1. Persistence of nitrogen limitation over terrestrial carbon uptake Galina Churkina, Mona Vetter and Kristina Trusilova Max-Planck Institute for Biogeochemistry churkina@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/N wood ~600, C/N herbacious ~20- 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 CO 2 (after Keeling) increasing nitrogen deposition

7. BIOME-BGC structure: carbon and nitrogen cycles autotrophic respiration heterotrophic respiration nitrogen leaching photosynthesis Atmospheric CO 2 nitrogen deposition Atmospheric N Litter Soil Canopy Stem Root Assumption: constant C/N ratios of plants, litter, and soil

8. Distribution of Nitrogen Deposition kgN/ha MOGUNTIA (Dentener & Crutzen, 1994) Pre-industrial Industrial

9. Scenarios CDEF N deposition (kg/ha/yr) 116Preindustrial level [1][1] Increasing to industrial level[2][2] Forest coverEach grid cell is forested 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][1] N deposition for each pixel is set to preindustrial level according to MOGUNTIA results [2][2] N deposition for each pixel is set to industrial level according to MOGUNTIA results

10. Explanation of residual land carbon sink? Reference1980’s (Pg C/yr) 1990’s (Gt C/yr) Residual land carbon sink IPCC 2001-0.3-3.8incomplete House et al 20030.3-4.01.6-4.8 Additional land carbon uptake due to increased nitrogen deposition Townsend et al. 1996, Holland et al. 1997n/a0.1-2.3 Nadelhoffer et al. 1999n/a0.25 This study0.210.26

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 DBF ENF Increase in nitrogen deposition is 15 kg/ha D-C: each grid cell is forested and has the same N deposition Relative change in NPP

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