Does N limit C sequestration in terrestrial ecosystems? If so, how?

1. Does N limit C sequestration in terrestrial ecosystems?If so, how? Yiqi Luo Department of Botany and Microbiology University of Oklahoma USA

2. Key points Mineral N regulates plant growth and its responses to global change N capital in organic form determines long-term carbon sequestration

3. Working hypotheses • CO2 stimulation of carbon sequestration will be down-regulated by limited N supply over time. • Climate warming stimulates N mineralization and increases N availability, which will enhance C sequestration • N deposition increases mineral N availability, stimulate plant growth, and thus will enhance C sequestration

4. Plant assimilation Nitrogen cycle Carbon cycle N deposition Soil Mineral N Warming Elevated CO2

5. Effects of nitrogen on plant growth, overall and grouped by biome LeBauer and Treseder 2008

7. Atm CO2 Internal fast External slow photosynthesis denitrification N deposition respiration litterfall & mortality Litter / CWD (i) N fixation decomposition mineralization Soil Organic Matter N leaching Thornton et al. 2009 Nitrogen cycle Carbon cycle Plant assimilation Soil Mineral N

8. Effects of N addition on C and N cycles Meta-analysis of data from 206 papers Lu et al. 2011 New Phytologist (N cycle) Lu et al. 2011 Agricultural Ecosystems & Environment (C cycle)

9. Aboveground plant N 44% Belowground plant N 53% N-Min 25% Microbial biomass N 5.8% Litter N 24% Litter/OH decomposition Soil N pool 6.2% Organic Horizon N 6.1% DON 21% Extremely leaking system N addition N2O 134% Den 84% NH4+ 47% NO3- 429% N uptake Nit. 154% Leaching 461% Lu et al. 2011a

10. Once N fertilization stops, mineral N gradually reset to the control level O’Sullivan et al. 2011 GCB

11. R:S14.5 Rs 4.3% Litter C 20.9% Litter/OH decomposition Microbe C 6.4% Soil organic C 2.2% Organic Horizon C 1.8% DOC 11% Deep layer SOM N additions Ps Aboveground plant C 35.7% Belowground plant C 23% • Reduce C input into soil systems • Little contributions of aboveground biomass and litter production to soil C • Increased C loss via decomposition and respiration • Increased C loss via DOC Lu et al. 2011b

12. Mack et al. 2004 Nature

13. Mineral N does not set the level of soil N capital over time

14. “dummy” heater Infrared heater clip unclip unclip clip Long-term (12 years) warming and clipping

15. C and N interactions under experimental warming Phenology Growing season Leaf Ps Respiration Plant growth Plant & soil C Microbial community Fungi/bacteria Plant community C4/C3 species NUE Quality of bulk litter Plant N uptake Litter Decomposition Available N Luo, 2007. Ann. Rev. Ecol. Evol. System

16. Phenology Growing season Leaf Ps Respiration Plant growth Plant & soil C Microbial community Fungi/bacteria Plant community C4/C3 species NUE Quality of bulk litter Plant N uptake Litter Decomposition Available N Sherry et al. 2007, PNAS Zhou et al. 2007a, JIPB Zhou et al. 2006, GBC; 2007b, GCB Luo et al. 2001, Nature Sherry et al. 2008, GCB Luo et al. 2009, GCB-E An et al. 2005, GCB Zhang et al. 2005 GCB Zhou et al. In review Wan et al. 2005, GBC Niu et al. 2010, Ecology Cheng et al. 2010 Agric Ecosystems

17. NUE is the main mechanism underlying warming-induced increases in plant C storage Niu et al. 2010 Ecology

18. Lu et al. In preparation

19. Warming effects on carbon processes Lu et al. In preparation

20. Progressive Nitrogen Limitation N sequestered in biomass & litter CO2 NPP C:N labile soil N C input to soil N sequestered in SOM N uptake N availability Luo et al. 2004 BioScineces

21. PNL may not occur if N fixation N loss N sequestered in biomass & litter CO2 NPP C:N labile soil N C input to soil N sequestered in SOM N uptake N availability Luo et al. 2004 BioScineces

22. 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 No complete down-regulation Luo et al. 2006 Ecology

23. Working hypotheses N capital increased by ~1 g N m-1 yr-1 to alleviate N limitation • CO2 stimulation of carbon sequestration will be down-regulated by limited N supply over time. • Climate warming stimulates N mineralization and increases N availability, which will enhance C sequestration • N deposition increases mineral N availability, stimulate plant growth, and thus will enhance C sequestration Increased N mineralization enhances biomass growth but not soil C sequestration. Yes for plant pools, not for soil pools

24. Soil mineral N availability regulates plant growth but does not determine long-term C sequestration Which N processes determine long-term C sequestration?

25. Rastteter et al. 1997

26. N capital in organic form  Long-term C sequestration Redistribution of N among pools  intermediate C sequestration Adjustment in C/N ratio  short-term C sequestration Rastteter et al. 1997

27. N capital Binkley et al. 2000 Ecosystems

28. Net change in organic N capital (the key variable to determine long-term C sequestration) Adding inorganic N Lu et al. 2011, New Phytologist Fire Wan et al. 2001, Ecological Appl Plant invasion Liao et al. 2008, New Phytologist Forest succession Yang et al. 2011, New Phytologist Forest plantation Liao et al. 2010, PloS One Elevated CO2 Luo et al. 2006, Ecology Experimental warming Lu et al. In preparation

29. Carbon and nitrogen coupling during forest succession A database of 124 published papers from the literature Yang et al. 2011 New Phytologists

30. The rates of C pool changes declined with forest age and approached an equilibrium state Yang et al. 2011 New Phytologists

31. The rate of relative N change was positively associated with the rate of relative C change with different slopes among various ecosystem components Yang et al. 2011 New Phytologists

32. The rate of absolute N change increased linearly with that of C pool change Yang et al. 2011 New Phytologists

33. Yang et al. Unpublished

34. The relative change in C: N ratio was larger than 1.0 in both aboveground plant and woody tissues, but close to 1.0 in other ecosystem components Yang et al. 2011 New Phytologists

35. Conclusions Mineral N limits plant growth but does not regulate long-term carbon sequestration Organic N capital determines long-term carbon sequestration

36. Acknowledgement Financial support: U.S. National Science Foundation US Department of Energy 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 Meta analysis collaborators: Dafeng Hui, Chengzhang Liao, Meng Lu, Shuli Niu, Shiqiang Wan, Yuanhe Yang, Deqiang Zhang, Xuhui Zhou http://ecolab.ou.edu