Soil CO2 efflux in a beech forest: the contribution of root respiration

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Soil CO2 efflux in a beech forest: the contribution of

root respiration

Daniel Epron, Laetitia Farque, Eric Lucot, Pierre-Marie Badot

To cite this version:

Original

article

Soil

_CO

₂

_efflux

in

a

beech forest:

the

contribution of

root

_respiration

Daniel

_Epron

Laetitia

_Farque

Eric Lucot

Pierre-Marie Badot

a

a

Équipe

sciences

_végétales,

laboratoire

_biologie

et

_{écophysiologie,}

institut des sciences et des

_techniques

de l’environnement,

université de Franche-Comté,

pôle

universitaire, BP 71427, 25211 Montbéliard cedex, France

b

Équipe

1975, sciences

_végétales,

laboratoire

_biologie

et

_{écophysiologie,}

institut des sciences et des

_techniques

de l’environnement,

université de Franche-Comté,

place

Leclerc, 25030

_Besançon

cedex, France

(Received 24

_September

1998;

_accepted

17 December 1998)

Abstract - The contribution of root

_respiration

to soil carbon efflux in a _young beech stand was estimated

_{by comparing}

soil _CO,

efflux from small trenched

_plots

to efflux from undisturbed areas _(main

_plot).

Soil

_CO

₂

efflux was _{measured every 2-4 weeks} in

1997. An

_empirical

model _(y = A

q

v

e

BT

)

was fitted to the soil

_CO,

efflux data and was used to calculate annual soil carbon efflux from soil _{temperature (T)} and soil volumetric water content

_(q

_v

_).

The annual soil carbon efflux were 0.66

_kg

_C

m-2

_year

_-1

in the main

plot

and 0.42

_kg

_C

m-2

_year

_-1

in the trenched

_plots.

The difference between these two estimations was corrected for the

_{decomposition}

of roots that were killed

_{following trenching.}

The

_{heterotrophic}

_component of soil carbon efflux accounts for 40 % of total soil

car-bon efflux (0.26

_kg

_C

m-2

_year

_-1

₎

_while

root

_respiration

accounts for 60 % of soil C release (0.40

_kg

_C

m-2

_year

_-1

_).

(© Inra/Elsevier,

Paris.)

carbon _cycle / _Fagus

_sylvatica

L. /

_respiration

/ root / soil

_CO

₂

efflux

Résumé - Flux de

_CO

₂

_provenant

du sol dans une hêtraie : la contribution de la

_respiration

des racines. La contribution de la

respiration

des racines au tlux de carbone _provenant du sol d’une

_jeune

hêtraie a été estimée en _comparant le flux de _{CO, provenant}

du sol sur des

_{petites placettes}

isolées par une tranchée au flux de _{CO, provenant} du sol mesuré sur la

_placette

principale.

Le flux de

CO,

provenant du sol a été mesuré toutes les 2 à 4 semaines en 1997. Un modèle

_empirique

_(y = _A

_&thetas;

_v

e

BT

)

a été

_ajusté

sur les

don-nées de flux de

_CO

₂

_provenant du sol, et _{utilisé pour calculer le flux annuel de carbone provenant} du sol à

_partir

de la

_température

du sol _(T) et de la teneur en eau

_volumique

du sol

_(&thetas;

_v

_).

_Les flux annuels de carbone _provenant du sol étaient de _0,66

_kg

_C

m-2

_y

_-1

_pour

la

placette principale

et de _0,42

_kg

_C

m-2

_-1

_y

pour les

petites placettes

isolées par une tranchée. La différence entre les deux estimations a

été

_corrigée

_pour

_prendre

en _compte la

_{décomposition}

des racines tuées lors de l’établissement de la tranchée. La _composante

hétéro-trophe représente

40 % du flux total de carbone _provenant du sol (0,26

_kg

_C

m-2

₎

_y

_-1

alors

que la

respiration

des racines

représente

60 % du

_dégagement

de carbone (0,40

_kg

_C

m-2

_y

_-1

_).

(© Inra/Elsevier, Paris.)

cycle

du carbone / _Fagus

_sylvatica

L.

_{/ respiration}

/ racine / flux de

_CO,

_provenant du sol.

1.

Introduction

Soil carbon efflux is an

_important

_component

of the

carbon

_cycle

in _temperate forests and is

_thought

to repre-sent 60-80 % of _ecosystem

_respiration

_{[12, 23, 27].}

Soil

*

Correspondence

and

_reprints

depron@pu-pm.univ-fcomte.fr

carbon efflux includes both

_CO

₂

_released

_during

decom-position

of leaf and root litters and

_CO

₂

from root

respi-ration.

_Respiration

rates of

_{plant organs and}

leaf and fine

car-bon

_through

an increase in

_productivity

_[22].

Direct

mea-surements of fine root

_respiration

have

_highlighted

the

high specific respiration

rates of fine roots of forest trees

[6, 8, 9, 24, 28]. Therefore,

root

_respiration

is

_thought

to

be an

_important

_component

of the carbon balance of

trees in forest ecosystems. But available estimations of

the contribution of root

_respiration

to soil

_CO

₂

efflux are

still rather scarce, and the most reliable ones _vary

con-siderably

from 30 to 60 %

_{[4, 11, 17, 18].}

Since both

root and

_{heterotrophic}

_respiration

are

_thought

to

_depend

on site characteristics

_(species,

_climate,

_{stand age,}

man-agement

practices,

etc.

_[23].),

estimations of the contri-bution of root

_respiration

to soil

_CO

₂

efflux are still

required

to

_provide

a better

knowledge

of carbon

budgets

of forest _ecosystems.

However, direct measurements of root

_respiration

are

rather difficult in situ and

_digging

to access the roots is

thought

to have a

_large

influence on root

_respiration

because of

_wounding

effects. In

addition,

instantaneous

measurements of root

_respiration

are difficult to scale to

the stand-level because

_CO

₂

concentration within the soil pores

changes greatly

with time and

depth

[24].

A reduc-tion in root

_respiration

at

_high

_CO

₂

has been

_reported

but its

_importance

is still controversial

[3, 7, 9, 21].

Indirect methods have been

_proposed

to

_quantify

both

het-erotrophic

and

_autotrophic

contributions to total soil

_CO

₂

efflux. Data obtained

_{by comparing}

in situ soil

_CO

₂

efflux and

_respiration

of soil

_samples

from which roots

were removed are

_questionable

because of

_high

soil dis-turbance

_during

soil

_sampling

and

_processing.

Root

res-piration

can be estimated

_{by subtracting}

litter,

root and soil

_organic

matter

_{decomposition}

rates from soil

_CO

₂

efflux

_[11]

or

_by

_comparing

soil

_respiration

before and

after

_{clear-felling}

[17, 18].

Root

_respiration

can be

esti-mated in a similar fashion

_{by comparing}

soil

_CO

₂

efflux

recorded on small trenched

_plots

to the one recorded on

the main

_plot

_{[4, 11].}

In this

_study,

we

_adapted

this latter

_approach

to

esti-mate the contribution of root

_respiration

to soil

_CO

₂

efflux in a _young beech stand in north-eastern

France,

a

site that

_belongs

to a network of 15

_{representative}

forests

extending

over a

_large

climatic _range in

_Europe.

2.

Materials

and methods

2.1.

_Study

site

The

_study

site is located in the Hesse forest

(north-eastern France, 48°40

N,

7°05

E,

elevation 305 m,

7

_km

₂

₎

and is one of the Euroflux sites

_(European

_project

ENV4-CT95-0078).

The

_{experimental plot}

covers

6

10

-3

km

2

and is

_{mainly composed}

of

_30-year-old

beeches

_{(Fagus sylvatica).}

Herbaceous

_understory

vege-tation is _{rather sparse. Leaf} area index was 5.7 in 1996

and 5.6 in

1997,

which

_corresponds

to a leaf litter fall of

0.14

_kg

_C

m

-2

_year

_-1

_(Granier,

_pers.

_comm.).

_Average

annual

_{precipitation}

and air _temperature are 820 mm and

9.2

°C,

_{respectively.}

Soil is a

_gleyic

luvisol

_according

to

the F.A.O. classification. The

_pH

of the _top soil

(0-30 cm)

is 4.9 with a C/N ratio of 12.2 and an _apparent

density

of 0.85

_kg

dm

-3

.

and is covered with a mull

_type

humus

_{(see [10]).}

Six

_sub-plots

of about 100

m

2

each were

_randomly

chosen within the

_experimental

_plot

for soil

_CO

₂

efflux

measurements. Two

3-m

2

_sub-plots

(2

x 1.5

_m)

with no

trees were established in June 1996

_{by digging}

a trench

(1

m

_deep)

around each,

lining

the trench with a

polyeth-ylene

film and

_filling

it back. The nearest trees were 1 m away from the trenches.

Soil _temperature was measured at -10 cm

_by

copper/constantan thermocouples.

Data

_acquisition

was

made with a CR7

_{datalogger (Campbell}

Scientific

Inc.,

USA)

at 10-s. time interval.

_{Thirty-minute}

_averages were

stored. In

addition,

soil _temperature was also monitored

simultaneously

with soil

_CO

₂

efflux with a

copper/con-stantan

_{thermocouple penetration probe}

inserted in the soil to a

_depth

of 10 cm in the

_vicinity

of the soil

respira-tion chamber. Volumetric water content of the soil

_(&thetas;

_v

₎

was _{measured every} 10 cm in

_depth

on the main

_plot

with a neutron

_probe

_{(NEA, Denmark)}

in

_eight

alumini-um access tubes

₍₁₆₀

cm or 240 cm

_deep)

at 1-week to

3-week intervals. Two distinct calibration curves were

used for sub-surface

_(-10

and -20

_cm)

and

_deeper

mea-surements. In

addition,

a

_polyethylene

reflector was used

for sub-surface measurements. Between two measure-ments, the volumetric water content of the soil was

assumed to

_{change linearly}

with time. This

_assumption

can _{be wrong if rainfalls} occurs

_during

that

_period.

Simulations of

_daily

soil carbon efflux would be

overes-timated before the rainfall event and underestimated after it.

However,

it would not

_strongly

affect our annual esti-mation of soil carbon efflux as overestimations would

counterbalance underestimations on an annual basis. A

TDR device

_{(Trase system,}

Soil Moisture

_Equipment

Corp.,

Santa

_{Barbara, USA)}

was used for additional

measurements of soil water content

_{using 40-cm-long,}

vertically

installed,

stainless steel wave

_guides.

Measurements were made on the six

_sub-plots

of the

main

_plots

and on the two trenched

_plots

_(two

measure-ments on each

_sub-plot)

on several occasions between June and October 1997. A

_comparison

between TDR and

neutron

_probe

data on the main

_plot

from June to

2.2. Soil

_CO

₂

efflux

Soil

_CO

₂

efflux was measured

_using

the Li 6000-09

(LiCor Inc., USA)

soil

_respiration

chamber in which the increase of the

_CO

₂

concentration was recorded with the

Li 6250 infrared _gas

_{analyser (LiCor}

_Inc.,

_USA)

as

already

described

_[10].

_Every

2 to 4

weeks,

12

measure-ments were recorded on each

_{sub-plots during}

an 8-h

period

from 8 am to _{4 pm.}

_Daily

_averages

_(n

= 72 for the

main

_plot

and n = 24 for the trenched

_plots)

and

confi-dence intervals at P = 0.05 _were calculated. An

empirical

model was fitted to the soil

_CO

₂

efflux data:

with

_&thetas;

_v

the soil volumetric water content at -10 cm, T

the soil _temperature at -10 cm and A and B two fitted

parameters. The correlation between soil water content

and soil

_CO

₂

efflux was less

_significant

for

_deeper

soil

layer

[10].

The model was then used to calculate annual soil carbon efflux from 1 December 1996 to 30 November 1997.

2.3. Root

_biomass,

root

_growth

and root

_decay

Root biomass was determined from vertical

_profiles

of root densities of 11

_{representative}

trees. Trenches

were

_dug

at a distance 150,

100,

50 and 25 cm from the trunks. Roots were counted

_by

diameter classes from the

soil surface to a

_depth

of 100 cm

_using

a 10 x 10-cm

_grid

affixed to the smoothed wall of the trench

_{[5, 14]}

The number of roots in each diameter class was converted into root volume

_knowing

the _average root

_length

of

roots.

_Average

root

_lengths

were calculated from

ramifi-cation

_patterns

of roots of each diameter

class,

which

were deduced from excavated root

_systems.

Root volume

was converted into root biomass

_using

a root mass _per

unit volume of 0.8

_kg

_DM

dm

-3

_(unpublished

_data)

The

relationships

observed between root _{biomass per} tree and trunk circumference were then used to estimate the mean

root biomass

_knowing

the distribution of trunk circum-ference. Fine root biomass

_(diameter

< 2

_mm)

was also estimated from

_eight

soil cores

₍₈

cm in

diameter,

12 cm

high)

collected

_monthly

from March to

_July

1997. Cores

were stored in

_{plastic bags}

at 4 °C until fine roots were

washed free of

_soil,

sorted into live and dead fractions

and dried at 60 °C for 48 h. Fine root biomass calculated from soil cores

_(0.31

_kg

_DW

m

-2

)

accounts for 45 % of the total fine root biomass in this site

_(0.69

_kg

_DW

_m

_-2

₎

according

to the vertical

_profiles

of root

_impacts.

Fine root

_growth

into 14 root-free cores was used to

estimate annual fine root

_production

from the number of

roots _{grown into the} cores over 1 _year

_{[19, 20].}

Soil

cores

₍₈

cm in

_diameter,

12 cm

_high)

were taken in

March 1997, all roots were

_carefully

_removed,

and the

sifted soil was

_replaced

within the hole. In

_April

1998,

these

_ingrowth

cores were retrieved and

_processed

as

above. This estimation of annual fine root

_production

was corrected for the

_spatial

and vertical variations of

fine root

biomass,

assuming

that fine root biomass in soil

cores _represents 45 % of the total fine root biomass. Fine root

_{decomposition}

was estimated

_{by coring}

and

sorting remaining

dead roots in the trenched

_plots

2 years after

trenching.

The

_remaining

fine root necromass was then

_compared

to initial fine root biomass and

necromass in soil cores. Coarse roots

_(2-10

mm in

diam-eter)

excavated

_during

the installation of the trenched

plots

were washed free of

soil,

cut into

_pieces

of 4-6 cm

long

and

_placed

into 10 x 15-cm litter

_bags

₍₁

mm mesh

size).

Bags

were then

_placed

at a soil

_depth

of 10-15 cm.

On five occasions

_during

a 20-month

_period,

14 litter

bags

were collected. Roots were

_carefully

washed free of soil and dried at 60 °C for 5

_{days. Simple exponential}

decay

functions

_(M

_t

=

M

0

e

-kt

)

were fitted to the

data,

_M

_t

and

_M

₀

_being

the

_remaining

and the initial root

_dry

mass,

respectively,

t

_being

time and k the

_decay

constant.

Carbon loss as

_CO

₂

_during

root

_{decomposition}

was

cal-culated as

_{(1 - a)}

c

_M

₀

_{(1 -}

e

-kt

).

_c, the initial carbon

concentration in root was set at 44

%;

a is the fraction of carbon which is

_incorporated

into soil

_organic

matter

while 1 -

a is the fraction lost as

_CO

₂

_by

microbial

respi-ration

_during

initial

_belowground

litter

decay;

a was set to 0.22

_[13].

3. Results

On the main

_plot,

soil

_CO

₂

efflux varied

_greatly

dur-ing

the _{year, from} less than 0.5

_&mu;mol

m

-2

s

-1

in

winter to

more than 4

_&mu;mol

m

-2

s

-1

in summer

_(figure

_1C).

Changes

in soil

_CO

₂

efflux were

_mainly

related to

changes

in soil _temperature, but a decrease in soil water content

_strongly

affected late summer values.

Therefore,

soil

_CO

₂

_efflux

was best described with an

_empirical

model

_including

_&thetas;

_v

the soil volumetric water content at

- 10

cm and T the soil

_temperature

at -10 cm

_(y

= A

&thetas;

v

e

BT

, figure

2).

Soil

_CO

₂

efflux was lower on the trenched

plots

than on the main

_plot

from

_May

to

October,

except

in

_September

when soil

_CO

₂

efflux on the main

plot

was

inhibited

_by

a

_pronounced

decline in soil water content.

Elimination of tree

_{transpiration}

_by

_{trenching clearly}

influenced soil water content

_{(figure 1A)}

while soil

tem-peratures were almost the same on the main

_plot

and on

the trenched

_{plots (figure}

_1B).

and soil volumetric water content at -10 cm recorded on

the main

_plot.

These

_predictions

were then used to calcu-late annual soil carbon efflux from 1 December 1996 to

30 November 1997

_{(table I).}

The annual soil carbon efflux were 0.66

_kg

_C

m

-2

_year

_-1

on the main

_plot

and 0.42

_kg

_C

m

-2

_year

_-1

on the trenched

_plots.

The difference

between the two estimations has to be corrected for the

decomposition

of roots that were killed

_{by trenching}

to

account for the

_{heterotrophic}

_component

of soil carbon efflux.

The

_comparison

of

_remaining

fine root necromass

2 _{years after}

_trenching

to initial fine root biomass and

necromass indicated that 53 % of killed fine roots

disap-peared

within _{2 years}

_(k

=

0.38,

table

II).

The

decay

con-stant obtained

_{by fitting exponential decay}

model over

coarse roots

(r

2 =

0.90,

table

_II).

Fine and coarse root

bio-masses deduced from root

_profiles

and root cores were

0.69 and 2.06

_kg

_Dw

_m

_-2

_,

_{respectively,}

for the main

_plot.

However, since trenched

_plots

were established at least 1 m _away from trees, fine and coarse root biomasses in trenched

_plots

at the time of

_trenching

were 1.10 and

1.03

_kg

_DW

_m

_-2

_,

_{respectively.}

The

_CO,

released

_during

the

_{decomposition}

of killed

roots from

I

December 1996 to 30 November 1997 was

estimated as 0.10

_kg

_C

m

-2

year

-1

and 0.06

_kg

_C

m

-2

year

-1

for fine and coarse roots,

_{respectively.}

The

_{heterotrophic}

component of soil carbon efflux was therefore 0.42

-(0.10

+ 0.06), i.e. 0.26

_kg

_C

m-2

_year

_-1

and accounted for 40 % of total soil carbon efflux while root

_respiration

was

_thought

to represent 60 % of soil C release

(i.e.

0.40

_kg

_C

m

-2

_year

_-1

_).

Fine root biomass in

_ingrowth

core _{after 1 year}

_ranged

from 0.06 to 0.63

_{g with}

an _average value of 0.29 g

(n =

14).

Fine _root biomass in soil cores

₍₈

cm in

diame-ter, 12 cm

_high)

is

_thought

to _represent 45 % of the total fine root

_biomass,

_taking

into account both the

_spatial

and vertical distribution of fine roots deduced from root

profiles.

We therefore calculated that fine root

produc-tion was 0.13

_kg

_DM

_m

_-2

_year

_-1

_,

which

_correspond

to

0.06

_kg

_C

m

-2

_year

_-1

_assuming

a carbon concentration in

root of 44 %.

4. Discussion

Our estimation of the contribution of root

_respiration

to soil carbon efflux in a

_30-year-old

beech stand in

north-eastern France

(60 %)

is similar to the one

report-ed

_by

Ewel et al.

_{[11] for}

a

_29-year-old

slash

_pine

planta-tion in Florida and

_{slightly higher}

than those

_{reported by}

Nakane et al.

_[17,

_18]

who estimated that root

_respiration

contributes about half of soil carbon efflux in a

80-year-old

_Japanese

red

_pine

stand and in a

_102-year-old

oak

forest

_{(table III).}

In contrast, Bowden et al.

_[4]

calculated that root

_respiration

accounted for 33 % of soil carbon

efflux in a

_temperate

mixed hardwood forest in

Massachusetts. However,

they neglected

in their calcula-tion the release of carbon from

_{decomposition}

of roots

killed

_{by trenching}

because

_{they postulated}

that

decom-position

of

_freshly

killed root was

_negligible

at the time

of their measurements. If their

_assumption

was untrue,

they

estimated that root

_respiration

would account for

about one half of soil carbon efflux.

Our estimation of the

_partitioning

between root and

heterotrophic

contributions to soil carbon efflux is

sensi-tive to the

_assumption

we have made to account for the

decomposition

of

_freshly

killed root. The

_decay

con-stants we used are within the range of

_published

values

for both fine and coarse roots of

_{woody species}

_{[1, 15,}

25]

. McClaugherty

et al.

_[15]

_argued

that fine roots were

still connected to

_larger

roots and therefore that

carbohy-drates and nutrients stored in coarse roots _may

_delay

the

decomposition

of fine roots in trenched

_{plots. They}

also

showed that fine root

_{decomposition}

followed a

two-phased

pattern, and that

_dry

matter loss was more

_rapid

during

the first _stage than

_during

the second. Our k value

for fine roots, which was

_simply

obtained

_{by comparing}

the

_remaining

fine root mass 2 _{years after}

_trenching

to

fine root mass in soil cores, should therefore be

consid-ered as a

_rough

estimation.

_{Nevertheless,}

we calculated

that a 20 % variation in the values of the

_decay

constants

would

_change

our estimation of the contribution of root

In other _reports

_using

trenched

_plots

to

_partition

root

and

_{heterotrophic}

contributions to soil carbon efflux

[4,

11],

differences in soil water content between normal

and trenched

_plots

have been

_neglected.

In our

_study,

trenching strongly

influences soil water content

_by

elimi-nating

tree

_{transpiration.}

In late summer and

_early

autumn 1997, the soil water content was twice as

_high

in the trenched

_plots

than in the main

_plot.

In a

_previous

paper

[10],

we showed that a decrease in soil water

con-tent

_strongly

influenced soil

_CO

₂

efflux in summer. If we

had

_neglected

the differences in seasonal courses of soil

water content between the main and the trenched

_plots,

the contribution of root

_respiration

to soil carbon efflux would have been underestimated

₍₅₂

% instead of 60

_%).

Fine root

_production

in our stand

_(0.13

_kg

_DM

m

-2

year

-1

)

is lower than those

_reported

for older beech forests

_(0.44

in a

_120-year-old

stand

[26]

and 0.39 in a

145-year-old

stand

_[2]),

but falls within _{the range} of

val-ues

_compiled

_by

Nadelhoffer and Raich

[ 16]

and Persson

[20]

for forest

_ecosystems.

In _many

studies,

fine root

production

were not corrected for the

_spatial

variations of fine root biomass. In our case, it would have led to an

overestimation of fine root

_production

_(0.21

instead of

0.13

_kg

_DM

m

-2

_year

_-1

₎

since there is less fine root in the

vicinity

of trunks than at 1 m _away. Our estimation of

fine root

_production

_{may be underestimated} since distur-bances associated with soil

_coring

_may have restricted

root

_ingrowth

in

_{early spring}

_[19].

Whatever the case,

the difference between the maximum and the minimum fine root biomass in soil cores collected

_monthly

from

March to

_July

_{1997 gave} a similar estimation of fine root

production

(i.e.

0.12

_kg

_DM

m

-2

_year

_-1

_).

In order to test the accuracy of our estimation of the

contribution of root

_respiration

to soil carbon

_efflux,

we

used soil carbon

_budget

to calculate the carbon allocation

to root

_respiration

and turnover. This calculation assumed that

_changes

in soil carbon content and

_changes

in fine root biomass are

_negligible

in

_comparison

with carbon

fluxes,

i.e. that soil

_organic

matter and fine root

biomass are in

_steady

state. Another

_assumption

is that soil

_CO

₂

efflux is the

_only

_output

_pathway

of soil carbon.

Therefore,

leaching

of dissolved carbon is

_neglected.

Carbon allocation to root

_respiration

and turnover as

esti-mated

_by

the difference between soil

_respiration

and lit-terfall

_(i.e.

0.52

_kg

_C

m

-2

_year

_-1

₎

_{is within the range of}

published

values for _temperate broad-leaves forests

[22],

but is

_higher

than the sum of fine root

_production

(0.06

_kg

_C

m

-2

_year

_-1

₎

and root

_respiration

(0.40

_kg

_C

m

-2

_year

_-1

_).

However,

the former calculation of carbon allocation to root

_respiration

and turnover is

thought

to be overestimated as it

_ignored

the

decomposi-tion of coarse

_woody

detritus

[22].

In our

site,

the

input

of carbon in the soil from dead branches that were left in the stand after a recent

_thinning

_may at least

_partly

account for the difference between our two estimations of carbon allocation to root

_respiration

and turnover

[22].

In

addition,

an underestimation of fine root

_production

cannot be

_excluded,

as mentioned earlier.

This

_study

_highlights

the

_important

contribution of

root

_respiration

to total soil

_CO

₂

efflux in a

_30-year-old

beech stand. Further works are needed to characterise the influence of

_{species composition,}

site location

_(both

cli-matic and

_edaphic

_conditions),

_{stand ages and}

manage-ment

_practices

on the

_partitioning

of root and

het-erotrophic

contributions to soil carbon efflux.

Acknowledgements:

Soil _temperature and water

con-tent data were

_{provided by}

André Granier

_(Inra

_Nancy,

unité

_{d’écophysiologie}

_{forestière).}

This work was

sup-ported by

the

_European

_{programme Euroflux}

(ENV4-CT95-0078)

and

_by

Office national des forêts

(ONF).

The District urbain du

_Pays

de Montbéliard

(DUPM)

is also

_acknowledged

for financial

_supports.

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