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 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

Atm CO2

Plant

Litter / CWD

Soil Organic Matter

Carbon cycle

Soil Mineral N

N deposition

N fixation

denitrification

N leaching

Nitrogen cycle

respiration

Internal(fast)

External(slow)

photosynthesis

litterfall & mortality

decompositionmineralization

assimilation

Thornton et al., 2009

Land carbon cycle sensitivity to increasing atmospheric CO2

Offline CLM-CN Fully-coupled CCSM3.1

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)

C-only

C-N

low Ndep

high Ndep

1980s

(TAR)

1990s

(AR4)

2000-2009

(AR4)

Atmospheric increase

3.3 ± 0.1 3.2 ± 0.1 4.1 ± 0.1

Emissions 5.4 ± 0.3 6.4 ± 0.4 7.2 ± 0.3

Net ocean-to-atm

-1.8 ± 0.8 -2.2 ± 0.4 -2.2 ± 0.5

Net land-to-atm -0.3 ± 0.9 -1.0 ± 0.6 -0.9 ± 0.6

Land partitioning:

Land use flux 1.7

(0.6 to 2.5)

1.6

(0.5 to 2.7)

n.a.

Residual land flux

-1.9

(-3.4 to 0.2)

-2.6

(-4.3 to -0.9)

n.a.

Global C-cycle component estimates from IPCC AR4, 2007

Influence of rising CO2 on NEE and N availability

(CO2 – control)

N a

vail

. (i

nd

ex)

Single and combined effects on NEE

LULCC

N dep

CO2

All combined

Shevliakova 2009 (LM3V model result)

Interaction effects for total land C

N x LULCC

C x LULCC

C x N

All effects

(3-way)

• 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.

Land components of climate-carbon cycle feedback

low Ndephigh Ndep

Thornton et al., 2009

CC CC+Ndep

Ctrl Ndep

Preind.

Rad CO2

N dep

Trans.

Prog.

Fixed

Lower N Higher N

cooler / drier

warmer / wetter

N availability hypothesis

Higher due to N deposition

Higher due to climate change

Higher due to deposition and climate change

All simulations with prescribed transient fossil

fuel emissions

Does climate change mimic the effects of increased N

deposition?

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.

ND effectCC effect

Thornton et al., 2009

• 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).

ND effectCC effect

Thornton et al., 2009

GPP

Gross N mineralization

• 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.

ND effectCC effect

Thornton et al., 2009

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

SoilMineral

N

N Immobilization

N Mineralization

Plant N uptake

Photosynthesis

Potential GPP sets N demand

Plants and microbescompete for N on basis of

relative demand

C-N Coupling Schematic

GPP downregulated by N supply

CLM-CN, GPPMulti-site comparison

Mid-summer mean diurnal cycle

Obs Model

0 6 12 18 24hour

0 6 12 18 24hour

obsmodel

obsmodel

Original model: no plant N storage pool

Revised model: plant N storage pool

GP

PG

PPSoil mineral N Plant allocated N

mineralization

immob.

Soil mineral N Plant allocated N

Pre-allocationplant N storage

N from storage (demand, storage)

N to storage (demand, availability)

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