Carbon-Nitrogen Interactions in the LM3 Land Model

1. Carbon-Nitrogen Interactions in the LM3 Land Model Stefan Gerber Department of Ecology and Evolutionary Biology Princeton Universitysgerber@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: • If the Fixation of N [conversion from N2 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 CO2?

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

4. Models Predict a Big Sink From CO2 Fertilization 2050

5. Uncertainty about the magnitude of CO2 fertilization is the key factor determining whether vegetation is a net carbon source or sink Change in Vegetation Biomass, kgC/m2 No CO2 fertilization CO2 Fertilization at 700 ppm -460Pg +200 Pg • GFDL Slab-Ocean Climate Model SM2.1coupled to Dynamic Land model LM3V • Atmospheric CO2 concentration: 700 ppm Shevliakova et al. 2006

6. CO2 fertilization and N limititation:N supply does not support predicted CO2 uptake Hungate et al., 2003

7. Nitrogen Cycling ? fertilizer combustion fertilizer

8. 4 1 5 3 2 4 The coupled terrestrial C-N cycle CO2, N2, reactive N Fire Deposition Photosynthesis (+) Respiration 5 Fixation Litterfall Mineralization (+) Uptake Litter Mineral N Stabilization (+) Immobilization Soil organic matter Mineralization Inorganic C Leaching/Denitrification Mineral N Leaching Organic C/N

9. 1 Leaves ~30:1 Sapwood 150:1 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 Heartwood 500:1 Tissue turnover Tissue turnover Storage Roots ~50:1 < Plant uptake > Nitrate and Ammonium

10. The coupled terrestrial C-N cycle CO2, N2, reactive N Fire Deposition Photosynthesis (+) Respiration Fixation Litterfall Mineralization (+) Uptake Litter 2 Mineral N Stabilization (+) Immobilization Soil organic matter Mineralization Inorganic C Leaching/Denitrification Mineral N Leaching Organic C/N

11. Litter Decomposition Microbial N limitation 2 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. Increasing N – demand for microbial growth Litter

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

13. Internal N-Cycle and feedbacks on C-Cycle CO2, N2, reactive N 1 Photosynthesis (+) Respiration Litterfall Mineralization (+) Uptake Litter 3 2 Mineral N Stabilization (+) Immobilization Soil organic matter Mineralization Inorganic C Mineral N Leaching/Denitrification Organic C/N

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

15. Primary succession experiment with fixed external N input:From bare soil to temperate forest C-N is N limitation in 1, 2, and 3 Carbon only C-N 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 Organic losses via hydrological leaching Fire / Disturbance

17. A more fully coupled terrestrial C-N cycle CO2, N2, reactive N Fire 4 Deposition Photosynthesis (+) 1 Respiration Fixation Litterfall Mineralization (+) Uptake Litter 3 2 Mineral N Stabilization (+) Immobilization Soil organic matter Mineralization Inorganic C Leaching/Denitrification Mineral N Leaching 4 Organic C/N

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

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. Modeled Soil Nitrogen: Details 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.

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 N inventories as deviation from steady state Tropical Site N fluxes N inventories as deviation from steady state Temperate Site N fluxes

27. CO2 fertilization and N limitation

28. Full Land Model Study Drivers • Atmospheric CO2 • 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

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 CO2 fertilization • The required nitrogen for CO2 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

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

40. DIN export at Hubbard Brook following Manipulation