Persistence of nitrogen limitation over terrestrial carbon uptake

1
Persistence of nitrogen limitation over
terrestrial carbon uptake

• Galina Churkina, Mona Vetter and Kristina
  Trusilova
• Max-Planck Institute for Biogeochemistry
• churkina_at_bgc-jena.mpg.de

2
Increase in Global Reactive Nitrogen Fixation
BNF biological nitrogen fixation
(after Galloway et al. 2004)
3
Land Ecosystems and Nitrogen

• Productivity of many land ecosystems is control
  by nitrogen availability (Vitousek, 2002, Reich
  et al. 1997, FACE results, etc.)

Reich et. al. PNAS, 1997.
4
Is carbon uptake of land increased because of
acceleration of nitrogen cycle?
5
Methods

• Largest increases in nitrogen deposition are
  occurring in Europe, North America, and Asia
• A most significant effect of nitrogen
  fertilization is expected in forests because of
• high C/N ratios of wood (e.g. C/Nwood600,
  C/Nherbacious20-40)
• long lifetime of carbon in wood
• Maximum sensitivity to nitrogen deposition was
  found in old forests (Vetter et al. 2005)
• Our study focused on forests in Northern
  Hemisphere

6
Model Simulations

• Simulations with terrestrial ecosystem BIOME-BGC
  model (carbon, water, nitrogen cycles)
• validated for deciduous and evergreen forests in
  North America and Europe (Law et al. 2001,
  Thornton et al. 2002, Churkina et al. 2003, etc.)
• Input Drivers
• climate data from NCEP reanalysis (1948-2002)
• increasing atmospheric CO2 (after Keeling)
• increasing nitrogen deposition

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BIOME-BGC structure carbon and nitrogen cycles
Assumption constant C/N ratios of plants,
litter, and soil
8
Distribution of Nitrogen Deposition
Pre-industrial
Industrial
kgN/ha
MOGUNTIA (Dentener Crutzen, 1994)
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Scenarios
C D E F
N deposition (kg/ha/yr) 1 16 Preindustrial level 1 Increasing to industrial level2
Forest cover Each grid cell is forested Each grid cell is forested According to vegetation map According to vegetation map
D-C - differentiating effect of nitrogen
fertilization on forest productivity in different
climates F-E - potential of forests with lifted
N limitation to decelerate CO2 concentration rise
in the atmosphere
1 N deposition for each pixel is set to
preindustrial level according to MOGUNTIA
results 2 N deposition for each pixel is set to
industrial level according to MOGUNTIA results
10
Explanation of residual land carbon sink?
 Reference 1980s (Pg C/yr) 1990s (Gt C/yr)
Residual land carbon sink Residual land carbon sink Residual land carbon sink
IPCC 2001 -0.3-3.8 incomplete
House et al 2003 0.3-4.0 1.6-4.8
Additional land carbon uptake due to increased nitrogen deposition   Additional land carbon uptake due to increased nitrogen deposition   Additional land carbon uptake due to increased nitrogen deposition  
Townsend et al. 1996, Holland et al. 1997 n/a 0.1-2.3
Nadelhoffer et al. 1999 n/a 0.25
This study 0.21 0.26
11
Compensation of the fossil fuel emissions?

• 8.1 Pg of carbon was additionally sequestered in
  N. H. forests during 1950-2000
• evergreen needleaf (3.9 Pg)
• deciduous broadleaf (4.0 Pg)
• deciduous needleaf (0.21 Pg) forests
• 300 Pg of carbon has been emitted into the
  atmosphere from fossil fuels over the same time
  period (EDGAR-HYDE 1.4, Van Aardenne et al.
  2001, EDGAR 3.2)

12
Sensitivity of forests to elevated nitrogen
deposition
ENF
DBF
Relative change in NPP
Increase in nitrogen deposition is 15 kg/ha D-C
each grid cell is forested and has the same N
deposition
13
Summary

• Forests fertilized through enhanced nitrogen
  deposition
• may partially explain missing carbon sink on land
• might have sequestered 2.7 of carbon emitted by
  the industry during the last 55 years
• Elevated nitrogen deposition is unlikely to
  enhance vegetation carbon sink significantly
  because of
• climatic limitations
• differentiating effects on carbon sequestration
  of uneven aged forests