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

Catena 114 (2014) 37–44

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Effect of rice husk biochar and coal fly ash on some physical properties of expansive clayey soil (Vertisol) Sheng-Gao Lu ⁎, Fang-Fang Sun, Yu-Tong Zong Zhejiang Provincial Key Laboratory of Subtropical Soil and Plant Nutrition, Ministry of Education Key Laboratory of Environmental Remediation and Ecosystem Health, College of Environmental and Resource Sciences, Zhejiang University, Hangzhou 310058, PR China

a r t i c l e

i n f o

Article history: Received 2 February 2013 Accepted 29 October 2013 Available online xxxx Keywords: Expansible clayey soil Rice husk biochar Coal fly ash Soil aggregate Pore size distribution Tensile strength

a b s t r a c t The objective of this work is to evaluate the effect of rice husk biochar (RHB) and coal fly ash (CAF) on the formation 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 clayey soil at four levels of 0, 2, 4, and 6% by weight, and incubated for 180 days in a glasshouse. Results indicate that the RHB significantly increases macroaggregates with a diameter larger than 0.25 mm and reduces microaggregates with a diameter of b 0.25 mm. Whereas CFA does not significantly affect the formation of macroaggregates. The RHB- and CFA-amended soils have significantly higher mean weight diameter (MWD) and geometric mean diameter (GWD) as compared with the control soil. The enhanced aggregate stability is attributed to a decrease in the aggregate breakdown by differential swelling and an increase in the aggregate resistance to mechanical breakdown. 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 6 and 45 μm. In the measured pore range (0.003–360 μm), the amended soils are found to have considerably higher porosity than the control soil. The RHB and CAF affect the PSD of clayey soil by binding microaggregates together to form macroaggregate and combining carbon and fly ash particles with clay mineral phases to form a larger complex. Meanwhile, the RHB and CFA significantly decrease the tensile strength and coefficient of linear extensibility (COLE) of clayey soil. For example, adding a 6% RHB can reduce the tensile strength from 936.8 to 353.6 kPa and COLE from 0.63 to 0.56, respectively. The RHB and CFA also decrease the plasticity index of clayey soil. 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 characteristics of the clayey soil. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Vertisol is a soil containing a large amount of expansive clay minerals. Because expansive clay causes swelling–shrinkage and stickiness, the Vertisol has high swelling pressure, exceptionally low hydraulic conductivity, poor soil structure, and deep crack cutting when it is dry and sticky when it is wet (Brierley et al., 2011; Murthy, 1988; Wilding and Puentes, 1988). These characteristics prevent Vertisol from agricultural and engineering use and make management difficult (Basma et al., 1996; Brierley et al., 2011; Cook et al., 1992; Dasog et al., 1988; Dink et al., 2013; Kishne et al., 2009, 2012; Millan et al., 2012). In spite of these disadvantages, the Vertisol is still used in agriculture due to its high 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; ⁎ Corresponding author. Tel.: +86 571 88982061. E-mail address: [email protected] (S.-G. Lu). 0341-8162/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catena.2013.10.014

Pal et al., 2001, 2012; Wilding and Puentes, 1988). In China, the Vertisol is estimated to cover approximately 4 × 106 ha, which are mainly distributed in semi-arid areas of Northern China (Li et al., 2011; Liu, 1991). Due to the poor physical properties, most Vertisol belongs to the middle and low-yield soils. In order to improve the physical conditions of Vertisol, several techniques have been developed, such as soil amendment using organic manures, industrial wastes, and synthetic polymer, and soil management by drying/wetting cycle and cropping system (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). Organic amendment 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; Husein Malkawi et al., 1999; Pillai and McGarry, 1999). Other materials suitable for soil amendment are the by-products of industrial processes. Previous studies indicated that adding industrial waste, such as fly ash, lime, gypsum, zeolites, and silica fume, into the Vertisol improved soil's structure and reduced the swelling of expansive clayey soils (Blissett

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and Rowson, 2012; Kalkan, 2009, 2011; Punthutaecha et al., 2006; Razmi and Sepaskhah, 2012). For example, fly ash has been reported to improve the texture, structure, water holding capacity, hydraulic properties, and aeration of the soils (Adriano and Weber, 2001; Chang et al., 1977; Lu and Zhu, 2004). Although many successful ameliorative practices have been reported, the mechanism on the improvement by various 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 open environment. Now, the rice husks have been used as a bio-fuel for electricity generation (Shackley et al., 2012). Rice husk biochar (RHB), a byproduct of the thermochemical conversion from the rice husk to biofuel, has been proposed to be a new soil amendment. RHB can be recycled easily in the rice–wheat system without adverse effect on the soil's health (Oguntunde et al., 2008; Shackley et al., 2012). As a new member of the soil amendments, the effect of the RHB on the soil's physical properties 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 production, has been used as the soil amendment for many years, and the relative works have been reviewed by several authors (Blissett and Rowson, 2012; Jala and Goyal, 2006). It has been reported that adding CFA into soils increases the water-holding capacity of soil and reduces the swelling potential of soil. Other benefits of adding CFA into the soils include the following: improving the texture of the soil, reducing the bulk density of the soil, improving the soil's aeration, and minimizing 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 soil physical properties has been long studied, however, its effect on the pore structure and mechanical strength of soils is little known in spite of the fact that the pore structure and stiffness property of the clayey soils greatly affect the production of crops. Sustainable crop production requires good physical properties of Vertisol. Large quantities of rice husk biochar (RHB) and coal fly ash (CFA) are available in China. Use of these large quantities of RHA and CFA in agricultural land not only improves the physical and chemical fertility of soils but also resolves waste disposal problem. Therefore, in this work we determine the effect of RHB and CFA on the formation and stability of aggregates, pore structure, swelling and mechanical properties of the Vertisol, and evaluate the possible benefits of amendment 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 the topsoil (0–20 cm) of a typical Vertisol in northern China, and was classified as the Typical Calci-Aquic Vertisol according to the Chinese Soil Taxonomy (Li et al., 2011). The soil, characterized by the stiffness and rigidity structure, was poor in organic matter and has deep shrinkage cracks upon desiccation. Typical clay minerals are montmorillonite and hydromica. Before use, the soil was air-dried, sieved to pass a 2-mm sieve and homogenized. Basic properties of soil were determined using the routine methods (Zhang and Gong, 2012). RHB was produced by pyrolysis of rice husk in low oxygen condition and CFA was collected from a coalburning 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) were prepared by weighing the calculated amounts of clayey soil and amendments, and mixing them in dry state with the content of amendments equaling to 2%, 4%, and 6%, respectively. The clayey soil without any amendments was used as the control. Each treatment was repeated

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

Clayey soil

Rice husk biochar

Coal fly ash

pH Sand (2–0.02 mm, %) Silt (0.02–0.002 mm, %) Clay (b0.002 mm, %) Total carbon (g kg−1) CEC (cmol kg−1) Porosity (%)

7.6 26.0 30.7 43.3 7.60 31.80 38.5

7.8 – – – 609.8 – 80.8

11.1 44.9 44.6 2.3 – – 51.7

The pH was determined in the ratio of solid to water of 1:2.5; particle size distribution was determined by sieving and the pipette method; cation exchangeable capacity was determined using the ammonium saturation and distillation methods; total carbon was estimated by potassium dichromate oxidation and titration with ferrous sulfate; –, not determined.

four times. The clayey soils with and without amendment were moisturized using deionized water and incubated in a glasshouse for 180 days. During incubation, water content was constantly maintained at 70% of water-holding capacity by weekly adjustments based on the weight of samples. After incubation, physical properties of the RHB- and CFAamended soils were analyzed. 2.3. Size distribution and stability of soil aggregate Size distribution of soil aggregates was determined using the dryand wet-sieving methods developed by Kemper and Rosenau (1986). Air-dried aggregates were separated by placing 100 g of air-dried soils on 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 aggregates remaining on each sieve were collected and weighed. Water-stable aggregates were estimated following the standard wet-sieving method. Briefly, 50 g composite soil samples representing each dry aggregate size class were placed on the topmost of a nest of sieves with diameters equaling to 2, 1, 0.5, and 0.25 mm, respectively. The sieves were placed 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 h and 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 was determined. The mass of b0.25 mm aggregate was calculated by difference between the initial sample weight and the sum of sample weights collected on the 2, 1, 0.5, and 0.25 mm sieve nest. The water stable indices, i.e., the mean weight diameter (MWD) and geometric mean diameter (GMD), were calculated according to the method of Kemper and Rosenau (1986). Percentage of aggregate disruption (PAD) was calculated by the formula below: PAD ¼ 100  ðA−BÞ=A where A is the weight of dry-sieved aggregates larger than 0.25 mm and B is the weight of wet-sieved aggregates larger than 0.25 mm. The 5–2 and 2–1 mm dry aggregates were selected to evaluate the aggregate stability according to the method used by Le Bissonnais (1996). This method used three disruptive tests according to the wetting conditions and energies: fast wetting (FW), slow wetting (SW), and mechanical breakdown by shaking after pre-wetting (wet-stirring, WS). The aggregates were dried at 40 °C for 24 h prior to test. In the fast wetting test, 5 g of aggregates was immersed into 50 ml deionized water for 10 min. In the slow wetting test, the same amount of aggregates was wetted on a tension plate at a potential of −0.3 kPa for 30 min. To perform wet-stirring, the aggregates were first wetted using ethanol, then added into 250 ml of deionized water and agitated by a rapid end over end movement. After each test, the fragmented aggregates were collected and transferred into a 0.05 mm sieve that was previously immersed in ethanol. The sieve was gently moved five times. The aggregates remaining on the sieve were collected, dried at 105 °C and gently dry-sieved

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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 and MWDSW were calculated from the mass fraction of soils remaining 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 for 24 h and equilibrated on a sand box. A full range of the soil water retentions were measured using a combination of the tension table and pressure plate apparatus (Soil Moisture Equipment Corp., Santa Barbara, CA, USA). At each pressure level, the sample was weighed before each increment in pressure was applied. At the end, the gravimetric water content of the soils was determined by drying at 105 °C for 48 h. The plant available water (PAW) was calculated from the difference between volumetric water content at field capacity (the matric potential of −33 kPa) and wilting point (the matric potential of −1500 kPa). 2.5. Pore size distribution of soil Pore size distribution (PSD) of soil was determined on the vacuum dried samples using a Mercury intrusion porosimetry (MIP) (Autopore IV 9500, Micromeritics Inc., USA). In the MIP method, the mercury pressure was increased step by step and the intruded volume of mercury was monitored for each pressure increment in a range from 0.0036 to 310 MPa. The applied pressure allowed to determine the pore diameter ranges from 0.003 μm to 360 μm. 2.6. Soil's consistency limit The consistency limits (liquid limit and plastic limit) of soils were determined according to the ASTM D4318 procedure (American Society for Testing and Materials, 1995). The plasticity index (PI) was defined to be 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 the potential volume change of soil upon wetting or drying, was determined on ground remolded soils according to Schafer and Singer (1976). About 100 g of b2 mm air-dried soils was mixed with deionized water to form a paste slightly drier than saturation, and then equilibrated by leaving the paste for 24 h. The resulting paste was loaded into a syringe and extruded into ten rods with varying lengths from 5 to 10 cm on a flat and smooth surface. The rods were trimmed into two ends perpendicular to the drying surface and their lengths were measured using a digital micrometer (accuracy ±0.01 mm). After air drying, the length of the rods was re-measured and the COLE was calculated by the formula below:

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Direct shear tests (DST) were conducted using a quadruplex strain controlled direct shear apparatus (Nanjing Soil Instrument Factory Co. Ltd.) to measure the shear strength of clayey soils. The apparatus consists of a soil shear box, a loading head, a weight hanger, and weights to generate normal loads. The soil cores (6.18 cm in diameter and 2 cm in height) were saturated slowly for 24 h and then drained at 300 hPa matric potential for 12 h. Samples were placed in a shear testing device, and normal loads of 50, 100, 200, and 400 kPa were applied and sheared immediately. A lateral displacement was applied at a speed of 0.8 mm min−1 until failure occurred and the peak shear force was recorded. The cohesion (c) and the angle of internal friction (φ) were obtained by the Mohr–Coulomb theory. 2.9. Statistical analysis Analysis of variance (ANOVA) was performed by a SPSS 13.0 Statistical 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 soils are presented in Fig. 1. It is clearly shown that RHB altered the aggregation characteristics of clayey soil. The aggregates in the control soil are dominated by the microaggregates with diameters less than 0.25 mm, which account for 70% of the total mass. The large percentage of microaggregates may cause problems of surface sealing and soil loss in the dry-land cropping system of clayey soil (Amezketa, 1999). The RHB-amended soils had significantly higher amounts of water-stable macroaggregates with diameters in 5–2, 2–1, 1–0.5 and 0.5–0.25 mm as compared with the control, and accordingly had a lower content of the b 0.25 mm microaggregates. Fig. 1 shows that at 6% content, the RHB has the strongest effect on the formation of aggregates, and that the 5–2 and 0.5–0.25 mm aggregates increased by 87 and 99%, respectively, in comparison with the control. Addition of 2, 4, and 6% RHB reduced the percentage of b0.25 mm microaggregates by 3%, 20%, and 31%, respectively. The CFA significantly affected the formation of 0.25–

COLE ¼ ðLm −Ld Þ=Ld where Lm and Ld are the length of dry and moist soils, respectively. 2.8. Soil strength Tensile strength of soil was determined by using the crushing method. Remolded soil cores were saturated by capillarity for 24 h, equilibrated on a pressure plate apparatus for 12 h, and dried at 105 °C for 2 h. The dried soil cores were placed horizontally between two parallel plates of a digital unconfined compression apparatus (YYW-2, Nanjing Soil Instrument Factory Co. Ltd.), and the pressure was gradually increased through the plates by a motor at a constant speed of 2 mm min−1 approaching the soil core. The maximum reading was recorded before the core was fractured by the load plate.

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

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0.5 mm aggregates (p b 0.05), however, had no significant effect on the formation 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 values of the water-stable aggregates and Le Bissonnais (1996) method. RHB significantly increased the values of MWD and GMD as compared with the control (Fig. 2). There were statistically significant differences in the 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 significantly reduced percentage of aggregate disruption (PAD) indicated that RHB was 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 aggregates are given in Table 2. The MWDFW values were 0.85 and 0.44 mm for 5–2 and 2–1 mm aggregates, respectively, corresponding to the initial heterogeneity in aggregate stability of clayey soil. The RHB- and CFAamended soils showed significantly higher MWDFW values (p b 0.05) than the control, indicating that all treatments increased aggregate resistance to the strong disaggregating energy in the fast wetting test. Similar to the results of the MWDFW test, a certain extent of heterogeneity in the initial MWDSW values of 5–2 and 2–1 mm aggregates was observed. Results showed that both RHB and CFA significantly improved the aggregate breakdown by differential swelling. The most pronounced effect was observed from the 6% RHB and CFA treatments (Table 2). On average, the MWDSW values for 2–1 mm aggregates were increased by 8% for 2%, by 25% for 4%, and by 36% for 6% RHB treatment, respectively. The significant increases in MWDSW values (p b 0.05) by the RHB- and CFA-treatments indicated a significant improvement in the aggregate resistance to mechanical breakdown. 3.3. Effect of RHB and CFA on pore size distribution

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

RHB

CFA

T0 T2 T4 T6 T0 T2 T4 T6

5–2 mm aggregates

2–1 mm aggregates

FW

SW

WS

FW

SW

WS

0.85c 0.83d 0.97b 1.03a 0.85d 0.86c 1.01b 1.04a

0.92c 0.91d 1.07b 1.14a 0.92c 0.86d 1.06b 1.07a

1.11d 1.22c 1.33b 1.44a 1.11c 1.07d 1.15b 1.17a

0.44c 0.44c 0.54b 0.57a 0.44c 0.41d 0.51b 0.54a

0.60d 0.62c 0.701b 0.80a 0.609c 0.55d 0.65b 0.73a

0.59d 0.64c 0.74b 0.80a 0.56d 0.62b 0.61c 0.68a

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

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 the range of investigated pores (Fig. 3b). Three peaks appeared in the pore 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 to the ultramicropores, micropores, and macropores of soils. The CFAamended soils exhibited multi-modal structure with three PSD peaks in the pore size range of 0.5–2 μm, 6–10 μm and 60–90 μm, respectively. The pore volume of the amended soils was characterized by an increased pore volume in the diameter ranges of 0.15–2.5 and 6–45 μm for the RHB-amended soil, and in the diameter ranges of 0.2–2.0 and 6–25 μm for the CFA-amended soils. The small pores having diameters less than 0.2 μm for the RHB- and CFA-amended soils had similar PSD shapes as that of the control, indicating that the CFA and RHB did not affect the small pores (b0.1 μm). The pore structure parameters of RHB- and CFA-amended soils are shown 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

The pore size distribution (PSD) of the RHB- and CFA-amended soils is shown in Fig. 3. The differential PSD curves in Fig. 3 clearly showed that the clayey soils had multi-modal PSDs with representative peaks at 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 of 90 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 and CFA on the mean weight diameter (MWD), geometric mean diameter (GWD) of aggregate, and percentage of aggregate disruption (PAD), in which error bars represent a standard deviation and different letters indicate a significant difference (p b 0.05) between amendment treatments and control.

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

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

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 the control soil. According to the equivalent pore diameter (EPD), the pores could be classified into the following five categories: macropores (75–100 μm), mesopores (30–75 μm), micropores (5–30 μm), ultramicropores (0.1–5 μm) and crytopores (0.1–0.007 μm) (Cameron and Buchan, 2006). The percentage of each pore category for the RHB- and CFA-amended soils is given in Fig. 5. The porosity of the RHB-amended soils was composed of 32% macropores, 27% ultramicropores, and 16% micropores, suggesting that the CFA mainly promoted the formation of ultramicropores. 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 observed to be contributed nearly equally by the macropores, mesopores, micropores and crytopores. In a 6% RHB amended soil, the pore volume of the ultramicropores was significantly higher than that of the control soil. The largest difference in the pore-size between the CFA-amended soil and control soil was often found from the mesopores, micropores, and ultramicropores. The difference in the pore volume between the RHB- 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 RHB was shown to increase the soil's water retention in terms of water 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% higher water-holding capacity, respectively. The observed improvement in the soil's water retention could be attributed to the increased porosity by 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 pore diameter (EPD) classes following the criteria proposed by Cameron and Buchan (2006).

on the contrary reduced the WHC, WP and AW values of the clayey soils. These results disagreed with those of Adriano and Weber (2001) and Chang et al. (1977), who reported that the CFA increased the water-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 increased the plastic and liquid limits, and decreased the plasticity index of the clayey soils. The liquid limit value of the soils was significantly decreased with the addition of CFA and the trend inversed for the plastic limit. The plastic index of the control soil was 17.6%, which was decreased to 11.5% after adding 6% CFA. The consistency limits of the soil are crucial in 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. The control soil had a COLE value of 0.06, which cracked in a dry condition and expanded extensively in a wet condition. Therefore, there were difficulties in managing and tilling the Vertisol. According to the classification of Schafer and Singer (1976), the soil falls into the group of severe shrinkage–swelling hazard rating. The introduction of RHB and CFA significantly decreased the COLE value of the clayey soil (Fig. 8). The COLE value 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 represent a standard deviation and different letters indicate a significant difference (p b 0.05) between amendment treatment and control.

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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 letters indicate a significant difference (p b 0.05) between amendment treatment and control.

6% RHB- and CFA-amended soils, respectively, which was probably because the RHB and CFA changed the swelling–shrinking properties of the clay minerals. Thus, a decrease in the swelling–shrinking properties of the clayey soil would prevent the clayey soils from cracking.

4. Discussion 4.1. Improvement in the aggregate stability

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

The formation and stability of the soil aggregates play an important role in the crop production and soil degradation prevention (Amezketa, 1999). An increase in the formation of macroaggregates by the addition of RHB indicates that the RHB is able to increase the soil aggregation. Addition of a 6% RHB reduced the percentage of microaggregates from 70.9% to 50.4%, suggesting that the macroaggregates were formed by the coalescence of many microaggregates. The carbon introduced by the RHB may act like a glue to cement microaggregates into macroaggregates in which larger pore spaces are present between micro-aggregates. The aggregate stability not only affects the movement of water and air in the soil but also influences the water holding capacity, root penetration, seedling emergence, runoff and erosion. The soil aggregate stability decreases in an order of SWNWSNFW. Table 2 indicates that the slaking of the aggregates is a major mechanism in the aggregate breakdown of clayey soils. Fast wetting leads to more entrapped air and greater differential movement of particles due to swelling (Le Bissonnais, 1996; Zaher et al., 2005). The high clay content increases the extent of differential swelling and the volume of entrapped air in pore space, which further increases the aggregate breakdown. The MWD values of three aggregate stability tests indicated that the RHB and CFA were able to improve the cohesion of soil particles. The RHB increases the aggregates' resistance to slaking and to differential swelling of clays by increasing internal cohesion of the mineral particles through the carbon polymers or the physical enmeshment of the particles. Zaher et al. (2005) reported that large quantities of soil organic carbon could reduce the pore pressure and swelling during rewetting and hence improved the aggregating ability of the soils. Another cause for the increased aggregate stability is that the RHB and CFA increase the soil hydrophobicity, which reduces the extent of the clay swelling and the aggregate

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

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

3.7. Effect of RHB- and CAF on the tensile strength The effect of RHB and CFA on the soil tensile strength (TS) is displayed in Fig. 9. Results showed that the soil tensile strength was significantly decreased (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 was decreased to 353.6 kPa from the initial 936.8 kPa of the clayey soil. In a similar manner, the TS value of the 6% CFA-amended soil was decreased to 651.9 kPa. The above results well agreed with our previous result that the CFA decreased the soil's strength (Lu and Zhu, 2004). The tensile strength of soil clods mainly resulted from the inter-particle bonds within soil clods, which were associated with the properties such as the force to hold water in soil, cohesion, and the internal friction force. The RHB and CFA might be able to weaken the inter-particle bonding strength through a lubricating effect, and as a result decreased the soil's tensile strength.

3.8. Effect of RHB and CAF on the shear strength

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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 significant difference (p b 0.05) between amendment treatment and control.

disruption. An increase in the aggregate stability enhances the soil resistance against the wind and water erosion. 4.2. Improvement of the soil pore structure In Vertisol, the swelling and aggregate slaking cause a reduction in the size of large pores, which decrease the water and air permeability. Several authors (Dink et al., 2013; Razmi and Sepaskhah, 2012; Zaher et al., 2005) explained how the clay dispersion and subsequent plugging of conducting pores by the dispersed clay particles were responsible for a drastic reduction in the hydraulic conductivity. The PSD of clayey soils indicated that clayey soils were featured with larger micropores. These larger micropores enhance soil strength and decrease the available water content. A significant increase (p b 0.05) was detected from the RHB- and CFA-amended soils in the pores with a size range from 100 to 0.005 μm. This increase is likely because the larger pores have a partial function of the RHB and CFA (e.g. a wider pore size distribution). Many authors have demonstrated that the organic waste-derived biochar contained numerous macropores with diameters larger than 10 μm (Brodowski et al., 2005). The CFA exists in a powder form with a granulometry of 0.5–100 μm. As a typical porous media, the CFA is composed of pores having a broad size distribution from nanometers to micrometers. Our previous SEM images indicated that the CFA contained many hole cenospheres in a diameter range of 20–120 μm (Lu and Zhu, 2004). These spherical shape cenospheres in the CFA made larger surface area available for holding water. The inner-hole structure of cenospheres enhanced the porosity of treated soils, and hence reduced the tensile strength. The macropores were more responsible for the cohesiveness of soil particles. Another possible cause was the formation of macroaggregates in the RHB- and CFA-amended soils. The PSD of the amended soil showed that the RHB and CFA did not change the concentration of the textural pore space of soils, which roughly corresponded to the pore space between the clay particles or between intra-aggregate pores. The formation of macroaggregates increased the macropores between microaggregates or between aggregates, which therefore improved the pore structure. The improvement of pore structure in clayey soils also increased the water retention of soil. The water retention is mainly affected by the pore size distribution and porosity. Water holding capacity and field capacity of soil are more related to the large pores, suggesting that the large pores (greater than 0.1 μm in diameter) in the RHB and CFA particles were responsible for water storage. This hypothesis is consistent with the pure biochar that holds water more than ten times its own mass (Kinney et al., 2012). The maximum pore water storage capacity of biochar was reported to be 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 sustainable agriculture. Both RHB and CFA have been proven to improve the soil structure, tensile strength, and swelling characteristics. In the RHB- and CFA-amended soils, the swelling is reduced through two

processes of (i) replacing the swelling clay with non-swelling amendments and (ii) resisting swelling, which depends upon the clay–carbon contact area. An effective contact between the clay and carbon particles would result in a high resistance to swelling. Decreased tensile strength offers greater potential for root growth because the roots can bypass the zones of high mechanical impedance. Soil strength is affected by a number of factors including the properties of the particle surfaces. The reduction of tensile strength in the CFA-amended soils suggested a less cohesiveness of soil particles, which therefore enhances the potential of crop root penetration. The decrease in the TS values of the amended soils is due to the addition of low-plastic materials and the interaction between the clay and carbon particles. Soil mechanical properties are also affected by the soil structure because soil strength is a function of the contact properties between the primary particles and soil compound particles (Amezketa, 1999; Zhang and Hartge, 1995). The hollow structure of CFA with central and well pores may reduce the soil particle cohesions. In the RHBamended soils, majority of carbon is present in the form of clay–carbon complexes and small amount of carbon in the form of a discrete material. The clay–carbon complexes influence the behavior of particles at a colloidal level, which not only causes a fundamental change in the micro-structural level but also affects the type and strength of bonds. The organic particles are stiff when being compressed, act as rigid particles 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 support capacity and has been taken as a measure for the soil erodibility and resistance to seedling emergence and root growth (Hemmat et al., 2010). The soil c value is partially controlled by the properties of the particle surfaces which are in contact or approach contact. After adding RHB, a larger portion of the mineral surfaces are coated by carbon particles, which may block the mineral particles from contacting with each other. Another source for the decrease of the c value with biochar is due to an increase in the soil water repellency. Soil mineral particles are covered by the adsorbed organic molecules with low surface free energy, resulting in a weak attraction between the solid and liquid phases. The decline in c value is partially attributed to the lower surface tension force at the air/water interface between the water films around the soil particles at the high degree of water saturation. 5. Conclusions The RHB is able to significantly enhance the formation of macroaggregates and reduce the fraction of microaggregates less than 0.25 mm, whereas the CFA did not affect the formation of macroaggregate significantly. The RHB and CFA significantly (p b 0.05) increased the stability of the soil aggregates. The percentage of aggregate disruption (PAD) significantly (p b 0.05) decreased with increasing of RHB. The RHB and CFA significantly affect the pore size distribution of clayey soils, but do not significantly affect the soil crytopores (b 0.1 μm). The volume of macropores was found to increase with the content of RHB and CFA because of their rich macropore distribution. Addition of RHB into clayey soil significantly

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(p b 0.05) increased the soil moisture content at water-holding capacity, field capacity, and available water because the RHB increases the porosities of the mesopores and micropores. On contrary, the CFA has no significant effect on the water content of soil at field capacity. Application of 6% CFA reduces the moisture content of soils at water-holding capacity and available water content. Application of RHB and CFA reduced the swelling potential and tensile strength of clayey soils, and consequently resulted in a less energy for breaking soil clod. Therefore, application of the RHB and CFA can effectively solve the problem of clod formation in clayey soil. The results of this work demonstrated that the RHB and CFA are able to significantly increase the aggregate stability, reduce the COLE and tensile strength, and improve several other physical properties. In conclusion, the RHB and CFA are good soil amendment agents for the improvement of the expansive clayey soils. Acknowledgments This research was supported by the National Key Basic Research Support Foundation of China (973) (2011CB100502). References Adriano, D.C., Weber, J.T., 2001. Influence of fly ash on soil physical properties and turfgrass stabilization. J. Environ. Qual. 30, 596–601. Akbulut, S., Arasan, S., Kalkan, E., 2007. Modification of clayey soils using scrap tire rubber and synthetic fibers. Appl. Clay Sci. 38, 23–32. Aksakal, E.L., Angin, I., Oztas, T., 2012. Effects of diatomite on soil physical properties. Catena 88, 1–5. Amezketa, E., 1999. Soil aggregate stability: a review. J. Sustain. Agric. 14, 83–151. ASTM D4318, 1995. Standard test method for liquid limit, plastic limit and plasticity index of soils. Annual Book of ASTM Standards, vol. 04.08. American Society for Testing and Materials. Atkinson, C.J., Fitzgerald, J.D., Hipps, N.A., 2010. Potential mechanisms for achieving agricultural benefits from biochar application to temperate soils: a review. Plant & Soil 337, 1–18. Attom, M.F., Al-Sharif, M.M., 1998. Soil stabilization with burned olive waste. Appl. Clay Sci. 13, 219–230. Bandyopadhyay, K.K., Mohanty, M., Painuli, D.K., Misra, A.K., Hati, K.M., Mandal, K.G., Ghosh, P.K., Chaudhary, R.S., Acharya, C.L., 2003. Influence of tillage practices and nutrient management on crack parameters in a Vertisol of central India. Soil & Tillage Research 71, 133–142. Basma, A.A., Al-Homoud, S.A., Malkavi, H., Al-Bashabshah, M.A., 1996. Swelling–shrinkage behavior of natural expansive clays. Appl. Clay Sci. 11, 211–227. Blissett, R.S., Rowson, N.A., 2012. A review of the multi-component utilisation of coal fly ash. Fuel 97, 1–23. Bravo-Garza, M.R., Bryan, R.B., Voroney, P., 2009. Influence of wetting and drying cycles and maize residue addition on the formation of water stable aggregates in Vertisols. Geoderma 151, 150–156. Brierley, J.A., Stonehouse, H.B., Mermut, A.R., 2011. Vertisolic soils of Canada: genesis, distribution, and classification. Can. J. Soil Sci. 91, 903–916. Brodowski, S., Amelung, W., Haumaier, L., Abetz, C., Zech, W., 2005. Morphological and chemical properties of black carbon in physical soil fractions as revealed by scanning electron microscopy and energy-dispersive X-ray spectroscopy. Geoderma 128, 116–129. Cai, Y., Shi, B., Ng, C.W.W., Tang, C., 2006. Effect of polypropylene fibre and lime admixture on engineering properties of clayey soil. Eng. Geol. 87, 230–240. Cameron, K.C., Buchan, G.D., 2006. Porosity and pore size distribution. In: La, R. (Ed.), Encyclopedia of Soil Science. CRC Press, Boca Raton, FL, pp. 1350–1353. Chang, A.C., Lund, L.J., Page, A.L., Warneke, J.E., 1977. Physical properties of fly ashamended soils. J. Environ. Qual. 6, 267–270. Cook, G.D., So, H.B., Dalal, R.C., 1992. Structural degradation of two Vertisols under continuous cultivation. Soil & Tillage Research 24, 47–64. Dasog, G.S., Acton, D.I., Mermut, A.R., de Jong, E., 1988. Shrink–swell potential and cracking in clay soils of Saskatchewan. Can. J. Soil Sci. 68, 251–260. Dink, T.M., Morgan, C.L.S., McInnes, K.J., Kishné, A.Sz., Daren Harmel, R., 2013. Shrink– swell behavior of soil across a Vertisol catena. J. Hydrol. 476, 352–359.

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