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Linking hydrophobicity of biochar to the water repellency and water holding capacity of biochar-amended soil
Environmental Pollution 253 (2019) 779e789
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Linking hydrophobicity of biochar to the water repellency and water holding capacity of biochar-amended soil* Jiefei Mao a, b, c, d, e, Kun Zhang a, b, Baoliang Chen a, b, * a
Department of Environmental Science, Zhejiang University, Hangzhou, 310058, China Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou, 310058, China c State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, 830011, China d Research Center for Ecology and Environment of Central Asia, Chinese Academy of Sciences, Urumqi, 830011, China e University of Chinese Academy of Sciences, Beijing, 100049, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 April 2019 Received in revised form 6 July 2019 Accepted 10 July 2019 Available online 11 July 2019
Biochar addition to soil may change the hydrophobicity of amended soil and influence soil hydraulic properties. Soil hydrophobicity, i.e. soil water repellency (SWR) can interrupt water infiltration and form preferential flow leading to a potential risk of soil erosion or groundwater pollution. Up to date, the effect of different biochars on soil hydrophobicity remains unclear and the association of SWR with soil hydraulic properties is still unknown. To link the biochar hydrophobicity to SWR and soil water holding capacity (WHC), the surface structure and chemical composition of 27 biochars with different feedstocks and pyrolysis temperatures were characterized, and the SWR and soil WHC of biochar-added soil were investigated. Carboxylic groups on the biochar surface, surface area and pore volume were mostly influenced by pyrolysis temperature, which suggested the dominant factor determining the severity of biochar hydrophobicity was pyrolysis temperature. Hydrophilic soil became hydrophobic after biochar amendment. A higher addition rate led to a stronger SWR of hydrophilic soil. Biochar addition increased soil WHC of hydrophilic soil with low total organic carbon (TOC) content. Biochar did not have significant influence on SWR and soil WHC of hydrophobic soil with high TOC content. It implied that the influence of biochar on SWR and soil hydraulic properties mainly depended on soil original hydrophobicity and TOC content. Therefore, the properties of biochar and influence on soil hydrophobicity and hydraulic properties should be considered before processing biochar application. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Biochar hydrophobicity Soil water repellency Soil hydraulic properties Pyrolysis temperature Surface properties
1. Introduction The application of biochar is a common soil amendment that improves soil fertility, positively affects agricultural production and even potentially mitigates climate change by soil carbon sequestration (Lehmann et al., 2006; Woolf et al., 2010). Biochar is obtained from biomass pyrolysis under oxygen-limited conditions (Chen et al., 2008; Lehmann, 2007; Lehmann and Joseph, 2009). The alkyl functional groups on the surface of biochar correlate to its hydrophobicity (Kinney et al., 2012). Biochar produced at low temperature contains more aliphatic compounds in the biochar pores that increase the hydrophobicity (Chen et al., 2008; Gray
* This paper has been recommended for acceptance by Dr. Yong Sik Ok. * Corresponding author. Department of Environmental Science, Zhejiang University, Hangzhou, 310058, China. E-mail address: [email protected] (B. Chen).
https://doi.org/10.1016/j.envpol.2019.07.051 0269-7491/© 2019 Elsevier Ltd. All rights reserved.
et al., 2014), while high temperature pyrolysis allows a much smaller number of aliphatic compounds remaining in the pores (Chen et al., 2008). The addition of hydrophobic biochar can turn hydrophilic soil into water repellent. With the ability to interrupt water infiltration, the existence of soil water repellency (SWR) in soil potentially increases the risk of soil erosion (Bisdom et al., 1993; Dekker and Ritsema, 2000; Mao et al., 2016). The distribution of hydrophobic parts in water-repellent soil is heterogeneous, which forces water to flow in perfectional pathways while hydrophobic zones are mostly excluded from flow. The occurrence of preferential flow would easily transport contaminants from top to deep soil and even into the underlying seepage and groundwater (Miyata et al., 2007; Hamlett et al., 2011). Kinney et al. (2012) observed that soil mixed with a biochar pyrolyzed at 300 C showed water repellency while the same soil mixed with a biochar pyrolyzed at 500 C was wettable. Compared to pyrolysis techniques, Ojeda et al. (2015)
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demonstrated that feedstock of biochar is more important to influence soil functions, e.g., soil wettability and soil water retention. Addition of hydrophobic biochar to soil could change soil hydrophobicity that influences soil hydraulic properties (Jeffery et al., 2015). In contrast, Abel et al. (2013) did not observe any increase of water repellency after addition of biochar to soil that increased water content by increasing the total pore volume. The influence of biochar addition on the severity of SWR mainly depends on the properties of biochar determined by its feedstock and pyrolyzed temperatures (Briggs et al., 2012; Gray et al., 2014; Kinney et al., 2012). Therefore, biochar hydrophobicity should be considered for biochar application to soil amendment. The effects of biochar on soil hydrophobicity concerning physical structure change of soil have been discussed. However, regarding to chemical properties of soil and characteristics of biochar, the effects of biochar on soil hydrophobicity remain unclear. Research concerning the influence of biochar addition on soil hydraulic properties have been widely conducted (Castellini et al., 2015). Addition of biochar produced at high temperature can increase soil moisture retention (Ajayi et al., 2016). The soil hydraulic conductivity decreased with increasing addition ratio of biochar to soil (Liu et al., 2016; Trifunovic et al., 2018). High amounts of biochar added to soil affected soil wettability that influenced soil water retention (Ojeda et al., 2015). One common reason to explain changes of soil hydraulic properties is the total porosity change after adding biochar. As seen in Villagra-Mendoza and Horn (2018), pore size distribution changed in sandy soil by the addition of biochar that increased meso porosity to improve hydraulic condition. However, up to now the direct influence of biochar hydrophobicity on soil hydraulic properties is rarely studied. The possible association of surface structure and chemical composition altered under different pyrolysis conditions and feedstock with the hydrophobicity of biochar is also not well known. In this study, we aim to discuss the difference of surface
structure and chemical composition of biochar produced at different temperatures (250, 500 and 700 C) that derived from various sources including plants and manures, to investigate the relationship between both surface structure and chemical composition and hydrophobicity of biochar, and to explore the direct influence of biochar on SWR and one of the soil hydraulic properties (i.e. the soil water holding capacity (WHC)) of biochar-amended soil with respect to biochar hydrophobicity. 2. Materials and methods 2.1. Biochar preparation, soil sampling and soil-biochar mixture Six plant tissues (pine needle, pine wood, rice straw, rice bran, wood sawdust, and orange peel) and three animal manures (fowl manure, dairy manure and pig manure) were selected as the source of biochar. Prior to pyrolysis, all the feedstocks were passed through a sieve with 0.15 mm diameter, filled and compressed in the crucibles covered by lid. The volumes of crucibles were from 100 to 300 mL depending on the density of the dried feedstock. Each sample was put in the muffle furnace under limited oxygen and pyrolyzed at 250, 500 and 700 C for six hours. The products were passed through a sieve with 0.15 mm diameter and named referring to their feedstock and pyrolysis temperature (Table 1). Three types of soils were selected and named according to their colours as red soil (Ferralsol), yellow soil (Regosol) and black soil (Phaeozems) (FAO, 2015). The red, yellow and black soils were collected from Guangxi, Shanxi and Liaoning Provinces, respectively. All three soils were air-dried at room temperature and passed a 100 mesh-sized sieve. The pH of red, yellow and black soil was 5.095, 7.735 and 5.005, respectively, which was determined by a 1:2.5 (w/w) soil to water ratio. The red soil was composed of 5.66% sand, 81.292% silt and 13.05% clay. The yellow soil was composed of 17.13% sand, 79.41% silt and 3.47% clay. The black soil was composed
Table 1 Properties of biochar pyrolyzed at different temperatures. Sample name
Feedstock type
Temperature, C
TOC content, %
Ash content, %
Surface area (N2), m2/g
Surface area (CO2), m2/g
Mesopore volume, cm3/g
Micropore volume, cm3/g
PN250 PN500 PN700 PW250 PW500 PW700 RS250 RS500 RS700 RB250 RB500 RB700 WB250 WB500 WB700 OP250 OP500 OP700 FM250 FM500 FM700 DM250 DM500 DM700 PM250 PM500 PM700
Pine needle Pine needle Pine needle Pinewood Pinewood Pinewood Rice straw Rice straw Rice straw Rice bran Rice bran Rice bran Wood sawdust Wood sawdust Wood sawdust Orange peel Orange peel Orange peel Fowl manure Fowl manure Fowl manure Dairy manure Dairy manure Dairy manure Pig manure Pig manure Pig manure
250 500 700 250 500 700 250 500 700 250 500 700 250 500 700 250 500 700 250 500 700 250 500 700 250 500 700
57.96 76.47 83.63 53.26 79.40 81.60 39.14 40.53 37.59 48.94 50.11 50.32 55.28 70.80 70.27 60.14 73.51 74.14 27.84 22.37 19.20 37.30 33.07 39.30 38.93 36.29 32.89
2.03 4.74 5.52 0.09 1.35 1.62 27.71 41.20 47.90 15.29 31.07 36.04 4.02 5.78 10.06 0.16 1.94 1.27 47.81 67.53 73.33 32.60 51.95 46.02 25.37 46.40 54.61
1.69 20.23 5.92 3.35 263.18 425.11 4.17 4.10 125.05 1.69 67.24 282.66 2.45 279.75 443.85 0.96 6.59 185.41 2.90 26.68 69.94 1.49 33.91 173.00 3.02 16.13 122.63
17.79 98.20 134.15 21.04 134.41 175.47 29.50 71.24 79.83 30.97 94.50 121.85 47.52 130.38 167.30 33.77 118.35 159.21 17.70 42.34 40.59 21.84 59.95 92.77 13.14 57.72 73.83
0.0023 0.0072 0.0022 0.0059 0.0000 0.0787 0.0120 0.0093 0.0387 0.0057 0.0175 0.0465 0.0055 0.0044 0.0700 0.0016 0.0025 0.0160 0.0093 0.0224 0.0496 0.0046 0.0196 0.0312 0.0056 0.0129 0.0428
0.0018 0.0090 0.0152 0.0019 0.0121 0.0183 0.0035 0.0113 0.0103 0.0000 0.0097 0.0135 0.0046 0.0141 0.0159 0.0026 0.0153 0.0167 0.0021 0.0061 0.0053 0.0021 0.0106 0.0127 0.0017 0.0078 0.0108
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of 47.06% sand, 51.13% silt and 1.82% clay. Accordingly, the texture of red, yellow and black soil was silt loam. The biochar was mixed with soil at 1% (low) and 5% (high) (w/w) addition rates. The mixing process followed the steps that were first to put the soil into a steel dish to make a thin soil layer, then to add biochar evenly on the soil layer and at last gently and well mix them using glass rod. All the samples were used for contact angle and soil WHC measurements. 2.2. Total organic carbon and ash content Total organic carbon content (TOC) of biochar and soil was determined by the subtraction of total carbon content and inorganic carbon content. The total carbon content and total inorganic carbon content of biochar was measured using TOC-V CPH total organic carbon analyser (SSM-5000A, Shimazdzu). The ash of biochar was the residual of biochar pyrolyzed at 800 C in a muffle furnace for four hours.
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feedstock, 2) soil types and biochar with different pyrolysis temperatures, 3) soil types and rates of biochar addition. A two-way ANOVA was used to test the significance of interaction of factors in each scenario. Followed by interaction test, Tukey test or Dunnett T3 test was selected to interpret the significant differences between biochar treatments. The differences in contact angle of biochar and the WHC of biochar-added soil were compared by Duncan’s Multiple Range test within the analysis of variance (ANOVA). Principle component analysis (PCA) was applied by converting correlated properties of biochar into uncorrelated components to explore possible link between factors concerning surface structure and chemical properties and the severity of water repellency of biochar. Linear regression was applied to test the linear relation between properties of biochar and hydrophobicity of biochar. 3. Results and discussion 3.1. Basic properties of biochar
2.3. Characterization of biochar The surface functional groups of biochar were analysed using a Fourier transform infrared (FTIR) spectrometer (Nicolet 6700, Thermo Scientific). The range of FTIR spectra was recorded from 400 to 4000 cm1 with a resolution of 1 cm1. The ratios of peak area of functional groups of biochar were used to investigate the correlation of functional groups with the biochar surface area and the level of biochar hydrophobicity. The surface area and pore volume of biochar were measured with the gas adsorptiondesorption method using surface area and pore size analyser (NOVA200e, Quantachrome Instruments). Nitrogen and carbon dioxide gas were applied individually on multipoint BET analysis to measure the surface area of biochar, where mesopore and micropore volume of biochar were analysed by N2 and CO2 gas application, respectively. 2.4. Water repellency and water holding capacity measurement Contact angle method was applied to assess the severity of water repellency of biochar and biochar-added soil. In this method the angle of a water droplet and the material surface is measured, of which the surface is considered as hydrophobic when the initial contact angle is larger than 90 (Wessel, 1988). The biochar and soil were stuck as a thin and flat layer on a glass slide with double-sided adhesive tape (5.5 cm 1.8 cm) (Bachmann et al., 2000). The contact angle measurement was duplicated and four distilled water droplets were placed on each slide. Soil WHC was measured using a volumetric method adapted from Yargicoglu et al. (2015). A known amount of air-dried soil was put in a funnel connected with a tube with a clip, while enough distilled water was added to completely submerge the soil and left for 30 min to saturate the soil. The clips on the tubes were opened and water was drained from each funnel for 30 min that was collected. Both adding and draining water was weighed to get the weight of the water held in soil for soil WHC calculation. Soil WHC experiment was duplicated. 2.5. Statistics analysis The correlations between surface structure and chemical properties and hydrophobicity of biochar were obtained by Pearson correlation and Spearman correlation (IBM SPSS statistics 20). The former method is used for the data following a normal distribution while the latter method is for the data non-normally distributed. During the hydrophobicity analysis of biochar-treated soil, three scenarios including 1) soil types and biochar with different
3.1.1. TOC content General properties of biochar changed with pyrolysis temperature and feedstock (Table 1). For the biochar with different feedstocks and pyrolyzed at the same temperature, the TOC of the biochar from manures was the lowest. The biochar of rice straw was slightly higher than the biochar of manures but much lower than other biochar from plant biomass. For biochar pyrolyzed at 250 C, the biochar of orange peel had the highest TOC (60.14%), while for the biochar pyrolyzed at 500 C and 700 C, the TOC of biochar of pine wood (79.40%) and pine needle (83.63%) was the highest, respectively. With the same feedstock, when pyrolysis temperature changed, the TOC of the biochar originated from plant biomass (pine needle, pine wood, rice straw, wood sawdust and orange peel) increased with pyrolyzed temperature. By contrast, the TOC of the biochar from fowl and pig manures decreased while pyrolyzed temperature increased, being in line with the findings in Song et al. (2019). It can be explained that for feedstock the organic content of manure was much lower than plants, during pyrolysis process, then carbon was condensed in plant-derived biochar while cellulose and lignin containing H and O were pyrolyzed. However, for manurederived biochar, remained organic matter released as volatile compounds declined the TOC of those biochars. Among these 27 biochars, PN700 and FM700 had the highest (83.63%) and the lowest (19.20%) TOC, respectively. The TOC contents of plantderived biochar were higher than the biochar with manure origin. 3.1.2. Ash content Similar to the TOC, the ash contents of biochar showed difference resulting of different feedstocks and pyrolysis temperatures (Table 1). Compared to the other biochar with the same pyrolysis temperature and different feedstocks, the ash content of the biochar of fowl manure was the highest, while the biochar of pine wood and orange peel had the lowest. For the biochar originated from plants with the same pyrolysis temperature, the ash contents of rice (straw and bran) were obviously higher than other biochars. For biochars had the same feedstock but different pyrolysis temperatures, rather than the biochar of orange peel and dairy manure, the ash content of other biochar increased with increased temperature. Fowl biochar FM700 (73.33%) and pine wood biochar PW250 (0.09%) had the highest and lowest ash contents, respectively. Differ from TOC content, the ash contents of manure-derived biochar were much higher than other plant-derived biochar. 3.1.3. Surface area and pore distribution According to N2 gas adsorption analysis, when pyrolysis temperature rose, the surface area of most of the biochar from the same
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feedstock increased. For the biochar at 250 C, the biochar of rice straw and orange peel had the highest (4.17 m2/g) and lowest (0.96 m2/g) surface area, respectively. The lowest surface area of the biochar at 500 and 700 C were rice straw (4.10 m2/g) and bran (5.92 m2/g), respectively. The biochar of wood sawdust had the highest surface area at both 500 and 700 C. The surface area measured based on the CO2 gas adsorption presented was different from N2 gas adsorption analysis. The biochar of wood sawdust and pig manure showed the highest (47.52 m2/g) and lowest (13.14m2/ g) surface area for the biochar at 250 C, respectively. The biochar of pinewood and fowl manure at both 500 and 700 C presented the highest and lowest surface area, respectively. Besides surface area, pore volume is also an important property of biochar. Mesopore with a diameter ranged from 2 to 50 nm and the micropore of biochar with a diameter smaller than 2 nm. The same as surface area, for the most of biochar having the same feedstock, the volumes of both mesopore and micropore increased with increasing pyrolysis temperature. For the biochar at 250 C, the biochar of rice straw had the highest meso- and micropore volume. The biochar of orange peel had the lowest mesopore volume while none of micropore was found in the biochar of rice bran. For the biochar at 500 C, the biochar of dairy manure had the highest mesopore and lowest micropore volume. None of the mesopore was found in the biochar of pinewood that was the lowest while the biochar having
the highest micropore volume was derived from orange peel. The biochar of pinewood had the highest meso- and micropore volume for the biochar at 700 C. The biochar of pine needle and fowl manure at 700 C had the lowest mesopore and micropore volume, respectively.
3.2. FTIR data According to the FTIR spectra of 27 biochar, low-temperature pyrolyzed biochar had more functional groups on the surface than higher temperature pyrolyzed biochar (Fig. 1). The C]O for ester (1700-1710cm1) and C]C in aromatic ring (1600-1615cm1) were observed at the surface of biochar pyrolyzed at each temperature. Rather than the biochar at 700 C, the aliphatic CH2 (2920, 2858, 1450, 1400, and 1380cm1) were obviously observed in the biochar produced at 250 C and 500 C. Meanwhile, only the biochar produced at 250 C presented C]C ring stretching vibration of lignin (1515cm1) and C]O in carboxylic acid (1318cm-1). However, those two products were not in the biochar produced at higher temperatures. The CeOeC (1159-1175nm1) peaks on the biochar surface were commonly found in plant-derived biochar, of which for manure-derived biochar, only PM250 showed this product.
Fig. 1. FTIR spectra of nine biochar produced at three different temperatures (250, 500 and 700 C).
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3.3. Water repellency of biochar 3.3.1. Hydrophobicity of biochar The hydrophobicity of biochar derived from different feedstock was distinguishable (Fig. 2). Larger initial contact angle suggests stronger water repellency. Compared to other biochar, at each pyrolysis temperature, the biochar of pinewood had the lowest hydrophobicity. The biochar at 250 C, the most water repellent biochar was manure-derived DM250 and PM250. The hydrophobicity of the biochar of wood sawdust and orange peel was the highest at 500 and 700 C, respectively. The biochar of pine needle and wood sawdust showed insignificant difference of hydrophobicity with different temperature. For pine wood and rice bran the difference of hydrophobicity between biochar at 250 and 500 C was insignificant. For orange peel, fowl manure and pig manure, the water repellency of biochar at 500 C and 700 C were also insignificantly differed. For the biochar of rice straw, the initial contact angle of RS700 was similar to RS250 but significantly higher than RS500. Even though the insignificant difference of hydrophobicity of biochar was found, the average value of initial contact angle of biochar still significantly decreased with increasing pyrolysis temperature (p ¼ 0.002) (Fig. 3). In short, the most of biochar followed the tendency that for the biochar with the same feedstock, the water repellency of biochar produced at high temperature was lower than that produced at low temperature. 3.3.2. Correlation of factors with hydrophobicity of biochar Both TOC content (p ¼ 0.478) and ash content (p ¼ 0.662) did not correlate to the water repellency of biochar significantly. With the same feedstock, the TOC content of biochar did not have any
Fig. 3. Average contact angle of biochar significantly decreased with increasing pyrolysis temperature. Small letters show the significance of average contact angle of the biochar produced at different temperatures.
significant correlation with their hydrophobicity (Fig. 4a). Oppositely, the hydrophobicity of most biochar decreased with increasing ash content of biochar with the same feedstock (Fig. 4b). Both surface area measured using N2 (r ¼ 0.565, p ¼ 0.002, n ¼ 27) and CO2 (r ¼ 0.494, p ¼ 0.009, n ¼ 27) had a negative correlation with biochar hydrophobicity (Fig. 5a and b). Regarding to pore volume, mesopore (r ¼ 0.496, p ¼ 0.009, n ¼ 27) and micropore (r ¼ 0.492, p ¼ 0.009, n ¼ 27) also significantly
Fig. 2. The initial contact angle of nine biochar produced at different temperatures. Small letters show the significance of contact angle of the biochar produced at different temperatures.
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Fig. 4. The scatter plot of relationship between TOC content (a, p ¼ 0.478, n ¼ 27) and ash content (b, p ¼ 0.662, n ¼ 27) with initial contact angle of biochar, each colour and mark refer to one kind of feedstock. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
correlated to water repellency of biochar (Fig. 5c and d), respectively. Here instead of absolute peak area of each functional group of biochar, the ratio of peak area of every two functional groups meaning the value of the peak area of one functional group divided by the peak area of another functional group was used to compare the relative abundance of functional groups between different biochar. The correlation of the ratio of peak area of every two functional groups of biochar with water repellency of biochar was also investigated. Most of the ratios of peak area did not have significant correlation with hydrophobicity of biochar. However, the ratio of summed aliphatic CH2/C]C ring stretching vibration of lignin (r ¼ 0.667, p ¼ 0.050, n ¼ 27), the ratio of summed aliphatic
CH2/C]O in carboxylic acid (r ¼ 0.783, p ¼ 0.013, n ¼ 27), the ratio of C]O for ester/C]O in carboxylic acid (r ¼ 0.667, p ¼ 0.050, n ¼ 27) and the ratio of C]C stretching in the aromatic ring/C]O in carboxylic acid (r ¼ 0.883, p ¼ 0.002, n ¼ 27) was correlated to the water repellency of biochar. Where, the ratio of CH2/C]C ring stretching vibration of lignin meant the ratio between aliphatic part and aromatic part of biochar surface, more aliphatic functional groups led to a higher biochar hydrophobicity. The positive correlations between the other ratios and biochar hydrophobicity demonstrated that carboxylic acids were more hydrophilic than CH2, ester and aromatic ring, which caused a lower hydrophobicity of biochar. To better identify the characteristic of factors influencing
Fig. 5. The correlation of different factors with water repellency of biochar: surface area measured using N2 (a, r ¼ 0.565, p ¼ 0.002, n ¼ 27) and CO2 (b, r ¼ 0.494, p ¼ 0.009, n ¼ 27), mesopore volume (c, r ¼ 0.496, p ¼ 0.009, n ¼ 27) and micropore volume (d, r ¼ 0.492, p ¼ 0.009, n ¼ 27).
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hydrophobicity of biochar, principal component analysis (PCA) was used. The Kaiser-Meyer-Olkin measure of sampling adequacy was 0.70 and the significance of Bartlett's Test of sphericity was 0.00, suggesting PCA analysis is applicable in this study. The communality values of all properties of biochar was larger than 0.50 (Table S1). The first, second and third principal component (eigenvalue > 1) explained ca. 58.0%, 22.4% and 11.7% of the variance in the dataset of the properties of biochar (Table S2), respectively. The high factor loadings of properties mostly shown at the first two components; therefore, Fig. 6 plotted the factor loadings of properties at component 1 and 2. The factor loading of surface area and pore volume and four functional group ratios was high positive and high negative on component 1, respectively (Fig. 6). All these properties had significant correlations with the level of biochar hydrophobicity, while the correlation of those three functional groups was positive but the surface area and pore volumes had negative correlations. The factor loading of TOC and ash content was high positive and high negative on component 2, respectively. Both surface area measured by N2 and the volume of mesopore had high positive factor loadings at component 3. Based on the linear regression analysis (Table S5), among all the properties of biochar, except surface area measured by N2 and mesopore volume, the others were not linearly related to biochar hydrophobicity. The PCA scatter plot shows a clear separation of all properties. The factors with high loading on component 1 changed mainly depending on pyrolysis temperature. TOC and ash content of biochar with high loading on component 2 were mainly determined by feedstock of biomass.
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separated according to soil types and biochar addition rates (Fig. 7). After adding biochar, red and yellow soil remained hydrophilic as the contact angles were still smaller than 90 . The 1% biochar addition did not increase soil hydrophobicity of red soil but significantly increased hydrophobicity of yellow soil (Fig. 7). Where 5% biochar addition significantly increased the soil hydrophobicity
3.4. Hydrophobicity of biochar-amended soil The TOC content of black soil (22.58%) was much higher than the red (0.14%) and yellow soil (0.78%). Based on a two-ANOVA, soil types and rates of biochar addition had a significant interaction when influencing water repellency of biochar-amended soil (Tables S3 and S4). Therefore, during the analysis of soil hydrophobicity, initial contact angles of biochar-added soil were
Fig. 6. Principle Component analysis (PCA) scatter of the factor loading of surface structure and chemical composition of biochar at component 1 and 2. Here SAN2 and SACO2 present surface area (based on N2 and CO2), micro- and mesopore mean the pore volumes, four functional group ratios from up to down were aliphatic CH2/C]O in carboxylic acid, C]C stretching in the aromatic ring/C]O in carboxylic acid, C]O for ester/C]O in carboxylic acid and aliphatic CH2/C]C ring starching vibration of lignin, respectively.
Fig. 7. The comparison between hydrophobicity of three types of soil before and after biochar addition with low (1%) and high (5%) rates. Small letters show the significance of average initial contact angle of soil.
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of both red yellow soils. Oppositely, soil hydrophobicity of black soil did not significantly change after 5% biochar addition but decreased at 1% biochar addition. The addition of biochar significantly changed the SWR depended on both biochar rates and soil properties. General, the red soil added biochar at low temperature had the lower SWR than the soil added biochar at higher temperature. As depicted in Fig. S1, the red soil added with 1% RB500 showed the highest SWR, while the addition of WB250 and DM700 in red soil presented the lowest SWR. However, for 1% biochar-added yellow soil, RS250- and RB500-added soil had the highest and lowest hydrophobicity, respectively. For both red and yellow soil with a 5% biochar addition rate, the soil added with WB500 and WB700 had the highest SWR while the soil added with DM700 had the lowest SWR. For the black soil, adding PN250 and OP500 at 1% rate lead the soil to be the most and least hydrophobic, respectively. However, except that two biochar the other biochar had a similar effect on the water repellency of black soil. When adding 5% biochar into black soil, the soil containing PN250 and PN500 had the highest SWR while the soil having RB700 showed the lowest. 3.5. Soil water holding capacity To link SWR to soil hydraulic properties, biochar-added soil with the highest and lowest SWR were selected to identify their soil water holding capacity (Fig. 8). The order of increment of soil water holding capacity by addition of WB700 to soil followed yellow soil (29.5%) > red soil (25.6%) > black soil (0.8%), while the order for addition of DM700 was the same: yellow soil (59.7%) > red soil (11.2%) > black soil (3.3%). Biochar addition significantly increased the soil WHC of red and yellow soil. The addition of WB700 increased relatively higher WR than adding DM700 into both red and yellow soil; however, the soil WHC of WB700-added soil was higher than DM700-added red soil. In contrast, adding DM700 to yellow soil lead to higher soil WHC than adding WB700. The
addition of biochar to black soil did not show any significant influence on soil WHC even from different biochar. 3.6. Properties of biochar changing with feedstock and pyrolysis temperature The biochar derived from fowl manure had the lowest TOC content, which was logical that manure is the residual part after digestion containing less carbon and nutrients than original biomass. For plant-derived biochar, the biochar originated from pine needle/wood and wood sawdust had a higher TOC content than the biochar from rice bran/straw, suggesting that the biochar derived from woody plants generally have a higher TOC content than grass-derived biochar (Chiou et al., 2015; Wang et al., 2018). In contrast to TOC content, the ash content of biochar depended on the feedstock that leads to a much high ash content in manurederived biochar rather than in plant-derived biochar (Whitely et al., 2006). However, the biochar from rice (e.g. bran and straw) also had a relatively high ash content, which is due to the high silicon content in rice tissues, especially in rice straw (Xu and Chen, 2013). The majority of biochar followed the tendency that the ash content increased with increasing pyrolysis temperature, demonstrating a condensation process during the pyrolysis (Xu and Chen, 2013). In agreement with previous studies (Chen et al., 2008; Zhang et al., 2018), both the surface area measured using N2 and CO2 gas, the biochar produced at high temperature had a higher surface area than the biochar produced at low temperature. The reason is that a large part of organic matter pyrolyzed and turned into CO2 during pyrolysis process that creates pores on biochar. The removal of organic matter and minerals from pores explained the reason that the micropore volume of the most biochar increased with € tter and Marschner, 2015). Gray increasing temperatures (Heitko et al. (2014) observed a dramatic increment in the pore volume of biochar when pyrolysis temperature increases. For the biochar
Fig. 8. Soil water holding capacity of soil with and without adding 5% biochar. Small letters showed the significance between the soil WHC of each sample for the same soil. WB7R and DM7R represented the biochar of wood sawdust and dairy manure pyrolyzed at 700 C added to red soil, respectively; WB7Y and DM7Y represented the biochar of wood sawdust and dairy manure pyrolyzed at 700 C added to yellow soil, respectively; WB7B, DM7B and RB7B represented the biochar of wood sawdust, dairy manure and rice bran pyrolyzed at 700 C added to black soil; PN2B represented the biochar of pine needle pyrolyzed at 250 C. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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produced at the same temperature, the major difference of surface structure of biochar including surface area and pore structure was mainly determined by the feedstock of biochar (Kinney et al., 2012; Qiu et al., 2017). It was commonly found in previous studies that more types of functional groups were found in the biochar produced at low temperature than in the biochar produced at high temperature (Chen et al., 2008; Keiluweit et al., 2010). C]C ring stretching vibration of lignin and C]O in carboxylic acid were typical functional groups in plant tissues (Bustin and Guo, 1999; Chen et al., 2012), which were only observed in the biochar produced at low temperature. It suggests that those functional groups were destroyed by pyrolysis process. The relatively high abundance of SieOeSi peak in the rice-derived biochar matched with their high ash content, demonstrating that silicon components are the main fraction in the ash content of biochar (Guo and Chen, 2014).
3.7. Properties of biochar influencing biochar hydrophobicity The TOC content as the basic chemical composition of biochar did not significantly correlated to the hydrophobicity of biochar, implying that not all the carbon component in biochar contributed to biochar hydrophobicity. The similar result was observed in Mao et al. (2014). The TOC content of soil did not significantly correlate to soil hydrophobicity, resulting from a part of soil organic matter contributing to SWR. With the same feedstock, the hydrophobicity of biochar decreased with increasing ash content, suggesting that the reduction of hydrophobicity is due to the existence of hydrophilic mineral components in the ash of biochar (Hartmann et al., 2010). As a result, regarding to all the biochar from different feedstock, the ash content had an insignificant correlation with the hydrophobicity of biochar, implying that the ash content cannot be the only factor influencing biochar hydrophobicity (Kinney et al., 2012). The TOC and ash content present the characteristic of the whole part of biochar. When water drops contact biochar, they first meet the surface of biochar; therefore, the properties of surface on biochar may affect biochar hydrophobicity more than the characteristic of the whole part. Functional groups detected by FTIR presented the chemical property on the biochar surface (Chen et al., 2008; Rechberger et al., 2017). Except four ratios of the abundance of functional groups, the other ratios insignificantly correlated to biochar hydrophobicity. Three ratios in those four ratios contained the same functional group C]O in carboxylic acid demonstrating the close relation of carboxylic compounds with biochar hydrophobicity. It matched with the observation of Rechberger et al. (2017), in which an increase in the abundance of carboxylic group on the surface of aged biochar which became more hydrophilic. Suliman et al. (2017) demonstrated that more oxygenated functional groups on the surface of biochar rendered biochar more hydrophilic. Since the peak of C]O in carboxylic acid was not found in the biochar produced at 500 and 700 C but only in 250 C, implying that the functional group ratios strongly correlated to the hydrophobicity of biochar produced at low temperature. In short, rather than the whole part of biochar, the surface chemical composition of biochar had more influence on the hydrophobicity of biochar. Compared to chemical composition of biochar, both surface area and pore volume as the important part of structure of biochar had negatively significant correlations with the hydrophobicity of bio€ tter and Marschner, 2015). More pores char (Gray et al., 2014; Heitko produced with increasing pyrolysis temperature caused the higher surface area of biochar. The higher pore volume caused high capillary force that leads the biochar to be more hydrophilic (Gray et al., 2014). Therefore, both surface area and pore volume had a
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good correlation with the hydrophobicity of biochar.
3.8. Linking biochar hydrophobicity to SWR and soil WHC Higher biochar addition rate leading to a stronger SWR was clearly observed in red and yellow soil but not in black soil (Fig. 8). Both red and yellow soil was hydrophilic and black soil was hydrophobic, demonstrating that hydrophobic biochar with a high addition rate causes stronger SWR on hydrophilic soil rather than on hydrophobic soil. It has been discussed that adding non-water repellency biochar to hydrophobic soil can decrease soil hydrophobicity (Hallin et al., 2015). Here we investigated that the soil hydrophobicity increased with the increasing addition rate of hydrophobic biochar. Ajayi et al. (2016) also reported that high amendment rate (100 g/kg) lead the hydrophilic soil to be more hydrophobic compared to the addition effect from a medium amendment rate at 50 g/kg. A significant difference was found between the hydrophobicity of the soil that were added the biochar with different feedstock at the same addition rate. Rather than 1% biochar addition rate, the SWR level of three soils added a higher rate of biochar was more distinguishable according to the biochar feedstock. For both red and yellow soil with a high biochar addition rate, the SWR of WB700-added soil was higher than DM700-added soil. The hydrophobicity of WB700 was relatively higher than DM700, suggesting more hydrophobic biochar added to red soil can increase soil hydrophobicity, while it showed the same effect for yellow soil. Therefore, the addition of biochar significantly changed the SWR mainly depended on soil hydrophobicity, biochar feedstock and addition rate. The soil WHC of black soil was more than two times higher than the WHC of either red or yellow soil. The soil with high organic carbon content presented a long water retention time (Rawls et al., 2003). Ojeda et al. (2015) discussed the association between the velocities of water absorption and desorption and the ability of soil organic matter retaining water that emphasise the important role of soil organic water on controlling soil moisture. Here black soil contained abundant humus substance and a certain amount of plant debris leading to a high TOC content and therefore resulting in a high soil WHC. Like the effect of biochar on SWR of black soil, the biochar addition did not have significant influence on changing soil WHC of black soil. It could be explained that compared to the dominant effect of large amount of organic matter in black soil on WHC, the contribution of biochar addition was difficult to be observed. Both the red and yellow soil did not have a high TOC content, and it is clear to observe the significant increase in WHC of red and yellow soil after biochar addition. Omondi et al. (2016) used meta-analysis and found out that the addition of biochar reduced soil bulk density leading to higher soil porosity and therefore increased soil hydraulic properties. Herath et al. (2013) also observed that compared to the soil with high organic carbon content, biochar addition reduced soil bulk density in the soil with low organic carbon content, suggesting the effect of biochar on the hydraulic properties of soil with low TOC content is more significant. The WB700-added red and yellow soil became more hydrophobic than DM700-added soil. However, for red soil, adding WB700 increased the ability to holding more water than adding DM700. In contrast, the soil WHC of WB700-added was significantly lower than DM700-added yellow soil. To sum up, for the soil with low TOC content, adding biochar would increase their soil WHC, but for high TOC soil, the influence of biochar is insignificant. The soil WHC could be influenced by soil TOC, organic matter and pore distribution and to what extent by activities of microorganisms (Atkinson et al., 2010; Gul et al., 2015; Jeffery et al., 2015), therefore, it had a direct link with soil hydrophobicity.
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4. Conclusions The characteristics of biochar largely depended on pyrolysis temperature and feedstock. However, the TOC and ash content of biochar was more determined by feedstock rather than pyrolysis temperature. In contrast, surface functional groups, surface area and pore distribution were mostly influenced by temperature. The hydrophobicity of biochar significantly correlated to factors including carboxylic groups on biochar surface, surface area and pore volume. It demonstrated that pyrolysis temperature was the dominant factor determining the severity of biochar hydrophobicity, while the feedstock of biochar was a minor factor influencing biochar hydrophobicity. The hydrophobicity of biochar affected the SWR of biochar-amended soil as long as the original soil was hydrophilic. The biochar at a high addition rate lead to a stronger SWR of hydrophilic soil. The biochar addition with a high rate did not significantly change the SWR of hydrophobic soil. It implied that the influence of biochar on SWR of biochar-added soil mainly depends on soil original hydrophobicity and to what extent on soil TOC content. Biochar addition to soil can significantly increase the soil water holding capacity; however, biochar did not have significant influence on water holding capacity of hydrophobic soil with high TOC content. Addition of biochar did not significantly change the soil WHC of hydrophobic soil with high TOC content, but significantly increased the soil WHC of hydrophobic soil with low TOC content. More hydrophobic biochar added to soil did not accordingly turned the soil into more water repellent. Therefore, the influence of biochar addition on SWR and hydraulic properties of different types of soil should be considered for soil amendment. The findings of this study mainly depended on the investigation from laboratory test, thus the influences of biochar on soil hydraulic properties including biochar aging would be more complicated once applying to field sites. Author disclosure statement The authors declare no conflict of interest. Acknowledgments This study is funded by the National Science Foundation for Young Scientists of China (Grant No. 21707118), National Natural Science Foundation of China (Grant No.21425730 and 21621005), and the National Key Technology Research and Development Program of China (Grant No. 2018YFC1800705). The study is also supported by the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDA20020101). The authors thank Xiaoyu Zhou in Philip Brookes Group in the Department of Environmental Science in Zhejiang University for helping with soil water holding capacity measurements. The authors also thank Shengyu Li’s Group in Laboratory of Desert Environment and Engineer in Xinjiang Institute of Ecology and Geography Chinese Academy of Sciences for helping with particle composition measurement. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.envpol.2019.07.051. References Abel, S., Peters, A., Trinks, S., Schonsky, H., Facklam, M., Wessolek, G., 2013. Impact of biochar and hydrochar addition on water retention and water repellency of sandy soil. Geoderma 202e203, 183e191. https://doi.org/10.1016/j.geoderma.
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Linking hydrophobicity of biochar to the water repellency and water holding capacity of biochar-amended soil* Jiefei Mao a, b, c, d, e, Kun Zhang a, b, Baoliang Chen a, b, * a
Department of Environmental Science, Zhejiang University, Hangzhou, 310058, China Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou, 310058, China c State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, 830011, China d Research Center for Ecology and Environment of Central Asia, Chinese Academy of Sciences, Urumqi, 830011, China e University of Chinese Academy of Sciences, Beijing, 100049, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 April 2019 Received in revised form 6 July 2019 Accepted 10 July 2019 Available online 11 July 2019
Biochar addition to soil may change the hydrophobicity of amended soil and influence soil hydraulic properties. Soil hydrophobicity, i.e. soil water repellency (SWR) can interrupt water infiltration and form preferential flow leading to a potential risk of soil erosion or groundwater pollution. Up to date, the effect of different biochars on soil hydrophobicity remains unclear and the association of SWR with soil hydraulic properties is still unknown. To link the biochar hydrophobicity to SWR and soil water holding capacity (WHC), the surface structure and chemical composition of 27 biochars with different feedstocks and pyrolysis temperatures were characterized, and the SWR and soil WHC of biochar-added soil were investigated. Carboxylic groups on the biochar surface, surface area and pore volume were mostly influenced by pyrolysis temperature, which suggested the dominant factor determining the severity of biochar hydrophobicity was pyrolysis temperature. Hydrophilic soil became hydrophobic after biochar amendment. A higher addition rate led to a stronger SWR of hydrophilic soil. Biochar addition increased soil WHC of hydrophilic soil with low total organic carbon (TOC) content. Biochar did not have significant influence on SWR and soil WHC of hydrophobic soil with high TOC content. It implied that the influence of biochar on SWR and soil hydraulic properties mainly depended on soil original hydrophobicity and TOC content. Therefore, the properties of biochar and influence on soil hydrophobicity and hydraulic properties should be considered before processing biochar application. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Biochar hydrophobicity Soil water repellency Soil hydraulic properties Pyrolysis temperature Surface properties
1. Introduction The application of biochar is a common soil amendment that improves soil fertility, positively affects agricultural production and even potentially mitigates climate change by soil carbon sequestration (Lehmann et al., 2006; Woolf et al., 2010). Biochar is obtained from biomass pyrolysis under oxygen-limited conditions (Chen et al., 2008; Lehmann, 2007; Lehmann and Joseph, 2009). The alkyl functional groups on the surface of biochar correlate to its hydrophobicity (Kinney et al., 2012). Biochar produced at low temperature contains more aliphatic compounds in the biochar pores that increase the hydrophobicity (Chen et al., 2008; Gray
* This paper has been recommended for acceptance by Dr. Yong Sik Ok. * Corresponding author. Department of Environmental Science, Zhejiang University, Hangzhou, 310058, China. E-mail address: [email protected] (B. Chen).
https://doi.org/10.1016/j.envpol.2019.07.051 0269-7491/© 2019 Elsevier Ltd. All rights reserved.
et al., 2014), while high temperature pyrolysis allows a much smaller number of aliphatic compounds remaining in the pores (Chen et al., 2008). The addition of hydrophobic biochar can turn hydrophilic soil into water repellent. With the ability to interrupt water infiltration, the existence of soil water repellency (SWR) in soil potentially increases the risk of soil erosion (Bisdom et al., 1993; Dekker and Ritsema, 2000; Mao et al., 2016). The distribution of hydrophobic parts in water-repellent soil is heterogeneous, which forces water to flow in perfectional pathways while hydrophobic zones are mostly excluded from flow. The occurrence of preferential flow would easily transport contaminants from top to deep soil and even into the underlying seepage and groundwater (Miyata et al., 2007; Hamlett et al., 2011). Kinney et al. (2012) observed that soil mixed with a biochar pyrolyzed at 300 C showed water repellency while the same soil mixed with a biochar pyrolyzed at 500 C was wettable. Compared to pyrolysis techniques, Ojeda et al. (2015)
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demonstrated that feedstock of biochar is more important to influence soil functions, e.g., soil wettability and soil water retention. Addition of hydrophobic biochar to soil could change soil hydrophobicity that influences soil hydraulic properties (Jeffery et al., 2015). In contrast, Abel et al. (2013) did not observe any increase of water repellency after addition of biochar to soil that increased water content by increasing the total pore volume. The influence of biochar addition on the severity of SWR mainly depends on the properties of biochar determined by its feedstock and pyrolyzed temperatures (Briggs et al., 2012; Gray et al., 2014; Kinney et al., 2012). Therefore, biochar hydrophobicity should be considered for biochar application to soil amendment. The effects of biochar on soil hydrophobicity concerning physical structure change of soil have been discussed. However, regarding to chemical properties of soil and characteristics of biochar, the effects of biochar on soil hydrophobicity remain unclear. Research concerning the influence of biochar addition on soil hydraulic properties have been widely conducted (Castellini et al., 2015). Addition of biochar produced at high temperature can increase soil moisture retention (Ajayi et al., 2016). The soil hydraulic conductivity decreased with increasing addition ratio of biochar to soil (Liu et al., 2016; Trifunovic et al., 2018). High amounts of biochar added to soil affected soil wettability that influenced soil water retention (Ojeda et al., 2015). One common reason to explain changes of soil hydraulic properties is the total porosity change after adding biochar. As seen in Villagra-Mendoza and Horn (2018), pore size distribution changed in sandy soil by the addition of biochar that increased meso porosity to improve hydraulic condition. However, up to now the direct influence of biochar hydrophobicity on soil hydraulic properties is rarely studied. The possible association of surface structure and chemical composition altered under different pyrolysis conditions and feedstock with the hydrophobicity of biochar is also not well known. In this study, we aim to discuss the difference of surface
structure and chemical composition of biochar produced at different temperatures (250, 500 and 700 C) that derived from various sources including plants and manures, to investigate the relationship between both surface structure and chemical composition and hydrophobicity of biochar, and to explore the direct influence of biochar on SWR and one of the soil hydraulic properties (i.e. the soil water holding capacity (WHC)) of biochar-amended soil with respect to biochar hydrophobicity. 2. Materials and methods 2.1. Biochar preparation, soil sampling and soil-biochar mixture Six plant tissues (pine needle, pine wood, rice straw, rice bran, wood sawdust, and orange peel) and three animal manures (fowl manure, dairy manure and pig manure) were selected as the source of biochar. Prior to pyrolysis, all the feedstocks were passed through a sieve with 0.15 mm diameter, filled and compressed in the crucibles covered by lid. The volumes of crucibles were from 100 to 300 mL depending on the density of the dried feedstock. Each sample was put in the muffle furnace under limited oxygen and pyrolyzed at 250, 500 and 700 C for six hours. The products were passed through a sieve with 0.15 mm diameter and named referring to their feedstock and pyrolysis temperature (Table 1). Three types of soils were selected and named according to their colours as red soil (Ferralsol), yellow soil (Regosol) and black soil (Phaeozems) (FAO, 2015). The red, yellow and black soils were collected from Guangxi, Shanxi and Liaoning Provinces, respectively. All three soils were air-dried at room temperature and passed a 100 mesh-sized sieve. The pH of red, yellow and black soil was 5.095, 7.735 and 5.005, respectively, which was determined by a 1:2.5 (w/w) soil to water ratio. The red soil was composed of 5.66% sand, 81.292% silt and 13.05% clay. The yellow soil was composed of 17.13% sand, 79.41% silt and 3.47% clay. The black soil was composed
Table 1 Properties of biochar pyrolyzed at different temperatures. Sample name
Feedstock type
Temperature, C
TOC content, %
Ash content, %
Surface area (N2), m2/g
Surface area (CO2), m2/g
Mesopore volume, cm3/g
Micropore volume, cm3/g
PN250 PN500 PN700 PW250 PW500 PW700 RS250 RS500 RS700 RB250 RB500 RB700 WB250 WB500 WB700 OP250 OP500 OP700 FM250 FM500 FM700 DM250 DM500 DM700 PM250 PM500 PM700
Pine needle Pine needle Pine needle Pinewood Pinewood Pinewood Rice straw Rice straw Rice straw Rice bran Rice bran Rice bran Wood sawdust Wood sawdust Wood sawdust Orange peel Orange peel Orange peel Fowl manure Fowl manure Fowl manure Dairy manure Dairy manure Dairy manure Pig manure Pig manure Pig manure
250 500 700 250 500 700 250 500 700 250 500 700 250 500 700 250 500 700 250 500 700 250 500 700 250 500 700
57.96 76.47 83.63 53.26 79.40 81.60 39.14 40.53 37.59 48.94 50.11 50.32 55.28 70.80 70.27 60.14 73.51 74.14 27.84 22.37 19.20 37.30 33.07 39.30 38.93 36.29 32.89
2.03 4.74 5.52 0.09 1.35 1.62 27.71 41.20 47.90 15.29 31.07 36.04 4.02 5.78 10.06 0.16 1.94 1.27 47.81 67.53 73.33 32.60 51.95 46.02 25.37 46.40 54.61
1.69 20.23 5.92 3.35 263.18 425.11 4.17 4.10 125.05 1.69 67.24 282.66 2.45 279.75 443.85 0.96 6.59 185.41 2.90 26.68 69.94 1.49 33.91 173.00 3.02 16.13 122.63
17.79 98.20 134.15 21.04 134.41 175.47 29.50 71.24 79.83 30.97 94.50 121.85 47.52 130.38 167.30 33.77 118.35 159.21 17.70 42.34 40.59 21.84 59.95 92.77 13.14 57.72 73.83
0.0023 0.0072 0.0022 0.0059 0.0000 0.0787 0.0120 0.0093 0.0387 0.0057 0.0175 0.0465 0.0055 0.0044 0.0700 0.0016 0.0025 0.0160 0.0093 0.0224 0.0496 0.0046 0.0196 0.0312 0.0056 0.0129 0.0428
0.0018 0.0090 0.0152 0.0019 0.0121 0.0183 0.0035 0.0113 0.0103 0.0000 0.0097 0.0135 0.0046 0.0141 0.0159 0.0026 0.0153 0.0167 0.0021 0.0061 0.0053 0.0021 0.0106 0.0127 0.0017 0.0078 0.0108
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of 47.06% sand, 51.13% silt and 1.82% clay. Accordingly, the texture of red, yellow and black soil was silt loam. The biochar was mixed with soil at 1% (low) and 5% (high) (w/w) addition rates. The mixing process followed the steps that were first to put the soil into a steel dish to make a thin soil layer, then to add biochar evenly on the soil layer and at last gently and well mix them using glass rod. All the samples were used for contact angle and soil WHC measurements. 2.2. Total organic carbon and ash content Total organic carbon content (TOC) of biochar and soil was determined by the subtraction of total carbon content and inorganic carbon content. The total carbon content and total inorganic carbon content of biochar was measured using TOC-V CPH total organic carbon analyser (SSM-5000A, Shimazdzu). The ash of biochar was the residual of biochar pyrolyzed at 800 C in a muffle furnace for four hours.
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feedstock, 2) soil types and biochar with different pyrolysis temperatures, 3) soil types and rates of biochar addition. A two-way ANOVA was used to test the significance of interaction of factors in each scenario. Followed by interaction test, Tukey test or Dunnett T3 test was selected to interpret the significant differences between biochar treatments. The differences in contact angle of biochar and the WHC of biochar-added soil were compared by Duncan’s Multiple Range test within the analysis of variance (ANOVA). Principle component analysis (PCA) was applied by converting correlated properties of biochar into uncorrelated components to explore possible link between factors concerning surface structure and chemical properties and the severity of water repellency of biochar. Linear regression was applied to test the linear relation between properties of biochar and hydrophobicity of biochar. 3. Results and discussion 3.1. Basic properties of biochar
2.3. Characterization of biochar The surface functional groups of biochar were analysed using a Fourier transform infrared (FTIR) spectrometer (Nicolet 6700, Thermo Scientific). The range of FTIR spectra was recorded from 400 to 4000 cm1 with a resolution of 1 cm1. The ratios of peak area of functional groups of biochar were used to investigate the correlation of functional groups with the biochar surface area and the level of biochar hydrophobicity. The surface area and pore volume of biochar were measured with the gas adsorptiondesorption method using surface area and pore size analyser (NOVA200e, Quantachrome Instruments). Nitrogen and carbon dioxide gas were applied individually on multipoint BET analysis to measure the surface area of biochar, where mesopore and micropore volume of biochar were analysed by N2 and CO2 gas application, respectively. 2.4. Water repellency and water holding capacity measurement Contact angle method was applied to assess the severity of water repellency of biochar and biochar-added soil. In this method the angle of a water droplet and the material surface is measured, of which the surface is considered as hydrophobic when the initial contact angle is larger than 90 (Wessel, 1988). The biochar and soil were stuck as a thin and flat layer on a glass slide with double-sided adhesive tape (5.5 cm 1.8 cm) (Bachmann et al., 2000). The contact angle measurement was duplicated and four distilled water droplets were placed on each slide. Soil WHC was measured using a volumetric method adapted from Yargicoglu et al. (2015). A known amount of air-dried soil was put in a funnel connected with a tube with a clip, while enough distilled water was added to completely submerge the soil and left for 30 min to saturate the soil. The clips on the tubes were opened and water was drained from each funnel for 30 min that was collected. Both adding and draining water was weighed to get the weight of the water held in soil for soil WHC calculation. Soil WHC experiment was duplicated. 2.5. Statistics analysis The correlations between surface structure and chemical properties and hydrophobicity of biochar were obtained by Pearson correlation and Spearman correlation (IBM SPSS statistics 20). The former method is used for the data following a normal distribution while the latter method is for the data non-normally distributed. During the hydrophobicity analysis of biochar-treated soil, three scenarios including 1) soil types and biochar with different
3.1.1. TOC content General properties of biochar changed with pyrolysis temperature and feedstock (Table 1). For the biochar with different feedstocks and pyrolyzed at the same temperature, the TOC of the biochar from manures was the lowest. The biochar of rice straw was slightly higher than the biochar of manures but much lower than other biochar from plant biomass. For biochar pyrolyzed at 250 C, the biochar of orange peel had the highest TOC (60.14%), while for the biochar pyrolyzed at 500 C and 700 C, the TOC of biochar of pine wood (79.40%) and pine needle (83.63%) was the highest, respectively. With the same feedstock, when pyrolysis temperature changed, the TOC of the biochar originated from plant biomass (pine needle, pine wood, rice straw, wood sawdust and orange peel) increased with pyrolyzed temperature. By contrast, the TOC of the biochar from fowl and pig manures decreased while pyrolyzed temperature increased, being in line with the findings in Song et al. (2019). It can be explained that for feedstock the organic content of manure was much lower than plants, during pyrolysis process, then carbon was condensed in plant-derived biochar while cellulose and lignin containing H and O were pyrolyzed. However, for manurederived biochar, remained organic matter released as volatile compounds declined the TOC of those biochars. Among these 27 biochars, PN700 and FM700 had the highest (83.63%) and the lowest (19.20%) TOC, respectively. The TOC contents of plantderived biochar were higher than the biochar with manure origin. 3.1.2. Ash content Similar to the TOC, the ash contents of biochar showed difference resulting of different feedstocks and pyrolysis temperatures (Table 1). Compared to the other biochar with the same pyrolysis temperature and different feedstocks, the ash content of the biochar of fowl manure was the highest, while the biochar of pine wood and orange peel had the lowest. For the biochar originated from plants with the same pyrolysis temperature, the ash contents of rice (straw and bran) were obviously higher than other biochars. For biochars had the same feedstock but different pyrolysis temperatures, rather than the biochar of orange peel and dairy manure, the ash content of other biochar increased with increased temperature. Fowl biochar FM700 (73.33%) and pine wood biochar PW250 (0.09%) had the highest and lowest ash contents, respectively. Differ from TOC content, the ash contents of manure-derived biochar were much higher than other plant-derived biochar. 3.1.3. Surface area and pore distribution According to N2 gas adsorption analysis, when pyrolysis temperature rose, the surface area of most of the biochar from the same
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feedstock increased. For the biochar at 250 C, the biochar of rice straw and orange peel had the highest (4.17 m2/g) and lowest (0.96 m2/g) surface area, respectively. The lowest surface area of the biochar at 500 and 700 C were rice straw (4.10 m2/g) and bran (5.92 m2/g), respectively. The biochar of wood sawdust had the highest surface area at both 500 and 700 C. The surface area measured based on the CO2 gas adsorption presented was different from N2 gas adsorption analysis. The biochar of wood sawdust and pig manure showed the highest (47.52 m2/g) and lowest (13.14m2/ g) surface area for the biochar at 250 C, respectively. The biochar of pinewood and fowl manure at both 500 and 700 C presented the highest and lowest surface area, respectively. Besides surface area, pore volume is also an important property of biochar. Mesopore with a diameter ranged from 2 to 50 nm and the micropore of biochar with a diameter smaller than 2 nm. The same as surface area, for the most of biochar having the same feedstock, the volumes of both mesopore and micropore increased with increasing pyrolysis temperature. For the biochar at 250 C, the biochar of rice straw had the highest meso- and micropore volume. The biochar of orange peel had the lowest mesopore volume while none of micropore was found in the biochar of rice bran. For the biochar at 500 C, the biochar of dairy manure had the highest mesopore and lowest micropore volume. None of the mesopore was found in the biochar of pinewood that was the lowest while the biochar having
the highest micropore volume was derived from orange peel. The biochar of pinewood had the highest meso- and micropore volume for the biochar at 700 C. The biochar of pine needle and fowl manure at 700 C had the lowest mesopore and micropore volume, respectively.
3.2. FTIR data According to the FTIR spectra of 27 biochar, low-temperature pyrolyzed biochar had more functional groups on the surface than higher temperature pyrolyzed biochar (Fig. 1). The C]O for ester (1700-1710cm1) and C]C in aromatic ring (1600-1615cm1) were observed at the surface of biochar pyrolyzed at each temperature. Rather than the biochar at 700 C, the aliphatic CH2 (2920, 2858, 1450, 1400, and 1380cm1) were obviously observed in the biochar produced at 250 C and 500 C. Meanwhile, only the biochar produced at 250 C presented C]C ring stretching vibration of lignin (1515cm1) and C]O in carboxylic acid (1318cm-1). However, those two products were not in the biochar produced at higher temperatures. The CeOeC (1159-1175nm1) peaks on the biochar surface were commonly found in plant-derived biochar, of which for manure-derived biochar, only PM250 showed this product.
Fig. 1. FTIR spectra of nine biochar produced at three different temperatures (250, 500 and 700 C).
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3.3. Water repellency of biochar 3.3.1. Hydrophobicity of biochar The hydrophobicity of biochar derived from different feedstock was distinguishable (Fig. 2). Larger initial contact angle suggests stronger water repellency. Compared to other biochar, at each pyrolysis temperature, the biochar of pinewood had the lowest hydrophobicity. The biochar at 250 C, the most water repellent biochar was manure-derived DM250 and PM250. The hydrophobicity of the biochar of wood sawdust and orange peel was the highest at 500 and 700 C, respectively. The biochar of pine needle and wood sawdust showed insignificant difference of hydrophobicity with different temperature. For pine wood and rice bran the difference of hydrophobicity between biochar at 250 and 500 C was insignificant. For orange peel, fowl manure and pig manure, the water repellency of biochar at 500 C and 700 C were also insignificantly differed. For the biochar of rice straw, the initial contact angle of RS700 was similar to RS250 but significantly higher than RS500. Even though the insignificant difference of hydrophobicity of biochar was found, the average value of initial contact angle of biochar still significantly decreased with increasing pyrolysis temperature (p ¼ 0.002) (Fig. 3). In short, the most of biochar followed the tendency that for the biochar with the same feedstock, the water repellency of biochar produced at high temperature was lower than that produced at low temperature. 3.3.2. Correlation of factors with hydrophobicity of biochar Both TOC content (p ¼ 0.478) and ash content (p ¼ 0.662) did not correlate to the water repellency of biochar significantly. With the same feedstock, the TOC content of biochar did not have any
Fig. 3. Average contact angle of biochar significantly decreased with increasing pyrolysis temperature. Small letters show the significance of average contact angle of the biochar produced at different temperatures.
significant correlation with their hydrophobicity (Fig. 4a). Oppositely, the hydrophobicity of most biochar decreased with increasing ash content of biochar with the same feedstock (Fig. 4b). Both surface area measured using N2 (r ¼ 0.565, p ¼ 0.002, n ¼ 27) and CO2 (r ¼ 0.494, p ¼ 0.009, n ¼ 27) had a negative correlation with biochar hydrophobicity (Fig. 5a and b). Regarding to pore volume, mesopore (r ¼ 0.496, p ¼ 0.009, n ¼ 27) and micropore (r ¼ 0.492, p ¼ 0.009, n ¼ 27) also significantly
Fig. 2. The initial contact angle of nine biochar produced at different temperatures. Small letters show the significance of contact angle of the biochar produced at different temperatures.
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Fig. 4. The scatter plot of relationship between TOC content (a, p ¼ 0.478, n ¼ 27) and ash content (b, p ¼ 0.662, n ¼ 27) with initial contact angle of biochar, each colour and mark refer to one kind of feedstock. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
correlated to water repellency of biochar (Fig. 5c and d), respectively. Here instead of absolute peak area of each functional group of biochar, the ratio of peak area of every two functional groups meaning the value of the peak area of one functional group divided by the peak area of another functional group was used to compare the relative abundance of functional groups between different biochar. The correlation of the ratio of peak area of every two functional groups of biochar with water repellency of biochar was also investigated. Most of the ratios of peak area did not have significant correlation with hydrophobicity of biochar. However, the ratio of summed aliphatic CH2/C]C ring stretching vibration of lignin (r ¼ 0.667, p ¼ 0.050, n ¼ 27), the ratio of summed aliphatic
CH2/C]O in carboxylic acid (r ¼ 0.783, p ¼ 0.013, n ¼ 27), the ratio of C]O for ester/C]O in carboxylic acid (r ¼ 0.667, p ¼ 0.050, n ¼ 27) and the ratio of C]C stretching in the aromatic ring/C]O in carboxylic acid (r ¼ 0.883, p ¼ 0.002, n ¼ 27) was correlated to the water repellency of biochar. Where, the ratio of CH2/C]C ring stretching vibration of lignin meant the ratio between aliphatic part and aromatic part of biochar surface, more aliphatic functional groups led to a higher biochar hydrophobicity. The positive correlations between the other ratios and biochar hydrophobicity demonstrated that carboxylic acids were more hydrophilic than CH2, ester and aromatic ring, which caused a lower hydrophobicity of biochar. To better identify the characteristic of factors influencing
Fig. 5. The correlation of different factors with water repellency of biochar: surface area measured using N2 (a, r ¼ 0.565, p ¼ 0.002, n ¼ 27) and CO2 (b, r ¼ 0.494, p ¼ 0.009, n ¼ 27), mesopore volume (c, r ¼ 0.496, p ¼ 0.009, n ¼ 27) and micropore volume (d, r ¼ 0.492, p ¼ 0.009, n ¼ 27).
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hydrophobicity of biochar, principal component analysis (PCA) was used. The Kaiser-Meyer-Olkin measure of sampling adequacy was 0.70 and the significance of Bartlett's Test of sphericity was 0.00, suggesting PCA analysis is applicable in this study. The communality values of all properties of biochar was larger than 0.50 (Table S1). The first, second and third principal component (eigenvalue > 1) explained ca. 58.0%, 22.4% and 11.7% of the variance in the dataset of the properties of biochar (Table S2), respectively. The high factor loadings of properties mostly shown at the first two components; therefore, Fig. 6 plotted the factor loadings of properties at component 1 and 2. The factor loading of surface area and pore volume and four functional group ratios was high positive and high negative on component 1, respectively (Fig. 6). All these properties had significant correlations with the level of biochar hydrophobicity, while the correlation of those three functional groups was positive but the surface area and pore volumes had negative correlations. The factor loading of TOC and ash content was high positive and high negative on component 2, respectively. Both surface area measured by N2 and the volume of mesopore had high positive factor loadings at component 3. Based on the linear regression analysis (Table S5), among all the properties of biochar, except surface area measured by N2 and mesopore volume, the others were not linearly related to biochar hydrophobicity. The PCA scatter plot shows a clear separation of all properties. The factors with high loading on component 1 changed mainly depending on pyrolysis temperature. TOC and ash content of biochar with high loading on component 2 were mainly determined by feedstock of biomass.
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separated according to soil types and biochar addition rates (Fig. 7). After adding biochar, red and yellow soil remained hydrophilic as the contact angles were still smaller than 90 . The 1% biochar addition did not increase soil hydrophobicity of red soil but significantly increased hydrophobicity of yellow soil (Fig. 7). Where 5% biochar addition significantly increased the soil hydrophobicity
3.4. Hydrophobicity of biochar-amended soil The TOC content of black soil (22.58%) was much higher than the red (0.14%) and yellow soil (0.78%). Based on a two-ANOVA, soil types and rates of biochar addition had a significant interaction when influencing water repellency of biochar-amended soil (Tables S3 and S4). Therefore, during the analysis of soil hydrophobicity, initial contact angles of biochar-added soil were
Fig. 6. Principle Component analysis (PCA) scatter of the factor loading of surface structure and chemical composition of biochar at component 1 and 2. Here SAN2 and SACO2 present surface area (based on N2 and CO2), micro- and mesopore mean the pore volumes, four functional group ratios from up to down were aliphatic CH2/C]O in carboxylic acid, C]C stretching in the aromatic ring/C]O in carboxylic acid, C]O for ester/C]O in carboxylic acid and aliphatic CH2/C]C ring starching vibration of lignin, respectively.
Fig. 7. The comparison between hydrophobicity of three types of soil before and after biochar addition with low (1%) and high (5%) rates. Small letters show the significance of average initial contact angle of soil.
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of both red yellow soils. Oppositely, soil hydrophobicity of black soil did not significantly change after 5% biochar addition but decreased at 1% biochar addition. The addition of biochar significantly changed the SWR depended on both biochar rates and soil properties. General, the red soil added biochar at low temperature had the lower SWR than the soil added biochar at higher temperature. As depicted in Fig. S1, the red soil added with 1% RB500 showed the highest SWR, while the addition of WB250 and DM700 in red soil presented the lowest SWR. However, for 1% biochar-added yellow soil, RS250- and RB500-added soil had the highest and lowest hydrophobicity, respectively. For both red and yellow soil with a 5% biochar addition rate, the soil added with WB500 and WB700 had the highest SWR while the soil added with DM700 had the lowest SWR. For the black soil, adding PN250 and OP500 at 1% rate lead the soil to be the most and least hydrophobic, respectively. However, except that two biochar the other biochar had a similar effect on the water repellency of black soil. When adding 5% biochar into black soil, the soil containing PN250 and PN500 had the highest SWR while the soil having RB700 showed the lowest. 3.5. Soil water holding capacity To link SWR to soil hydraulic properties, biochar-added soil with the highest and lowest SWR were selected to identify their soil water holding capacity (Fig. 8). The order of increment of soil water holding capacity by addition of WB700 to soil followed yellow soil (29.5%) > red soil (25.6%) > black soil (0.8%), while the order for addition of DM700 was the same: yellow soil (59.7%) > red soil (11.2%) > black soil (3.3%). Biochar addition significantly increased the soil WHC of red and yellow soil. The addition of WB700 increased relatively higher WR than adding DM700 into both red and yellow soil; however, the soil WHC of WB700-added soil was higher than DM700-added red soil. In contrast, adding DM700 to yellow soil lead to higher soil WHC than adding WB700. The
addition of biochar to black soil did not show any significant influence on soil WHC even from different biochar. 3.6. Properties of biochar changing with feedstock and pyrolysis temperature The biochar derived from fowl manure had the lowest TOC content, which was logical that manure is the residual part after digestion containing less carbon and nutrients than original biomass. For plant-derived biochar, the biochar originated from pine needle/wood and wood sawdust had a higher TOC content than the biochar from rice bran/straw, suggesting that the biochar derived from woody plants generally have a higher TOC content than grass-derived biochar (Chiou et al., 2015; Wang et al., 2018). In contrast to TOC content, the ash content of biochar depended on the feedstock that leads to a much high ash content in manurederived biochar rather than in plant-derived biochar (Whitely et al., 2006). However, the biochar from rice (e.g. bran and straw) also had a relatively high ash content, which is due to the high silicon content in rice tissues, especially in rice straw (Xu and Chen, 2013). The majority of biochar followed the tendency that the ash content increased with increasing pyrolysis temperature, demonstrating a condensation process during the pyrolysis (Xu and Chen, 2013). In agreement with previous studies (Chen et al., 2008; Zhang et al., 2018), both the surface area measured using N2 and CO2 gas, the biochar produced at high temperature had a higher surface area than the biochar produced at low temperature. The reason is that a large part of organic matter pyrolyzed and turned into CO2 during pyrolysis process that creates pores on biochar. The removal of organic matter and minerals from pores explained the reason that the micropore volume of the most biochar increased with € tter and Marschner, 2015). Gray increasing temperatures (Heitko et al. (2014) observed a dramatic increment in the pore volume of biochar when pyrolysis temperature increases. For the biochar
Fig. 8. Soil water holding capacity of soil with and without adding 5% biochar. Small letters showed the significance between the soil WHC of each sample for the same soil. WB7R and DM7R represented the biochar of wood sawdust and dairy manure pyrolyzed at 700 C added to red soil, respectively; WB7Y and DM7Y represented the biochar of wood sawdust and dairy manure pyrolyzed at 700 C added to yellow soil, respectively; WB7B, DM7B and RB7B represented the biochar of wood sawdust, dairy manure and rice bran pyrolyzed at 700 C added to black soil; PN2B represented the biochar of pine needle pyrolyzed at 250 C. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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produced at the same temperature, the major difference of surface structure of biochar including surface area and pore structure was mainly determined by the feedstock of biochar (Kinney et al., 2012; Qiu et al., 2017). It was commonly found in previous studies that more types of functional groups were found in the biochar produced at low temperature than in the biochar produced at high temperature (Chen et al., 2008; Keiluweit et al., 2010). C]C ring stretching vibration of lignin and C]O in carboxylic acid were typical functional groups in plant tissues (Bustin and Guo, 1999; Chen et al., 2012), which were only observed in the biochar produced at low temperature. It suggests that those functional groups were destroyed by pyrolysis process. The relatively high abundance of SieOeSi peak in the rice-derived biochar matched with their high ash content, demonstrating that silicon components are the main fraction in the ash content of biochar (Guo and Chen, 2014).
3.7. Properties of biochar influencing biochar hydrophobicity The TOC content as the basic chemical composition of biochar did not significantly correlated to the hydrophobicity of biochar, implying that not all the carbon component in biochar contributed to biochar hydrophobicity. The similar result was observed in Mao et al. (2014). The TOC content of soil did not significantly correlate to soil hydrophobicity, resulting from a part of soil organic matter contributing to SWR. With the same feedstock, the hydrophobicity of biochar decreased with increasing ash content, suggesting that the reduction of hydrophobicity is due to the existence of hydrophilic mineral components in the ash of biochar (Hartmann et al., 2010). As a result, regarding to all the biochar from different feedstock, the ash content had an insignificant correlation with the hydrophobicity of biochar, implying that the ash content cannot be the only factor influencing biochar hydrophobicity (Kinney et al., 2012). The TOC and ash content present the characteristic of the whole part of biochar. When water drops contact biochar, they first meet the surface of biochar; therefore, the properties of surface on biochar may affect biochar hydrophobicity more than the characteristic of the whole part. Functional groups detected by FTIR presented the chemical property on the biochar surface (Chen et al., 2008; Rechberger et al., 2017). Except four ratios of the abundance of functional groups, the other ratios insignificantly correlated to biochar hydrophobicity. Three ratios in those four ratios contained the same functional group C]O in carboxylic acid demonstrating the close relation of carboxylic compounds with biochar hydrophobicity. It matched with the observation of Rechberger et al. (2017), in which an increase in the abundance of carboxylic group on the surface of aged biochar which became more hydrophilic. Suliman et al. (2017) demonstrated that more oxygenated functional groups on the surface of biochar rendered biochar more hydrophilic. Since the peak of C]O in carboxylic acid was not found in the biochar produced at 500 and 700 C but only in 250 C, implying that the functional group ratios strongly correlated to the hydrophobicity of biochar produced at low temperature. In short, rather than the whole part of biochar, the surface chemical composition of biochar had more influence on the hydrophobicity of biochar. Compared to chemical composition of biochar, both surface area and pore volume as the important part of structure of biochar had negatively significant correlations with the hydrophobicity of bio€ tter and Marschner, 2015). More pores char (Gray et al., 2014; Heitko produced with increasing pyrolysis temperature caused the higher surface area of biochar. The higher pore volume caused high capillary force that leads the biochar to be more hydrophilic (Gray et al., 2014). Therefore, both surface area and pore volume had a
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good correlation with the hydrophobicity of biochar.
3.8. Linking biochar hydrophobicity to SWR and soil WHC Higher biochar addition rate leading to a stronger SWR was clearly observed in red and yellow soil but not in black soil (Fig. 8). Both red and yellow soil was hydrophilic and black soil was hydrophobic, demonstrating that hydrophobic biochar with a high addition rate causes stronger SWR on hydrophilic soil rather than on hydrophobic soil. It has been discussed that adding non-water repellency biochar to hydrophobic soil can decrease soil hydrophobicity (Hallin et al., 2015). Here we investigated that the soil hydrophobicity increased with the increasing addition rate of hydrophobic biochar. Ajayi et al. (2016) also reported that high amendment rate (100 g/kg) lead the hydrophilic soil to be more hydrophobic compared to the addition effect from a medium amendment rate at 50 g/kg. A significant difference was found between the hydrophobicity of the soil that were added the biochar with different feedstock at the same addition rate. Rather than 1% biochar addition rate, the SWR level of three soils added a higher rate of biochar was more distinguishable according to the biochar feedstock. For both red and yellow soil with a high biochar addition rate, the SWR of WB700-added soil was higher than DM700-added soil. The hydrophobicity of WB700 was relatively higher than DM700, suggesting more hydrophobic biochar added to red soil can increase soil hydrophobicity, while it showed the same effect for yellow soil. Therefore, the addition of biochar significantly changed the SWR mainly depended on soil hydrophobicity, biochar feedstock and addition rate. The soil WHC of black soil was more than two times higher than the WHC of either red or yellow soil. The soil with high organic carbon content presented a long water retention time (Rawls et al., 2003). Ojeda et al. (2015) discussed the association between the velocities of water absorption and desorption and the ability of soil organic matter retaining water that emphasise the important role of soil organic water on controlling soil moisture. Here black soil contained abundant humus substance and a certain amount of plant debris leading to a high TOC content and therefore resulting in a high soil WHC. Like the effect of biochar on SWR of black soil, the biochar addition did not have significant influence on changing soil WHC of black soil. It could be explained that compared to the dominant effect of large amount of organic matter in black soil on WHC, the contribution of biochar addition was difficult to be observed. Both the red and yellow soil did not have a high TOC content, and it is clear to observe the significant increase in WHC of red and yellow soil after biochar addition. Omondi et al. (2016) used meta-analysis and found out that the addition of biochar reduced soil bulk density leading to higher soil porosity and therefore increased soil hydraulic properties. Herath et al. (2013) also observed that compared to the soil with high organic carbon content, biochar addition reduced soil bulk density in the soil with low organic carbon content, suggesting the effect of biochar on the hydraulic properties of soil with low TOC content is more significant. The WB700-added red and yellow soil became more hydrophobic than DM700-added soil. However, for red soil, adding WB700 increased the ability to holding more water than adding DM700. In contrast, the soil WHC of WB700-added was significantly lower than DM700-added yellow soil. To sum up, for the soil with low TOC content, adding biochar would increase their soil WHC, but for high TOC soil, the influence of biochar is insignificant. The soil WHC could be influenced by soil TOC, organic matter and pore distribution and to what extent by activities of microorganisms (Atkinson et al., 2010; Gul et al., 2015; Jeffery et al., 2015), therefore, it had a direct link with soil hydrophobicity.
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4. Conclusions The characteristics of biochar largely depended on pyrolysis temperature and feedstock. However, the TOC and ash content of biochar was more determined by feedstock rather than pyrolysis temperature. In contrast, surface functional groups, surface area and pore distribution were mostly influenced by temperature. The hydrophobicity of biochar significantly correlated to factors including carboxylic groups on biochar surface, surface area and pore volume. It demonstrated that pyrolysis temperature was the dominant factor determining the severity of biochar hydrophobicity, while the feedstock of biochar was a minor factor influencing biochar hydrophobicity. The hydrophobicity of biochar affected the SWR of biochar-amended soil as long as the original soil was hydrophilic. The biochar at a high addition rate lead to a stronger SWR of hydrophilic soil. The biochar addition with a high rate did not significantly change the SWR of hydrophobic soil. It implied that the influence of biochar on SWR of biochar-added soil mainly depends on soil original hydrophobicity and to what extent on soil TOC content. Biochar addition to soil can significantly increase the soil water holding capacity; however, biochar did not have significant influence on water holding capacity of hydrophobic soil with high TOC content. Addition of biochar did not significantly change the soil WHC of hydrophobic soil with high TOC content, but significantly increased the soil WHC of hydrophobic soil with low TOC content. More hydrophobic biochar added to soil did not accordingly turned the soil into more water repellent. Therefore, the influence of biochar addition on SWR and hydraulic properties of different types of soil should be considered for soil amendment. The findings of this study mainly depended on the investigation from laboratory test, thus the influences of biochar on soil hydraulic properties including biochar aging would be more complicated once applying to field sites. Author disclosure statement The authors declare no conflict of interest. Acknowledgments This study is funded by the National Science Foundation for Young Scientists of China (Grant No. 21707118), National Natural Science Foundation of China (Grant No.21425730 and 21621005), and the National Key Technology Research and Development Program of China (Grant No. 2018YFC1800705). The study is also supported by the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDA20020101). The authors thank Xiaoyu Zhou in Philip Brookes Group in the Department of Environmental Science in Zhejiang University for helping with soil water holding capacity measurements. The authors also thank Shengyu Li’s Group in Laboratory of Desert Environment and Engineer in Xinjiang Institute of Ecology and Geography Chinese Academy of Sciences for helping with particle composition measurement. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.envpol.2019.07.051. References Abel, S., Peters, A., Trinks, S., Schonsky, H., Facklam, M., Wessolek, G., 2013. Impact of biochar and hydrochar addition on water retention and water repellency of sandy soil. Geoderma 202e203, 183e191. https://doi.org/10.1016/j.geoderma.
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