Carbon-nitrogen cycle interactions, ozone stress, and N

emissions speciation

Peter Thornton, J.-F. Lamarque, M. Vertenstein, N. Rosenbloom

Retranslocation

Litterfall

PlantUptake

Immobilization

Fire Losses

Deposition/ Fixation

Nitrification/ Denitrification

Leaching

Internal N CyclingExternal N Cycling

C flux

Legend

Tempsensitivity

N flux

AtmosphericCO2

VegetationBiomass

SoilOrganicMatter

AtmosphericN species

Coupled Carbon-Nitrogen dynamics• Strong feedback between decomposition and plant growth: soil mineral N is the primary source of N for plant growth.

• Can result in a shift from C source to C sink under warming. P.E. Thornton, NCAR

CLM-CCLM-CN (CO2,Nfix,dep)CLM-CN (CO2,Nfix)CLM-CN (CO2)

C4MIP models C4MIP mean

Land biosphere sensitivity to increasing atmospheric CO2 (L)

Results from offline CLM-CN, driven with CAM climate, in carbon-only (CLM-C) and carbon-nitrogen (CLM-CN) mode, from present to 2100. Using SRES A2 scenario assumed CO2 concentrations.

CLM-CN (CO2,Nfix,dep)CLM-CN (CO2,Nfix)CLM-CN (CO2)

Land biosphere sensitivity to increasing atmospheric CO2 (L)

Evidence that increasing N-limitation under rising CO2 has an important effect on the transient behavior of L, and that consideration of anthropogenic N deposition reverses this trend by around 2060.

Total C uptake (PgC) Mean NEE (PgC/y)

Expt 1850-2000 2000-2100 1980-2000 2080-2100

N dep 16 50 -0.24 -0.73

CO2 fert 61 220 -0.98 -2.56

CO2+Ndep 79 301 -1.31 -4.13

CLM-C 223 843 -3.80 -10.75

Cumulative land carbon uptake and net ecosystem exchange, 1850-2100

Tair Prcp

NEE

sen

sitiv

ity to

Tai

r (Pg

C /

K)

0

1

2

3

4

5

NEE

sen

sitiv

ity to

Prc

p (P

gC /

mm

d-1

)

-25

-20

-15

-10

-5

0

CLM-CCLM-CN

NEE sensitivity to Tair and Prcp (interannual variability)

Coupling C-N cycles buffers the interannual variability of NEE due to variation in temperature and precipitation (global means, control simulations).

Potential for complex climate feedbacks depending on the spatial patterns of changing temperature and precipitation.

NPP variability dominates the Tair and Prcp response in most locations, but HR dominates that Prcp response in cold climates, due to feedback between snowpack, soil warming, and enhanced HR.

Tair Prcp

% c

hang

e fr

om c

ontr

ol

-40

-20

0

20

40

60

CLM-C: +CO2

CLM-CN: +CO2

CLM-CN: +CO2 +Nmin

NEE sensitivity to Tair and Prcp: effects of rising CO2 andanthropogenic N deposition

Carbon-only model has increased sensitivity to Tair and Prcp under rising CO2. CLM-CN has decreased sensitivity to both Tair and Prcp, due to increasing N-limitation.

Summary of plant response to ozone stress

S Sleaf cuticle

H2OO3

CO2

Chloroplast

Plant responses:

• Wound/defense response (VOCs)

• Reduced mesophyll conductance

• Reduced chloroplast function

• Reduced stomatal conductance

?

?

Problems:

• Species-specific responses

• Species mixes and competition important

• Few observations on native species

• Strong interaction with water stress response

Speciation of land N emissions

Nitrification vs. denitrification depends on aerobic state of soil, probably at the microscopic scale.

Sophisticated models already exist, and it should be possible to adapt them for use in CLM-CN.

Agricultural emissions could be tied to new efforts with crop modeling.