ResultsandDiscussion
Bulk density
Biochar had a statistically significant influence on the bulk density of coarse sand after amendment (p<0.05; Table 1). This decrease in bulk density following biochar incorporation has also been observed in other studies (e.g., Mukherjee et al., 2014; Pathan et al., 2003; Laird et al., 2010) and is expected due to the lower particle density of the biochar materials compared to soils (Laird et al., 2010; Brewer et al., 2014; Rogovska et al., 2014). The bulk density of the 6 to 13 mm non-spherical bead/sand mixture decreased similarly to the sand/biochar mixture; the 2.5 mm non-spherical bead/sand mixture is not different (Table 2). In contrast, the sand amended with spherical beads had a higher bulk density than that of the non-spherical bead/sand mixtures, likely linked to a possible improved packing and geometric packing arrangement.
Hydraulic Conductivity
The Ksat of coarse sand amended with spherical beads were significantly affected (p<0.05) as compared to the Ksat of coarse sand (Fig. 3). Increasing the size of spherical beads in the sand from 1.0 to 15 mm decreased the Ksat. For example, the Ksat values of coarse sand added with 1 mm and 15 mm spherical beads were decreased to 206, and 112 mm/h, respectively as compared to 229 mm/h for sand. Those reasons might be attributed to greater tortuosity and increased length of the water pathway following amendment of larger sized beads to sand as compared to the unamended sand, for which >85% of the particles were <1.0 mm, with a median size of 0.5 mm (McKeague et al., 1982; Kameyama et al., 2012). It should be noted for the case of 0.1 mm beads also reduced Ksat, possibly as the result of clogging existing pores. Keren et al. (1980) attributed the reduction in hydraulic conductivity of soils following gypsum additions was due to small gypsum particles mechanical plugging existing pores.
The Ksat was also significantly affected by nonspherical bead and its particle size representing what could be affected by particle shape. Those Ksat value showed similar patterns with the Ksat of biochars in the condition of 3 and 6 mm, corresponding intermediate sizes of from 2-4 mm and 4-8 mm, indicating do not following the Ksat curve of the spherical bead added into coarse sand. It might be attributed to similar non-spherical shapes as shown Fig. 1 and Fig. 2. It could be also inferred that it does not well match with those methods such as Campbell (1985), and Smettem and Bristow (1999), and Saxton et al. (1986) models in predicting of Ksat, presumably because application of biochar and the shape of biochar play an important role in predicting and modeling of Ksat.
In the soil treated with biochar, there are two possible theoretical pathways in water flow through soil profiles (Barnes et al., 2014). One is water migration through the pores within the biochar, the other is water migration through external space between sands or biochar-sand mixtures.
Firstly, there are potential water pathways through the pores within the biochar particle. The size distribution of pores of biochar appear a different range of from sub-nano (<1 nm) to macro (>50 nm), and their ability are well known for improving water holding capacity (Yu et al., 2006; Atkinson et al., 2010; Joseph et al., 2010). From soil capillary forces, a given height of water rise in a capillary column can be related to the pore radius by the following equation:
Where is the height of rise in the capillary column (pore, m), γ is the surface tension of water [@25℃=71.97 kg/sec2], θcontact is the contact angle (assumed=0° rad), g is the acceleration due to gravity (9.8 m/sec2), pwater is the density of water (999.97 kg/m3), and r is the radius of the pore (m) (Hillel, 1998). Therefore, the largest pore that will be holding water at a soil moisture potential at the wilting point (-1500 kPa) is 0.2 μm. In other words, soil pores <200 nm are not of agronomic significance, since this soil moisture will not be plant or microbe available, as well as not contributing significantly to saturated water flow.
According to pore classification in relation to pore function, it is mentioned that pore sizes of between 0.005 and 0.5 μm are recognized as retention and diffusion ions in solutions as residual pores, and pore sizes of between 0.5 and 50 μm are capable of holding of water against gravity and release as storage pores, and pore sizes above 50 μm play a drainage in excess water as transmission pores (Lal and Shukla, 2004). In other words, only pore sizes above 50 μm play a critical role for water transport.
Gray et al. (2014) found that the sizes of biochar’s macropore were centered in the low micrometer range and Shaaban et al. (2013) reported average pore diameter for rubber wood sawdust biochar treated at 300, 500, and 700℃ was 7, 13, and 7 nm, respectively. All these median pore sizes are significantly below those pores theoretically available at the soil wilting point. Besides, Barnes et al. (2014) mentioned water pathway within biochar has greater tortuosity and the restriction of water flow due to the size of the smallest pore as well as the lack of interconnectivity. These facts justify the lack of significant impacts of biochar’s intra-particle pores in water transport.
Secondly, there is also water pathway through external spaces between sands or biochar-sand mixtures. The size of external pores counts on particle size, particle morphology, and compaction (Juang and Holtz, 1985; Gray et al., 2014). Bigelow et al. (2004) found that the Ksat values in coarse sand increased 6 times due to the higher presence of macro-porosity in the coarse sand (0.347 cm3/cm3) compared to fine sand (0.182 cm3/cm3), although the total porosity in fine sand, was higher in the fine sand (0.45 cm3/cm3 compared to 0.38 cm3/cm3). This result highlights the importance of pore size in regards to controlling Ksat. For example, according to Jong et al. (2007), the required time for water to move 30 mm through a channel with a 5 cm water head water was 200 sec for a 200 μm channel compared to 1,400 sec for a channel of 50 μm of diameter. Thereby, Ksat depends on the presence and proportion of macropores (50 μm) existing within sands or biochar-sand and meanwhile the water pathway through internal pores of biochar was largely restricted or had a little impact on the Ksat values.
Particle shape was also important parameter in ground-water flow such as hydraulic conductivity (Coelho et al., 1997). Sperry and Peirce (1995) reported porous media consisted of irregular particles showed lower hydraulic conductivity for the larger (700 to 840 μm) particles, though particle shape had no observable influence for the smaller (150 to 180 μm) particles on hydraulic conductivity. Those results could be extrapolated on the basis of two things. The first thing is that non-spherical bead might put in denser configurations than spherical bead, creating smaller pore passage sizes and greater tortuosity. The other thing is when beads pour into a column, the shape the bead affects the angle of repose. For example, Friedman and Robinson (2002) found while the minimum and maximum angle of glass beads were 22.1 and 23.1 degrees respectively, those of soil grains were between 34 and 37 degrees. In other word, the angle of repose is influenced by the shape and roughness of the particlesand is greater for non-spherical particles, which makes the Ksat increase (Sperry and Peirce 1995; Yun et al., 2005).
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