1. Carbon-Nitrogen Interactions in the LM3 Land Model Stefan Gerber Department of Ecology and Evolutionary Biology Princeton University sgerber@princeton.edu GFDL, March, 2010 Lars Hedin, Steve Pacala, Michael Oppenheimer, Elena Shevliakova, Sergey Malyshev, Sonja Keel, Jack Brookshire, Susana Bernal …

2. My 2 Zero Order Nitrogen Cycle Questions: 1.If the Fixation of N [conversion from N 2 to available N] has more than doubled during modern times, what has happened to the N cycle and N balances? 2. How do the nitrogen and carbon cycles interact and how does 1. influence current and future levels of atmospheric CO 2 ?

3. 1.Roughly 90% of nitrogen was recycled every year in pre- industrial times. Losses were historically made up by natural nitrogen fixation [~100TgN/yr] 2.Humans now at least double these historic inputs by combustion and adding fertilizer [>100 TgN/yr]. Many land ecosystems now leak nitrogen. 3.Is the global Nitrogen cycle in Balance? Human Impact on the Nitrogen Cycle Midlatitude N Leaching

4. 2050 Models Predict a Big Sink From CO 2 Fertilization

5. Uncertainty about the magnitude of CO 2 fertilization is the key factor determining whether vegetation is a net carbon source or sink No CO 2 fertilizationCO 2 Fertilization at 700 ppm Shevliakova et al. 2006 -460Pg GFDL Slab-Ocean Climate Model SM2.1coupled to Dynamic Land model LM3V Atmospheric CO 2 concentration: 700 ppm Change in Vegetation Biomass, kgC/m2 +200 Pg

6. CO 2 fertilization and N limititation: N supply does not support predicted CO 2 uptake Hungate et al., 2003

7. Nitrogen Cycling fertilizer combustion ?

8. CO 2, N 2, reactive N The coupled terrestrial C-N cycle Litter Soil organic matter Mineral N Respiration Fire Litterfall Stabilization (+) Mineralization Immobilization Leaching Leaching/Denitrification Uptake Fixation Photosynthesis (+) Inorganic C Mineral N Organic C/N (+) Deposition 1 2 3 4 5 4 5

9. Leaves ~30:1 Sapwood 150:1 Heartwood 500:1 Storage Nitrate and Ammonium Roots ~50:1 Tissue turnover Plant nitrogen limitation/sufficiency Specify C:N ratio in tissue as a parameters Storage is worth 1 year of tissue regeneration. Depletion of storage causes reduction in photosynthesis A sufficient large storage reduces plant N uptake 1

10. CO 2, N 2, reactive N The coupled terrestrial C-N cycle Litter Soil organic matter Mineral N Respiration Fire Litterfall Stabilization (+) Mineralization Immobilization Leaching Leaching/Denitrification Uptake Fixation Photosynthesis (+) Inorganic C Mineral N Organic C/N (+) Deposition 2

11. Litter Increasing N – demand for microbial growth Litter Decomposition Microbial N limitation This suggests that microbes are N limited when C:N of litter exceeds ~10 (for bacteria) or ~30 (for fungi). A solution is fast microbial turnover, so overall microbial mass is small and N saturation achieved quickly. 2

12. Response to N addition as a function of Litter Quality (and N content, Knorr et al., 2005) Litter bag experiments: Higher the initial N lower the decomposition. Mellillo et al., 1982 Litter Quality and Decomposition Rates are Complex Soil organic matter Litter and soil organic matter N might stimulate litter processing, but increase the stabilization of organic matter in soils. Li et al., 2006 2

13. CO 2, N 2, reactive N Internal N-Cycle and feedbacks on C-Cycle Litter Soil organic matter Mineral N Respiration Litterfall Stabilization (+) Mineralization Immobilization Uptake Photosynthesis (+) Inorganic C Mineral N Organic C/N (+) 1 2 3 Leaching/Denitrification

14. Sinks of available N Immobilization / Uptake / Loss Available N Soil Immobilization and Stabilization Hydrological Leaching (and Denitrification) Plant Uptake Capacity (if N limited)

15. Primary succession experiment with fixed external N input: From bare soil to temperate forest Carbon only C-N C-N is N limitation in 1, 2, and 3 It takes much longer for C-N to reach equilibrium, but when reached, the system is not N limited. The system escapes N limitation because plants and soil retain any new N from deposition until they are saturated.

16. “Uncontrollable” losses Fire / Disturbance Organic losses via hydrological leaching

17. CO 2, N 2, reactive N A more fully coupled terrestrial C-N cycle Litter Soil organic matter Mineral N Respiration Fire Litterfall Stabilization (+) Mineralization Immobilization Leaching Leaching/Denitrification Uptake Fixation Photosynthesis (+) Inorganic C Mineral N Organic C/N (+) Deposition 1 2 3 4 4

18. Primary succession + fixed external N input + Dissolved Organic Nitrogen (DON) Carbon only C-N We now account for dissolved organic N losses. It takes much longer to reach steady state, and the system remains N limited, because DON losses scale roughly to biomass

19. Tropics Temperate Boreal time Ecosystem N-demand Early Succession Late Succession More Favorable Growth Conditions N fixers Non-Fixers A Powerful but Expensive Feedback from the C-Cycle on the N Cycle: Biological N fixation

20. Primary succession + fixed external N input and DON (previous experiment) Carbon only C-N

21. Primary succession + DON + biological N Fixation Carbon only C-N N fixation allows for faster biomass accumulation and steady state is reached much earlier.

22. N feedback on Net Primary Productivity (NPP) at Steady State: Relative change of Net Primary Productivity in a coupled C-N simulation vs. C only

23. Modeled Veg N [kg m -2 ] Global: 3.1 GtN (model) 3.5 GtN (obs/est.) Modeled Soil N [kg m -2 ] Global: 120 GtN (model) 95-140 GtN (obs/est.) Reconstructed Soil N [kg m -2 ] (Global Soil Data Task Group, 2000)

24. Simulated soil N agrees well with reconstructed inventories in high-productivity regions but is low in low-productivity and low-latitude regions. This discrepancy is a direct result of the model’s temperature sensitivity during decomposition, which is higher than suggested by the gradients of the global inventory [Ise and Moorcroft, 2006]. The model is less capable of resolving variations in C:N ratios between biomes which are between 10 and 15 in warm zones and 15–20 in cooler regions: mean modeled C:N ratio in soils is 15 with little latitudinal variations. Modeled Soil Nitrogen: Details

25. Recapitulation of Important Points C-N interactions are most important during transient changes (primary succession and/or disturbance) At (quasi-) steady state, N limitation in most ecosystems is small Exceptions: Biomes with frequent disturbances Biological N fixation is a powerful feedback mechanism that is highly adaptive in tropical forests

26. Transient Behavior (Wind-Throw) – the N Perspective Tropical Site Temperate Site N fluxes N inventories as deviation from steady state

27. CO 2 fertilization and N limitation

28. Drivers Atmospheric CO 2 Recent climate (Sheffield et al., 2006) N deposition rates (Dentener, 2006) Land-use transition rates (Hurtt et al., 2006) Setup Start in year 1500 with potential vegetation Include/exclude C-N feedbacks Include/exclude Environmental Drivers Full Land Model Study

29. LM3 is designed to diagnose and predict the land use sink

30. Effect of Shifting Cultivation and Forestry on C-N dynamics The time scales depend on initial conditions (previous human disturbances), overall biomass, and turnover of plants biomass relative to litter/soil pools.

31. Terrestrial Uptake [PgC yr -1 ] Budget based on ocean models (Sarmiento et al., 2009, IPCC94)

32. Carbon Sink – C vs. C-N (PgC/yr)

33. Residual terrestrial sink 1800 to 2000 Effects of N cycle on residual sink (C-only minus C-N) Effects of anthropogenic N deposition cycle on residual sink (C-N minus C-N-Natural Deposition)

34. NPP changes for temperate and tropical forests

35. Conclusions Including the N cycle improves the terrestrial C-cycle model by constraining CO 2 fertilization The required nitrogen for CO 2 sequestration is supplied via: –Tropics: adaptive biological nitrogen fixation –Temperate/Boreal: anthropogenic nitrogen deposition The next step: add Phosphorus

36. Can the terrestrial C budget reconciled when the C only land model is coupled to N? Khatiwala et al., 2009

37. Land Use Only Ocean based range (Sabine et al., 2004) Dynamic Vegetation Target

38. N limitiation

40. Residual Sink - N deposition 1800 2000 1900 + N deposition

41. DIN export at Hubbard Brook following Manipulation