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

Key Uncertainties

• Nutrient limitation effect on CO 2 • Nutrient – climate interactions fertilization – 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

Carbon cycle

respiration Atm CO 2 photosynthesis Plant litterfall & mortality

Nitrogen cycle

Internal (fast) External (slow) denitrification N deposition assimilation Litter / CWD Soil Mineral N decomposition Soil Organic Matter mineralization N leaching N fixation Thornton et al., 2009

Land carbon cycle sensitivity to increasing atmospheric CO 2

Offline

CLM-CN

Fully-coupled

CCSM3.1

C-only C-N high Ndep low Ndep Effect of C-N coupling is to

increase

atmospheric CO 2 by about compared to previous model results

150 ppm

by 2100, Thornton et al., 2007 (left), and Thornton et al., 2009 (right)

Atmospheric increase Emissions Net ocean-to atm Net land-to-atm Land partitioning: 1980s (TAR) 3.3 ± 0.1

5.4 -1.8 -0.3 ± 0.3

± 0.8

± 0.9

1990s (AR4) 3.2 ± 0.1

6.4 -2.2 -1.0 ± 0.4

± 0.4

± 0.6

Land use flux Residual land flux 1.7 (0.6 to 2.5) -1.9

(-3.4 to 0.2) 1.6

(0.5 to 2.7) -2.6

(-4.3 to -0.9) Global C-cycle component estimates from IPCC AR4, 2007 2000-2009 (AR4) 4.1 ± 0.1

7.2 ± 0.3

-2.2 ± 0.5

-0.9 ± 0.6

n.a.

n.a.

Influence of rising CO 2 availability on NEE and N (CO 2 – control)

Single and combined effects on NEE LULCC All combined N dep CO 2 Shevliakova 2009 (LM3V model result)

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

Land components of climate-carbon cycle feedback low Ndep high Ndep • Effect of C-N coupling on gamma_land is to

reduce

atmospheric 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

Preind.

N dep

Trans.

All simulations with prescribed transient fossil fuel emissions

Rad CO 2

Prog.

Fixed

CC

Ctrl CC+Ndep

Ndep

warmer / wetter cooler / drier Lower N Does climate change mimic the effects of increased N deposition?

Higher N N availability hypothesis Higher due to N deposition Higher due to climate change Higher due to deposition and climate change

Climate-carbon cycle feedback CO 2 -induced climate change (warmer and wetter) leads to

increased

land carbon storage  ND  CC effect effect • 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

 ND  CC effect effect

GPP Gross N mineralization

• 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). Thornton et al., 2009

 ND  CC effect effect • Increased N deposition carbon stocks causes increase in both SOM and vegetation • 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

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

C-N Coupling Schematic

Photosynthesis N Immobilization Potential GPP sets N demand Plants and microbes compete for N on basis of relative demand

Soil Mineral N

Plant N uptake N Mineralization GPP downregulated by N supply

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

Original model: no plant N storage pool obs model Soil mineral N immob.

mineralization Plant allocated N Revised model: plant N storage pool N to storage  (demand, availability) N from storage  (demand, storage) Soil mineral N Plant allocated N 0 6 12 hour 18 24 obs model Pre-allocation plant N storage 0 6 12 hour 18 24

Implications and Conclusions

• Additional empirical constraints are required to reduce prediction uncertainty – warming (x CO 2 ?) 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