ECOLOGIE ET
ECOPHYSIOLOGIE
FOREST IERES
UMR 1137 INRA UHP
CONTEXT
GP
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NEE => NEP => source or sink ?
CH4
RECO
=> GS
=> IRGA
=> MS
Rnew Rold
OBJECTIVE
 To understand shifts in processes occuring in the
carbon balance of peatlands for restoring their sink
activity.
 To highlight the contribution of ‘new peat’ (acrotelm)
respiration versus ‘old peat’ (catotelm) mineralisation to
total CO2 efflux.
 To quantify the contribution of both sources using stable
isotope signature (13C-CO2).
 To study climatic influences on these two sources and
their partitioning
 To determine the effects of key plant species on these
two sources
F, d
 Linear mixing model
CALCULATIONS
 Mass balance equations
Fn, dn
Fo, do
F = Fn + Fo
[1]
F.d = Fn.dn + Fo.do
[2]
Determination of dn and do by
incubating peat cores and
collecting evolved CO2
[3]
Determination of d by Keeling plot
Fn/F
=
(d - do)
(dn - do)
measurements
=> d 13C of living plant material, dead plant material, organic
matter in soil cores at different depth;
=> d 13C of CO2 evolved by these materials in lab incubation
(incubation conditions to be defined)
=> In situ peat respiration (gas exchange chambers and IRGA)
=> d 13C of in situ respired CO2 (Keeling plot methodology)
MATERIAL
LICOR Li-6200
By-pass
Teflon filter
Dessicant
(MgClO)
Sampling
system
(Plexiglas)
Butyl septum
Exetainer Tube (10 ml)
 Determination of d
1. KEELING PLOTS
-10 Soil respiration, Hesse
forest
-12
-14
d (‰)
-16
-18
-20
-22
d = 6196,836*1/[CO2] -24,84
-24
-26
R2 = 0,969
0
0,0005
0,0010
0,0015
1/[CO2]
0,0020
Ngao J., Epron D., Brechet C. and Granier A. Estimating the
contribution of leaf litter decomposition to soil carbon efflux in a
beech forest using 13C depleted litter. In prep.
QUESTIONS AND LIMITATION
 Do the isotopic signatures of ‘old’ and ‘new’ peats be
contrasted enough ?
=> long term change in atmospheric 13C abundance
(-6.5‰ to -8.0‰ over the last 50 years)
=> progressive enrichment of SOM by microbial
discrimination or preferential decomposition
=> photosynthetic refixation of ‘old’ CO2 in ‘new’
peat
=> CH4 oxidation
what we expect
Novák M., Buzek F., Adamová M. 1999. Vertical trends in
13C, 15N and 34S ratios in bulk Sphagnum peat.
Soil Biology and Biochemistry 31: 1343-1346.
EXPERIMENTAL DESIGN
1m
Depleted litter
Control litter
0.5 m
Ngao J., Epron D., Brechet C. and
Granier A. Estimating the contribution of
leaf litter decomposition to soil carbon
efflux in a beech forest using 13C
depleted litter. In prep.
CONTRIBUTION OF LITTER DECOMPOSITION
12
10
FL/F (%)
8
6
4
2
0
0
30
60
90
120
150
180
210
240
270
300
330
360
Julian days
 Maximum contribution : 10% of F
 Mean annual contribution : 5%
Ngao J., Epron D., Brechet C. and Granier A. Estimating
the contribution of leaf litter decomposition to soil carbon
efflux in a beech forest using 13C depleted litter. In prep.
FRACTION LOSS AS CO2
 Annual soil CO2 efflux
838 gC.m-2
 Litter mass loss
61.5% of initial DM
 Litter contribution (5%)
42 gC.m-2
 Litter mass loss
99 gC.m-2
 Fraction loss as CO2
42 %
Ngao J., Epron D., Brechet C. and Granier A. Estimating the
contribution of leaf litter decomposition to soil carbon efflux in a
beech forest using 13C depleted litter. In prep.