequestered
in SOM
N availability
Luo et al. 2004 BioScineces
PNL may not occur if
N sequestered in
biomass & litter
CO2
NPP
N fixation
C:N
C input
to soil
N uptake
labile soil N
N sequestered
in SOM
N loss
N availability
Luo et al. 2004 BioScineces
Soil pools
Litter pools
14
Soybean
a
12
Swiss 3 yrs
carbon
Florida
c: Carbon
10
Frequency
Sorghum
Duke 6 yrs
Duke 3 yrs
Swiss 2 yrs
Swiss 3 yrs
Mean = 0.054
Se = 0.0117
n = 40
P < 0.001
8
6
4
P. nigra
2
Ca grassland
Swiss 1 yr
P. x euram
Soybean
b
Florida
Nitrogen
Duke 6 yrs
Duke 3 yrs
Mean = 0.227
Se = 0.0666
n=7
P = 0.011
Sorghum
Oak Ridge
Swiss 6 yrs
0.0
0.2
0.4
0.6
Response Ratio
Luo et al. 2006 Ecology
14
12
Frequency
P. alba
-0.2
0
Mean = 0.187
Se = 0.0376
n = 14
P < 0.001
Oak Ridge
10
d: Nitrogen
Mean = 0.106
Se = 0.0322
n = 36
P = 0.002
• 21% increase in
litter C
• 25% increase in
litter N
• 5.6% increase in
soil C
• 11.2% increase
in soil N
8
6
4
2
0
-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5
Response Ratio
• 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
CO2 stimulation of carbon
sequestration will be downregulated by limited N supply
over time.
N capital increased by ~1 g N
m-1 yr-1 to alleviate N
limitation
Climate warming stimulates N
mineralization and increases N
availability, which will enhance
C sequestration
Increased N mineralization
enhances biomass growth but
not soil C sequestration.
N deposition increases mineral
N availability, stimulate plant
growth, and thus will enhance
C sequestration
Yes for plant pools, not for soil
pools
Soil mineral N availability regulates
plant growth but does not determine
long-term C sequestration
Which N processes determine longterm 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  shortterm C
sequestration
Rastteter et al. 1997
N capital
Binkley et al. 2000 Ecosystems
Net change in organic N capital
(the key variable to determine long-term C sequestration)
Adding inorganic N
Fire
Plant invasion
Forest succession
Forest plantation
Elevated CO2
Experimental warming
Lu et al. 2011, New Phytologist
Wan et al. 2001, Ecological Appl
Liao et al. 2008, New Phytologist
Yang et al. 2011, New Phytologist
Liao et al. 2010, PloS One
Luo et al. 2006, Ecology
Lu et al. In preparation
Carbon and nitrogen coupling during forest succession
A database of 124 published papers from the literature
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. 2011 New
Phytologists
Yang et al.
Unpublished
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
Yang et al. 2011 New
Phytologists
Conclusions
1. Mineral N limits plant growth but
does not regulate long-term
carbon sequestration
2. Organic N capital determines
long-term carbon sequestration
Acknowledgement
Financial 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
http://ecolab.ou.edu