Land Carbon Sink and Nitrogen Regulation under Elevated
Land Carbon Sink and Nitrogen Regulation under Elevated CO 2: 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 Mc. Murtrie, Ram Oren, William Parton, Diane Pataki, Rebecca Shaw, Bo Su, Donald Zak Other collaborators: Dafeng Hui and Deqiang Zhang
Probing mechanism toward predictive understanding
Meta-analysis to reveal central tendency
Meta analysis 104 published papers, 940 lines Category variables: • • sources of data experimental facilities ecosystem types, field sites, • • • exposure times, nitrogen treatments CO 2 concentrations of treatments Response variables (18): 1. Biomass in shoot, root, and whole plant; 2. C pools in shoot, root, whole plant, litter, and soil 3. N pools in shoot, root, whole plant, litter, and soil; 4. Ratios of C and N in shoot, root, litter, and soil pools; 5. Root/shoot ratio.
Luo et al. 2006 Ecology • 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
As atm CO 2 is rising, productivity usually increases CO 2 How does nitrogen regulates ecosystem responses to rising CO 2? NH 4+ NO 3
NCEAS Working group Progressive N limitation in plant and ecosystem responses to elevated CO 2
Progressive Nitrogen Limitation CO 2 N sequestered in biomass & litter NPP C: N C input to soil N uptake labile soil N N sequestered in SOM N availability Luo et al. 2004 Bio. Scineces
Two Approaches to Study C and N Coupling in Land Ecosystems 1. Global assessment 2. Meta-analysis of site-specific data from CO 2 experiments
Hungate et al. 2003 Science Ecosystem models with N cycling processes incorporated predict carbon sinks more realistically that models without N cycling.
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
• 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
Implications 1. Complete downregulation of CO 2 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 CO 2.
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 CO 2 stimulation of plant growth and C sequestration Luo et al. Ecology In press
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 CO 2. 4. Stochastic modeling may be the only viable approach to account for diverse C and N responses to elevated CO 2.
Acknowledgement The Terrestrial Carbon Program, the Office of Science (BER), U. S. Department of Energy, Grant No. DEFG 03 -99 ER 62800 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.
CO 2 Facility Variable Shoot C Root C Plant C FACE 11. 59* 47. 23* 4. 57* OTC 13. 87* 1. 33 7. 94* GC 16. 22* 36. 47* 21. 22* Soil C Shoot N Root N Plant N Soil N 5. 75* 21. 11* 27. 73* 26. 25* 3. 52 6. 62* 12. 58* 19. 41* 12. 80* 11. 52* 4. 35 12. 27* 14. 66* Little systematic biases caused by facility Luo et al. Ecology In press
Ecosystem Type Variable Shoot C Root C Plant C cropland 14. 21* 22. 54* 15. 72* forest 21. 50* 48. 76* 26. 72* grassland 9. 80* 40. 49* 0. 54 Soil C Shoot N Root N Plant N Soil N 2. 81 -1. 6 5. 56* 31. 28* 24. 49* 15. 08* 18. 29* 10. 49* 20. 46* 26. 76* 25. 67* 5. 71* desert 9. 66 11. 4 24. 60* wetland 3. 43 -12. 97* -8. 51 2. 9 -3. 64 -0. 91 -0. 73 -10. 5* -0. 60 8. 98* -8. 52 Desert, wetland cropland have different responses, largely due to small sample sizes Luo et al. Ecology In press
CO 2 If NPP is stimulated? Yes No N demand Can N supply meet demand? No Types Examples PNL may not develop Nevada Desert Alaska Tundra PNL occurs Texas grassland Florida woodland Yes PNL may not occur Kansas prairie Duke Forest Oak Ridge
Nitrogen Treatment Variable Shoot C Root C Plant C control 2. 98 38. 98* 12. 26* +N 22. 42* 51. 96* 28. 35* Soil C Shoot N Root N Plant N Soil N -4. 28 20. 45* 14. 07* 24. 90* -9. 18* 13. 35* 31. 02* 30. 73* 27. 71* 13. 35* N addition stimulates more C and N accumulation Luo et al. Ecology In press
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