Land Carbon Sink and Nitrogen Regulation under Elevated CO2: Central Tendency

1
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
2
Probing mechanism toward predictive understanding
3
Meta-analysis to reveal central tendency
4
Meta analysis

• 104 published papers, 940 lines
• Category variables
• Response variables (18)
• Biomass in shoot, root, and whole plant
• C pools in shoot, root, whole plant, litter, and
  soil
• N pools in shoot, root, whole plant, litter, and
  soil
• Ratios of C and N in shoot, root, litter, and
  soil pools
• Root/shoot ratio.

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

• exposure times,
• nitrogen treatments
• CO2 concentrations of treatments

5

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

• 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

6
As atm CO2 is rising, productivity usually
increases
7
NCEAS Working group
Progressive N limitation in plant and ecosystem
responses to elevated CO2
8
(No Transcript)
9
Two Approaches to Study C and N Coupling in Land
Ecosystems

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

10
(No Transcript)
11
Hungate et al.2003 Science
Ecosystem models with N cycling processes
incorporated predict carbon sinks more
realistically that models without N cycling.
12
Results of meta-analysis

• 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)

Luo et al. Ecology In press
13

• 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

Luo et al. Ecology In press
14
Implications

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.

15
Stoichiometrical Flexibility

• C/N increases by
• 11.6 in shoot
• 10.8 in root
• N.S. in litter
• 2.9 in soil

Flexible C/N can support short-term CO2
stimulation of plant growth and C sequestration
Luo et al. Ecology In press
16
Concluding Remarks

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.

17
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.
18
(No Transcript)
19
CO2 Facility
Variable FACE OTC GC
Shoot 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 4.35
Root N 27.73 19.41 12.27
Plant N 26.25 12.80 14.66
Soil N 3.52 11.52
Little systematic biases caused by facility
Luo et al. Ecology In press
20
Ecosystem Type
Variable cropland forest grassland desert wetland
Shoot C 14.21 21.50 9.80 9.66 3.43
Root C 22.54 48.76 40.49 11.4 -12.97
Plant C 15.72 26.72 0.54 24.60 -8.51
Soil C 2.81 5.56 10.49 -0.73
Shoot N -1.6 31.28 20.46 2.9 -10.5
Root N 24.49 26.76 -3.64 -0.60
Plant N 15.08 25.67 -0.91 8.98
Soil N 18.29 5.71 -8.52
Desert, wetland and cropland have different
responses, largely due to small sample sizes
Luo et al. Ecology In press
21
Types
22
Nitrogen Treatment
Variable control N
Shoot 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
Luo et al. Ecology In press