1. Nitrogen Controls on Carbon Sequestration J. Melillo 1, D. Kicklighter 1, H. Tian 1, A. McGuire 2, J. Clein 2, B. Moore III 3 [jmelillo@mbl.edu] 1 Marine Biological Laboratory 2 University of Alaska 3 University of New Hampshire

2. The Global Nitrogen (N) Cycle and Carbon Sequestration Fact: Humans have more than doubled the rate at which reactive N is added to the biosphere annually. Fact: Through fertilizer production, the planting of legumes, and high temperature combustion, humans transform about 150 TgN annually from unreactive N to reactive N. Fact: A fraction of this reactive N enters ecosystems in which carbon sequestration is N limited. Question: Is N-stimulated carbon sequestration currently a significant component of the global carbon budget?

3. Approach We used a process-based biogeochemistry model running with an open N cycle to estimate the annual N- stimulated carbon sequestration rate in North America. The current spatially explicit N- deposition pattern for the region was estimated using the results of Dentner and Crutzen.

4. Nitrogen Deposition in North America (>15 0 N)

5. Annual N-deposition Rates Tg N/yr All land Forest only North America7.33.2 USA4.81.9 Canada1.81.1 Other0.70.2

6. N-Stimulated Carbon Storage g C /m 2 /yr

8. Carbon Sequestration in North America (PgC) N- Other All stimulated Mechanisms USA +0.051 +0.259 +0.310 Canada +0.023 -0.019 +0.004 Other +0.010 +0.038 +0.048 North America +0.084 +0.278 +0.362

9. Conclusions We estimate that N-stimulate carbon sequestration accounts for about ¼ of the current C sink in North America. Other important mechanisms include land cover and land use changes and CO 2 fertilization. Biogeochemistry models used in carbon-cycle studies must include coupled C-N cycles.