Stefan GerberLars Hedin, Sonja Keel, Stephen
Pacala, Elena Shevliakova
Soil and Water Science Department University of FloridaPrinceton University
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?