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Critical needs for new understanding of nutrient dynamics in Earth System Models

Critical needs for new understanding of nutrient dynamics in Earth System Models. Peter Thornton Oak Ridge National Laboratory Collaborators:

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Critical needs for new understanding of nutrient dynamics in Earth System Models

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  1. Critical needs for new understanding of nutrient dynamics in Earth System Models Peter Thornton Oak Ridge National Laboratory Collaborators: Gautam Bisht, Jiafu Mao, Xiaoying Shi, Forrest Hoffman, Keith Lindsay, Scott Doney, Keith Moore, Natalie Mahowald, Jim Randerson, Inez Fung, Jean-Francois Lamarque, Johannes Feddema, Yen-Huei Lee NASA GSFC, 22 Feb 2011

  2. Key Uncertainties • Nutrient limitation effect on CO2 fertilization • Nutrient – climate interactions • Is the “nitrogen as phosphorus proxy” hypothesis useful in the tropics? • Nutrient dynamics in a warming Arctic • Mechanisms and time scales for plant nutrient dynamics: • Competition (with microbes and other plants) • Uptake and storage (across days and seasons) • Deployment

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

  4. Land carbon cycle sensitivity to increasing atmospheric CO2 Offline CLM-CN Fully-coupled CCSM3.1 C-only high Ndep C-N low Ndep Effect of C-N coupling is to increase atmospheric CO2 by about 150 ppm by 2100, compared to previous model results Thornton et al., 2007 (left), and Thornton et al., 2009 (right)

  5. Global C-cycle component estimates from IPCC AR4, 2007

  6. Influence of rising CO2 on NEE and N availability N avail. (index) (CO2 – control)

  7. Single and combined effects on NEE LULCC All combined N dep CO2 Shevliakova 2009 (LM3V model result)

  8. Interaction effects for total land C C x N (3-way) N x LULCC C x LULCC All effects

  9. low Ndep high Ndep Land components of climate-carbon cycle feedback • Effect of C-N coupling on gamma_land is to reduceatmospheric CO2 by about 130 ppm by 2100, compared to previous model results • Net climate-carbon cycle feedback gain (including ocean response) is nearly neutral or negative, compared to positive feedback for previous models. Thornton et al., 2009

  10. All simulations with prescribed transient fossil fuel emissions N dep Preind. Trans. Prog. warmer / wetter Rad CO2 Fixed cooler / drier N availability hypothesis Lower N Higher N Higher due to N deposition Higher due to climate change Does climate change mimic the effects of increased N deposition? Higher due to deposition and climate change

  11. ND effect CC effect Climate-carbon cycle feedback CO2-induced climate change (warmer and wetter) leads to increased land carbon storage • Both climate change (red curve) and anthropogenic nitrogen deposition (blue curve) result in increased land carbon storage. • Climate change producing uptake of carbon over tropics, opposite response compared to previous (carbon-only) results. Thornton et al., 2009

  12. ND effect CC effect GPP • GPP response is highly correlated with gross N mineralization • Relationship between GPP and N min is similar for effects of climate change and direct N fertilization (anthropogenic N deposition). Gross N mineralization Thornton et al., 2009

  13. ND effect CC effect • Increased N deposition causes increase in both SOM and vegetation carbon stocks • Radiatively-forced climate change causes a decline in SOM and an increase in vegetation carbon stocks. • Consistent with the hypothesis that increased GPP under climate change is due to transfer of nitrogen from SOM to vegetation pools. Thornton et al., 2009

  14. Does warming-induced carbon uptake in the tropics make sense if the most limiting nutrient is P instead of N?

  15. Photosynthesis C-N Coupling Schematic GPP downregulated by N supply Potential GPP sets N demand Plants and microbes compete for N on basis of relative demand N Immobilization Plant N uptake Soil Mineral N N Mineralization

  16. CLM-CN, GPP Multi-site comparison Mid-summer mean diurnal cycle Model Obs

  17. Original model: no plant N storage pool obs model Soil mineral N Plant allocated N GPP immob. mineralization 0 6 12 18 24 hour Revised model: plant N storage pool obs model N to storage  (demand, availability) N from storage  (demand, storage) GPP Soil mineral N Plant allocated N Pre-allocation plant N storage 0 6 12 18 24 hour

  18. Implications and Conclusions • Additional empirical constraints are required to reduce prediction uncertainty • warming (x CO2?) x nutrient manipulations • Tropical forest (areal extent, C stocks, C fluxes) • Arctic tundra and boreal forest • Brave new models • Introduce the known important mechanisms • Get the wrong answer for the right reasons • … to eventually get the right answer

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