Stefan GerberLars Hedin, Sonja Keel, Stephen

Pacala, Elena Shevliakova

Soil and Water Science Department University of FloridaPrinceton University

sgerber@ufl.edu

Hungate et al., 2003

Stoichiometry remains fixed in vegetation andsoilsN fixation and N deposition are primary inputpathways5 to 10 % of N deposited contributes to CstorageN fixation increases between 10 and 45 % perdoubling of CO2

Leaching losses decline between 0 and 10 % perdoubling of CO2

CO2, N2, reactive N

Litter

Soil organicmatter

Mineral N

Respiration

Fire

Litterfall

Stabilization(+)

Mineralization

Mineralization

Immobilization

Leaching

Leaching/ Denitrification

Uptake

Fixation

Photosynthesis(+)

Inorganic CMineral NOrganic C/N

(+)

Deposition

1

2 3

4

5

41) Plant regulation of Cand N intake2) N dependent humi-fication and decompo-sition3) Hierarchical systemfor N sink4) N losses fromorganic pool

5) Biological Nfixation: Response toecosystem N demand

Costly ProcessOpportune, if other N sources areexhaustedPossibly limited by light, particularly inextra-tropicsDynamics of regulation likely varies, fastadjustment in tropics, slow adjustment(succession) in temperate

Fixa

tion

rate

(g N

m-2

yr-1

)

N fixing trees adjustrate of biological Nfixation according totheir demand

N fixing tree species present

Shade Tolerance IndexLess tolerant

Nitr

ogen

ase

Act

ivity

Temperature (oC)Ph

osph

atas

eac

tivity

Trop

ics

Tem

pera

teBo

real

time

Ecos

yste

mEc

osys

tem

NN--d

eman

dde

man

d

Early Succession Late Succession More Favorable GrowthConditions

N fixersNon-Fixers

N demand of aunconstrained system

(C-only)

Realized N fixation

C-N

kg N ha-1 yr-1

Net change in land carboninventory based on ocean-atmosphere budgeta

(Sabine et al., 2004)

Residualterrestrial

sinkNETLU

NETLU

Residual terrestrialsink 1800 to 2000

Effects of N cycle onresidual sink (C-only minus C-N)

Effects of anthropogenicN deposition cycle onresidual sink (C-N minusC-N-Natural Deposition)

kg N ha-1 yr-1

N demand of aunconstrained system

(C-only)

Realized N fixation

C-N

N demand of aunconstrained system

(C-only)

Realized N fixation

C-N

kg N ha-1 yr-1

N limitation

Static N fixation

C-onlyC-NC-N, N fixation constant =>

Hungate et al., 2003

Understanding biological N fixation is central tounderstand the land’s response to CO2We are only at the beginning of mechanistictreatment of biological N fixation in models

Questions / Future directionsP cycleDecoupled fixation: heterotrophs, lichens,epiphytesN fixation responses to demand, can we detect aCO2 response in N fixation?

sgerber@ufl.edu