A first approach to assessing soil oxygenation in the field using the Tensionic ceramic tensiometer

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A first approach to assessing soil oxygenation in the field

using the Tensionic ceramic tensiometer

A. Vigouroux

To cite this version:

A. Vigouroux. A first approach to assessing soil oxygenation in the field using the Tensionic ceramic

tensiometer. Agronomie, EDP Sciences, 1997, 17 (8), pp.389-394. �hal-02684515�

Note

A

first

_approach

to

_assessing

soil

_oxygenation

in the

field

_using

the

Tensionic ceramic tensiometer

A

_Vigouroux*

Laboratoire de _biologie et _{pathologie végétales,} Ensa, Inra, 2, place Viala, 34060 _Montpellier cedex 1, France

(Received 23 _September 1996; accepted 15 December 1997)

Summary -

This _paper describes a _prototype method for

_assessing

_oxygen

_availability

in soils

using

water

_injected

into the ceramic cup of the Moutonnet Tensionic tensiometer. The water is recovered after several

_days

of

equilibra-tion with the soil _{solution and the dissolved oxygen it contains is}

_immediately

measured with a field _oxymeter. The easy installation, low cost of each device and

_rapid

measurement allow numerous

_replications

in the same

_plot,

mak-ing

it

_possible

to obtain

_{representative}

results. Certain

_metrological

refinements are still needed, but the

_consistency

of the results

_already

obtained _suggests that the device offers

_{interesting possibilities}

in various areas of

agronomic

stud-ies. _{(© Inra/Elsevier)}

soil / aeration / _oxymeter / ceramic cup tensiometer

Résumé - Une tentative d’estimation commode de

_{l’oxygénation}

des sols à l’aide du tensiomètre à

_bougie

poreu-se Tensionic. Il est décrit un

_principe

d’estimation du niveau

_{d’oxygénation}

des sols utilisant l’eau

injectée

dans la

bou-gie

du tensiomètre Tensionic de Moutonnet. Cette eau,

après

plusieurs jours

de mise à

_{l’équilibre}

avec la solution du

sol, est

_récupérée

et on _{y dose} immédiatement

_l’oxygène

dissous à l’aide d’un

_oxymètre

de terrain. La facilité de _pose, la modicité du

_prix

de

_{chaque appareil}

et la

_rapidité

de la mesure autorisent un nombre

_important

de

_{répétitions}

_par par-celle devant _permettre d’obtenir des valeurs

_{représentatives}

des conditions

_globales

du sol

_exploré

par les racines. Divers

_{ajustements métrologiques}

sont encore nécessaires mais la cohérence des résultats

_déjà

obtenus semble confir-mer les

_{possibilités}

intéressantes de ce

_système

_{pour conforter divers types} d’études

_{agronomiques.}

_{(© Inra/Elsevier)}

sol / aération /

_oxymètre

/ tensiomètre

INTRODUCTION

To

_analyse

the mechanisms

_by

which

coarse-tex-tured soils

_predispose

stone fruit trees to certain bacterial diseases

_(Vigouroux

and

Bussi,

1994b),

it was _necessary to determine in what conditions

tree roots function in winter. Matric

_potential

and

oxygenation

could be critical factors and have to

be assessed under field conditions. Matric

poten-tial is easy to measure but the different methods

available for

_oxygenation

are

_generally

unwieldy

and sometimes unreliable

_(see

_synthesis

_{by Levy}

and

Toutain, 1979;

Glinsky

and

_Stepniewski,

1985;

or the paper

by

Faulkner et

al,

1989).

Other

*

Correspondence and _reprints

methods

_(Renault

and

_Stengel,

₁₉₉₄₎

are devoted

to

_extremely

fine

_analyses

and not

_adapted

to

pro-viding

a

global

estimation at the level of the

hori-zon

_{exploited by}

roots. An

_adapted

use of the

Tensionic,

a ceramic tensiometer

perfected by

Moutonnet

_(Moutonnet

et

_al,

_1993),

_provided

us

with a series of values

_suggesting

that this device

might

offer

_{interesting possibilities}

for

_achieving

this

_{objective (Vigouroux}

and

_Bussi,

_1994a).

In

this paper we

_simply

describe the measurement

principle

involved and

_present

a few results in

order to draw attention to what

_might

be a

conve-nient and

_{fairly inexpensive}

method of

_monitoring

global

soil

_oxygenation.

MATERIALS AND METHODS

Principle

Moutonnet’s Tensionic

_(fig

₁₎ is a modified

tensiome-ter with a _system of

_capillary

tubes

_leading

to the ceramic cup, which remains in

_place

in the soil. Water

is

_injected

into the device and recovered about 10 d

later, after

_{equilibration}

with the soil solution

_by

diffu-sion

_through

a

_{’high-flow’}

ceramic _{cup. Moutonnet}

used the device to monitor nitrate levels in fertilised soils and

_groundwater,

but it is

_obviously

also

_possible

to monitor a

_large

number of other elements in the col-lected

_{liquid sample,}

which can be considered as

_being

relatively

similar to the soil solution itself. If the dis-solved oxygen in the solution is in

_equilibrium

with the soil

_atmosphere,

it should be

_possible,

with a minimum of

_precautions,

to estimate the

_oxygenation

conditions available to roots

_by

_measuring

_{the dissolved oxygen in}

the

_sample

with an _oxymeter.

Description

of the

_apparatus

A _{porous ceramic cup with} a

_sample

volume of 12 mL

is fixed to the end of a PVC tube of the same external

diameter

_{(approximately}

25 _mm). Different

_lengths

of tube are used so that the ceramic cup can be

_positioned

at the

_required

_depth.

A

_preliminary

hole is

_dug

with a

special

auger the size of the ceramic cup. In stony soil,

it is often _necessary to drive an iron rod the same size as the Tensionic into the

_ground.

As with

ordinary

ten-siometers, the usual

_precautions

are taken to ensure a continuum between the soil and the ceramic cup and to

isolate the latter from the direct effect _{of any free air} around the outside of the tube. A

_capillary

PVC tube

(E) connects the _top of the device and the bottom of the ceramic cup and is used to fill and _empty the _cup.

A second

_capillary

tube _(P) is connected at the _top of the ceramic _{cup and is used} to _{purge the system.} A

third tube _(T) can be used in

_parallel

to measure the

soil water matric

_potential,

if

_required.

Measurement _{of the dissolved oxygen}

Distilled water was used to

_improve

standardisation. The ceramic _cup was filled via the first tube

_by

means of a

_syringe

until the water _{overflowed via the purge}

tube, and the ends of the two tubes were then sealed.

For

_monitoring

nitrates, Moutonnet et al ₍₁₉₉₃₎ have established a 10-d

_{equilibration}

time with the soil solu-tion.

_Taking

this as a minimum time

₍₁₂

d were used in

practice),

we extracted the

_liquid

_{from the ceramic cup,}

still

_using

a

_syringe

and

_{recovering only}

12 of the 13 mL

_injected

₍₁₂ in the ceramic _{cup +} 1 in the _tubes) to ensure no bubbles were _present at the end of the

sam-pling.

The

_liquid

was then

_injected

into a small

narrow-necked

_glass

_phial

and into the bottom of the

_phial

to reduce

_oxygenation

of the

_{sample by}

air to a minimum. An _oxymeter

_probe

was then

_slowly

introduced and

gently

moved around to make the measurement. A WTW 0X

_320-type

_oxymeter and a Cellox 325

_probe

were _used, and the measurement stabilised after 40-50

s. The _oxymeter had a resolution of 0.01

_mg

_{of 0}

₂

_/L

and

automatically

took the

_sample

_temperature into account. The soil _temperature varied between 9 and 14 °C

_throughout

the

_experiment

and, for a

_{given day,}

differed between different locations from 0.2 to 0.4 °C

at the

_{depth surveyed.}

Protocol for

_installing

Tensionics

After

_{promising preliminary}

_{investigations,}

an

experi-ment was conducted

_during

the winter of 1995-1996. The soils from four

_peach

orchards were monitored from

_January

to

_{early May. They corresponded}

to two

pairs

of fine- and coarse-textured soils, each

_pair

(fine-textured/coarse-textured) was

_planted

with the same

variety

of

_peach

tree

_grafted

onto the same rootstock of

the same _age and at the same

_planting

_density.

The

fine-textured soil was

_sandy

loam

_consisting

of recent allu-vial

_{deposits by}

the Rhône river and the coarse-textured soil was a _{very stony quaternary}

_{fluvial-glacial deposit}

(75 %

_pebbles)

with a _very

sandy

fine fraction (70 %

sand).

At the

_beginning,

four Tensionics were installed

in the

_plots

with fine-textured soil and five in the

_plots

with coarse-textured soil _(these were more difficult to

install).

Another one was installed in all

_plots

after 1 month because the first

_readings

obtained were

hetero-geneous, as will be seen hereafter. As the roots colonised

_mainly

the -10- to -50-cm soil

_layer,

all ceramic cups were

_placed

at a

_depth

of 30 cm. Each

Tensionic was marked and monitored

_{individually.}

RESULTS

In order to

_give

an idea of the distribution and

_type

of

_variability

of the data

_collected,

the whole data

set is shown in

_figures

2 and 3. Even when

_taking

into account the usual

_variability

of _oxygen

day

may appear scattered. In fact a more

_thorough

analysis

shows that

_they

are

_{relatively grouped}

except

one. When the Tensionics are considered

individually, they

exhibit the same

temporal

vari-ation and the

_markedly

different value is

_always

obtained from the same Tensionic

(ie,

No 5 in the

coarse-textured soil and No 2 in the fine-textured soil in the case of cv Red

Diamond).

Examination

of the ceramic cups in situ at _{the end of the}

exper-iment

_provided

a

_{straightforward explanation}

for the

_divergent

values

_(earthworm

_burrow,

bundle of hair roots, clod

_containing

a

_large

amount _of

poor-ly

decomposed

organic

matter).

Very

similar observations were made in

_preliminary

investiga-tions.

DISCUSSION

Apparatus

Given the

_precautions

taken when the ceramic cups were installed and the restricted diameter of

the hole

_required

to install

_them,

interference from the

_atmosphere

seemed

_negligible

and the soil was

disturbed very little. The Tensionics should

prefer-ably

be installed a little in advance to ensure that

the

_equilibria

that have been disturbed can be

re-established. The mud used as a seal and to ensure

continuity

with the soil nevertheless created a het-erogeneous

layer

relative to the

_surrounding

soil,

and this could alter the conditions for gas circula-tion at this level. In

fact,

like the ceramic _cup

itself,

this

_{heterogeneous}

element

_probably

has a

limited effect on the diffusion of oxygen in its

dis-solved state because of the

_{long equilibration}

time. The minimum time

_chosen,

based on that deter-mined

_by

Moutonnet for

nitrates,

is somewhat

arbitrary,

but the

_differing

time intervals between

measurement dates did not _{indicate any}

systemat-ic effect.

However,

this

_{point requires}

further

investigation

and work is at

_present

_{in progress}

(Bienfait

and

_Benkhellouf,

_1995).

As

_implied

above,

equilibration

time affects the extent to

which the

_{liquid sample}

_{in the ceramic cup is} rep-resentative of the solution in the

_immediately

sur-rounding

soil and even of that of a certain volume

of soil around it. This cumulative

_aspect

allows the

_{equilibration}

time to be considered as the ’data

recording period’

for the

_liquid.

This constitutes a

major

advantage

in

_estimating

_{average conditions} for root

_functioning,

as

_compared

with

instanta-neous measurements _{of gas. The latter} are more

likely

to reflect

_{heterogeneous}

rates of diffusion in

a medium

subjected

to constant

_changes.

On the other

hand,

a minimum soil water content is

prob-ably

necessary to achieve

_satisfactory

_O

₂

diffu-sion. This

_point

needs to be evaluated and could limit the use of this method.

In

addition,

the

_capillary

tubes

_leading

to the ceramic cup contain 1 mL of

liquid

per linear

meter. Their effect is thus limited at shallow

depths

but is also

_being

studied at _greater

_depths

(Bienfait

and

Benkhellouf,

1995).

O

2

measurement

in the

_{liquid sample}

The _oxymeter used has a _very

satisfactory

resolu-tion relative to the measurable differences in the

soil,

and it is reliable if maintained and used in accordance with the standard

_procedures

for this

type

of

equipment.

The use of a

_syringe

to remove

the

_sample

_{from the ceramic cup avoids}

_pollution

from the

_atmosphere.

The

_liquid

has to be

_injected

gently

into the bottom of the collection

_phial.

The neck of the

_phial

is

_only

_very

_slightly

_larger

than the diameter of the

_oxymeter

_probe,

to

_keep

exchanges

with the

_atmosphere

to a minimum.

Moreover,

the

_probe

is inserted and moved around

slowly

in the

_{liquid being}

examined. The

_phial

itself also has a small internal diameter

_(approx

25

_mm)

so that the head of the

_probe

is

complete-ly

covered

_by

the

_{liquid despite}

the small

_quantity

collected and the

_liquid-air

contact surface is reduced. There is nevertheless a contact surface

between the

_liquid

_sample

and the air

_during

injec-tion into the

_phial.

A series of tests conducted to

evaluate the level of interference revealed a

noticeable increase in

_O

₂

_(0.4-0.8

_mg

_O

₂

_/L),

mainly

for low soil values since the difference in

O

2

content between soil and

_atmosphere

is there-fore

_higher.

This

_aspect

constitutes a

_major

bias. It can be

_{theoretically}

estimated but new

_procedures

for

_collecting

the

_sample

and

_measuring

_O

₂

_are

currently being developed

in order to reduce this interference.

Values obtained

It was mentioned above that the values observed

in a

_{given plot}

and on a

_{given day}

seemed

scat-tered but were in fact

_{relatively grouped}

_except

one. This was true for each notation date

through-out the measurement

_campaign

and the

_marginal

values

_always

concerned the same sensor. The

phenomenon

was observed more or less

_clearly

in

each

_plot

and a clear circumstantial

_explanation

was found for this

_particular

behaviour in each orchard. Under these

_conditions,

these ’abnormal’ values

_{might complicate}

the task of

_comparing

and

_interpreting

soil conditions between

_plots

_but,

on the other

hand,

_they

constitute

_good

criteria of

reliability

for the method itself: the same causes

systematically

induce the same effects which are

recorded

_by

the Tensionics as such. In

_parallel,

in

each

_plot,

the method

_provided

a batch of

relative-ly grouped

values,

whose mean can be considered as a reflection of the overall state of the soil. The oxygen distribution in the soil is known to _{be very}

heterogeneous.

The values obtained

_give

an idea

which seems

_{representative}

of this

_{heterogeneity}

even for the very different

_examples

of soils

com-pared.

Moreover,

more accurate information can

be

_easily

obtained

_{by increasing}

the number of

sensors and

_{by prolonging}

the survey since sen-sors are

_inexpensive

and the

_experimental

proce-dure is

_{simple. Unfortunately,}

since no similar

techniques

for

_measuring

_{soil solution} oxygena-tion

_exist,

there was no basis for

_comparison.

CONCLUSION

There is no convenient method for

_estimating

global

soil

_oxygenation

at the scale of the root sys-tem of

_plants

and,

in

_particular,

trees.

_Moreover,

using

the soil solution to evaluate

_oxygenation

enables more relevant

_integrated

values to be

The Tensionic is _easy to install and enables the soil solution to be

_conveniently

and

_efficiently

extracted.

_Surveys

can

_easily

be

_performed

over

variable durations. On the other

hand,

the

mea-surement of the

_O

₂

_in

the

_{liquid sample}

has to be

improved.

Values obtained from well-aerated soils

are

_fairly

_reliable,

but the bias is

_large

for low

_O

₂

levels and another more

_precise

measurement sys-tem is in

_preparation.

In

fact,

this last

_point

is somewhat

_secondary

and

_{purely metrological.}

The

most

_interesting

_aspect

of this method is the col-lection of

_{representative samples}

of the soil solu-tion. The

_proposed

device

_(with

an

_adapted

distri-bution in the studied

_plots)

seems to fulfil this role

provided

that

_{equilibration}

between soil and _cup is

correct. The time needed for

_{equilibration}

and the effect of soil

_humidity

on

_O

₂

_diffusion

are

current-ly being

studied. In

_conclusion,

further

improve-ments to the

_O

₂

_{measurement by}

more

_specialised

scientists are needed but the

_principle

of the

sys-tem should be of value to several areas of

agrono-my.

Acknowledgements:

We had useful discussions with D Bienfait of the Ctifl in

_Carpentras

who is also

inter-ested in the _system, and wish to thank V Valles and P Renault of the Station de sciences du sol _(Inra) in

Avignon

and P

_Saglio

of the Station de

_physiologie

végétale

(Inra) in _{Bordeaux, among others,} for their advice. We also thank T Girard - Inra _-SRIV, domaine de Gotheron _(Valence) for his

_help

in

_installing

the

Tensionics and

_carrying

out some of the measurements.

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_{Dépérissement}

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_{asphyxie : approche}

de

_l’oxygène

et du gaz

carbonique

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Expérimentation

Cerise. Dom

_Exper

La

Tapy-Carpentras,

89-99

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for

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I,

Stepniewski

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