EC for measuring soil CO2 efflux and disturbances introduced by the measurements systems

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HAL Id: hal-02826914

https://hal.inrae.fr/hal-02826914

Submitted on 7 Jun 2020

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introduced by the measurements systems

Bernard Longdoz

To cite this version:

Bernard Longdoz. EC for measuring soil CO2 efflux and disturbances introduced by the measurements systems. ESF-summer school on ”Integrated methodology on soil carbon flux measurements”, Sep 2004, Monte Bondone, Italy. 34 p. �hal-02826914�

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Bernard LONGDOZ

UMR Écologie et Écophysiologie Forestière

Institut National de la Recherche Agronomique (INRA) Centre de Nancy

FRANCE

E-mail : longdoz@nancy.inra.fr

Eddy Covariance, Forest soil CO₂ efflux, fluxes canopy model

EC for measuring soil CO2 efflux &

Disturbances introduced by the

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CO₂ budget for a control air volume: • Source(s)

• Sink(s)

• Accumulation

• Transport by wind (advection, turbulence) • Molecular diffusion

Eddy Covariance

• Storage

• vertical & horizontal turbulence

• vertical & horizontal advection

• Molecular diffusion

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• Storage

• vert. & hor. turbulence

• vert. & hor. advection

• molecular diffusion • horizontal homogeneity • No mean vert. w at soil surface • few cm height: turb. >>> diffusion

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height cm tens few at flux turbulent vertical Rs =

w : vertical wind component

C : [CO₂]

‘ : variance, high frequency oscillations around the time average (turbulence)

_

(

w ⋅'C'

)

at few tenscm height

=

(vert. turbulence + molecular diffusion)_{soil surf}

= (vert. turbulence + vert. advection)_{few cm}+ storage

= Rs Usually at few tens cm height :

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Main material composing EC system : • High frequency 3D sonic anemometer • High frequency gas analyser

Measurements of

• 3 wind speed components

• [CO₂]

at high frequency (above 10 Hz):

(

w'C'

)

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sonic anemometer

Closed path IRGA

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Open path IRGA

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Limitations

Usually: vertical turbulent flux >>>

vertical advection + storage

Except during low turbulence period (low u*)

(quiet nights, below closed canopy) 0 0.5 1 1.5 0 0.2 0.4 0.6 0.8 1 1.2 FRICTION VELOCITY u* [m/s] N O R M A L IS E D F L U X [ -]

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Disturbances introduced by the

measurement system

Introduction of measurement system

disturbances of natural process

Soil CO₂ efflux measurement system

disturbances of CO₂ flux going out of soil surface

How production and diffusion can be disturbed ?

Which variables controls soil CO₂ efflux ?

How experimental system change variables values ? How reduce and correct these changes ?

(reduce the impact, correction)

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Variables controlling the soil CO

₂

efflux

∆C<<< Rs Rs = Fc

Fc : wind transport (turbulence) +

molecular diffusion

Momentum conservation equation

v_z depends on δP/δz and v_h(2)

(

)

mol c z V z C D C v Fc Rs δ δ − = =

v_z : vertical wind speed

C : [CO₂]

δC/δz : vertical [CO₂] gradient

D_c : CO₂ molecular diffusivity

V_mol : molar volume

(Lund et al., 1999)

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(1) + (2) Rs function of δP/δz, δC/δz and v_h

Conclusion :

Ideal measurement system should reproduce the natural

distributions of the δP/δz, δC/δz and v_h

Perturbations of the δP/δz, δC/δz and v_h by chambers

(open, closed dynamic/static), EC and gradient method

δP/δz : pumping effect, wind transport

δC/δz : diffusion

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δ

P/δz

No perturbations by EC, gradient method and closed static chambers

Possible perturbations with closed dynamic and open chambers (pump)

Closed dynamic chambers

• Pump creates overpres. and depres.

• Air circulates along pressure gradient • Pressure in chamber fct(chb. position

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PDC = P_ch - P_atm 0 pumping effect PDC = ± few Pa ??? Important ??? 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 -3 -2 -1 0 1 2 3 PDC (Pa) N or m al is ed s oi l C O2 e ff lu x exp {-0.41 PDC} Data from the Vielsalm forest site

(Belgium) PDC = -0.5 Pa Rs +20% PDC = +0.2 Pa Rs -9% Yes Sensitivity depend on

Soil porosity Soil productivity

Sensitivity increase with Sensitivity increase with

Fang & Moncrieff

(1998)

Rs +510% Rs -44%

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Grassland & crops : potential disturbance Solution P < P_atm P > P_atm Leak <<< soil CO₂ efflux

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Another leak important PDC

Check regularly PDC

High frequency oscillations of P_atm not reproduced

Study of the impact in progress (Takle & al 2004 AFM) Another source PDC :

• fan for [CO2] homogeneity

PDC > 1 Pa in the centre PDC < -0.5 Pa around solution : metal grid at the chamber bottom • heating of transparent chamber (up to 15°C)

air dilatation positive PDC

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PDC in open chamber • 1 pump

PDC exist but

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• 2 pumps

Solution : 2 mass flow controllers

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δ

C/δz

No perturbations by EC

Possible perturbations with gradient method, closed static chambers, closed dynamic and open chambers

Gradient method

Bad insertion of tube in soil

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Closed dynamic chamber 0 0.2 0.4 0.6 0.8 1 1.2 0 50 100 150 200 250 300 350 Time (s) C O 2 e ff lu x ( g m -2 h -1 ) 360 365 370 375 380 385 390 C O 2 (µ m o l m o l -1 )

[CO₂]_ch > [CO₂]_atm

or

[CO₂]_ch from below to

above [CO₂]_atm (scrub)

Efflux increase if [CO₂]_ch < [CO₂]_atm

Efflux decrease if [CO₂]_ch > [CO₂]_atm

δ

C/δz

disturbance

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Recommendation :

Determine the [CO₂] thresholds to neglect slope variations

(work in this interval or correction)

Closed static chamber

If chemical traps: [CO₂]_ch

↗

until Rs = trap absorption

Rs underestimated If sample same that dynamic without scrub

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0 0.2 0.4 0.6 0.8 1 1.2 0 50 100 150 200 250 300 350 Time (s) C O2 e ff lu x ( g m -2 h -1 ) 360 365 370 375 380 385 390 C O 2 (µ m o l m o l -1 ) Open chamber

Inlet: [CO₂]_atm

In chamber [CO₂]

↗

[CO₂]_ch > [CO₂]_atm

Same

recommendation

For all chambers

Insertion of the collar roots cut C_soil (

δ

C/δz

_{) perturbed}

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v

_h

No perturbations by EC and gradient method

Unable to reproduce

v

_h in closed static/dynamic and open

chambers

closed static chamber

v

_h = 0

closed dynamic and open chambers

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Difficult to separate from pressure disturbance !!! Longdoz et al., 2000 :

PDC and [CO₂]_ch corrections

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Experimental Design

Experimental design adapted to :

• Ecosystem (grassland , forest, crops) • Spatial scale

• Temporal scale Grassland

EC & all chambers : no separation grass

↮

soil

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Forest

Chambers, gradient method, EC (with selection on u*) Crops

EC : no separation vegetation

↮

soil

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Spatial scale 0 0.5 1 1.5 2 2.5 3 3.5 3 5 7 9 LAI R 1 0 Interplot variability

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0 10 20 30 40 50 60 0.5-1.0 1.0-1.5 1.5-2.0 2.0-2.5 2.5-3.0 3.0-3.5 3.5-4.0 4.0-4.5 4.5-5.0 5.0-5.5 5.5-6.0 Fs10 (µmol m-2 s-1) F re qu en cy ( % ) Beech Douglas fir Intraplot variability

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EC : few m2

gradient : few dm2

open chamber : few dm2

manual closed dynamic chamber : plots moved by experimenter

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0 1 2 3 4 5 6 15/8 14/9 14/10 14/11 14/12 14/1 13/2 15/3 15/4 15/5 15/6 15/7 S pa ti al m ea n so il C O2 e ff lu x ( µm ol m -2 s -1 ) simulated measured Temporal scale

Inter-annual (long term) Seasonal

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Année 2002 50 100 150 200 250 300 350 0 1 2 3 4 0 5 10 15 20 Flux Température du sol Jours juliens F lu x d e C O2 ( µ m o l. m -2 .s -1 ) T e m p é ra tu re m o y e n n e d u s o l ( °C ) Day to day

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-0.5000 0.0000 0.5000 1.0000 1.5000 2.0000 2.5000 3.0000 3.5000 4.0000

3-juil-2001 4-juil -2001 5-juil -2001 6-juil -2001 7-juil-2001 8-juil-2001 9-juil-2001 10-juil-2001 11-juil-2001 12-juil-2001 13-juil-2001

Jours F lu x d e C O 2 ( µ m o l. m -2 .s -1 ) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 H u m id it é d u s o l (m 3 .m -3 ) e t p lu ie ( l. m -2 ) Half-hour

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EC : half-hour on long term

open chamber : half-hour on short term (days) gradient : half-hour on long term

manual closed dynamic chamber :

day to day seasonal variation automatic closed dynamic chamber :

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Topic for discussion (proposal)

automatic closed dynamic chamber + gradient

separation of CO₂ production and diffusion