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