The CLM5 Nitrogen cycle

Rosie Fisher, Dave Lawrence, Will Wieder, Gordon Bonan, Keith Oleson, Peter Lawrence, Sean Swenson,

Danica Lombardozzi, Ahmed Tawfik, Justin Perket, Erik Kluzek, Ben Andre, Bill Sacks, Mariana Vertenstein

Charlie Koven, Bill Riley, Bardan Ghmire (LBNL) Anthony Walker (ORNL), Chonggang Xu, Ashehad Ali (LANL)

Mingjie Shi & Josh Fisher (NASA-JPL) Eddie Brzostek (WVU), Quinn Thomas (VT),

NCAR-CLM4(CN)

Impact of Nitrogen on simulated carbon

fertilization

Issues raised with the CLM4.0(CN)

“Plants get Nitrogen for free”

The FUN* Model A marketplace for Nitrogen Uptake*Fixation and Uptake of Nitrogen

Hypothesis: Plants will take up N from the

cheapest sources

Solution to FUN model

Cgrowth = Cnpp - Cnuptake

Ngrowth = Nuptake

Nuptake = Cnuptake / CNuptake_cost Ngrowth = Cgrowth / CNtarget

Solve for maximum growth

GPP - Mresp

CNtarget/CNuptake_cost +1Cnuptake =

The FUN* Model A marketplace for Nitrogen Uptake*Fixation and Uptake of Nitrogen

FUNMod.F90

“Leaf Nitrogen content is static”

The FlexCN Model Variable carbon:nitrogen ratios

Increase in productivity due to change C:N ratio

Hypothesis: Plants will vary their tissue Carbon:Nitrogen ratio as N availability varies in

space and time

Zaehle et al. 2014

‘FlexCN’ allows for tissue-level variation in C:N ratio relative to target parameter.

Standalone FlexCN model tested by Ghmire et al. (BGC)

Increase in productivity due to increased NUE (fertilization)Increase in productivity due to increased leaf allocation

N Uptake

“N pool”

C Allocation to -leaves

-fine roots -stems

-coarse roots

Fraction of Allocation to -leaves -fine roots -stems -coarse roots

Actual N Allocation to

-leaves -fine roots

-stems -coarse roots

x

If N uptake is too low, C:N ratios will increase

GPP - MResp - GRresp - cost of N uptake

Fixation

Active Uptake

RetranslocationFlexCN N allocation scheme

NutrientCompetitionFlexibleCNMod.F90

“Photosynthetic capacity does not respond to the environment”

The LUNA* Model How best to use the Nitrogen you have?

Predicted optimal photosynthetic capacity

Hypothesis: Leaf Nitrogen is distributed so that light capture, carboxylation and respiration are co-limiting

*Leaf Use of Nitrogen for Assimilation

LUNA performance vs. observations

0 50 100 150

050

150

Predicted Vc,max25

Observed Vc,max25 (a) r2=0.59

ME=0.46

0 20 40 60 80 120

050

100

Predicted V c,max25

Observed Vc,max25 (b) r2=0.63

ME=0.52

0 20 40 60 80 100

050

150

Predicted V c,max25

Observed Vc,max25 (c) r2=0.46

ME=0.46

0 100 200 300 400

0200

400

Predicted J max25

Observed Jmax25 (d) r2=0.5

ME=0.33

0 50 100 200

0100

300

Predicted Jmax25

Observed Jmax25 (e) r2=0.85

ME=0.75

0 50 100 150 200

0100

200

300

Predicted J max 25Observed Jmax25 (f ) r2=0.45

ME=0.45

Herbs Shrubs Trees

Ali et al. 2015

LunaMod.F90

FUN flex-CN reconciliation

FUN-FlexCN coupling• The FUN model targets a fixed C/N ratio

• This intrinsically does not allow flexible CN ratio.

• We thus need to change Cnuptake to allow for this

Cgrowth = Cnpp - Cnuptake

Ngrowth = Nuptake

Nuptake = Cnuptake / CNuptake_cost Ngrowth = Cgrowth / CNtarget

Solve for maximum growth

GPP - Mresp

CNtarget/CNuptake_cost +1Cnuptake =

C allocation to uptake responds to CNuptake-cost and CNactual

Cnuptake = Cadj x (GPP-MR)

(CNtarget / CNuptake-cost) + 1.0

Cadj = 1.0 - (CNuptake-cost-Pa) / Pb

Cadj = Cadj + (1.0 - Cadj) x (CNactual- CNtarget)/ Pc

FUN equationAdjustment factor

Reduce C allocation with cost

Increase C allocation with high C:N

Offline FlexCN-FUN feedback behavior

Years of simulation Years of simulation

Pa = 5 : Pb= 200 : Pc= 80

Ncost=100 C/N

Ncost=10 C/N

Ncost=100 C/N

Ncost=10 C/N

Ncost=100 C/N

Ncost=10 C/N

Ncost=100 C/N

Ncost=10 C/N

Retranslocation

Cost of retrans-location gC/gN

C:N ratio of remaining leaf tissue (decreasing N content)

Cost of below ground extraction (fixation or active)

‘Free’ retranslocation

threshold ‘Paid-for’ retranslocation

limit

Iteration for each litter fall timestep

Remaining N goes to litter

Schematic of Retranslocation Algorithm

N limitation in CLM5Nitrogen is not abundant, for some reason:

slow decomposition? high leaching or denitrification? low productivity & fixation rates? lower deposition?

N uptake becomes more expensive

A higher fraction of NPP is spent on uptake. Tissue C:N ratios increase

N available for photosynthesis declinesNPP for growth decreases

Metrics of N limitationA higher fraction of NPP is spent on uptake. Tissue C:N ratios increase

N available for photosynthesis declines

NPP for growth decreases

Things I need Plot of CN ratios Plot of N spent

NutrientCompetitionFlexibleCNMod.F90

LeafC:N ratio

Fraction of NPP spent on N uptake

New parameters of N model• Nitrogen cost factors

• Fixation

• Active uptake

• Retranslocation

• Target leafCN ratio

• Flexible leafCN parameters

• LUNA parameters (only one is tunable)

Sensitivity Analysis of N cycle parameters

ITERATION (1-6 = 0.2 0.5 1.0 1.5 2.0)

Sensitivity Analysis of non-N parameters

ITERATION (1-6 = 0.2 0.5 1.0 1.5 2.0)

Conclusions• The new CLM5 nitrogen cycle model is substantially different to

the CLM4.5 and CLM4.0.

• We are making progress on understanding the behavior and interactions in the new model

• Much remains to be tested and understood (see parameterization talk)

• The model allows comparisons with many new data streams (N fixation, CN ratio, Vcmax variation)

• …and also fixes numerous theoretical problems with the existing CLM N cycle model