Catena 114 (2014) 37–44

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Effect of rice husk biochar and coal fly ash on some physical propertiesof expansive clayey soil (Vertisol)

Sheng-Gao Lu ⁎, Fang-Fang Sun, Yu-Tong ZongZhejiang Provincial Key Laboratory of Subtropical Soil and Plant Nutrition,Ministry of EducationKey Laboratory of Environmental Remediation and EcosystemHealth, College of Environmental andResource Sciences, Zhejiang University, Hangzhou 310058, PR China

⁎ Corresponding author. Tel.: +86 571 88982061.E-mail address: lusg@zju.edu.cn (S.-G. Lu).

0341-8162/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.catena.2013.10.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 February 2013Accepted 29 October 2013Available online xxxx

Keywords:Expansible clayey soilRice husk biocharCoal fly ashSoil aggregatePore size distributionTensile strength

The objective of this work is to evaluate the effect of rice husk biochar (RHB) and coal fly ash (CAF) on the for-mation and stability of aggregates, pore size distribution, water retention, swell–shrinkage, consistency limit,and tensile strength of an expansive clayey soil (Vertisol). For this purpose, RHB and CAF are added to the clayeysoil at four levels of 0, 2, 4, and 6% byweight, and incubated for 180 days in a glasshouse. Results indicate that theRHB significantly increasesmacroaggregates with a diameter larger than 0.25 mm and reduces microaggregateswith a diameter of b0.25 mm. Whereas CFA does not significantly affect the formation of macroaggregates. TheRHB- and CFA-amended soils have significantly higher mean weight diameter (MWD) and geometric mean di-ameter (GWD) as comparedwith the control soil. The enhanced aggregate stability is attributed to a decrease inthe aggregate breakdown by differential swelling and an increase in the aggregate resistance to mechanicalbreakdown. The RHB-amended soil has a greater water-holding capacity and higher available water content.Pore size distribution (PSD) of RHB- and CFA-amended soils, determined by the mercury intrusion porosimetry(MIP), indicates that the amendment enhances the formation of mesopores having a pore size range between 6and 45 μm. In the measured pore range (0.003–360 μm), the amended soils are found to have considerablyhigher porosity than the control soil. The RHB and CAF affect the PSD of clayey soil by binding microaggregatestogether to formmacroaggregate and combining carbon and fly ash particles with clay mineral phases to form alarger complex. Meanwhile, the RHB and CFA significantly decrease the tensile strength and coefficient of linearextensibility (COLE) of clayey soil. For example, adding a 6% RHB can reduce the tensile strength from 936.8 to353.6 kPa and COLE from 0.63 to 0.56, respectively. The RHB and CFA also decrease the plasticity index of clayeysoil. The above results indicate that the RHB and CFA are able to improve the physical quality and swelling–shrinkage status of expansive clayey soils, being a potential soil amendment for improving poor physical charac-teristics of the clayey soil.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Vertisol is a soil containing a large amount of expansive clay min-erals. Because expansive clay causes swelling–shrinkage and stickiness,the Vertisol has high swelling pressure, exceptionally low hydraulicconductivity, poor soil structure, and deep crack cutting when it is dryand sticky when it is wet (Brierley et al., 2011; Murthy, 1988; Wildingand Puentes, 1988). These characteristics prevent Vertisol from agricul-tural and engineering use andmakemanagement difficult (Basma et al.,1996; Brierley et al., 2011; Cook et al., 1992; Dasog et al., 1988; Dinket al., 2013; Kishne et al., 2009, 2012; Millan et al., 2012). In spite ofthese disadvantages, the Vertisol is still used in agriculture due to itshigh natural nutrient fertility.

Vertisol is an important soil in many countries such as Australia,China, Canada, Egypt, India, Jordan, Saudi Arabia, South Africa, Sudan,and the United States (Brierley et al., 2011; Liu, 1991; Murthy, 1988;

ghts reserved.

Pal et al., 2001, 2012;Wilding and Puentes, 1988). In China, the Vertisolis estimated to cover approximately 4 × 106 ha, which are mainly dis-tributed in semi-arid areas of Northern China (Li et al., 2011; Liu,1991). Due to the poor physical properties, most Vertisol belongs tothe middle and low-yield soils. In order to improve the physical condi-tions of Vertisol, several techniques have been developed, such as soilamendment using organic manures, industrial wastes, and syntheticpolymer, and soil management by drying/wetting cycle and croppingsystem (Akbulut et al., 2007; Aksakal et al., 2012; Attom and Al-Sharif,1998; Bandyopadhyay et al., 2003; Cai et al., 2006; Kalkan, 2011;Wallace and Terry, 1998; Yazdandoust and Yasrobi, 2010). Organicamendment is a traditional option to improve the structure of Vertisol.Typical organic materials used include animal manure, sewage sludge,city refuse, compost, and crop residues (Bravo-Garza et al., 2009; HuseinMalkawi et al., 1999; Pillai andMcGarry, 1999). Othermaterials suitablefor soil amendment are the by-products of industrial processes. Previ-ous studies indicated that adding industrial waste, such as fly ash,lime, gypsum, zeolites, and silica fume, into the Vertisol improved soil'sstructure and reduced the swelling of expansive clayey soils (Blissett

Table 1Selected physicochemical properties of clayey soil, rice husk biochar and coal fly ash.

Parameter Clayey soil Rice husk biochar Coal fly ash

pH 7.6 7.8 11.1Sand (2–0.02 mm, %) 26.0 – 44.9Silt (0.02–0.002 mm, %) 30.7 – 44.6Clay (b0.002 mm, %) 43.3 – 2.3Total carbon (g kg−1) 7.60 609.8 –

CEC (cmol kg−1) 31.80 – –

Porosity (%) 38.5 80.8 51.7

The pH was determined in the ratio of solid to water of 1:2.5; particle size distributionwas determined by sieving and the pipette method; cation exchangeable capacity wasdetermined using the ammonium saturation and distillation methods; total carbon wasestimated by potassium dichromate oxidation and titration with ferrous sulfate; –, notdetermined.

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and Rowson, 2012; Kalkan, 2009, 2011; Punthutaecha et al., 2006;Razmi and Sepaskhah, 2012). For example, fly ash has been reportedto improve the texture, structure, water holding capacity, hydraulicproperties, and aeration of the soils (Adriano and Weber, 2001; Changet al., 1977; Lu and Zhu, 2004). Although many successful ameliorativepractices have been reported, the mechanism on the improvement byvarious soil amendments still remains unclear.

Rice husk is an agro-industrial waste product of the rice production.Very large quantities of rice husks are produced every year in China,which have become a waste problem and have to be burnt in an openenvironment. Now, the rice husks have been used as a bio-fuel for elec-tricity generation (Shackley et al., 2012). Rice husk biochar (RHB), a by-product of the thermochemical conversion from the rice husk to biofuel,has been proposed to be a new soil amendment. RHB can be recycledeasily in the rice–wheat system without adverse effect on the soil'shealth (Oguntunde et al., 2008; Shackley et al., 2012). As a newmemberof the soil amendments, the effect of the RHB on the soil's physical prop-erties has not been studied yet (Atkinson et al., 2010; Van Zwieten et al.,2010).

Coal fly ash (CFA), a combustion product of the coal in energy pro-duction, has been used as the soil amendment for many years, and therelative works have been reviewed by several authors (Blissett andRowson, 2012; Jala and Goyal, 2006). It has been reported that addingCFA into soils increases the water-holding capacity of soil and reducesthe swelling potential of soil. Other benefits of adding CFA into thesoils include the following: improving the texture of the soil, reducingthe bulk density of the soil, improving the soil's aeration, and minimiz-ing the crust's formation, runoff and soil erosion (Adriano and Weber,2001; Chang et al., 1977; Lu and Zhu, 2004). The effect of CFA on soilphysical properties has been long studied, however, its effect on thepore structure and mechanical strength of soils is little known in spiteof the fact that the pore structure and stiffness property of the clayeysoils greatly affect the production of crops.

Sustainable crop production requires good physical properties ofVertisol. Large quantities of rice husk biochar (RHB) and coal fly ash(CFA) are available in China. Use of these large quantities of RHA andCFA in agricultural land not only improves the physical and chemicalfertility of soils but also resolves waste disposal problem. Therefore, inthis work we determine the effect of RHB and CFA on the formationand stability of aggregates, pore structure, swelling and mechanicalproperties of the Vertisol, and evaluate the possible benefits of amend-ment on this “problematic soil”.

2. Materials and methods

2.1. Clayey soil, rice husk biochar and coal fly ash

The soil, locally known as Shajiang black soil, was collected from thetopsoil (0–20 cm) of a typical Vertisol in northern China, and was classi-fied as the Typical Calci-Aquic Vertisol according to the Chinese Soil Tax-onomy (Li et al., 2011). The soil, characterized by the stiffness and rigiditystructure,was poor in organicmatter andhas deep shrinkage cracks upondesiccation. Typical clay minerals are montmorillonite and hydromica.Before use, the soil was air-dried, sieved to pass a 2-mm sieve and ho-mogenized. Basic properties of soil were determined using the routinemethods (Zhang and Gong, 2012). RHB was produced by pyrolysis ofrice husk in low oxygen condition and CFA was collected from a coal-burning power plant. Their basic properties were given in Table 1.

2.2. Incubation of RHB- and CAF-amended soils

The mixtures of clayey soil and amendments (RHB and CFA) wereprepared byweighing the calculated amounts of clayey soil and amend-ments, and mixing them in dry state with the content of amendmentsequaling to 2%, 4%, and 6%, respectively. The clayey soil without anyamendments was used as the control. Each treatment was repeated

four times. The clayey soils with andwithout amendment weremoistur-ized using deionized water and incubated in a glasshouse for 180 days.During incubation, water content was constantly maintained at 70% ofwater-holding capacity by weekly adjustments based on the weight ofsamples. After incubation, physical properties of the RHB- and CFA-amended soils were analyzed.

2.3. Size distribution and stability of soil aggregate

Size distribution of soil aggregates was determined using the dry-and wet-sieving methods developed by Kemper and Rosenau (1986).Air-dried aggregates were separated by placing 100 g of air-dried soilson the top of a stack of five sieves (5, 2, 1, 0.5 and 0.25 mm in diameter).The soils were sieved for 10 min on a ro-tap sieve. Dry aggregatesremaining on each sieve were collected and weighed. Water-stable ag-gregates were estimated following the standard wet-sieving method.Briefly, 50 g composite soil samples representing each dry aggregatesize class were placed on the topmost of a nest of sieves with diame-ters equaling to 2, 1, 0.5, and 0.25 mm, respectively. The sieves wereplaced in a sieve holder of the Yoder type aggregate analysis machine(DM200-II) and sieved in water for 30 min at a rate of 30 cycle/min.The resultant aggregates on each sieve were dried at 105 °C for 24 hand weighed. According to the size range of 5–2, 2–1, 1–0.5, and 0.5–0.25 mm, respectively, the percentage of water-stable aggregate wasdetermined. The mass of b0.25 mm aggregate was calculated by dif-ference between the initial sample weight and the sum of sampleweights collected on the 2, 1, 0.5, and 0.25 mm sieve nest. The waterstable indices, i.e., the mean weight diameter (MWD) and geometricmean diameter (GMD), were calculated according to the method ofKemper and Rosenau (1986). Percentage of aggregate disruption(PAD) was calculated by the formula below:

PAD ¼ 100 � A−Bð Þ=A

where A is the weight of dry-sieved aggregates larger than 0.25 mmand B is the weight of wet-sieved aggregates larger than 0.25 mm.

The 5–2 and2–1 mmdry aggregateswere selected to evaluate the ag-gregate stability according to the method used by Le Bissonnais (1996).This method used three disruptive tests according to the wetting condi-tions and energies: fast wetting (FW), slow wetting (SW), and mechani-cal breakdown by shaking after pre-wetting (wet-stirring, WS). Theaggregates were dried at 40 °C for 24 h prior to test. In the fast wettingtest, 5 g of aggregates was immersed into 50 ml deionized water for10 min. In the slow wetting test, the same amount of aggregates waswetted on a tension plate at a potential of −0.3 kPa for 30 min. To per-form wet-stirring, the aggregates were first wetted using ethanol, thenadded into 250 ml of deionized water and agitated by a rapid end overendmovement. After each test, the fragmented aggregateswere collectedand transferred into a 0.05 mm sieve that was previously immersed inethanol. The sieve was gently moved five times. The aggregates remain-ing on the sieve were collected, dried at 105 °C and gently dry-sieved

Fig. 1. Effect of RHB and CFA on the aggregate size distribution of clayey soils, in whicherror bars represent a standard deviation and different letters indicate a significant differ-ence (p b 0.05) between amendment treatments and control.

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by hand through a nest of six sieves (2, 1, 0.5, 0.25, 0.125 and 0.05 mm).The mass of aggregates in each size was determined. The MWDFW,MWDSW andMWDSW were calculated from the mass fraction of soils re-maining on each sieve, as described by Le Bissonnais (1996).

2.4. Water retention of soil

The soil cores (100 cm3 in volume) were saturated by capillarity for24 h and equilibrated on a sand box. A full range of the soil water reten-tions weremeasured using a combination of the tension table and pres-sure plate apparatus (Soil Moisture Equipment Corp., Santa Barbara, CA,USA). At each pressure level, the samplewasweighed before each incre-ment in pressurewas applied. At the end, the gravimetric water contentof the soilswas determined bydrying at 105 °C for 48 h. The plant avail-ablewater (PAW)was calculated from the difference between volumet-ricwater content atfield capacity (thematric potential of−33 kPa) andwilting point (the matric potential of −1500 kPa).

2.5. Pore size distribution of soil

Pore size distribution (PSD) of soil was determined on the vacuumdried samples using a Mercury intrusion porosimetry (MIP) (AutoporeIV 9500,Micromeritics Inc., USA). In theMIPmethod, themercury pres-sure was increased step by step and the intruded volume of mercurywas monitored for each pressure increment in a range from 0.0036 to310 MPa. The applied pressure allowed to determine the pore diameterranges from 0.003 μm to 360 μm.

2.6. Soil's consistency limit

The consistency limits (liquid limit and plastic limit) of soils weredetermined according to the ASTMD4318 procedure (American Societyfor Testing andMaterials, 1995). The plasticity index (PI) was defined tobe the difference between the liquid limit and the plastic limit.

2.7. Coefficient of linear extensibility (COLE)

The coefficient of linear extensibility (COLE) of soil, a measure of thepotential volume change of soil upon wetting or drying, was deter-mined on ground remolded soils according to Schafer and Singer(1976). About 100 g of b2 mmair-dried soilswasmixedwith deionizedwater to form a paste slightly drier than saturation, and then equilibrat-ed by leaving the paste for 24 h. The resulting paste was loaded into asyringe and extruded into ten rods with varying lengths from 5 to10 cm on a flat and smooth surface. The rods were trimmed into twoends perpendicular to the drying surface and their lengths were mea-sured using a digital micrometer (accuracy ±0.01 mm). After air dry-ing, the length of the rods was re-measured and the COLE wascalculated by the formula below:

COLE ¼ Lm−Ldð Þ=Ld

where Lm and Ld are the length of dry and moist soils, respectively.

2.8. Soil strength

Tensile strength of soilwas determinedbyusing the crushingmethod.Remolded soil cores were saturated by capillarity for 24 h, equilibratedon a pressure plate apparatus for 12 h, and dried at 105 °C for 2 h. Thedried soil cores were placed horizontally between two parallel plates ofa digital unconfined compression apparatus (YYW-2, Nanjing Soil Instru-ment Factory Co. Ltd.), and the pressure was gradually increased throughthe plates by a motor at a constant speed of 2 mm min−1 approachingthe soil core. The maximum reading was recorded before the core wasfractured by the load plate.

Direct shear tests (DST) were conducted using a quadruplex straincontrolled direct shear apparatus (Nanjing Soil Instrument Factory Co.Ltd.) to measure the shear strength of clayey soils. The apparatus con-sists of a soil shear box, a loading head, a weight hanger, and weightsto generate normal loads. The soil cores (6.18 cm in diameter and2 cm in height) were saturated slowly for 24 h and then drained at300 hPa matric potential for 12 h. Samples were placed in a shear test-ing device, and normal loads of 50, 100, 200, and 400 kPa were appliedand sheared immediately. A lateral displacementwas applied at a speedof 0.8 mm min−1 until failure occurred and the peak shear forcewas re-corded. The cohesion (c) and the angle of internal friction (φ) were ob-tained by the Mohr–Coulomb theory.

2.9. Statistical analysis

Analysis of variance (ANOVA) was performed by a SPSS 13.0 Statis-tical Package. Means were compared by least significant difference(LSD) at p b 0.05 level.

3. Results

3.1. Effect of RHB and CAF on the size distribution of aggregates

Results of aggregate size analyses for RHB- and CAF-amended soilsare presented in Fig. 1. It is clearly shown that RHB altered the aggrega-tion characteristics of clayey soil. The aggregates in the control soil aredominated by the microaggregates with diameters less than 0.25 mm,which account for 70% of the total mass. The large percentage ofmicroaggregates may cause problems of surface sealing and soil loss inthe dry-land cropping system of clayey soil (Amezketa, 1999). TheRHB-amended soils had significantly higher amounts of water-stablemacroaggregates with diameters in 5–2, 2–1, 1–0.5 and 0.5–0.25 mmas compared with the control, and accordingly had a lower content ofthe b0.25 mm microaggregates. Fig. 1 shows that at 6% content, theRHB has the strongest effect on the formation of aggregates, and thatthe 5–2 and 0.5–0.25 mm aggregates increased by 87 and 99%, respec-tively, in comparison with the control. Addition of 2, 4, and 6% RHBreduced the percentage of b0.25 mm microaggregates by 3%, 20%, and31%, respectively. The CFA significantly affected the formation of 0.25–

Table 2Effect of RHB and CFA on the mean weight diameter (MWD) by three aggregate stabilitytests.

Treatment 5–2 mm aggregates 2–1 mm aggregates

FW SW WS FW SW WS

RHB T0 0.85c 0.92c 1.11d 0.44c 0.60d 0.59dT2 0.83d 0.91d 1.22c 0.44c 0.62c 0.64cT4 0.97b 1.07b 1.33b 0.54b 0.701b 0.74bT6 1.03a 1.14a 1.44a 0.57a 0.80a 0.80a

CFA T0 0.85d 0.92c 1.11c 0.44c 0.609c 0.56dT2 0.86c 0.86d 1.07d 0.41d 0.55d 0.62bT4 1.01b 1.06b 1.15b 0.51b 0.65b 0.61cT6 1.04a 1.07a 1.17a 0.54a 0.73a 0.68a

FW: fast wetting; SW: slow wetting; WS: wet-stirring. Means in a column followed by adifferent letter differ significantly at 5% level of significance.

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0.5 mmaggregates (p b 0.05), however, had no significant effect on theformation of 5–2 and 2–1 mm aggregates.

3.2. Effect of RHB and CAF on aggregate stability

The aggregate stability was evaluated by the MWD and GMD valuesof the water-stable aggregates and Le Bissonnais (1996) method. RHBsignificantly increased the values of MWD and GMD as compared withthe control (Fig. 2). There were statistically significant differences inthe MWD and GMD values between the control and those having a 4%or 6% RHB treatment. On the contrary, the addition of 2, 4 and 6%CFA did not cause statistically significant differences. The significantlyreduced percentage of aggregate disruption (PAD) indicated that RHBwas able to increase the aggregate stability of clayey soil (Fig. 2).

The MWD values of Le Bissonnais' tests for 5–2 and 2–1 mm aggre-gates are given in Table 2. The MWDFW values were 0.85 and 0.44 mmfor 5–2 and 2–1 mmaggregates, respectively, corresponding to the initialheterogeneity in aggregate stability of clayey soil. The RHB- and CFA-amended soils showed significantly higher MWDFW values (p b 0.05)than the control, indicating that all treatments increased aggregate resis-tance to the strong disaggregating energy in the fast wetting test. Similarto the results of theMWDFW test, a certain extent of heterogeneity in theinitial MWDSW values of 5–2 and 2–1 mm aggregates was observed. Re-sults showed that bothRHBandCFA significantly improved the aggregatebreakdown by differential swelling. The most pronounced effect was ob-served from the 6% RHB and CFA treatments (Table 2). On average, theMWDSW values for 2–1 mm aggregates were increased by 8% for 2%, by25% for 4%, and by 36% for 6% RHB treatment, respectively. The significantincreases in MWDSW values (p b 0.05) by the RHB- and CFA-treatmentsindicated a significant improvement in the aggregate resistance to me-chanical breakdown.

3.3. Effect of RHB and CFA on pore size distribution

The pore size distribution (PSD) of the RHB- and CFA-amended soilsis shown in Fig. 3. The differential PSD curves in Fig. 3 clearly showedthat the clayey soils had multi-modal PSDs with representative peaksat pore diameters of around 0.007 μm, 1–3 μm, 6–10 μm and 90 μm.The RHB exhibited a single, sharply defined peak at a pore diameter of90 μm and a small peak in the diameter range of 0.2–0.4 μm (Fig. 3a).The CFA exhibited a bimodal pore structure with a first pore class in

Fig. 2. Effect of RHB andCFA on themeanweight diameter (MWD), geometricmean diam-eter (GWD) of aggregate, and percentage of aggregate disruption (PAD), in which errorbars represent a standard deviation and different letters indicate a significant difference(p b 0.05) between amendment treatments and control.

the range of 2–3 μm and a second pore class in the range of 6–7 μm.PSDs of the RHB-amended soils exhibited three distinct peaks in therange of investigated pores (Fig. 3b). Three peaks appeared in thepore size ranges from 1 to 2 μm (first peak), 6 to 9 μm (second peak)and 60 to 90 μm (third peak), respectively, which are attributed tothe ultramicropores, micropores, and macropores of soils. The CFA-amended soils exhibited multi-modal structure with three PSD peaksin the pore size range of 0.5–2 μm, 6–10 μm and 60–90 μm, respec-tively. The pore volume of the amended soils was characterized byan increased pore volume in the diameter ranges of 0.15–2.5 and6–45 μm for the RHB-amended soil, and in the diameter ranges of0.2–2.0 and 6–25 μm for the CFA-amended soils. The small pores hav-ing diameters less than 0.2 μm for the RHB- and CFA-amended soilshad similar PSD shapes as that of the control, indicating that the CFAand RHB did not affect the small pores (b0.1 μm).

The pore structure parameters of RHB- and CFA-amended soils areshown in Fig. 4. Total MIP porosity (for pore diameter range of 0.003–360 μm) of the 6% CFA amended soil was 46.2%, being higher(p b 0.01) than that (38.5%) of the control soil. Total intrusion volumes

Fig. 3. Effect of RHB and CFA on the pore size distribution (PSD) of clayey soils. a. PSDcurves of RHB and CFA; b. PSD curves of RHB-amended soils; c. PSD curves of CFA-amended soils.

Fig. 4. Effect of RHB and CFA on the total pore volume and porosity of clayey soil measured by MIP method.

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of the 6% RHB- and CFA-amended soils were 28.28 and 35.47 cm3

100 g−1, respectively, being greater than 25.43 cm3 100 g−1 of thecontrol soil. According to the equivalent pore diameter (EPD), thepores could be classified into the following five categories: macropores(75–100 μm), mesopores (30–75 μm), micropores (5–30 μm), ultra-micropores (0.1–5 μm) and crytopores (0.1–0.007 μm) (Cameron andBuchan, 2006). The percentage of each pore category for the RHB- andCFA-amended soils is given in Fig. 5. The porosity of the RHB-amendedsoils was composed of 32% macropores, 27% ultramicropores, and 16%micropores, suggesting that the CFA mainly promoted the formation ofultramicropores. The pore volumes of macropore, mesopore, micropore,and ultramicropore were increased with the content of RHB and CFA.On the other hand, the pore volume of the RHB-amended soils was ob-served to be contributed nearly equally by the macropores, mesopores,micropores and crytopores. In a 6% RHB amended soil, the pore volumeof the ultramicropores was significantly higher than that of the controlsoil. The largest difference in the pore-size between the CFA-amendedsoil and control soil was often found from the mesopores, micropores,and ultramicropores. The difference in the pore volume between theRHB- and CFA-amended soils may be attributed to the flocculation/aggregation processes induced by the amendments.

3.4. Effect of RHB and CAF on water retention

The RHBwas shown to increase the soil's water retention in terms ofwater holding capacity (WHC), field capacity (FC), wilting point (WP)and available water (AW) (Fig. 6). In comparison with the control soil,the 2, 4, and 6% RHB-amended soils showed a 12, 20, and 31% higherwater-holding capacity, respectively. The observed improvement inthe soil's water retention could be attributed to the increased porosityby the incorporation of RHB into the clayey soils. On the other hand,the CFA showed less pronounced improvement on the FC value, and

Fig. 5. Pore volume of RHB- and CFA-amended soils corresponding to equivalent porediameter (EPD) classes following the criteria proposed by Cameron and Buchan (2006).

on the contrary reduced the WHC, WP and AW values of the clayeysoils. These results disagreed with those of Adriano and Weber (2001)and Chang et al. (1977), who reported that the CFA increased thewater-holding capacity and available water content of soils.

3.5. Effect of RHB and CAF on the consistency limit

Fig. 7 shows the effect of RHB and CAF on the plastic and liquid limits,and plasticity index of the clayey soils. Addition of RHB significantly in-creased the plastic and liquid limits, and decreased the plasticity indexof the clayey soils. The liquid limit value of the soils was significantly de-creased with the addition of CFA and the trend inversed for the plasticlimit. The plastic index of the control soilwas 17.6%,whichwas decreasedto 11.5% after adding 6% CFA. The consistency limits of the soil are crucialin managing soils, especially for the case of irrigation agriculture.

3.6. Effect of RHB and CAF on COLE

The effect of RHB and CFA on the soil's COLE is shown in Fig. 8. Thecontrol soil had a COLE value of 0.06, which cracked in a dry conditionand expanded extensively in a wet condition. Therefore, there were dif-ficulties in managing and tilling the Vertisol. According to the classifica-tion of Schafer and Singer (1976), the soil falls into the group of severeshrinkage–swelling hazard rating. The introduction of RHB and CFA sig-nificantly decreased the COLE value of the clayey soil (Fig. 8). The COLEvalue was decreased from 0.063 of the control to 0.056 and 0.032 of the

Fig. 6. Effect of RHB and CFA on the soil's water retention, in which error bars representa standard deviation and different letters indicate a significant difference (p b 0.05)between amendment treatment and control.

Fig. 7. Effect of RHB and CFA on the plastic (PL) and liquid limits (LL), and plasticity index (PI) of the clayey soils, in which error bars represent a standard deviation and different lettersindicate a significant difference (p b 0.05) between amendment treatment and control.

42 S.-G. Lu et al. / Catena 114 (2014) 37–44

6% RHB- and CFA-amended soils, respectively, which was probably be-cause the RHB and CFA changed the swelling–shrinking properties ofthe clayminerals. Thus, a decrease in the swelling–shrinking propertiesof the clayey soil would prevent the clayey soils from cracking.

3.7. Effect of RHB- and CAF on the tensile strength

The effect of RHB and CFA on the soil tensile strength (TS) is displayedin Fig. 9. Results showed that the soil tensile strength was significantlydecreased (p b 0.05) with increasing of the RHB and CFA contents.With the addition of RHB, the TS value of the 6% RHB-amended soil wasdecreased to 353.6 kPa from the initial 936.8 kPa of the clayey soil. In asimilar manner, the TS value of the 6% CFA-amended soil was decreasedto 651.9 kPa. The above results well agreed with our previous resultthat the CFA decreased the soil's strength (Lu and Zhu, 2004). The tensilestrength of soil clodsmainly resulted from the inter-particle bondswithinsoil clods, which were associated with the properties such as the force tohold water in soil, cohesion, and the internal friction force. The RHB andCFAmight be able toweaken the inter-particle bonding strength througha lubricating effect, and as a result decreased the soil's tensile strength.

3.8. Effect of RHB and CAF on the shear strength

The shear strength of a soil can be expressed by two parameters of co-hesion (c) and internal friction angle (φ). The effect of RHB andCFAon theshear strength parameters (c andφ) is presented in Fig. 10. Mean c andφof the studied clayey soil were 14.33 ± 0.62 kPa and 20.72 ± 0.77°, re-spectively. As compared with the control soil, the RHB-amended soilshad significantly smaller c values, and the 2% and 4% CFA amended soilshad significantly higher c values. However, both RHB and CFA increasedthe φ value of the soils as indicated in Fig. 10.

Fig. 8. Effect of RHB and CFA on the COLE of clayey soils, in which error bars representa standard deviation and different letters indicate a significant difference (p b 0.05)between amendment treatment and control.

4. Discussion

4.1. Improvement in the aggregate stability

The formation and stability of the soil aggregates play an importantrole in the crop production and soil degradation prevention (Amezketa,1999). An increase in the formation of macroaggregates by the additionof RHB indicates that the RHB is able to increase the soil aggregation. Ad-dition of a 6%RHB reduced the percentage ofmicroaggregates from70.9%to 50.4%, suggesting that the macroaggregates were formed by the coa-lescence of many microaggregates. The carbon introduced by the RHBmay act like a glue to cement microaggregates into macroaggregates inwhich larger pore spaces are present between micro-aggregates.

The aggregate stability not only affects the movement of water andair in the soil but also influences the water holding capacity, root pene-tration, seedling emergence, runoff and erosion. The soil aggregate sta-bility decreases in an order of SWNWSNFW. Table 2 indicates that theslaking of the aggregates is a major mechanism in the aggregate break-downof clayey soils. Fastwetting leads tomore entrapped air and great-er differential movement of particles due to swelling (Le Bissonnais,1996; Zaher et al., 2005). The high clay content increases the extent ofdifferential swelling and the volume of entrapped air in pore space,which further increases the aggregate breakdown. The MWD values ofthree aggregate stability tests indicated that the RHB and CFA wereable to improve the cohesion of soil particles. The RHB increases the ag-gregates' resistance to slaking and to differential swelling of clays by in-creasing internal cohesion of the mineral particles through the carbonpolymers or the physical enmeshment of the particles. Zaher et al.(2005) reported that large quantities of soil organic carbon could reducethe pore pressure and swelling during rewetting and hence improvedthe aggregating ability of the soils. Another cause for the increased ag-gregate stability is that the RHB and CFA increase the soil hydrophobic-ity, which reduces the extent of the clay swelling and the aggregate

Fig. 9. Effect of RHB and CFA on the tensile strength of clayey soils, inwhich error bars rep-resent a standard deviation and different letters indicate a significant difference (p b 0.05)between amendment treatment and control.

Fig. 10. Effect of RHB and CFA on the cohesion (c) and the internal friction angle (φ) of clayey soils, in which error bars represent a standard deviation and different letters indicate a sig-nificant difference (p b 0.05) between amendment treatment and control.

43S.-G. Lu et al. / Catena 114 (2014) 37–44

disruption. An increase in the aggregate stability enhances the soil resis-tance against the wind and water erosion.

4.2. Improvement of the soil pore structure

In Vertisol, the swelling and aggregate slaking cause a reduction inthe size of large pores, which decrease the water and air permeability.Several authors (Dink et al., 2013; Razmi and Sepaskhah, 2012; Zaheret al., 2005) explained how the clay dispersion and subsequent pluggingof conducting pores by the dispersed clay particles were responsible fora drastic reduction in the hydraulic conductivity. The PSD of clayey soilsindicated that clayey soils were featured with larger micropores. Theselarger micropores enhance soil strength and decrease the availablewater content. A significant increase (p b 0.05) was detected from theRHB- and CFA-amended soils in the pores with a size range from 100to 0.005 μm. This increase is likely because the larger pores have a par-tial function of the RHB and CFA (e.g. a wider pore size distribution).Many authors have demonstrated that the organic waste-derived bio-char contained numerous macropores with diameters larger than10 μm (Brodowski et al., 2005). The CFA exists in a powder form witha granulometry of 0.5–100 μm. As a typical porous media, the CFA iscomposed of pores having a broad size distribution from nanometersto micrometers. Our previous SEM images indicated that the CFAcontained many hole cenospheres in a diameter range of 20–120 μm(Lu and Zhu, 2004). These spherical shape cenospheres in the CFAmade larger surface area available for holding water. The inner-holestructure of cenospheres enhanced the porosity of treated soils, andhence reduced the tensile strength. Themacroporesweremore respon-sible for the cohesiveness of soil particles. Another possible cause wasthe formation of macroaggregates in the RHB- and CFA-amended soils.The PSD of the amended soil showed that the RHB and CFA did notchange the concentration of the textural pore space of soils, whichroughly corresponded to the pore space between the clay particles orbetween intra-aggregate pores. The formation of macroaggregates in-creased the macropores between microaggregates or between aggre-gates, which therefore improved the pore structure. The improvementof pore structure in clayey soils also increased the water retention ofsoil. The water retention is mainly affected by the pore size distributionand porosity. Water holding capacity and field capacity of soil are morerelated to the large pores, suggesting that the large pores (greater than0.1 μm in diameter) in the RHB and CFA particles were responsible forwater storage. This hypothesis is consistent with the pure biochar thatholds water more than ten times its own mass (Kinney et al., 2012).The maximum pore water storage capacity of biochar was reported tobe 2.4 g g−1 with an upper boundary biochar porosity of 80%.

4.3. Improvements in soil swelling and mechanical strength

Improvement of soil mechanical properties is a key goal for sustain-able agriculture. Both RHB and CFA have been proven to improve thesoil structure, tensile strength, and swelling characteristics. In theRHB- and CFA-amended soils, the swelling is reduced through two

processes of (i) replacing the swelling clay with non-swelling amend-ments and (ii) resisting swelling, which depends upon the clay–carboncontact area. An effective contact between the clay and carbon particleswould result in a high resistance to swelling. Decreased tensile strengthoffers greater potential for root growth because the roots can bypass thezones of highmechanical impedance. Soil strength is affected by a num-ber of factors including the properties of the particle surfaces. The re-duction of tensile strength in the CFA-amended soils suggested a lesscohesiveness of soil particles, which therefore enhances the potentialof crop root penetration. The decrease in the TS values of the amendedsoils is due to the addition of low-plastic materials and the interactionbetween the clay and carbon particles.

Soil mechanical properties are also affected by the soil structure be-cause soil strength is a function of the contact properties between theprimary particles and soil compound particles (Amezketa, 1999;Zhang and Hartge, 1995). The hollow structure of CFA with centraland well pores may reduce the soil particle cohesions. In the RHB-amended soils, majority of carbon is present in the form of clay–carboncomplexes and small amount of carbon in the form of a discretemateri-al. The clay–carbon complexes influence the behavior of particles at acolloidal level, which not only causes a fundamental change in themicro-structural level but also affects the type and strength of bonds.The organic particles are stiff when being compressed, act as rigid parti-cles when dry, and become soft and sponge-like after absorbing water(Hemmat et al., 2010; Husein Malkawi et al., 1999; Kinney et al., 2012).

The shear strength of soil is essential in predicting the load supportcapacity and has been taken as a measure for the soil erodibility and re-sistance to seedling emergence and root growth (Hemmat et al., 2010).The soil c value is partially controlled by the properties of the particlesurfaces which are in contact or approach contact. After adding RHB, alarger portion of the mineral surfaces are coated by carbon particles,which may block the mineral particles from contacting with eachother. Another source for the decrease of the c value with biochar isdue to an increase in the soil water repellency. Soil mineral particlesare covered by the adsorbedorganicmoleculeswith low surface free en-ergy, resulting in a weak attraction between the solid and liquid phases.The decline in c value is partially attributed to the lower surface tensionforce at the air/water interface between the water films around the soilparticles at the high degree of water saturation.

5. Conclusions

The RHB is able to significantly enhance the formation ofmacroaggre-gates and reduce the fraction of microaggregates less than 0.25 mm,whereas the CFA did not affect the formation of macroaggregate signifi-cantly. The RHB and CFA significantly (p b 0.05) increased the stabilityof the soil aggregates. The percentage of aggregate disruption (PAD) sig-nificantly (p b 0.05) decreased with increasing of RHB. The RHB and CFAsignificantly affect the pore size distribution of clayey soils, but do not sig-nificantly affect the soil crytopores (b0.1 μm). The volume ofmacroporeswas found to increase with the content of RHB and CFA because of theirrichmacropore distribution. Addition of RHB into clayey soil significantly

44 S.-G. Lu et al. / Catena 114 (2014) 37–44

(p b 0.05) increased the soil moisture content at water-holding capacity,field capacity, and available water because the RHB increases the porosi-ties of themesopores andmicropores. On contrary, the CFA has no signif-icant effect on thewater content of soil at field capacity. Application of 6%CFA reduces the moisture content of soils at water-holding capacity andavailablewater content. Application of RHB andCFA reduced the swellingpotential and tensile strength of clayey soils, and consequently resulted ina less energy for breaking soil clod. Therefore, application of the RHB andCFA can effectively solve the problem of clod formation in clayey soil. Theresults of thiswork demonstrated that the RHB and CFA are able to signif-icantly increase the aggregate stability, reduce the COLE and tensilestrength, and improve several other physical properties. In conclusion,the RHB and CFA are good soil amendment agents for the improvementof the expansive clayey soils.

Acknowledgments

This research was supported by the National Key Basic ResearchSupport Foundation of China (973) (2011CB100502).

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