influence of biochar incorporation on tdr-based soil water content measurements

European Journal of Soil Science, January 2014, 65, 105–112 doi: 10.1111/ejss.12083

Influence of biochar incorporation on TDR-based soilwater content measurements

K . K a m e y a m a , T . M i y a m o t o & T . S h i o n oAgricultural Environment Engineering Research Division, National Institute for Rural Engineering, National Agriculture and FoodResearch Organization (NARO), 2-1-6 Kannondai, Tsukuba, Ibaraki 305-8609, Japan

Summary

The incorporation of biochar (BC) into agricultural soil changes the soil’s physical properties, which leads tochanges in the soil’s hydraulic properties, such as water retention and permeability, and alters the soil moistureenvironment in agricultural fields. To elucidate the effects of the incorporation of BC on the soil moistureenvironment, measurements of the soil water in biochar-amended agricultural fields are needed. Time domainreflectometry (TDR) is a widely used and established technique for the continuous measurement of the soilwater content (SWC) in agricultural fields. However, TDR measurements are affected by the conductivityof soils. Biochar formed at higher pyrolysis temperatures is known to be very conductive. Therefore, weinvestigated the influence of the incorporation of BC on TDR-based SWC measurements. We examinedcalcaric dark red soil and the BC produced by pyrolysis of sugarcane bagasse at 400, 600 and 800◦C. Theapparent relative permittivity (εa ) of the BC (800◦C)-amended soil was greater than that of the non-amendedsoil at a given water content, whereas the εa values of the soils amended with the BC (400◦C) and BC(600◦C) were the same as that of the non-amended soil at a given water content. We concluded that when acalibration curve obtained from a non-amended soil is used, TDR-based measurements tend to over-estimatethe SWC containing the BC formed at higher pyrolysis temperatures because of conductive and dielectriclosses. Therefore, the use of the real component (ε′

r ) of the soil’s complex relative permittivity instead of εa

is effective when making TDR-based water content measurements of soils that contain BC formed at higherpyrolysis temperatures.

Introduction

Biochar (BC), charcoal produced by the pyrolysis of biomass, is

highly resistant to microbial decomposition (Baldock & Smernik,

2002). Hence, its incorporation into farmlands may have the

potential to mitigate CO2 emissions (Glaser et al., 2009). In

addition, the incorporation of BC into agricultural soil changes

the soil’s porosity, pore size distribution and bulk density

because of BC’s porous structure (Lehmann & Joseph, 2009).

As a consequence, the soil’s hydraulic properties, such as water

retention and permeability, are also changed (Kameyama et al.,

2012) and alter soil moisture characteristics in agricultural fields.

To apply BC appropriately to agricultural land, more detailed

information is required regarding the effects of the incorporation

of BC on the soil moisture. A step towards understanding these

effects would be to measure soil water in BC-amended agricultural

fields.

Correspondence: K. Kameyama. E-mail: [email protected]

Received 29 July 2013; revised version accepted 29 July 2013

Time domain reflectometry (TDR) is a widely used andestablished technique for the measurement of soil water content(SWC) in agricultural fields. Volumetric water content (θ ) of soilis estimated from TDR-measured apparent permittivity (εa) byusing calibration curves (Topp et al., 1980). However, TDR iswell known to over-estimate θ for soils with conductive surfaces,such as clay soils, and for soils with large concentrations ofelectrolytes, such as saline soils (Bridge et al., 1996; Wyseureet al., 1997; Bittelli et al., 2008). In addition, Robinson et al.(1994) reported that TDR over-estimates θ for soils containingiron minerals, especially magnetite. They suggested that this over-estimation might be caused by the conductivity and magnetism ofiron minerals. These results indicate that TDR measurements inhighly conductive soils are challenging.

The physicochemical properties of BC depend on the feedstockand pyrolysis conditions, such as temperature and holding time(Lehmann & Joseph, 2009). In particular, pyrolysis temperaturegoverns the electrical properties of BC (Ishihara, 1996). Nishimiyaet al. (1995), Sugimoto & Norimoto (2004) and Xiao et al.(2012) showed that the conductivity of BC drastically increases

© 2013 British Society of Soil Science 105

106 K. Kameyama et al.

at pyrolysis temperatures greater than 500–600◦C. These studieshave brought much-needed attention to the application of TDR forsoils amended with BC formed at higher pyrolysis temperatures.

In addition, pyrolysis temperature controls water repellency(Kinney et al., 2012). From the viewpoint of improving theavailable water content in field soil, BC formed at higher pyrolysistemperatures may be desirable because that formed at lowertemperatures is water repellent (Kinney et al., 2012). Therefore,we need to determine the effects of the incorporation of BCformed at higher pyrolysis temperatures on εa and bulk electricalconductivity (σ dc) prior to conducting TDR measurements in BC-amended agricultural fields.

The objective of this study was to determine the effects of BCincorporation on TDR measurements and we hypothesized thatthe incorporation of BC formed at higher pyrolysis temperatureshas effects on measurements of εa and σ dc. To this end, the θ –εa

and θ –σ dc relationships for BC-amended soils were determined.Moreover, a correction method developed for conductive soils wasapplied to BC-amended soils and its applicability was discussed.

Materials and methods

Soil and biochar samples

A soil sample was collected from the surface layer of a sugarcanefield on Miyako Island, Japan. The soil was calcaric dark redsoil, termed ‘Shimajiri Maji’. It is classified as a Typic Hapludalfaccording to the United States Department of Agriculture (USDA)Soil Taxonomy (Soil Survey Staff, 2010). The soil was air-dried for one month and passed through a 2-mm sieve. Its

physicochemical properties are shown in Table 1. Its high pHvalue results from the presence of numerous coral limestonefragments that contribute calcium ions. This soil is rich in clay,and its texture corresponds to a clay soil in the USDA texturalsoil classification system. Clay minerals of the soil are mostlycomposed of a chlorite-vermiculite intermediate, illite and kaolin(Tokashiki, 1993).

Sugarcane bagasse was used as the BC feedstock. The feedstockwas air-dried and heated in a batch-type carbonization furnace atthree different pyrolysis temperatures of 400, 600 and 800◦C witha holding time of 2 hours. The resultant BC was sieved through a2-mm mesh. The physicochemical properties of the BC (400◦C),BC (600◦C) and BC (800◦C) are shown in Table 2. Each valuerepresents an average of duplicate measurements. The solution pHvalue of the BC varied from 5.0 at 400 to 9.8 at 800◦C. Withincreasing pyrolysis temperature, the cation exchange capacity(CEC), oxygen content (O), hydrogen content (H) and nitrogencontent (N) decreased, whereas the carbon content (C) and particledensity increased. Each physicochemical property exhibited adifferent extent of dependence on the pyrolysis temperature,except for the solution electrical conductivity (EC), which didnot vary with the pyrolysis temperature.

The solution EC of biochar gives an indication of the totalsoluble N, P, Ca, Mg, K and S contents (Lehmann & Joseph,2009). In general, high-temperature BC exhibits a greater solutionEC relative to low-temperature BC (Gundale & DeLuca, 2006).However, the solution EC of sugarcane-bagasse-derived BC isrelatively small because it has very few soluble nutrients (Uras,2011). Therefore, differences in the solution EC between thesamples pyrolysed at different temperatures were not found.

Table 1 Physicochemical properties of the Shimajiri Maji soil (n = 2)

Exchangeable cationsb Sand Silt ClayCa Mg K Na

pH (H2O)a

ECa

/ dSm−1

T-C/ g kg−1 C:N

CEC (pH7)b

/ cmolc kg−1 / cmolc kg−1

Particle density/ g cm−3 / %

Shimajiri Maji 8.0 0.6 13.4 8.4 11.2 31.3 1.35 0.66 0.21 2.75 8 19 73

aSoil:solution = 5 g:25 ml.bObtained by using the ammonium acetate (pH 7) method (Sumner & Miller, 1996).

Table 2 Physicochemical properties of sugarcane bagasse-derived biochar (n = 2)

C H Ob N

Pyrolysistemperature pH (H2O)a

ECa

/ dS m–1 / %H/C

/ mol : molO/C

/ mol : molCEC (pH 7)c

/ cmolc kg−1

Particle density/ g cm−3

BET surfacearea (SBET)

/ m2 g–1

Electricalresistivityd

/ � cm

BC (400◦C) 5.0 0.2 65.4 3.57 16.3 1.0 0.66 0.19 12.2 1.50 14.4 5.0 × 107

BC (600◦C) 7.7 0.2 75.3 1.65 3.8 0.69 0.26 0.04 10.4 1.57 218 1.2 × 106

BC (800◦C) 9.8 0.2 79.4 0.36 3.6 0.66 0.05 0.03 4.4 1.86 219 12.4

aBiochar:solution = 1 g:25 ml.bO = 100–C–H–N–S.cObtained by using the Shollenberger method.dMeasured with a digital multimeter.

© 2013 British Society of Soil Science, European Journal of Soil Science, 65, 105–112

TDR measurements of biochar-amended soil 107

Electrical resistivity of the BC was drastically decreased between600 and 800◦C. This result agreed with previous studies byNishimiya et al. (1995), Sugimoto & Norimoto (2004) and Xiaoet al. (2012). In addition, this result indicates that conductivity ofthe BC (800◦C) is much greater than that of the BC (400◦C) andBC (600◦C).

Measurements of apparent permittivity and bulk electricalconductivity

The air-dried soil and BC were prepared by passing through a2-mm sieve. The soil samples were then mixed with a givenamount of the BC using a spoon for 5 minutes to prepare BC-amended soils. The soil samples amended by the BC (400◦C),BC (600◦C) and BC (800◦C) were prepared with a gravimetricBC content of 3% (w/w) in order to investigate the effects ofthe pyrolysis temperature on the θ –εa relationship of the soils.In addition, the BC (800◦C)-amended soils were prepared withgravimetric BC contents of 1, 3 and 5% (w/w) to examinethe effects of the BC content on the θ –εa relationship of thesoils. The incorporation rate was equivalent to approximately 20,60 and 100 Mg BC ha−1, respectively, at a depth of 0–20 cm.Biochar application experiments have been conducted for a widerange of application rates (1–135 Mg ha–1) in both pot and fieldexperiments (Jeffery et al., 2011).

The soil samples (soil–BC mixtures) were placed in acylindrical container (inside diameter, 7 cm; height, 10 cm) withmanual tapping. The heights of the soil samples were up to 7.5 cm.The dry bulk density, porosity, pH and EC values of the samplesused for the TDR measurements are shown in Table 3.

To measure the εa of the samples, a TDR cable tester(TDR100, Campbell Scientific, Inc., Logan, Utah, USA) and athree-rod TDR probe (Mini-TDR Model T-3, East 30 Sensors,Pullman, Washington, USA; diameter, 1.4 mm; length, 60 mm;space between centre and outside rods, 6 mm; cable length,2 m) were used throughout the experiments. Shorter probes areeffective for TDR measurements in highly conductive soilsbecause reflection losses due to EC decrease with the probe lengthreduction (Jones & Or, 2004). The TDR cable tester was connectedto a personal computer, which was used to collect and analyse thewaveforms. The TDR probe was inserted vertically into the soil

Table 3 Bulk density, porosity and solution pH and EC of soil samplesused for TDR measurements

Pyrolysis temperature − 400◦C 600◦C 800◦C

Gravimetric biocharcontent / % (w/w) 0 3 3 1 3 5

Sample dry bulk density / g cm−3 1.07 0.91 0.92 1.01 0.93 0.85Porosity / cm3 cm−3 0.62 0.65 0.65 0.63 0.66 0.68pHa 8.0 8.0 8.1 8.0 8.0 8.0ECa / dS m−1 0.6 0.6 0.6 0.6 0.6 0.7

aSoil:solution = 5 g:25 ml.

samples. Five waveforms were collected with only one sample thatcontained a specific level of the BC to determine the εa and σ dc

of the soil sample with the PCTDR software (Campbell Scientific,Inc.). Five εa and σ dc measurements were then averaged. For theθ measurements, the soil samples were weighed gravimetricallyusing an electronic balance. To prepare samples with largerwater contents, the soils were removed from the cylinders andapproximately 20 ml distilled water was added. The soils were leftundisturbed for at least 24 hours to allow the moisture to becomeevenly redistributed. The wet soils were repacked into the samecylinders at the same density, and measurements of θ , εa and σ dc

were conducted as mentioned above.

Separation of the real and imaginary parts

The dielectric permittivity of soil is a complex variable charac-terized by a real part (ε′

r ), which accounts for the energy storedin the dielectrics, and an imaginary part (ε′′

r ), which describesthe dielectric and conductive losses. The imaginary component isgenerally assumed to be negligible at the TDR bandwidth; hence,the TDR-measured εa represents only the real part (εa ≈ ε′

r ).However, when TDR measurements are performed on conduc-tive materials, the imaginary component cannot be neglected. εa

is then determined by both the real and imaginary parts (Bittelliet al., 2008). The complex dielectric permittivity (ε∗

r ) is writtenas follows (Robinson et al., 2003):

ε∗r = ε′

r − i

(ε′′r + σdc

ε0ω

), (1)

where σ dc is the electrical conductivity at zero frequency, ε0 isthe permittivity of free space and ω is the angular number (2π f ),where f is the frequency (Hz).

Topp et al. (1988) proposed a method of separating ε′r and ε′′

r

on the basis of the TDR waveform. Using this method, Toppet al. (2000) determined the influence of ε′

r and ε′′r on TDR

measurements in soils. Recently, Bittelli et al. (2008) proposed asimple correction method based on measured εa and σ dc values.They compared the propagation velocity of electromagnetic wavesin dielectric materials with the velocity obtained from travel-timeanalysis. They obtained equations for estimating ε′

r and ε′′r using

the effective conductivity (σ e) introduced by Topp et al. (1988).σ e includes both σ dc and dielectric losses.

ε′′r = σe

ε0ω, (2)

ε′r = εa − σ 2

e

4εaε20ω2

. (3)

The TDR waveforms analysis performed with the PCTDRsoftware provides both εa and σ dc ; however, the values of σ e

and ω are unknown. Hence, we obtained the values of σ e and



ω from the TDR waveforms according to the methods of Toppet al. (1988, 2000). In brief, σ e and ω were estimated from thereflection coefficient of one return trip of a pulse and pulse risetime, respectively, in the TDR waveforms.

Results and discussion

TDR waveforms for biochar-amended soils

The TDR waveforms for the soils amended with 3% (w/w) BC(400◦C), BC (600◦C) and BC (800◦C), as well as the non-amendedsoil, are shown in Figure 1. The θ values of the soil sampleswere 0.1 and 0.2 m3 m−3. The travel time of the reflected signalincreased and the reflection coefficient substantially decreasedwith the incorporation of the BC (800◦C). The difference betweenthe non-amended soil and the BC (800◦C)-amended soil wasgreater in terms of θ . On the other hand, with incorporation of BC(400◦C) and BC (600◦C), the travel times slightly decreased andthe reflection coefficients increased more than those for the non-amended soil. These results indicate that the pyrolysis temperatureaffected the TDR waveforms of the BC-amended soils.

The reflection coefficient substantially decreased for the BC(800◦C)-amended soil in comparison with the non-amended soil(Figure 1). The waveforms of the conductive soils exhibited a

decrease in the reflection coefficient with increasing σ dc (Nadleret al., 1991; Topp et al., 2000; Munoz-Carpena et al., 2005). Theconductive soils usually contained large concentrations of salt inthe soil solution, whereas the BC (800◦C)-amended soil containsvery conductive material in the solid phase (Table 2). Therefore,the very conductive solid phase, as well as the soil solution with alarge EC, might play a role in decreasing the reflection coefficient.

The reflected signal was stretched to the right for the BC(800◦C)-amended soil when compared with the non-amendedsoil (Figure 1), although waveforms of conductive soils seldomdisplay this effect. Robinson et al. (1994) reported that thewaveform stretches to the right for soils that contain iron materials,particularly for magnetite, and suggested that this might be causedby the increased conductivity and magnetism of iron minerals.Therefore, the very conductive solid-phase might show differenteffects on the dielectric properties of soil to those of the conductiveliquid-phase. These factors would result in the differences betweenthe observed waveforms of conductive soils and the BC (800◦C)-amended soil.

In contrast, the discrepancies between the waveforms of thesoils amended with the BC (400◦C) and BC (600◦C) and thoseof the non-amended soil may be explained by decreases in thebulk density (Miyamoto & Chikushi, 2006). A decrease in the

(a) (1) θ = 0.1

0

0.5

1

1.5

2

0 2 4 6 8

Non-amended soilBiochar (400 °C)-amended soilBiochar (600 °C)-amended soilBiochar (800 °C)-amended soil


Ref

lect

ion

coef

fici

ent

Time / ns

0% (w/w)1% (w/w)3% (w/w)5% (w/w)

Biochar content

0% (w/w)1% (w/w)3% (w/w)5% (w/w)

Biochar content

10 12

(2) θ = 0.2

0

0.5

1

1.5

2

0 2 4 6 8

Ref

lect

ion

coef

fici

ent

Time / ns

10 12

(b) (1) θ = 0.1

0

0.5

1

1.5

2

0 2 4 6 8

Ref

lect

ion

coef

fici

ent

Time / ns

10 12

(2) θ = 0.2

0

0.5

1

1.5

2

0 2 4 6 8

Ref

lect

ion

coef

fici

ent

Time / ns

10 12

Figure 1 TDR waveforms for the biochar-amended soil samples at (1) θ = 0.1 and (2) θ = 0.2 with (a) different pyrolysis temperatures and (b) differentbiochar contents of the biochar pyrolysed at 800◦C.



bulk density results in increased air content in the soil sample. Inaddition, the dielectric permittivity of air is less than that of solidphases. These results indicate that the BC (800◦C) has a greatereffect on the TDR measurement than the BC (400◦C) and BC(600◦C).

The TDR waveforms of the soils amended with 1, 3 and 5%(w/w) BC (800◦C) and that of the non-amended soil are shownin Figure 1(b). The θ values of the soil samples were 0.1 and0.2 m3 m−3. The waveform was progressively elongated to theright and the reflections became less intense as the proportionof BC increased. In addition, the discrepancies were larger interms of the θ values. These results indicate that the effects of theincorporation of the BC (800◦C) on the waveform became morepronounced as more BC (800◦C) was incorporated into the soil.

Apparent permittivity and bulk electrical conductivity

The θ –εa relationships of the soils amended with 3% (w/w)BC (400◦C), BC (600◦C) and BC (800◦C) and that of the non-amended soil are shown in Figure 2(a). The θ –εa relationshipsfor the soils amended with the BC (400◦C) and BC (600◦C)were similar to that for the non-amended soil. Therefore, theincorporation of the BC (400◦C) and BC (600◦C) had a negligibleeffect on the measurement of SWC by TDR. In contrast, theθ –εa relationship for the BC (800◦C)-amended soil substantiallydiffered from that for the non-amended soil, and the differenceincreased with the θ value.

The θ –σ dc relationships for the soils amended with 3% (w/w)BC (400◦C), BC (600◦C) and BC (800◦C) and that for the non-amended soil are shown in Figure 2(b). Similarly to the θ –εa

relationships, those for θ –σ dc for the soils amended with theBC (400◦C) and BC (600◦C) did not differ from that for thenon-amended soil. However, that for BC (800◦C)-amended soildiffered substantially from that for the non-amended soil. Thedifferences between σ dc of the BC (800◦C)-amended soil andthat of the non-amended soil increased with the θ . As previouslymentioned, the BC (800◦C) had the greatest effect on the TDRmeasurement.

The θ –εa and θ –σ dc relationships for the soils amended with 1,3 and 5% (w/w) BC (800◦C) and those for the non-amended soilare shown in Figure 3(a,b). The differences between the εa andσ dc for the BC (800◦C)-amended soils and those for the non-amended soil increased with BC content.

The θ –εa relationships for the non-amended soil and thosefor the soils amended with the BC (400◦C) and BC (600◦C)matched those obtained with Topp’s equation (Topp et al., 1980),which is a universal calibration equation. In contrast, that forthe BC (800◦C)-amended soil did not match that obtained withTopp’s equation (Figure 3a). These results indicate that when thecalibration curve between εa and θ obtained for a non-amendedsoil is used, the TDR measurement tends to over-estimate θ ofsoils that contain BC formed at greater pyrolysis temperatures.

The conductivity of BC (800◦C) was much greater than thatof BC (400◦C) and BC (600◦C) (Table 2). Nishimiya et al.

(b)

(a)

0

0.05

0.1

0.15

0.2

0

10

20

30

40

50

60

70

80

0



Topp equation (Topp et al., 1980)

Soil

EC

, σ dc

/ S

m-1

Die

lect

ric

perm

ittiv

ity,

ε a

0.1 0.2 0.3 0.4

Volumetric water content, θ / m3 m−3

0 0.1 0.2 0.3 0.4


Figure 2 Relationship between the volumetric water content and (a)dielectric permittivity and (b) bulk electrical conductivity for the biochar-amended soil samples with biochar prepared at different pyrolysistemperatures.

(1998) suggested that the conductivity of biochar formed at greaterpyrolysis temperatures might result from the condensation ofaromatic rings induced by dehydration at temperatures of 600 to800◦C. The H:C ratio of the BC (800◦C) was close to zero (Table2), which suggests that dehydration had occurred and may explainwhy its conductivity was greater than that of BC (400◦C) and BC(600◦C).

In addition, Ishihara (1996) and Wang & Hung (2003) reportedthat charcoals (wood-derived BC) formed at greater temperatureshave the capacity to shield electromagnetic waves because of theirlarge conductivities; however, the electromagnetic-wave shieldingmechanism is still not clear. Therefore, BC (800◦C) might provideshielding of the electromagnetic waves in the TDR measurements.As a consequence of the large conductivity of BC (800◦C) (Table2), its presence in the soil would provide electromagnetic waveshielding in the TDR measurements and increase εa .

Separation of the real and imaginary parts

Because we used the same TDR equipment and waveform analysissoftware as Bittelli et al. (2008), we applied their correction



(b)

(a)

0

0.05

0.1

0.15

0.2

0

10

20

30

40

50

60

70

80

0

Soil

EC

, σ dc

/ S

m-1

Die

lect

ric

perm

ittiv

ity,

ε a

0.1 0.2 0.3 0.4


0 0.1 0.2 0.3 0.4


0% (w/w)1% (w/w)3% (w/w)5% (w/w)

Biochar content

0% (w/w)1% (w/w)3% (w/w)5% (w/w)

Biochar content


Figure 3 Relationship between the volumetric water content and (a)dielectric permittivity and (b) bulk electrical conductivity for the biochar-amended soil samples with different biochar contents for the biocharprepared at a pyrolysis temperature of 800◦C.

method to our experimental data and found that two modificationsof their method were needed. One modification was to simplifythe process of obtaining σ e values. Obtaining the values of σ e

directly using the PCTDR software was difficult. Equations (2)and (3) require the values of σ e instead of σ dc . Therefore, thevalues of σ e had to be obtained from the reflection coefficient ofone return trip of a pulse, according to the method of Topp et al.(1988). The other modification involved the identification of ω. Wefound that ω should be estimated independently from the reflectioncoefficient of one return trip of a pulse and pulse rise time. Bittelliet al. (2008) assumed a representative maximum frequency of1.5 × 108 Hz for calculating ω = 2πF max in Equations (2) and (3).However, the maximum frequency values varied between 8 × 107

and 4 × 108 Hz for the BC-amended soils. This modification mightbe necessary because of the conductivity of BC (800◦C) (Table 2).

The separation of the ε′r and ε′′

r components of complexdielectric permittivity is an effective means of correcting theTDR calibration curve for the BC-amended soils. The θ –ε′

r

relationships for the soils amended with 3% (w/w) BC (400◦C),BC (600◦C) and BC (800 ◦C) and that for the non-amended soilare shown in Figure 4(a). In addition, the θ –ε′

r relationships for

the soils amended with 1, 3 and 5% (w/w) BC (800◦C) and thatfor the non-amended soil are shown in Figure 4(b). The θ –ε′

r

relationships for all amended soils did not substantially differfrom that for the non-amended soil. As shown in Equation (3),the value of ε′

r was reduced by the value of σ e , which includesboth σ dc and dielectric loss. Therefore, the difference between εa

in Figure 2(a) and ε′r in Figure 4(a) depends on σ e . These results

suggest that the εa values obtained via TDR waveform analysisare affected by the conductive and dielectric losses of the BC(800◦C)-amended soil. Using σ e as defined by Topp et al. (1988),we were able to identify the combined dielectric and conductivelosses (ωε0εr ′′ + σ dc). On the other hand, the Giese and Tiemannequation reliably estimates the value of σ dc obtained from onlyTDR waveform analysis (Baker & Spaans, 1993; Topp et al.,2000). Therefore, we evaluated the contributions of both dielectricand conductive losses to εa of the BC-amended soil. The ε′

r –θ

relationship that was converted from the θ –εa relationships withσ e =ωε0εr ′′ + σ dc for the BC (800◦C)-amended soil was identicalto that obtained with Topp’s equation (Figure 4). However, theε′r –θ relationship that was converted from the θ –εa relationships

(b)

(a)

0

10

20

30

40

50

60

70

80

0

10

20

30

40

50

60

70

80

0 0.1 0.2 0.3 0.4


0 0.1 0.2 0.3 0.4


0% (w/w)1% (w/w)3% (w/w)5% (w/w)

Biochar content



Rea

l rel

ativ

e di

elec

tric

per

mitt

ivity

, ε r′

Rea

l rel

ativ

e di

elec

tric

per

mit

tivi

ty, ε

r′


Figure 4 Relationship between the volumetric water content and realrelative dielectric permittivity for the biochar-amended soil samples with(a) different pyrolysis temperatures and (b) different biochar contents ofthe biochar prepared at a pyrolysis temperature of 800◦C.



by using only σ dc (assuming that σ e = σ dc) did not match theresults obtained with Topp’s equation (results not shown).

These results indicate that irrespective of the source of increasein σ e , the dielectric loss contribution is greater than the conductiveloss contribution. However, with respect to the sources of thedielectric loss, Topp et al. (2000) pointed out that dielectricand conductive losses do not differ according to the source ofconductivity from the clay content in the soil matrix or theelectrolytes in the soil solution. Therefore, the source of thedielectric loss could not be identified using this method. However,with respect to the fact that the solution EC for the BC (800◦C)-amended soils was relatively small (Table 3), the electrolytesin the soil solution might contribute less to the dielectric loss.Further studies are needed to elucidate completely the electricalmechanism by which the incorporation of the BC (800◦C) intothe soil increases the εa .

Conclusions

In this study, we investigated the influence of BC incorporation onTDR-based SWC measurements. The θ –εa relationships for thesoils amended with the BC (400◦C) and BC (600◦C) were similarto that for the non-amended soil. Therefore, the incorporationof the BC formed at these temperatures has a negligible effecton the measurement of the SWC by TDR. In contrast, theθ –εa relationship for the BC (800◦C)-amended soil differedsubstantially from that for the non-amended soil. Therefore, theBC (800◦C) had the greatest effect on the TDR-based SWCmeasurements. These results could be caused by the greaterconductivity of BC (800◦C) than that of BC (400◦C) and BC(600◦C).

The θ –εa relationships for the non-amended soil and thosefor the soils amended with the BC (400◦C) and BC (600◦C)matched those obtained with Topp’s equation, which is a universalcalibration equation. In contrast, the θ – εa relationship for the BC(800◦C)-amended soil did not match that obtained using Topp’sequation. Therefore, TDR-based measurements tended to over-estimate the water content of soils containing the BC formed atgreater pyrolysis temperatures when the calibration curve betweenεa and θ obtained for the non-amended soil was used.

The θ –ε′r relationships for the BC (800◦C)-amended soils did

not substantially differ from that for the non-amended soil or thatpredicted by Topp’s equation when ε′

r was obtained using thevalue of σ e , which includes both conductive and dielectric loss.These results suggest that εa obtained by TDR waveform analysisare affected by the conductive and dielectric losses in the BC(800◦C)-amended soil. In addition, the use of ε′

r , in which thecontributions of the conductive and dielectric losses are removedfrom εa , instead of εa is effective when making TDR-based watercontent measurements of soils that contain the BC formed atgreater pyrolysis temperatures.

These results showed that pyrolysis temperatures, when BC isproduced, must be noted when making TDR-based water contentmeasurements in BC-amended agricultural lands. In addition,

θ –ε′r relationships instead of θ –εa relationships as calibration

curves should be used when making TDR-based water contentmeasurements in agricultural lands amended by BC formed atgreater temperatures. These will make it possible to measureSWC accurately by TDR in BC-amended agricultural land, andtherefore, they will be useful in understanding influences of BCincorporation on soil water-holding capacity, permeability andnutrient leaching, which are related to SWC, in agricultural land.

Acknowledgements

This work was partially supported by the Japan Society for thePromotion of Science (JSPS) Grant-in-Aid for Young Scientists(B) 23780254. We thank three anonymous reviewers for helpfuldiscussions and insightful comments.

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