Effect of tillage on bare soil energy balance and thermal regime: an experimental study

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Effect of tillage on bare soil energy balance and thermal

regime: an experimental study

Guy Richard, Pierre Cellier

To cite this version:

Guy Richard, Pierre Cellier. Effect of tillage on bare soil energy balance and thermal regime: an

experimental study. Agronomie, EDP Sciences, 1998, 18 (3), pp.163-181. �hal-00885878�

Original

article

Effect of

_tillage

on

bare

soil

_energy

balance

and thermal

_regime:

an

experimental

study

Guy

Richard

a

Pierre

Cellier

b

a

Inra, Unité _{d’agronomie} de Laon-Péronne, 02007 Laon Cedex, France

b

Inra, Unité de recherches en _{bioclimatologie,} 78850 Thiverval-Grignon, France

(Received 22 January 1998; accepted 26 _{May 1998)}

Abstract - The effects of

_tillage

on the _{energy balance and temperature} of bare soil were studied

using

three

plots

that had

different soil structures due to different times of seedbed

_preparation

and soil

_compaction.

The

_experiment

was

_performed

on a

_loamy

soil

_(Gleyic

_luvisol) in northern France

_during

the establishment of _{sugar beet} in

spring

1992.

Temperature,

water content, heat

capacity

and thermal

_conductivity

of the

_{ploughed layer,}

surface albedo and

_roughness,

net radiation, soil heat flux, sensible heat flux and

_evaporation

were all measured over a

_{spring-tilled}

_soil, an autumn-tilled soil and a

compacted

soil. Differences in soil heat fluxes were related to soil

_evaporation

and thermal

_{conductivity.}

Differences in soil _temperatures were related to heat

_capacity.

In

_spite

of a considerable

_evaporation,

the

_compacted

soil had the

_highest

soil heat flux because of its

_high

thermal

_{conductivity.}

Nevertheless, the

_{spring-tilled}

soil was the warmest because of its low heat

_capacity,

and _{sugar beet}

_germinated

more

_rapidly

with

_spring

soil

_tillage.

_{(© Inra/Elsevier, Paris.)}

soil

_tillage

/ soil structure / _temperature / energy balance / thermal

properties

Résumé - Effet du labour sur le bilan

_d’énergie

et le

_régime

_thermique

d’un sol nu : étude

expérimentale.

Une

expérimentation

a été conduite dans un sol limoneux

(Gleyic

luvisol)

dans le Nord de la France

pendant

la

phase

d’implantation

d’une culture de betterave sucrière au

_printemps

1992 pour étudier les effets du travail du sol sur le bilan

d’énergie

et sur le

_{régime thermique}

du sol. La

_{température,}

l’humidité, la

_capacité

_calorifique,

la conductivité

ther-mique

au sein de la couche labourée, l’albédo et la

_rugosité

de la _surface, le _rayonnement net, le flux de chaleur dans le sol et dans l’air et le flux de chaleur latente ont été mesurés dans un sol travaillé au

_printemps,

dans un sol travaillé à

l’automne ou dans un sol fortement

_compacté.

Les différences de flux de chaleur dans le sol entre traitements étaient liées aux niveaux

d’évaporation

et à la conductivité

_{thermique ;}

les différences de

_température

étaient liées à la

_capacité

calorifique.

Le sol fortement

_compacté

avait le flux de chaleur le

_plus

élevé

_malgré

un niveau

_{d’évaporation important,}

à cause de sa forte conductivité

_thermique.

Mais c’était le sol travaillé au

_{printemps qui}

était le

_plus

chaud à cause de sa

faible

_{capacité calorifique,}

entraînant une

_germination

des betteraves sucrières

_{plus rapide.}

_{(© Inra/Elsevier,}

Paris.)

travail du sol / structure du

_{sol / température}

/ bilan

_{d’énergie/}

_propriétés

_thermiques

Communicated by Jim Douglas (Penicuik, U.K.)

*

Correspondence and _reprints E-mail: richard@laon.inra.fr

1. INTRODUCTION

Soil

_temperature

is an

_important

variable in many processes. It influences

plant

growth

and

development (germination,

emergence, root

devel-opment

and

_{functioning),}

soil microbial

_activity

(decomposition

of

_organic

matter,

nitrogen

trans-formations),

physical

factors

_(viscosity

and surface tension of

_water)

and

_physical

_processes

_(transport

of water, gases and

solutes).

Soil

_temperature

_{depends mainly}

on the

climate,

but

_permanent

or

_temporary

soil characteristics can also cause

_temperature

differences of several K between soils under the same climate

_{[8, 24].}

For

example,

soil texture influences the albedo of the soil surface and the thermal

_regime

of the soil:

chalky

soils with a

_high

albedo warm more

_slowly

than

_loamy

soils with a lower albedo

[8].

Soil

tem-perature

can also be affected

_{by agricultural}

prac-tices which

_modify

the soil surface

_(natural

or

artificial

mulches,

roughness, ridges),

the soil

com-pactness

and the soil water

_regime.

Numerous studies have dealt with the effect of soil

_tillage

on soil

_temperature

_[29].

_{They generally compared}

the effects of conventional

_tillage

_(with

annual

_deep

tillage)

and minimum

_tillage

_(without

annual

_deep

tillage),

or different forms of conventional

_tillage.

It is not _easy to summarize their

_findings

because soil thermal

_regime

can be described

_by

_many vari-ables such as thermal time calculated with different base

_{temperatures, temperature}

and/or soil heat flux recorded once a

_day

or _every

_hour,

mean or

maximum and minimum

_temperatures

_during

a

day.

The measurements can also be made at differ-ent

_depths,

at different

_periods

_{of the year and with} or without a _crop.

Nevertheless,

it is

_generally

agreed

that minimum

_tillage

leaves the soil colder than conventional

_tillage

_(up

to 4 K

_[14]).

The

depressive

effect of minimum

_tillage

on soil

tem-perature

is

_generally

attributed to lower maximum

temperatures

[15]

linked to _{the presence of crop} residues at the soil surface which increase soil

albedo. But minimum

_tillage

can also reduce evap-oration because of the crop residues and

conse-quently

increase soil heat flux and soil

_temperature

[13].

The differences between various conventional

tillage techniques

(chisel/mouldboard/rotavator,

deep/shallow ploughing, fall/spring ploughing)

can reach 2 K

_{[ 14]}

and are

_generally

smaller than those

between conventional and minimum

_tillage.

Contradictory

results have also been

_reported.

Autumn

_tillage

can

_produced

a lower or a

higher

maize seed bed

_temperature

in

_spring

than did

spring tillage

[2, 28].

Soil _temperatures can be

higher

after

_{deep tillage}

with mouldboard

plough-ing

than with chisel

_[23]

or with

_no-tillage

without

surface residues

_[1].

In both _cases, the warmest soil had a lower thermal

conductivity

and heat

_capacity,

and a

_higher

soil heat flux. On the

_contrary,

a

com-pacted ploughed layer

can be warmer than an

uncompacted ploughed layer

and it had a

_higher

thermal

_conductivity

and a heat

_capacity,

and

final-ly higher

soil heat flux

_[3].

Soil thermal

_regime

_depends

on both the soil heat flux and its

_partitioning

in

_depth.

Soil heat flux is one _{of the four fluxes of the energy balance} and

_{consequently,}

its

_{magnitude depends}

on those of the other fluxes: net

_radiation,

latent heat

flux,

sensible heat flux towards the

_atmosphere.

These four fluxes

_depend

on the climatic conditions

(solar

and

_atmospheric

radiation,

air

_temperature,

air

_humidity,

wind

_speed),

the soil state variables

(surface

temperature

and water

_content),

and the soil

_physical

_parameters.

Soil

_parameters

can

directly modify

the fluxes of the energy balance

(albedo,

emissivity, roughness,

thermal

conductivi-ty),

or

_they

can

_modify

the soil state variables

(hydraulic conductivity,

heat

_capacity).

Contradictory

results could be due to the effect of

tillage

on soil

_parameters

other than the thermal

properties,

which are

_generally

the

_only

ones

mea-sured. Soil

_compactness

also affects soil

_hydraulic

properties

[18],

which can

_{greatly modify}

soil heat flux

_[26].

A smooth soil has a

_higher

albedo than a

rough

soil,

and

_consequently

_{less available energy} at the soil surface

_[2],

a smooth soil also has a lower

_{aerodynamic roughness}

which can limit the loss of energy

by

sensible and latent heat

_fluxes,

and

_consequently

enhance the soil heat flux

_[25].

It is therefore necessary to describe most of _the

para-meters _{and processes modified}

_by

soil

_tillage

to

obtain a better

_{understanding}

of the

_{relationships}

between soil

_tillage

and soil

_temperature

_[3].

Soil

_temperature

_{is very}

_important

_{for crop} emergence and

early growth, particularly

for crops such as _{sugar beet which is} sown at the

_beginning

of

_spring

in north-western

_Europe

when the soil is cold

_[4].

_{Secondary tillage}

can

_greatly

affect the

structure of the

_{ploughed layer}

at _{sugar beet}

sow-ing

[27].

It could be

_managed

to enhance soil

warming

in

_spring

and to reduce the time _{for sugar} beet emergence. The main variables in the

sec-ondary tillage

are the date of

_tillage,

which defines the

_length

of time the tilled soil is

_exposed

to frost and rainfall until

_sowing,

and the water content of the soil at

_tillage,

_{which governs soil}

_compaction

under tractor wheel tracks and soil

_loosening

after

tool

_operation.

We have therefore examined the effects of various _types of

_tillage

on soil

tempera-ture in order to

_improve

rules of decision

_making

for

_secondary

_tillage.

We measured most of the variables and _parameters which could influence the soil heat flux and its

_partitioning

in

_depth:

_energy balance

_{components, albedo,}

_roughness,

soil water content, soil

_hydraulic

and thermal characteristics.

Three different soil structures in the

_ploughed

layer,

simulating

a wide range of soil conditions at sugar beet

sowing,

were

_{compared. They}

differed in the

_timing

of

_{secondary tillage}

after

_ploughing

and before

_sowing

and in their

_degree

of

compact-ness.

2. MATERIALS AND METHODS

2.1.

_Experimental

sites

The field

_experiment

was conducted

_during

_sugar

beet establishment in the

_spring

of 1992 near Laon (49°34N, 3°38E) in France. Three

_experimental

treat-ments were studied:

S: mouldboard

_ploughing

in autumn _{(November 1991 )}

and

_secondary

_tillage

with a combined cultivator in

spring

(days

97 and ₁₀₁₎

_just

before _sugar beet

sow-ing (day

101);

A: mouldboard

_ploughing

and

_{secondary tillage}

with a

reciprocating

harrow in autumn

_(September

_1991);

C: treatment A which was

_compacted

in March 1992 in

wet conditions with a

_heavy

tractor whose _tyres were

inflated at 300 kPa.

For treatments A and C, the

_sowing

was carried out

with no

_tillage

in

_spring.

With treatments A and _S, we

wanted to vary the structural

discontinuity

between the seed bed and the

_sub-layers,

which

_might

affect soil

water

_evaporation

and soil heat flux. With treatments A and _C, we

_expected

no difference in the structural

conti-nuity

between the seed bed and the

_sub-layers

and we

wanted two levels of

_{ploughed layer}

bulk

_density.

The

_experimental

area was at the southern end of a

farm field _(about 10 _ha) where the

_{tillage operations}

described in treatment S were used. The

_plot

for treat-ment A was 100 m x 100 m and the

_plot

for treatment C

was and 10 m x 20 m. The

_plots

were

_{large enough}

for

micrometeorological

methods to be used to assess the sensible heat flux over treatments A and S. The

neigh-bouring

crops were _{sugar beet} on three sides and wheat on the eastern side, which ensured no

_large

flux

discon-tinuity

at the field boundaries. The soil was classified as a silt loam

_(Gleyic

luvisol, FAO _{classification).} It

con-tains 12 %

_clay,

81 % _silt, 7 % sand

_(percent

mineral fraction) and 2 %

_organic

matter.

_Sugar

beet was sown on

_day

101 and the sensors were installed between

_days

104 and 108. Measurements were continued until the sugar beet

plants

had two leaves

_(day

_137), so that

_plant

transpiration

was

_always

_{negligible compared}

to soil

evaporation.

2.2.

_{Measurements,}

fluxes and assessment of soil

_properties

Table I summarizes the measurements. Sensors were

placed

and soil

_samples

were extracted between the

sugar beet rows, as functions of the wheel tracks made

during secondary

tillage

and

_sowing.

The

thermocou-ples

used to measure surface _temperature were held on

the soil surface with a thin

_plastic

stem and coated with mud at the time of _{installation,} to

_give

them

_optical

properties

similar to the

_surrounding

soil _[17]. These

thermocouples

were inserted into the soil inside

25 x 5 mm stainless steel tubes _[17].

The _energy

_exchange

between the soil and the

atmos-phere

was

_{investigated by analysing}

its _energy balance:

where

_R

_n

is net radiation, G is the soil heat _flux, H is the sensible heat flux towards the

_atmosphere

and λE is the latent heat flux.

Net radiation can be

_partitioned

into four terms

describing

solar and thermal radiation:

where

_R,

and

_R

_a

are the solar

_(short

_wave) and

atmos-pheric

(long

wave) radiation, a is the soil _albedo, ϵ’and

e are the soil emissivities for

_absorption

and emission of

long

wave _radiation, σ is the Stefan-Boltzmann constant

(5.67 x 10-8W m-2

_K

_-4

_),

and

_To

is the soil surface

tem-perature. The _parameters ϵ’ and ϵ are

_generally

consid-ered to be

_equal

_[5]. Net radiation was measured

direct-ly using

a net radiometer.

The soil heat flux was estimated

_by

the calorimetric

method,

using

the formula:

where z is the

depth

and T is the temperature.

_G

_d

is the

soil heat flux at a

_depth

_zdwhere it is much smaller than

G and _C(z) is the volumetric heat

_capacity

of the soil. Soil _temperature was measured at six

_depths

(0.00, 0.02, 0.05, 0.10, 0.20 and 0.50 _m)

_using

_{thermocouples.}

The

depth

zd was taken as the arithmetic mean of the two

lowest measurements

_(i.e.

at 0.35 _m).

_G

_d

was calculated

from

_equation

(4)

assuming

that the heat is transferred in the soil

_{only by}

conduction:

where k

s

is the _apparent thermal

_conductivity

of the soil,

using

soil _temperatures measured at 0.20 and 0.50 m

and a thermal

_conductivity

estimated as 1.5 W m-1 K-1

[8].

The volumetric heat

_capacity

was calculated for two

soil

_{layers (0.00-0.02}

m and 0.05-0.10 _m) as a linear

relationship

between the soil bulk

_density

and its water content at different

_depths

_by

the method of de Vries [11]. The _apparent thermal

_{conductivity,}

_k

_s

_,

_was calcu-lated for the same soil

_layers

from the soil _temperature

gradient

and the heat flux

_by

_{inverting equation}

_(4).

Average

values from

_night-time

temperature

profiles

(0 a.m.-5 _a.m.) were used for this, because the soil heat

fluxes are more stable

_during

the

_night

than

during

the

day.

The sensible heat flux to the

_atmosphere

H was

esti-mated

_by

a

_simplified

_aerodynamic

method from the

temperature and wind

_gradients

between 0.5 and 1.5 m

[22]. It included

_stability

corrections and used a free

convection

_expression

when the Richardson number

was below -0.30. The latent heat flux λE was calculated as the residual term of the _{energy balance}

equation

(equation

(1)).

The sensible heat flux and the latent heat flux are

convective fluxes that

_depend

on turbulent transfers in

the lower _part of the

_atmosphere.

The latent heat flux also

_depends

on the

_availability

of water at the soil

sur-face. These two fluxes can therefore be

_{expressed using}

the resistance

_analogy

with the

_following

formula [7]:

where _{p is the air}

_{density (kg}

_m

_-3

_),

c is the

_specific

heat

capacity

of

_dry

air _(J

_kg

_-1

_),

L is the

latent

_{heat of} vapor-ization _(J

_kg

_-1

_),

_To

is the soil surface _{temperature (K),}

_T

_a

is the air _temperature at a reference level _(K),

_q

_sat

_(T

₀

_{) is}

the saturated water _vapour

_mixing

ratio

_(kg

_kg

_-1

_dry

_air)

at _temperature

_T

₀

_,

_qais the

_mixing

ratio of the air at the reference

_height

_(kg

_kg

_-1

_dry

_{air), rah}

_is

the

_aerodynamic

resistance to heat transfer _(s

_m

_-1

_),

_ravis the

_aerodynamic

resistance to water vapour transfer (s

_m

_-1

_{), r}

_s

is the soil resistance to

_evaporation

_(s

_m

_-1

_),

_ψ

₀

is the water suction

at the soil surface

_{(m), g}

is the acceleration of force fall (9.81 m2

_s

_-1

_),

and R is the ideal _gas constant

(8.314

J mol-1

K

-1

).

The

_aerodynamic

resistances to

heat and water _vapour transfer are assumed to be

_equal

when the transfers of heat and mass are turbulent

(r

a

=

rah=

r

av [7]). The

aerodynamic

resistance

depends

on wind

_speed,

surface

roughness

and the vertical

gradi-ents of _temperature and water _{vapour, the resistance} to

evaporation

rs

depends

on the

_availability

of water at the soil surface. The _parameter h is different from 1.0

only

in

_dry

conditions, for water suctions _greater than 0.3 MPa,

corresponding

to a water content of 0.12 _g

_g

_-1

in our soil.

Micrometeorological

and soil _temperature

measure-ments were recorded _{every 5} s

_by

a CR10

_datalogger

and

_averaged

over 30-min intervals. Soil water contents

were measured _{every 2} or 3

_{days. Dry}

bulk

_density

was

measured

_only

once, at the end of the

experiment.

All

the measurements were made in a

_sub-plot

of 20 m x

20 m in the centre of the

_plot.

The

_experimental

_plots

were small

_enough

to ensure that air _temperature and wind

_speed

at 1.5 m were similar for all treatments. The differences in the air _temperature and in the wind

_speed

between the two

_plots

were used to estimate the error of

measurement due to the

_probes

and

_{data-loggers.}

The differences in the air _temperature were

_generally

less than 0.2 K, sometimes around 0.3 K and

_they

varied in the same direction as the difference at the soil surface.

Consequently,

we considered _temperature difference

between two treatments at a

_given

_depth

to be

signifi-cant if

_they

were _greater than 0.3 K. The difference in

the

_daily

wind

_speed

was _very low _(± 0.05 m

s

-1

).

3. RESULTS

3.1.

_{Meteorological}

conditions

during

the

_experimental

_period

The

_{meteorological}

conditions varied

consider-ably,

as shown in

figure

1. Air

_temperature

varied from 3 to

_{20 °C,}

while the difference between air

temperature

and dew

_point

went from 2 K on a

rainy

day

to 10 K on a

_dry

one. Solar radiation was 5-30 MJ

m

-2

_day

_-1

_.

The wind varied from

_light

(1

m

s

-1

)

to _strong

₍₆

m

s

-1

).

There were three

rainy periods: days

105-108

_(cumulative

rainfall =

11 mm),

days

117-122

_(cumulative

rainfall =

22mm)

and

_days

130-132

_(cumulative

rainfall =

16

_mm),

and three

_dry

_periods

with different refer-ence

_{evapotranspiration}

levels:

days

109-116

_(E

₀

₌

2.4 mm

day

-1

),

days

123-129

(E

0

=

3.5 mm

day

-1

)

and

_days

133-137

_(E

₀

₌

5.1 mm

day

-1

).

3.2.

_{Ploughed layer}

structure

Plot A was tilled in the autumn, and winter

rain-fall

₍₂₄₀

mm from October to

_March)

had

com-pacted

the seed bed and

_degraded

the soil surface structure. A thick crust

_{developed during}

the win-ter

_(10-20

mm

_thick),

_leaving

no distinct

aggre-gates

at the

_{soil/atmosphere}

interface. The surface

was

_particularly

smooth,

with a random

_roughness

near zero

_{(corresponding}

to the standard deviation of the

_heights).

The

_dry

bulk

_density

of the seed bed

_(0-0.08

m

_layer)

was lower in

_plot

S than in

plot

A

_(figure

_2).

The

_spring

soil

_tillage

on

_plot

S

just

before sugar beet

sowing

gave a loose and

fragmentary

seed bed with a

_great

fraction of

aggregates

smaller than 20 mm

₍₇₈₀

_g

kg

-1

_dry

soil).

The surface random

_roughness

was 2.2 mm.

The mean

_dry

bulk densities in the tilled

_layer

under the seed beds of

_plots

A and S were similar. Plot C had a massive structure

_throughout

the

ploughed layer

and it was the densest soil

(figure

2)

because of the severe

_{compaction produced by}

the wheels of the tractor under wet conditions.

3.3. Albedo and thermal

_properties

Figure 3

shows the

_change

with time in the albe-do on the three

_plots.

Albedo was lower

_during

rainy periods,

when the soil surface was wet, than

during dry periods.

It was

_generally

S < A <

_C,

but it was A < S < C

_during

the first

_{dry period (days}

silt

_deposits

on

_plots

A and C

_probably

produced

the

_higher

albedos.

Figure 4

shows the

_changes

with time in the vol-umetric heat

_capacity

and thermal

_conductivity

for the three treatments. The volumetric heat

_capacity

and thermal

_conductivity

of the 0.00-0.02 m

_layer

were lower than those at 0.05-0.10 m in

_depth.

The

volumetric heat

_capacity

was

higher

on wet

_days

than on

dry days

for all three

plots, particularly

in

the 0.00-0.02 m

layer,

where the water content

changed rapidly.

Thermal conductivities varied

changed

less with time due to climatic conditions than did the heat

_capacity.

Volumetric heat

_capacity

and thermal

_conductivity

in the 0.00-0.02 m

_layer

were S < A < C. The maximum ratios between the heat

_capacities

of the three

_plots

were about 1.5 at the

_beginning

of the

_experiment.

The mean ratios between the thermal conductivities of

_plots

A and S was

_1.4,

that for

_plots

C and S was 2. The

volu-metric heat

_capacity

and thermal

_conductivity

in the 0.05-0.10 m

_layer

were much more

_similar,

except

for the thermal

conductivity

of

_plot

_C,

which was about twice that of

_plots

A and S. Plot

C

_always

had the

_highest

volumetric heat

_capacity

and thermal

_conductivity

because of its much

greater

bulk

_density

_(1.5

_Mg

_m

_-3

_).

3.4. Water

_regime

of the

_{ploughed layer}

The

_changes

in water content with time and

depth

for each

_{dry period}

are shown in

_figure

5. The first water content

_profile

was measured on

day

108,

_just

before rainfall

_(figure

_1).

The

_topsoil

(0.00-0.05

m)

of

_plot

S was

_dry,

while the

_topsoils

of A and C were wet,

_{corresponding}

to a water suction of 100 kPa. The 5 mm of rain

_during

the

evening

of

_day

108 rewetted

_topsoil

of

S,

_probably

giving

it a soil moisture similar to that of

_topsoils

A and C. The

_topsoil

of S was

_{already dry}

on

_day

112 and later on

day

115,

while the

topsoils

of A and C remained wet.

_Thus,

the S

_topsoil

dried

quicker

than the A and C

_{topsoils during}

the first

dry period. Conversely,

the

_topsoils

of

_plots

A and C also dried out

_during

the second and third

_dry

periods.

The effects of

_tillage

on the soil surface

drying

were also visible in the

_change

in albedo. The albedo of

_plot

A was lower than that of

_plot

S

during

the first

_{dry period (figure}

_3),

but

_higher

during

the other two

_{dry periods}

when the soil

sur-face water contents of the three

_plots

were similar.

Changes

in water content with time between

plots

below a

_depth

of 0.05 m were the

_opposite

of those in the

_topsoil.

Water content remained more constant within

_plot

S than within

_plots

A and C

during

the three

_{dry periods.}

3.5. Thermal

_regime

of the

_{ploughed layer}

Figure

6a shows the mean

_daily

_temperatures

on the three treatments at 0.02 m in

depth.

Plot S was warmer than

_plot

_A,

which was warmer than

_plot

C.

_During

the

_dry

_periods,

the absolute difference in the mean

_day

_temperatures between

_plots

A and S was 0.5-1.0

_K,

and it was 0.5-1.5 K between

plots

C and S. The three

_plots

had the same tem-perature

during

the

_{rainy periods. Figure}

6b,

c

shows the differences in the maximum and

mini-mum

_daily

_temperatures

between

_plots

A or C and

plot

S. Maximum soil _temperatures were

_generally

S > A >

_C,

while the minimum soil

_temperatures

were C > S > A. _{The range of maximum soil}

tem-perature was

_greater

than that of minimum soil

temperature.

The difference in maximum soil

tem-peratures

rose to a maximum

_immediately

after

rainy periods

and then decreased

_during

the

_dry

periods. Similarly,

the differences in minimum soil

temperature

changed

as the soil dried.

The overall effect of the treatments on the soil

thermal

_regime

is shown in table

II,

which indicates the overall means of the mean, maximum and

mini-mum soil

_temperature,

and

_daily

_amplitude

for

_rainy

and

_{dry periods. Dry periods}

were warmer than

rainy periods.

Mean soil

_temperature

decreased with

increasing depth only during dry periods.

The mean

soil

_temperatures

were S > A > C

_during

the

_dry

periods,

and were similar

_during

the

_rainy

_periods.

The maximum

_temperatures

were S > A > C at the soil

_surface,

and were similar on all three

_plots

at

depths >

0.05 m. The minimum

_temperatures

varied

tem-peratures.

The minimum

_daily

soil

_temperature

was

higher

on

_plot

C at

_depths

< 0.05 m, while it was

higher

on

_plot

S for

_{depths >}

0.05 m. Plot A was the

_coolest,

_except

at 0 m. The decrease in

ampli-tude with

_increasing

_depth

in the first 5 cm was

most marked in

_plot

S,

but at

_{depths >}

0.05 m it was rather similar in all three

_plots.

3.6.

_Energy

balance at the soil surface

The soil heat

_fluxes,

_averaged

over 24

h,

were similar for

_plots

A and

_S,

but

_higher

for

_plot

C

(figure

7a).

The fluxes

_averaged

over the

_day-time

plots,

as shown

_{in figure}

7b,

c, which

_displays

the differences in the

_day

₍₇

a.m.-5

_p.m.)

or

_night

(8

p.m.-5

a.m.)

soil heat fluxes between

_plots

A and

S,

and between

_plots

C and S

_(absolute

values

of fluxes for the

_{night period).}

The soil heat flux was _greatest in

_plot

C

_during

both the

_day

and the

night,

which indicates that more heat was stored

during

the

_day

and more heat was lost

_during

the

night.

In contrast,

_plot

A had the lowest soil heat fluxes

_(in

absolute

_value).

The difference in the soil heat fluxes for

_plots

A and S decreased as the soil dried

_during

the second and the third

_dry

peri-ods.

Figure

8 shows the

_changes

with time in net

radiation

_(8a),

sensible heat flux

_(8b)

and latent heat flux

_(8c)

for

_plots

A and

_S,

_averaged

over the

day period

(7

a.m.-5

_p.m.).

The latent heat flux

was the main flux

_during

the

_{rainy periods,}

when sensible heat fluxes were

_nearly

zero. It

progres-sively

decreased

_during

the

_dry

_periods

as the soil

surface dried. In contrast, soil heat flux and

atmos-pheric

sensible heat flux increased. Net radiation

was

_higher

on

_plot

S than on

_plot

A

(8

% difference

from the mean for the whole

_experimental

_period).

It was similar on the two

_plots

at the end of the first

_{dry period,}

at which time the albedo of

_plot

A

was lower than that of

_plot

S

_(figure

₃₎

because of the

_higher

water content of the soil surface

(figure

5).

The sensible heat flux in the

atmos-phere

was

_higher

on

_plot

S than on

_plot

A

_{(40 %}

difference from the mean for the whole

experimen-tal

_{period), particularly during}

the first

_{dry period.}

The latent heat flux was smaller on

_plot

S than on

plot

A, except

during

the third

_dry

_period

when it was similar on the two

_plots.

The difference was about 20 %

(relative

to the

_mean)

_during

the first

dry

period

(corresponding

to _{differences in}

evapo-ration of 0.5 mm

day

-1

).

4. DISCUSSION

We will examine how

_tillage

modified the soil

properties

that influenced the soil thermal

_regime,

the energy

balance,

the soil

_temperature

and

_finally

the effect of

_tillage

on _{sugar beet emergence.}

4.1. Soil

_properties

The difference in albedo of 0.05 between a smooth soil surface due to crust formation and a

freshly

tilled soil was similar to those

_{reported by}

Cipra

et al.

_[10]

and Idso et al.

_[21].

Soil

com-paction greatly

increased thermal

_conductivity

(about

200 % at 0.05 m

_deep),

and gave a smaller increase in heat

_capacity

_(about

20 % at 0.05 m

deep),

as observed

_by

Allmaras et al.

_[3].

This

con-firms the

_great

effect of soil bulk

_density

on

changes

in thermal

_{conductivity,}

as measured

_by

Bussière et al.

_[6]

_using

thermal

_probes.

The

_timing

of the

_{secondary tillage}

had a smaller effect on thermal

_conductivity

than did soil

_compaction.

The increase in thermal

_conductivity

and heat

_capacity

with the autumn soil

_{tillage by}

10-20 % was

simi-lar to the

_changes

observed

_by

Johnson and

Lowery

[23]

and Arshad and Azooz

_[1],

who

com-pared

the

_spring

thermal

_regimes

of conventional tilled soils

_(equivalent

to the

_{spring-tilled}

soil S in our

experiment)

and untilled soils

(without

_crop

residues,

equivalent

to the autumn-tilled soil A in our

_experiment).

We shall deal with net

_radiation,

_evaporation

and sensible heat fluxes to

_analyse

the _energy balance. Net _{radiation defines the energy available} at the soil

surface;

evaporation

is

_strongly

linked to

water

_availability

and so to

_previous

climatic

con-ditions

_(the

_days

_before),

while the sensible heat fluxes in the soil and in the

_atmosphere

reflect

mainly

instantaneous

_{meteorological}

conditions and are

_strongly

linked to the surface

_temperature

(equations

(3)

and

_(4)).

4.2. Net radiation

Net radiation

_depends

on the solar and

atmos-pheric

radiation,

the soil surface

_temperature,

and the soil albedo and

_emissivity

_(equation

_(2)).

Table III shows the radiation balance and some of its

_components

from instantaneous data

_(averaged

over 1 h for a

_day

with

high

solar

radiation,

when the differences in radiation balance and surface

temperature

between

plots

are

_likely

to be

great-est),

and from data

_averaged

over the whole exper-imental

_period.

The difference in net radiation from instantaneous data was

_mainly

due to a differ-ence in reflected

radiation,

but the difference in the soil surface

_temperatures

of the two soil

_tillage

timings compensated

for more than half the

differ-ence due to albedo. Similar results were obtained

by

Allmaras et al.

_[3]

for the net radiation on

ploughed (high

albedo and cold

_soil)

and

_ploughed

+ harrowed

_(low

albedo and warm

_soil)

_plots.

But

the

_temperature

differences

₍₆

K for a 12-h

aver-age)

were

_undoubtedly

due to the

_great

difference in soil

_roughness

in this case. The difference in soil surface

_temperature

was smaller when the whole

period

was

considered,

because it combined data with low and

_high

solar radiation and with

_dry

and

wet soil. The difference in reflected radiation

aver-aged

over the whole

_period

remained much

_larger

than the differences in emitted

_long-wave

radia-tion:

_consequently

the

_{spring-tilled}

soil had a

high-er net radiation

_input

than the autumn-tilled soil. The

_compacted

soil and the autumn-tilled soil

probably

had similar net radiations because of the small differences in their albedo and surface

tem-peratures.

4.3. Latent heat flux

Evaporation depends

on the vapour saturation deficit above the soil

surface,

the

_aerodynamic

resistance to water _{vapour transfer}

_(r

_a

₎

and the soil resistance to water transfer

_(r

_s

₎

_(equation

_(6)).

Evaporation

decreased

_steadily

each

_day

after rain-fall because the soil surface dried and soil resis-tance to

_evaporation

increased

_(figure

_9a,

_b),

as

reported by

Idso et al.

_[19].

The

_aerodynamic

resis-tance to water _vapour transfer had little effect on

evaporation: r

a

was much smaller than _rs, and it increased less than

_{did r}

_s

_(figure

9a,

b).

The decrease in

_evaporation

varied

_{greatly according}

to soil

_tillage

and the

_{dry period (figure}

_8c).

Soil

sur-face resistance to

_evaporation

increased much more

rapidly

after rain on the

_{spring-tilled}

_soil,

especial-ly during

the two first

_{dry periods (days}

108-117 and

_123-129)

when the soil surface dried more

quickly.

Surface water content decreased more

rapidly

on the

_{spring-tilled}

soil because water transfer to the soil surface was

_probably

less effi-cient in the

_{spring-tilled}

soil than in the

autumn-tilled soil. The structural

_{discontinuity}

between the seed bed and the

_sub-layer

caused

_by

the

_spring

tillage

and the _greater

_porosity

of the first few cen-timetres of the soil should have decreased the

hydraulic conductivity

within the

_{spring-tilled}

soil.

Such an increase in

_evaporation

with

_increasing

bulk

_density

and

_compaction

was mentioned

_by

Hadas

_[16],

_although

the effect of soil structure on

hydraulic conductivity

is not well known

_[18].

The soil surface of the autumn-tilled soil also dried out and the soil surface resistance to

_evaporation

increased

_during

the third

_dry

_period

when

poten-tial

_evaporation

was

_high

₍₅

mm

day

-1

).

In this

case, there was no difference in soil

_evaporation

between the two

_{tillage timings.}

The autumn-tilled soil counteracted

_{evaporation by supplying}

the soil surface with water

_{by capillarity,}

because of its

rel-atively high hydraulic

conductivity,

only

when the low climatic demand was low. The effect of soil

tillage

on

_evaporation

_appears to be

_closely

corre-lated with climatic conditions.

4.4. Soil heat flux

Net radiation was

_higher

on the

spring-tilled

soil

than on the autumn-tilled

soil,

and it was the

reverse for the latent heat flux.

Thus,

there was more _{energy available for the sensible heat fluxes}

(H

+ G =

R

n

-

&lambda;E)

on the

_{spring-tilled}

soil than on the autumn-tilled soil

_{(table IV).}

This

_produced

a

greater sensible heat flux and a _greater soil heat flux on the

_{spring-tilled}

soil than on the autumn-tilled soil. But the fraction of the available energy that went into the soil

_(G

/

_(R

_n

_-

_&lambda;E))

was smaller

on the

_{spring-tilled}

soil than on the autumn-tilled

for the ratio H

_{/ (R}

_n

_-

_&lambda;E).

On the _contrary, the dif-ferences in net radiation and latent heat flux for the

compacted

and the autumn-tilled soils were

proba-bly

very small

(the

similar

changes

in water

con-tent with time and

_depth

for the two treatments

(figure

5)

indicate similar

_{evaporation).}

In these

conditions,

soil heat flux and

_(G

/

_(R

_n

_-

_&lambda;E))

were

higher

on the

_compacted

soil than on the

autumn-tilled soil. The

_atmospheric

sensible heat flux

depends

on the thermal

_gradient

above the soil

sur-face and

_aerodynamic

resistance to heat transfer

(equation

(5)),

while the soil heat flux

_depends

on the thermal

_gradient

at the soil surface and the soil surface thermal

_{conductivity (equation}

_(4)).

The air resistance for the two

_{tillage timings}

was similar

(figure

9a),

while the thermal

_conductivity

was

lower on the

_{spring-tilled}

soil than on the autumn-tilled soil and it was

highest

on the

_compacted

soil

(figure

4).

The thermal

_conductivity

_probably

limit-ed the

_penetration

of heat into the soil on the

spring-tilled

soil,

despite

the

_greater

available ener-gy; but it enhanced heat

penetration

into the

com-pacted

soil. It was

already

observed that the soil heat flux and

_G/R

_n

ratios over the same soil with different water contents were first related to differ-ences in

_evaporation

and then to differences in soil thermal

_properties

_{[12, 20].}

4.5. Soil

_temperature

The

_change

in soil

_temperature

_{with time is}

pro-portional

to the soil heat flux G and

_inversely

pro-portional

to the soil heat

_capacity

C

_(equation

_(3)).

The differences in soil

_temperature

between the three treatments were more

_{pronounced during}

the

day

than

_during

the

_night,

because the soil heat flux is

_{higher during}

the

_day

than

_during

the

_night

in the

_{spring; they}

were about 0-2 K in the whole

ploughed layer,

in the same _range as in

_previous

studies. The soil

_temperatures

of the three

_plots

were in the reverse order from their volumetric heat

_{capacities, they}

were not in the order of soil heat fluxes: the differences in soil heat flux on the three

_plots

were counteracted

_by

the differences in heat

_capacity.

This was not so in the

_experiment

of Allmaras et al.

_[3],

where soil

_temperatures

were in the same _sequence as soil heat flux. The differ-ences in soil heat flux were due to the difference in random

_roughness

and flux towards the

atmos-phere, they

were much

_larger

₍₄₀

_%)

than those in

our

_experiment

_{(10 %).}

The differences in soil

tem-perature were transient and linked to the climatic

conditions;

they

occurred

_only

_during

the transition conditions

_following

a rainfall event. Soil heat

evaporation,

and similar on the three

_plots,

soil heat

_capacities

were

_high,

due to

_high

water con-tent, and similar. The

spring-tilled

soil became

warmer

_immediately

after

rainfall,

because it had a

high

soil heat flux - due to low

_evaporation,

and in

spite

of its low thermal

_{conductivity -}

and a low heat

_{capacity -}

due to its low water content.

Differences in soil

_temperature

_logically

decreased

as the soil was

_drying:

soil heat flux became less

different between the three treatments, because

evaporation

and thermal

_conductivity

became low even in the autumn-tilled and

_compacted

soils,

and heat

_capacity

also became less different between the three treatments.

4.6.

_Crop

establishment

The effect of the soil

_{tillage timing}

on _crop

estab-lishment was

_significant,

even

_though

the differ-ences in soil

_temperature

were small. The time nec-essary to reach 50 % of

_germinated

seeds was

shorter for the

_{spring-tilled}

soil

₍₈

_days)

than for the autumn-tilled soils

₍₁₀

_days).

_{The time necessary} to reach 50 % of

_{emerged seedlings}

was less

pro-nounced between the two

_{tillage timings,}

14

_days

for the

_{spring-tilled}

soil and 15

_days

for the autumn-tilled

soil,

because the

_{sowing depth}

was also modi-fied

_by

soil

_tillage.

The

_sowing

was more

_superficial

for the autumn-tilled soil

(0.010

m ± 0.006

_m)

than for the

_{spring-tilled}

soil

_(0.017

m ± 0.006

_m);

the seeder had

_difficulty

_penetrating

the autumn-tilled

plot

because of the

_degraded

soil surface.

5. CONCLUSION

This

_study

demonstrates that the effects of

tillage

on the soil thermal

_regime

are

_significant,

but

_complex

and

_{contradictory,}

and that it is

essen-tial to _{describe the energy balance of the soil}

sur-face to understand its effects.

_Tilling

the soil

_just

before

_sowing

_{increased the energy available} to warm the soil

_{by reducing evaporation,}

increased soil

_temperature

_{and enhanced sugar beet} emer-gence. But

tilling

the soil also modified albedo and

thermal

_properties.

The overall effect of

_tillage

depends

on its effect on each soil

_parameter,

and

there is also a _strong interaction with climatic con-ditions. For

_{example, tilling}

the soil before

_sowing

did not alter the soil _temperature when the soil was

dry.

The

_positive

effect of

_{spring tillage}

on soil

temperature can also be counteracted if the soil is

compacted during tillage. Consequently,

we need a soil

_temperature

simulation model to _define

pre-cisely

the soil structure

_required

to enhance soil

warming

under various climatic

_conditions,

and thus to define the best

_tillage

_strategy.

The

physi-cal-based model for

_predicting

soil

_temperature

should take into account heat transfer into the

soil,

and as this

_study

_shows,

the transfer of water into the

_soil,

so as to

_predict

soil

_evaporation

within the energy balance at the soil surface. We will use our

data to calibrate the model of

_Chanzy

and Bruckler

[9],

which describes these

_physical

_{processes, and} then

_analyse

the

_sensivity

of this model.

Acknowledgements:

The authors thank D. Boitez and C.

_Dominiarczyk

for technical _assistance, and O. Parkes for

_checking

the

_English

text.

REFERENCES

[1] Arshad M.A., Azooz R.H.,

Tillage

effects on soil thermal

_properties

in a semiarid

region,

Soil Sci. Soc.

Am. J. 60

₍₁₉₉₆₎

561-567.

[2] Allmaras _R.R., Nelson _W.W., Hallauer E.A., Fall

versus

_{spring plowing}

and related heat balance in the

western Corn _Belt, Minn.

_Agric.

Stn Tech. Bull. 283 (1972) 1-22.

[3] Allmaras R.R., Hallauer E.H., Nelson W.W., Evans S.D., Surface _{energy balance and soil thermal}

property modifications

by

tillage-induced

soil structure, Minn.

_Agric.

Stn Tech. Bull. 306

(1977)

1-40.

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Fleury

A., Marin Laflèche A., Maillet I.,

Analysis

of the

_variability

_{of sugar beet (Beta}

vulgaris) growth

during

the

_early

_stages. I Influence of various conditions on crop establishment,

_Agronomic

12 (1992) 515-525.

[5] Brutsaert W.H.,

Evaporation

into the

Atmosphere,

Reidel, Dordrecht, 1982.

[6] Bussière F., Cellier P.,

Dorigny

A., Estimation de la conductivité

_thermique

d’un sol in situ à l’aide d’une sonde à choc

_thermique,

_Agronomie

12 (1992) 69-78.

[7] Camillo _P.,

_Gurney

_R.J., A resistance _parameter for bare-soil

_evaporation

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[8] Cellier P., Richard G., Robin P., Partition of

sen-sible heat fluxes into bare soil and the

_atmosphere,

Agric.

For. Meteorol. 82 ₍₁₉₉₆₎ 245-265.

[9]

Chanzy

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