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 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 CO 2 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 decomposition mineralization assimilation Thornton et al., 2009

Land carbon cycle sensitivity to increasing atmospheric CO 2 Offline CLM-CNFully-coupled CCSM3.1 Effect of C-N coupling is to increase atmospheric CO 2 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) (AR4) Atmospheric increase 3.3 ± ± ± 0.1 Emissions5.4 ± ± ± 0.3 Net ocean-to- atm -1.8 ± ± ± 0.5 Net land-to-atm-0.3 ± ± ± 0.6 Land partitioning: Land use flux1.7 (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 CO 2 on NEE and N availability (CO 2 – control) N avail. (index)

Single and combined effects on NEE LULCC N dep CO 2 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 Ndep high Ndep Thornton et al., 2009

CC CC+Ndep CtrlNdep Preind. Rad CO 2 N dep Trans. Prog. Fixed Lower NHigher 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 CO 2 -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 effect  CC 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 effect  CC 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 effect  CC 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?

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

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

hour hour obs model obs model Original model: no plant N storage pool Revised model: plant N storage pool GPP Soil mineral NPlant allocated N mineralization immob. Soil mineral NPlant allocated N Pre-allocation plant 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 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