1. 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 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): 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. sources of data experimental facilities ecosystem types, field sites, exposure times, nitrogen treatments CO 2 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 Luo et al. 2006 Ecology

6. As atm CO 2 is rising, productivity usually increases How does nitrogen regulates ecosystem responses to rising CO 2 ? NH 4 + NO 3 - CO 2

7. NCEAS Working group Progressive N limitation in plant and ecosystem responses to elevated CO 2

8. NPP N sequestered in biomass & litter C input to soil N sequestered in SOM labile soil N N uptake N availability C:N CO 2 P rogressive N itrogen L imitation Luo et al. 2004 BioScineces

9. 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

11. Hungate et al.2003 Science Ecosystem models with N cycling processes incorporated predict carbon sinks more realistically that models without N cycling.

12. 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 ) Results of meta-analysis 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

14. 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. Implications

15. Stoichiometrical Flexibility C/N increases by 11.6% in shoot 10.8% in root N.S. in litter 2.9% in soil Luo et al. Ecology In press Flexible C/N can support short-term CO 2 stimulation of plant growth and C sequestration

16. 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. Concluding Remarks

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.

19. VariableFACEOTCGC Shoot C 11.59*13.87*16.22* Root C 47.23* 1.3336.47 * Plant C 4.57* 7.94*21.22* Soil C 5.75* 6.62* Shoot N21.11*12.58* 4.35 Root N27.73* 19.41*12.27* Plant N26.25*12.80*14.66* Soil N3.52 11.52* CO 2 Facility Little systematic biases caused by facility Luo et al. Ecology In press

20. Variablecroplandforestgrasslanddesertwetland Shoot C14.21*21.50* 9.80*9.663.43 Root C22.54*48.76*40.49*11.4-12.97* Plant C15.72*26.72* 0.5424.60*-8.51 Soil C2.81 5.56* 10.49* -0.73 Shoot N-1.631.28*20.46* 2.9 -10.5* Root N24.49*26.76*-3.64 -0.60 Plant N15.08*25.67*-0.918.98* Soil N18.29*5.71*-8.52 Ecosystem Type Desert, wetland and cropland have different responses, largely due to small sample sizes Luo et al. Ecology In press

21. PNL occurs PNL may not occur PNL may not develop CO 2 If NPP is stimulated? N demand Can N supply meet demand? Yes No Nevada Desert Alaska Tundra Kansas prairie Duke Forest Oak Ridge Texas grassland Florida woodland Examples Types

22. Variablecontrol+ N Shoot C 2.9822.42* Root C 38.98*51.96* Plant C 12.26*28.35* Soil C -4.2813.35* Shoot N 20.45*31.02* Root N 14.07*30.73* Plant N24.90*27.71* Soil N-9.18*13.35* N addition stimulates more C and N accumulation Nitrogen Treatment Luo et al. Ecology In press