Nitrogen-Carbon cycle interactions at the global scale

1. Nitrogen-Carbon cycle interactions at the global scale SönkeZaehle Max Planck Institute for Biogeochemistry, Jena, Germany with contributions from Quinn R Thomas, Pamela Templer, Belinda Medlyn, Rich Norby, Ram Oren, Peter Thornton contact: szaehle@bgc-jena.mpg.de

2. N cycle effects in terrestrial biosphere dynamics • Nitrogen affects Carbon cycling by: • Biochemical activity of tissues (enzymes) • Stoichiometry constraints of living tissues (both plants and microbes) • Major effects on: • Leaf-level photosynthesis • Plant growth and allocation • Decomposability due to N demand of decomposers

3. Why are Nitrogen-Carbon cycle interactions important? • Plant available nitrogen is scarce in the natural environment, and constrains ecosystem productivity • Adding anthropogenic Nr causes increased productivity in N limited ecosystem, enhancing C storage • Accelerated soil organic matter turnoverfrom soil warming or priming may increaseplant N availability and plant growth • Productivity response to elevated CO2is constrained by the available N, which may attenuate the response Net Primary Production (1990s) no nitrogen limitation nitrogen limitation Zaehle et al. , 2010

4. Land DGVMs + land use Terrestrial Biosphere Models (C + H2O ( + N ) cycling Carbon Climate Hadley C4MIP CNP(Climate) IPSL CN Climate Climate GISS UKMO CGCM3 ECHAM5 ‘94 ‘96 ‘98 ‘00 ‘02 ‘04 ‘06 ‘08 ‘10 ‘12 A short history ofland-atmospheremodelling Dynamic Global Vegetation Models CASA-CNP CLM-CN IGSM-CN LM3V-CN O-CN ISAM-CN

5. Atmospheric [CO2] Human N inputs (Deposition & fertilisers) C cycle N cycle N fertilisation effects on productivity and C sequestration Effects of Nr additions

6. Observed nitrogen addition responses Forest growth responses Nr additions generally • Stimulate aboveground biomass growth due to • Increased foliage area and foliar nitrogen • Reduction in root growth / respiration • reduce in soil respiration • reduced root biomass • reduced soil organic matter decomposition (heterotrophic respiration) ? • Depend on other co-limiting factors • Can saturate and show reverse effects Thomas et al. 2010 Soil carbon turnover responses Janssens et al. 2010

7. Response of C sequestration to Nr additions Zaehle & Friend, 2010 Thomas et al. 2010

8. Response of C sequestration to Nr additions (2) resulting from N deposition and co-occuring changes in climate and atm. [CO2]… resulting from N additionsonly (either deposition or fertiliser treatments) Adapted from Zaehle& Friend, 2010

9. Zaehle & Dalmonech, 2011 Integration of N deposition effect to the global scale • Extrapolation of observed response ratios (Lui & Graever, 2009): Response x Area x N deposition/area = 0.4±0.1 Pg C / yr • Projections by current global Nitrogen-Carbon cycle models All factors net land-atm. C flux [gC m-2 yr-1] Nitrogen Deposition Zaehle et al., 2010

10. Small role of anthropogenic Nr in 20th century net land CO2 exchanges 10 yr running mean fluxes Zaehle et al. , 2011

11. N feedbacks reduce C losses from enhanced decay, but by how much? Melillo et al. 2011

12. C cycle response in Harvard Forest after 7 years of warming Melillo et al. 2011

13. Geographic pattern of CO2 fertilisation effects on plant production Carbon-Nitrogen cycle model response to 5 +K soil temperature % enchancement of plant production Zaehle et al. 2010b

14. Atmospheric [CO2] C cycle N cycle N constrains on on productivity and C sequestration Effects of N availability on responses to elevated CO2

15. Progressive nitrogen limitation of CO2 fertilisation, does it exist? Luo et al. 2004

16. Evidence from Free Air CO2 Enrichment experiments Initial response depends on initial nitrogen status Long-term responses strongly depend on transient N status development -> are there additional N sources Rich source of observations Identification of key mechanisms possible Difficult to balance N budget from observations, particular due to uncertain soil organic matter changes Difficult to discern disctinct/common features across experiments However, clear in-between site variance due to N availability Norby et al. 2010 Norby et al. 2005 McCarthy, et al. 2010

17. Testing N-C models against FACE experiments Duke FACE ORNL FACE • Systematic study confrontingN-C models with FACE data • Nine N-C model responses with similar set-up • Main conclusion so far • Initial response reproduced • some, but not all models show ‘correct’ type of responses • long-term response vary • CO2enrichmentsince 1997 • in 3 paired rings • Control: ambient (~365 ppmv) • Elevated: ambient + 200 (~565 ppmv) • N fertiliser (+11.2 g N m-2 yr-1 ) Observations: Palmroth et al. 2006, PNAS mid end start end mid start Zaehle & NCEAS-FACE Group, in prep. Zaehle et al. 2010, GRL

18. Geographic pattern of CO2 fertilisation effects on plant production Carbon cycle model only response to 370+200 ppm CO2 % enchancement of plant production Carbon-Nitrogen cycle model response to 370+200 ppm CO2

19. Key uncertainties in modelling Nitrogen-Carbon cycle interactions • Modelling N effects on plant growth (downacclimation of photosynthesis) • Changes in carbon allocation patterns • Changes in biological nitrogen fixation • Degree of flexibility in ecosystem stoichiometry • Representation of priming effects and microbial dynamics in soil organic matter decay (and thus plant N availability) • Representation of N loss pathways to volatilisation and leaching

20. Uncertainties: Flexibility of vegetation and soil stoichiometry Response to elevated CO2 C only Flexible Vegetation C:N Flexible Soil C:N Fixed C:N Response to soil warming Fixedsoil C:N Response to Nr fertiliser Fixed C:N Flexible soil C:N Flexible C:N C only Zaehle, Norby, Fisher in prep.

21. Effects of Nitrogen-CarbonInteractions on carbon-cycle projections

22. N effects on future projected land carbon storage: O-CN results SRES A2 scenario with climate from IPSL-CM4 Blue: C-only Red: C-N Synergy CO2 fertilisation Nitrogen depos. Climate change Combined effect Zaehle et al. 2010, GRL

23. Carbon-cycle climate feedbacks + + Fossil fuel emissions Atmospheric [CO2] Radiative Forcing & Climatic Change _ _ C cycle _ + Soil Vegetation N cycle Mineral N Carbon-Concentration interaction Negative feedback: reduces growth rate of atm. [CO2] Carbon-Climate interaction Positive feedback: increases atm. [CO2] & climate change Carbon-Nitrogen interaction Negative feedback: dampens C uptake and reduces C loss

24. Carbon-concentration interaction Carbon-cycle only models generally show a strong negative interaction (ie land ecosystems reduce CO2 accumulation in the atmosphere) N cycling reduces land C uptake due to CO2 by 50-73% by the year 2100 D Land C/ D atm. [CO2] C4MIP: Friedlingstein et al. 2006 CLM-C/CLM-CN: Thornton et al. 2009 O-C/O-CN: Zaehle et al. 2010

25. Carbon-climate interaction Carbon-cycle only models generally show a strong positive interaction (ie land ecosystems loose C accumulation in response to climate change) N cycling increases land C uptake due to climate by the year 2100 with quite an uncertainty on the range C4MIP: Friedlingstein et al. 2006 CLM-C/CLM-CN: Thornton et al. 2009 O-C/O-CN: Zaehle et al. 2010

26. N cycle modulation of the terrestrial C cycle response • Reduction of carbon-concentration interaction • Reduction in climate-carbon interaction • Net effect is a slower accumulation of C in the terrestrial biosphere, i.e. a higher rate of climate change Zaehle & Dalmonech, 2011

27. Conclusions • Primary effect of accounting for N-C interactions is a strong reduction in land carbon storage due to CO2 fertilisation • This determines the net effect of the Nitrogen cycle on climate (via CO2), implying that C-N cycle climate models show higher rate of climate change than C4MIP • There is a positive effect of climate change on net N mineralisation that increases land carbon storage (~50 Pg C; SRES A2 from CLM-CN/O-CN) • In two of three models this leads to a small increase in total land C storage due to climate change, though this stimulation is transient and dependent on the extent of warming • This effect is the same order of magnitude as projected effects from N deposition (27-66 Pg C; from CLM-CN/O-CN, different deposition assumptions)