Response of the Thermal Conductivity as a Function of Water Content of a Burnt Mediterranean Loam Soil

doi:10.4236/ojss.2012.21001

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Open Journal of Soil Science, 2012, 2, 1-6

http://dx.doi.org/10.4236/ojss.2012.21001 Published Online March 2012 (http://www.SciRP.org/journal/ojss) 1

Response of the Thermal Conductivity as a Function of

Water Content of a Burnt Mediterranean Loam Soil

Carles M. Rubio1,2, Xavier Úbeda3, Francesc Ferrer2

1Department Agri-Food Engineering and Biotechnology, Polytechnic University of Catalonia, Barcelona, Spain; 2Department Envi-

ronmental Biophysics and Soils, Lab-Ferrer Soils and Environmental Consulting, Cervera, Spain; 3Department Physical Geography,

University of Barcelona, Barcelona, Spain.

Email: [email protected]

Received January 31st, 2012; revised February 29th, 2012; accepted March 10th, 2012

ABSTRACT

The purpose of this research is to explore the variability on the soil thermal conductivity -



- after a prescribe fire, and to

assess the effects of the ashes on the heat transfer once it’s were incorporated into the soil matrix. Sampling plot was

located in the Montgrí Massif (NE of Spain). A set of 42 soil samples between surface and 5 cm depth was collected

before and after the fire. To characterize the soil chemical and physical variables were analyzed. To determine the vari-

ability on the soil



a dry-out curve per scenario (before and after fire) was determined. SoilRho method based on

ASTM D-5334-08 which was validated by LabFerrer was used. Soil thermal conductivity has shown changes in their

values. Indeed, in all moisture scenarios the values of soil



decreased after soil was burnt. The critical point in the rela-

tionship



(



) for the soil after fire which always was stronger than soil before to be burnt. Soil with “white” ashes

showed a high thermal conductivity. An X-Ray diffractometry analysis allowed to clarify and to verify these results. To

sum up, we could say that thermal conductivity presents changes when the scenario changes, i.e. before and after to be

burnt. On the other hand, the volume of ashes incorporated on the soil increased the differences between no burnt and

burnt soil, showing even some improvements on the heat transfer when water con tent start e d to govern the p rocess.

Keywords: Thermal Properties; Soil Moisture; Prescribed Fire; Ashes; SoilRHO

1. Introduction

The impact of fire on soils can vary between highly be-

neficial, when the fire is not too intense and soil heating

brief, to irreversible damage, which occurs during deeply

penetrating heat pulses and long-term exposures [1,2].

Prescribed fires are used in Catalonia since 1999 as a tool,

among others, for managing forested areas with large

amounts of fuel in order to prevent high intensity fires.

The Montgrí prescribed fire main objective was reduced

the shrubland in an ancient and abandoned pine planta-

tion. On the whole of the literature, many researchers

have studied the variations on the chemical and physical

properties on or above burnt soils, but there are other

physical properties as are thermal properties that govern

the heat flow transport inside the soil, and affect the as-

pects mentioned above. Because of the severity of the

soil heating during a prescribed fire the impacts can be

significant and serious. They include the formation of a

hydrophobic layer on the surface of or within the soil,

destruction of most of the organic material in the upper

few centimeters of soil and the concomitant loss of soil

aggregate stability [3], changes in so il pH and soil chem-

istry, long-term differences in soil moisture amounts, in-

creases in soil bulk density with accompanying decreases

in soil porosity, therefore changes on soil structure [2,4,5].

Thus, when biomass on or above a soil surface burns, a

heat pulse penetrates the soil. The resulting high soil tem-

peratures can alter soil properties and kill roots and soil

microbes [6].

On the other hand, chang es in the soil th ermal conduc-

tivity (



, a measure a soil’s ability to conduct heat) are

less obvious, but is not less significant because it is di-

rectly to many of the other fire included changes in the

soil. Some of these changes would be changes in the struc-

ture, because soil thermal conductivity is strongly deter-

mined by soil structure [7], and soil composition [8,9],

that occur whenever soil organic matter is combusted,

therefore soil bulk density changes. Consequently, the

purpose of this research is to explore the variability on

the soil thermal conductivity -



- after a prescribed burn

for a natural and typical Mediterranean limestone soil to

laboratory scale. For achieving the main goal, it was

splited into up three operative objectives; 1) to observe

the relationship between soil thermal conductivity and

Response of the Thermal Conductivity as a Function of Water Content of a Burnt Mediterranean Loam Soil

water content, 2) to evaluate the influence of several per-

centages of ashes on the so il thermal conductivity, and 3)

to determine which was the impact of the ashes on ther-

mal conductivity when the ashes were incorporated into

the soil matrix, t aki ng account several moisture scenarios.

2. Materials and Methods

The study area is located in the north-eastern corner of

the Iberian Peninsula in the coastal mountains of Catalo-

nia, within the calcareous Montgrí Massif. The vegeta-

tion of this area is typically Mediterranean, composed of

Pinus plantation (Pinus halepensis) with shrubland of

Quercus coccifera, Cistus albidus, Rosmarinus officinalis

and Pistacea lentiscus. At the time of the fire, the air tem-

perature was 12.5˚C with an air relative moisture about

60%. In these cond itions a prescrib ed fire was carried out.

A set of 42 soil samples between surface and 5 cm depth

was collected before and after the fire (UTM coordinates

x: 514555 y: 4659552). The size of the plot is 18 × 4

meters, with a quadrangular structure (see Figure 1). Soil

samples were taken before, and just after the fire, from

42 points arranged in three transects and three crosses

across the central transect.

The fire temperature of the soil surface was measured

with a laser thermometer. To characterize the soil chemi-

cal and physical variables were analyzed. Particle-size dis-

tribution was determined using the wetting sieve method

for 2000 to 50 m and a device by dispersion laser beams

(Malvern Mastersizer/E) for particles smaller than 50 m.

Bulk density and porosity were determined from undis-

turbed sample volume. Calcium carbonate content was de-

termined based on Bernard calcimeter [10], whereas the

hygroscopic water content was determined by weight

differences after drying the samples at 105˚C during 24

hours the pH and conductivity was analysed following

extraction with pure water (1:2.5), and measured with a

pH-meter and conductimeter [11]. The organic matter con-

tent was measured according to the sulfochromic oxida-

tion method [12]. To determine the variability on the soil

thermal conductivity -



- a dry-out curve (understood as

the relationship between thermal conductivity and water

content) was calculated [13,14]. The type of sample used

for the experiment was a composed soil sample. 42 soil

samples for each scenario, i.e. befo r e and afte r pr escr ib ed

burnt were used. The samples were wetted up with dif-

ferent quantities of deionized water, and packed until to

Figure 1. Sampling plot designed. Black points are sampling

points.

achieve the target bulk d ensity into a soil column device.

Water content was calculated by dried sample in the oven.

To determine the thermal conductivity as a function of

water content we used the SoilRho® method, which is

based on ASTM D-5334-08. This methodology was de-

veloped according to the method described by Shiozawa

and Campbell [15]. A SH-1 thermal sensor combined with

KD2-Pro reader-logger allowed to obtain reliable and ac-

curacy soil thermal conductivity values, thus obtaining a

continuous large soil thermal data-set. In all cases, a set

of 5 measures per moistured scenario (n = 30) were taken

in both composed soil samples. The moisture process

gave from air dried up to close to saturation.

The experimental design with ashes was performed on

soil column devices, as well. For carrying out the ongo-

ing research, several volumes of two types of ashes (white

and black ashes) were used. The colour of ashes imply

differences on the temperature reached on the soil sur-

face, therefore, also imply differences in the own compo-

sition of ashes. The ashes were mixed with burnt soil on

the percentages of 10%, 20% ···, up to 90% in volume of

ashes per volume of burnt soil. The differences on soil

thermal conductivity for both types of ashes were evalu-

ated only when soil was air dried.

For determining the thermal stability in the most criti-

cal case (90% of ashes) a whole of dry-out curves were

constructed. In this case, only took into account the higher

percentage of two types of ashes (90% white ashes and

90% black ashes). The dry out curves had 8 moistured

scenarios with 5 thermal measur es per scenario (n = 40).

The moisture process gave from air dried up to close to

saturation.

3. Results and Discussion

The soil from this plot in the Montgrí massif was classi-

fied according to USDA as loam textural class [16]. Mean

bulk density was around 1.1 g·cm–3. The chemical and

physical properties values before and after prescribed

burnt were, respectively: mean total organic carbon con-

tent were about 14.7% and 17.2%. The mean electric

conductivity increased. On the other hand, the pH of the

media did not show any change, and hygroscopic water

content was similar, as well. All the values are the aver-

age of 42 samples taken before, and 42 samples taken after

prescribed burnt. The values are showed in Table 1.

Respect to soil thermal conductivity -



-, it showed

changes in its values (Figure 2). Indeed, in all cases the

values of the dry out curve for soil



decreased after soil

was burnt than native soil. The critical point in the rela-

tionship



(



) always was stronger when soil samples

were burnt than soil before prescribed fire, starting a

critical reaction at 8% of water content for samples not

burnt, and 6% of water content for burnt samples. Proba-

bly, this situation could be explained by the incorporation

Response of the Thermal Conductivity as a Function of Water Content of a Burnt Mediterranean Loam Soil 3

Table 1. Physical and chemical characteristics of the studied

soil before and after fire. OM = organic matter content;

CaCO3 = calcium carbonate content; EC = electrical con-

ductivity and Hw = hygroscopic water content.

Variables Before Fire After Fire

Sand (%) 39.3 41.7

Silt (%) 35.1 32.4

Clay (%) 25.6 25.9

E.C. (µs·cm–1) 330 520

pH 7.0 7.1

O.M. (%) 11.2 10.9

CaCO3 (%) <3 <3

Hw (%) 1.8 1.9

00.2 0.4 0.6



H2O

·g

soil-1

)

0.2

0.4

0.6

0.8



(W·m

-1

·C

-1

)

Before fire

After fire

Figure 2. Dry-out curves of the relationship between ther-

mal conductivity and gravimetric wa te r c ontent.

of burnt organic matter on the soil after prescribed fire,

such that organic matter behaviour did not transmit well

the heat pulse.

Also, the mean temperature values for the soil samples

before and after fire during the experiment were about

21.5˚C and 18.5˚C, respectively. The difference between

both values did not affect the soil thermal properties mea-

surements. On the other hand, the coefficient of variation

of the thermal conductivity measures indicated that the

values were between 0.4% and 2.2% for not burnt soil,

and 0.3% and 1.7% for burnt soil. Therefore, the values

were considered as acceptable, and very homogeneous.

Finally, a new experiment using the black and white

ashes found out over the soil surface after fire was car-

ried out. After the prescribed fire a different quantity of

ash patches were found out.

Some of the patches were black ashes where the tem-

perature of the fire was lower, and other patches were

white ashes where the temperatur e of the fir e was higher

(around 600˚C). The new test was used to find differ-

ences out between two types of ashes. The soil samples

after fire was used to amend and to repack with different

quantities of black ashes i n volum e percenta ge of soil (0%,

10%, 20%, 30% ··· 100%), and maintaining a similar bulk

density. The same test was performe d with white ashes.

The relationship between bulk density and porosity

using burnt soil and different ashes (black and white) is

showed in Figure 3. The linear relation between both va-

riables means a well-compacted soil samples, and a de-

creasing pattern of the macro-porosity when bulk den sity

increase, as well. In any case, two well-defined groups of

samples were determined. The soil mixture with black

ashes presented less bulk density values than soil with

white ashes, which it presented a lower porosity. [3] pre-

sented the results about differences in the bulk density

for a two soils after fire. Their conclusions were focused

in some changes about bulk density, but only within the

top section (0.05 m). This fact wou ld evidence the incor-

poration of a certain quantity of ashes in th e few top cen-

timeters of the soil surface, and it would involve a less

bulk density than below 0.05 m, such that the compacta-

tion effects are limited to upper 0.05 m only [3].

Figure 4, presents different quantities on percentages

of two types of ashes mixture with burnt soil, where soil

thermal conductivity was measured . The white ashes (do t

line) always shown a higher thermal conductivity, mean-

while soil with black ashes was a lower



. Probably, this

fact would be attributed by the large organic carbon con-

tent that was not burned during the prescribed fire [17],

such that the organic carbon content has a low thermal

0.8 0.911.1

(

·cm

-3

)

Porosity (%)

Black ashes

White ashes

Figure 3. Relationship between bulk density and porosity

for a burnt soil mixture with two different types of ashes.

Response of the Thermal Conductivity as a Function of Water Content of a Burnt Mediterranean Loam Soil

0 20406080

Volu m e ofashes 100

(

)

0.12

0.16

0.2



(W·m

-1

·C

-1

)

Bla ck ash e s

White ashes

2.9% for black ashes. Therefore, the overall data set was

acceptably homogeneous.

During a burn one may expect changes in soil biota,

soil chemistry, and soil minerals [5]. To verify in detail

the differences between both soil samples (before and

after burnt), an X-ray difractometry was performed. Fig-

ure 5, shows the results of the analysis. The soil before

fire (black line) presented a less quartz than soil after fire,

however, exist traces of mica muscovite, which it disap-

pear when soil was burnt.

The values of quartz element increased substantially

after burnt, probably the plagioclase were unstructured

when temperature rose over 500˚C, releasing the quartz

and some cations. In this case, the electric conductivity

of the soil would be increased, such as indicated the Ta-

ble 1.

Eventually, in Figure 6, can observe the relationship

between thermal conductivity as a function of water con-

tent for a soil sample after burnt with 90% of volume of

ashes (white and black ashes).

Figure 4. Effects of the different ashes on soil thermal con-

ductivity for a burnt soil. Clearly, differences exist between white and black ashes

when these were incorporated into the soil matrix. In all

cases, especia lly when the so il was closed to saturation the

differences were higher. An acceptable explanation could

be splited as follows: 1) white ashes presen ted a fin e par-

ticle size, hence it increased the water film around the

particle, and increased the bulk density as well, reducing

the air filled inside the samples. Therefore, a rather high

compaction than black ashes invol ved a better heat transfer;

2) mica muscovite presents dielectric properties, there-

fore it is a good resistivity material. When mica musco-

vite disappeared , the thermal conductivity increased.

conductivity, and bigger particle size than ashes [18]. Thus,

when organic matter content increase exhibit a decrease

of soil



[19]. However, the fine particle size of the white

ashes improved the target bulk density, therefore it con-

tributed to improve the soil thermal conductivity values,

except in the two peaks relatives to 30% and 50%, where

the quantity of black ashes decreased, it became similar

thermal conductivity values than white ashes. On the whole

of the experiments, the coefficient of variation indicated

that the dispersion was less of 2.4% for whites ashes, and

quartz

quartz quartz

plagioclase

mica muscoviteplagioclas

0 102030405060

le Thetaº

100

Count Intensity (%)

0 102030405060

le Thetaº

BEFORE FIRE

AFTER FIRE

Figure 5. X-Ray difractometry analysis for soil samples before and after prescribed fire.

Response of the Thermal Conductivity as a Function of Water Content of a Burnt Mediterranean Loam Soil 5

00.20.40.6



H2O

·g

soi l-1

)0.8

0.2

0.4

0.6

0.8



(W·m

-1

·C

-1

)

Black ashes

White ashes

Figure 6. Dry-out curves of the relationship between ther-

mal conductivity and gravimetric water content, when dif-

ferent types of ashes (white and black) were incorporated

into the soil matrix.

Finally, note that the particle size of the black ashes

were large than white ashes, and also the organic carbon

content did not works well when the heat flux transfer

was necessary. The measurements of the thermal con-

ductivity of ashes revealed that



presented a high influ-

ence due to water film around the ashes [3,20].

4. Conclusion

As summary, we could say that thermal properties can

present changes when the scenario changes, i.e. before

and after a prescribed fire. Soil after fire always pre-

sented a less thermal conductivity, and therefore a less

thermal diffusivity, and volumetric specific h eat capacity.

On the other hand, when the ashes provoked by the fire

were incorporated to the soil, the white ashes, which are

poorer in organic carbon content, provided a better heat

flow transfer. Also, the particle size was relevant in the

retention water content. Therefore, when soil is burned

its thermal properties change, and a natural or anthropic

addition of ashes, especially white ashes, could improve

the conductance of the heat flux into the soil, improving

the soil bulk density, and water retention content as well.

5. Acknowledgements

The research was founded by Lab-Ferrer Soils and Envi-

ronmental Consulting Center and University of Barce-

lona. Likewise, we want to mention the agreements be-

tween LabFerrer and the Department of Agri-Food En-

gineering and Biotechnology of the Universitat Politèc-

nica de Catalunya. We thank the GRAF team for the

fieldwork support during the prescribed fire.

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