Effects of biochar addition on soil hydraulic properties before and after freezing-thawing

Catena 176 (2019) 112–124

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Effects of biochar addition on soil hydraulic properties before and after freezing-thawing

T

Qiang Fua,b,c,1, Hang Zhaoa,1, TianXiao Lia,b,c, , Renjie Houa, Dong Liua,b,c, Yi Jia,b,c, ZhaoQiang Zhoua, LiYan Yanga ⁎

a

School of Water Conservancy & Civil Engineering, Northeast Agricultural University, Harbin 150030, China Key Laboratory of Effective Utilization of Agricultural Water Resources of Ministry of Agriculture, Northeast Agricultural University, Harbin, Heilongjiang 150030, China c Heilongjiang Provincial Key Laboratory of Water Resources and Water Conservancy Engineering in Cold Region, Northeast Agricultural University, Harbin, Heilongjiang 150030, China b

ARTICLE INFO

ABSTRACT

Keywords: Biochar Freezing-thawing period Soil pore size distribution Soil water retention Soil hydraulic conductivity

Biochar as a soil amendment has attracted wide attention worldwide. However, the study of biochar on soil hydraulic properties during freeze-thawing remains insufficient. The purpose of this study was to determine the effect of straw biochar on soil water retention and soil hydraulic conductivity during a freeze-thaw period. Specifically, the effects of different biochar application rates (0, 30, 60, 90 and 120 t·hm−2) on the soil water characteristic curve (SWRCs), saturated hydraulic conductivity (Ksat) and hydraulic characteristic parameters were analysed through field trials. The response relationship between changes in the soil pore size and hydraulic characteristics was explored. The results showed that the combination of biochar amendment and freezingthawing significantly increased the soil micro pore size (≥0.3–5 μm), soil voids (> 100 μm) and total porosity (TP) and thereby improved the soil water retention capacity during the melting period. In addition, the use of biochar can promote the Ksat before and after freezing-thawing. However, increases in the carbon added during the melting period decreased the value of Ksat by 3.89%, 8.12%, 13.02% and 18.14% (p < 0.05) due to blockage of the soil pores by fine carbon particles. Compared with the control treatment, biochar significantly increased the field capacity (FC) and available water content (AWC) in soil. The largest relative change in the FC was obtained with an applied carbon amount of 60 t·hm−2, and this effect would improve the drought resistance capacity of the soil in the spring season. The excessive application of biochar led to an imbalance between the liquid and gas phases in soil, and thus, suggested biochar should not be applied in excess amounts to avoid negative effects on the soil structure. This study revealed the response mechanism of the hydraulic characteristics of the carbon-soil mixture during the freeze-thaw period and can provide a reference for the efficient utilization of soil water resources and the prediction of soil moisture in cold and dry areas.

1. Introduction Global land degradation has seriously reduced the safety of soil, and healthy soils are critical for ensuring sufficient food production and the long-term supply of water resources (Rodríguez-Eugenio et al., 2018). According to the statistical data provided the Food and Agriculture Organization of the United Nations, the degraded land area accounts for approximately 24% of the global land area (Montanarella et al., 2015), and continuous land degradation has become a global crisis. The potential organic pollutants in soil pose a great threat to human health and soil ecosystems (Ye and Zeng, 2017a). In recent years, biochar has attracted much attention as a soil conditioner because it can improve

the physical and chemical properties of soil, the microbial activity in and soil fertility (Li et al., 2018; Kolb et al., 2009; Ameloot et al., 2013; Kloss et al., 2014). In addition, due to its rich pore structure, biochar can synthesize a new type of nanocomposite that can degrade organic pollutants (e.g., tetracycline hydrochloride (TC)) (Zeng and Ye, 2019), and thus, biochar has broad potential for use in the fields of agricultural production and environmental restoration (Uzoma et al., 2011; Trinh et al., 2017; Kleiner, 2009). Biochar has various properties, such as a low density and high specific surface area, pH value and cation exchange capacity (CEC) (He et al., 2011; Chen et al., 2011), and shows strong adsorption and carbon fixation abilities (Ye and Zeng, 2017b). The application of biochar to

Corresponding author at: School of Water Conservancy & Civil Engineering, Northeast Agricultural University, Harbin 150030, China. E-mail address: [email protected] (T. Li). 1 These authors contributed equally to the work and should be regarded as co-first authors. ⁎

https://doi.org/10.1016/j.catena.2019.01.008 Received 10 September 2018; Received in revised form 27 December 2018; Accepted 8 January 2019 0341-8162/ © 2019 Elsevier B.V. All rights reserved.

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soil will affect the soil structure, and previous studies have shown that the application of biochar reduces the soil bulk density, increases the soil porosity, and affects the distribution of aggregates in soil (Laird et al., 2010; Lu et al., 2014). Biochar amendment also induces changes in the soil pore size (Zhao et al., 2015). Some researchers have reported that the use of biochar results in significant improvements in crop yields, which are likely due to positive improvements in the physical properties and hydraulic parameters of soil (Uzoma et al., 2011; Maroušek et al., 2017; Yuan et al., 2018; Wang et al., 2017). Biochar is a kind of porous substances, applied to the soil can change the soil water holding capacity (Liu et al., 2016; Arthur and Ahmed, 2017) and thereby significantly change the hydraulic characteristic parameters and some physical indexes of soil (Laird et al., 2010; Castellini et al., 2015; Głąb et al., 2018). The mixing of smaller biochar particles with soil results in a narrower pore size and thus a lower soil saturated hydraulic conductivity (Zhao et al., 2015; Ibrahim et al., 2013). Especially in areas with high soil coarse fractions, the application of biochar reduces the saturated hydraulic conductivity (Lim et al., 2016). Previous studies of the effect of biochar on the hydraulic characteristics of soil have confirmed that this effect is not only related to the amount of biochar applied but also indirectly related to the soil texture, materials used to prepare the biochar, the preparation temperature and the biochar particle size. Moragues-Saitua et al. (2017) believe that the effects of the application of biochar to the hydraulic properties of temperate moist soils depend not only on the amount of biochar applied but also on the soil texture. Głąb et al. (2016) revealed that the effect of biochar on the hydrological and physical properties of sandy soils was related to the proportion of biochar applied and also depended on the size of the biochar particles, and biochar prepared from miscanthus has a higher available water content than biochar made from wheat. Kim and Shim (2013) found that a higher production temperature produces biochar with a richer specific surface area, which has a positive effect on soil water retention. However, Jeffery et al. (2015) found that the application of biochar produced at different temperatures had no significant effect on soil water retention, which is likely due to the hydrophobicity of biochar. Gray et al. (2014) believed that the hydrophobicity of biochar decreases with increasing production temperature. The main reason is that hydrophobic aliphatic functional groups will be lost or a small of retained in biochar pores at higher production temperature. However, all of the above-mentioned production temperatures did not exceed 600 °C. When use of an extremely high temperature (≥1000 °C) for the production of biochar, the soil water retention effect will be reduced (Andrenelli and Maienza, 2015). Kinney and Masiello, 2012 proposed that the optimal temperature range for the pyrolysis of biochar was between 400 and 600 °C, at this time, biochar has low hydrophobicity. Recently, some researchers considered the influence of environmental factors, including temperature (Liu et al., 2018), humidity (Mendoza et al., 2018), acidity/alkalinity (Burrell and Zehetner, 2016), and soil pollutants (Wu et al., 2017), on biochar. An increasing number of researchers have begun to pay attention to the application of biochar in response to the freeze-thaw period (Han et al., 2018; Zhou et al., 2017a; Liu et al., 2017). Zhou et al. (2017b) revealed that the application of biochar during the freeze-thaw period reduced nitrogen loss, and Liu et al. (2018) found that freezing-thawing led to changes in the size and shape of biochar particles, which have been predicted to affect the hydraulic characteristics of soil. Previous studies on the effects of biochar application on the hydraulic properties of soil have mainly focused on non-freezing and thawing periods, and few studies have investigated the effects during freezing and thawing periods. Unlike during the non-freezing period, the migration of moisture in soil and the soil hydraulic characteristics are greatly affected by seasonal freezing and thawing (Fu et al., 2016), which cause severe drought in soil during the spring. This study focuses on the black soil area of the Songnen Plain, China. The soil water retention curves (SWRCs), soil water

characteristic parameters and saturated hydraulic conductivity (Ksat) before and after freezing-thawing were determined through field experiments. The soil pore size distribution and total porosity (TP) were used to compare the changes in soil hydraulic characteristics before and after freezing-thawing, and the influence of the amount of applied biochar on the hydraulic properties of soil was explored. The response mechanism of seasonal freezing-thawing to the hydraulic characteristics of carbon-soil mixtures was expounded, and a suitable amount of carbon-soil mixtures is proposed. This study not only provides basic information regarding the effect of the application of biochar on the hydraulic characteristics of soil in cold and arid areas and further supports the notion that soil can be improved through the application of biochar and the practical significance of biochar application for soil conservation entropy and nutrient migration in cold and arid areas in spring. 2. Materials and methods 2.1. Study area The experimental area is located in the comprehensive test site of Northeast Agricultural University in the eastern Songnen Plain of China. The geographical location of the study area is 126°43′7″E and 45°44′24″N, and the area is 139 m above sea level. This area has a midtemperate continental monsoon climate with four distinct seasons and cold and dry winters. The location of the test area is shown in Fig. 1. The annual average temperature of the test site is 3.6 °C, the average temperature in winter is −14.2 °C, the annual average snowfall is 23.6 mm, the annual average evaporation is 1326 mm, and the snow cover period is approximately 110 d. The soil begins to freeze from midto-late November and gradually starts to melt in late April of the next year, and the maximum freezing depth is approximately 180 cm. The experimental area is set in idle farmland with a flat terrain. In the area, mainly dry-field crops (soybeans) are planted in summer, seeding starts at the end of April each year, and harvest occurs at the end of September or early October. During the winter freeze-thaw period, the experimental area exhibits a natural snow cover with no vegetation coverage. The fertile soil in the study area is a typical soil of the Songnen Plain. In this area, the soil at depths of 0–30 cm is black loam (Fu et al., 2018), and average clay (< 0.002 mm), average silt (0.002–0.02 mm) and average sand (0.02–2 mm) account for 14.27%, 35.89%, and 49.84%, respectively, of the area. 2.2. Test materials 2.2.1. Biochar properties The biochar used in this experiment was commercially available biochar produced by Liaoning Jinhefu Agricultural Development Co., Ltd., China. The biochar is prepared from corn straw under anaerobic conditions at 500 °C. Its basic physical and chemical properties are as follows (Chen et al., 2011): particle size range of 1.5–2.0 mm, C mass fraction of 70.38%, N mass fraction of 1.53%, H mass fraction of 1.68%, S mass fraction of 0.78%, ash content of 31.8%, and pH value 9.14. 2.2.2. Soil properties The basic data of the soil at the beginning of the experiment were determined as follows: the soil dry bulk density was measured using the ring knife method (Bao, 2008); the soil natural moisture content was determined with the drying method; the soil particles were detected using a Winner 801 laser particle size analyser (Winner particle, Jinan, China); the soil texture was discriminated; the soil organic matter content was measured using the external heating method involving potassium dichromate oxidation; and the soil porosity was measured using a Daiki-1130 soil three-phase instrument (Zequan Technology, China). The soil physical properties at different depths are shown in Table 1. 113

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Fig. 1. Map of the study area.

2.3. Test plan and sampling

centre of the plot. The undisturbed soil samples were stored in an artificial climate chamber at 3 °C before the SWRCs were determined.

The experiment was conducted between October 2017 and May 2018. In the field trial, five test plots (each consisting of 5 × 6 m, yielding an area of 30 m2) were set up next to each other, and three replicates of each treatment were included in the experiment. Biochar was evenly scatter on the soil surface of the experimental plot after autumn harvest in 2017, and by using conventional agricultural tillage machine and artificial mixing method to mix the biochar with the deep soil (0-30 cm) to achieve the same color everywhere, which resulted in the formation of a carbon-soil mixed layer. At amount of applied was 30, 60, 90, and 120 t·hm−2 (these treatments were denoted B30, B60, B90, and B120, respectively), not applied any biochar as the control treatment (this treatment was denoted CK). During the freeze-thaw period, all the experimental plots retained their original snow without any interference, and all the treated plots had the same snow thickness. Therefore, the effect of snow on the carbon-soil mixed layer was not considered in this study. To study the effect of biochar application on the hydraulic characteristics of soil at different depths. The carbon-soil mixed layer was divided into surface soil (L1: 0–7 cm), subsurface soil (L2: 7–15 cm) and plough bottom soil (L3: 15–30 cm). In the early stage of stable freezing, various points were randomly distributed in different experimental zones, and a soil profile with a depth of 30 cm was excavated. Undisturbed soil from different soil layers was collected using a 5-cmdiameter ring knife (volume of 100 cm3), and three replicate samples of each soil layer were obtained. After the soil was fully thawed, soil samples were collected as in the early freezing stage. To avoid an obvious compaction area, all processing samples were collected from the

2.4. Research methods 2.4.1. Soil water retention curves Soil water retention curves (SWRCs) are commonly used to analyse the water-holding capacity of soil. Each undisturbed soil sample was immersed in a water storage vessel for 12 h for saturation. The SWRCs of different soil layers at the early freezing stages and thawing stages were obtained using a high-speed freezing centrifuge (CR21G III, Japan) at room temperature (25 °C). A total of 12 matrix suctions (0, −0.01, −0.03, −0.05, −0.1, −0.33, −0.5, −1, −3, −5, −10, and −15 bar) were established. After the set equilibrium time was reached, the soil was weighed using an electronic balance and dried in an oven at 105 °C for 12 h. The volumetric water content and bulk density corresponding to each suction value were then calculated (hereafter, the term “water content” refers to the volumetric water content). The Van Genuchten model (Van Genuchten, 1980) can accurately reflect the relationship between matrix suction and the volumetric water content and constitutes the basis for further studies of soil hydraulic characteristics. The model is expressed as follows:

(h ) = f (x ) =

r

+

s

r

[1 + | ·h|n ]m

s

,h<0 ,h

0

where h is the soil negative pressure measured using H2O, cm; θ(h) is the soil volumetric water content under the corresponding suction, %; θr is the residual volumetric water content, %; θs is the saturated

Table 1 Soil physical index. Soil depth/cm 0–7 ≥7–15 ≥15–30

Soil dry bulk density/(g·cm−3)

Natural moisture content/%

Organic matter content/%

Porosity/(cm3·cm−3)

Soil texture

1.40 ± 0.013 1.39 ± 0.011 1.49 ± 0.015

29.62 ± 0.97 29.55 ± 0.37 33.33 ± 0.85

4.02 ± 0.08 3.78 ± 0.05 3.45 ± 0.18

0.47 ± 0.005 0.50 ± 0.004 0.44 ± 0.006

Loam Loam Loam

Note: The results are presented as the means ± S.D. 114

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volumetric water content, %; α is the reciprocal of the intake value, cm−1; and n and m are the parameters characterizing the shape of the curve, m = 1 − 1/n. According to the measured data, the above parameters were obtained using RETC software (Van Genuchten et al., 1991).

was spread evenly under the infiltration plate to ensure good contact during the measurement. 2.5. Statistical analysis SPSS 22 and Origin 2017 software were used for data processing, graphing and tabulation. Each data point was summarized by calculating the average value and standard deviation (S.D). Relative change values (RCs, %) were calculated to compare the results from the same treatment during the pre-freezing and thawing periods. One-way analysis of variance (ANOVA) was used to assess the effect of the different treatment conditions on the soil water characteristic parameters, pore size distribution, TP and Ksat. The least significant difference (LSD) method was used to test the significance of the differences between the different soil layers in the different treatments. A significance level of p = 0.05 was set.

2.4.2. Soil water characteristic parameters The soil water characteristic parameters were derived from the SWRCs. These parameters included the field capacity (FC; defined as the equilibrium volumetric soil water content (θ(h)) at a matric potential of −0.33 bar (Lu et al., 2014)), the permanent wilting point (PWP; volumetric soil water content θ(h) at a matric potential of −15 bar (Marshall and Holmes, 1981)), the available water content (AWC; calculated as the difference between the FC and PWP) and the relative field capacity (RFC; proportion between FC and θMS, where θMS is the measured saturated water content). To better evaluate the changes in soil hydraulic parameters before and after freezing and thawing, the following three soil physical indicators were selected: macroporosity (Pmac), air capacity (AC) and slope (Sinf) at the inflection point of the SWRC (Dexter, 2004). These parameters are calculated as follows:

Pmac = AC =

Sinf =

MS

3.1. Change in soil water retention curves The SWRCs from the same treatment before and after freezingthawing are shown in Fig. 2(a)–(e). As shown in the graph, although the CK treatment increased the θ(h) during the thawing period compared with that during the pre-freezing period, more obvious increases in θ(h) before and after freezing-thawing were obtained after the application of biochar. As the carbon content increased, the difference in θ(h) between the different soil layers decreased gradually, as shown in Fig. 2(b)–(e); nevertheless, this change was still greater than that found for the waterholding capacity with the CK treatment. Although the variation trend obtained with the B120 treatment before and after freezing-thawing was similar to that found with the CK treatment, the difference in slopes was obvious, as shown in Fig. 2(a) and (e). As shown in Fig. 2(a)–(e), the θ(h) during the melting period was higher than that before freezing, and thus, the water retention curves of the surface soils in the different treatments during the melting period were drawn, as shown in Fig. 2(f). As indicated in the figure, during the melting period, the slope of the SWRCs of surface soils showed an increased variation with increases in the amount of biochar applied, and this increased variation led to a corresponding change in the soil water storage capacity. The difference between the SWRCs is reflected not only in the higher suction range but also in the matrix suction, which also showed a significant change in the range of Log(|ψH2O cm|) = 2–3.5. To more intuitively analyse the effect of carbon application on the soil water storage performance under different suction ranges, we divided the soil suctions into three stages: the water-holding capacity of each treatment in the low-suction section, Log(|ψH2O cm|) < 2 (i.e., < 0.1 bar), was proportional to the applied carbon content; in the middle suction section Log(|ψH2O cm|) = 2–3.5 (i.e., 0.1–3 bar), the slope of the decrease in the SWRC with a high carbon application rate was notably larger, as shown for the B90 and B120 treatments in Fig. 2(f), and the gap in the θ(h) values obtained between the CK treatment and the B30 and B60 treatments was decreased; in the high-suction section Log(|ψH2O cm|) > 3.5 (i.e., > 3 bar), the water capacity showed significant changes among all the treatments. The water storage capacity under the final matrix suction could be ordered as CK < B60 < B30 < B90 < B120, which showed that biochar application increased the soil water storage capacity. We also obtained the same findings for the subsurface and plough bottom layers (data not shown). The SWRC was fitted using RETC software, and the degree of fitting was R2 > 0.99. The results, which are shown in Table 2, indicate that the soils subjected to the same treatment exhibited higher θs and θr values before and after freezing-thawing. Among the calculated values, θr showed the most obvious RC, and the B60 treatment resulted in relative increase in L1, L2 and L3 of 28.04%, 25.17% and 19.64%,

m

MS

n(

3. Results

FC

s

r)

2n n

1 1

1 n

2

where θm is the volumetric water content of the matrix suction at 0.01 bar, and θs, θr and n are the parameters of the SWRC fitted by the V-G model, respectively. 2.4.3. Soil pore size distribution SWRCs can indirectly reflect the pore distribution in soil (Lei et al., 1988). If the pores in the soil are assumed to be circular tubes of various pore sizes, the relationship between the suction s and the pore diameter d can be expressed as:

S=

4 d

where σ is the water surface tension coefficient, which is generally 75 × 10−5 N/cm at room temperature. If the units of s and d are Pa and mm, respectively, the relationship between d and s is d = 300/s, and d is called the equivalent pore diameter. According to the soil pore size detailed in the Soil Science Encyclopedia (Cameron and Buchan, 2006), the equivalent pore size was divided into six pore sizes based on the suction range used in this study: extremely micro pore size (< 0.3 μm), micro pore size (≥0.3–5 μm), small pore size (≥5–30 μm), medium pore size (≥30–75 μm), large pore size (≥75–100 μm), and soil voids (> 100 μm). The soil total porosity (TP) was calculated according to the bulk density (ρb) of the different treatments. The soil particle density (ρs) is 2.65 g·cm−3, and the relationship between these variables can be represented by the following formula:

TP = 1

b s

2.4.4. Soil saturated hydraulic conductivity The soil saturated hydraulic conductivity (Ksat) at different depths was measured using an SW080B tension infiltration instrument (Channel Technology, Beijing). When measuring the topsoil, soil with a diameter of 40 cm and a thickness of 2–3 cm was first removed with a spatula; when measuring the Ksat of the subsurface and plough bottom layer soils, a 60 × 60 cm soil plane was excavated, and the hydraulic conductivity was measured. Before each infiltration, wet siliceous sand 115

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Fig. 2. Soil moisture characteristic curve.

116

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Table 2 Van-Genuchten (V-G) model fitting parameters. Layer

L1

L2

L3

Treatments (t·hm−2) 0 30 60 90 120 0 30 60 90 120 0 30 60 90 120

Freezing period

Thawing period

θS(%)

θr(%)

α

n

R2

θS(%)

θr(%)

Α

n

R2

56.51 59.76 63.28 63.40 67.40 53.01 56.34 57.73 58.16 61.87 51.75 51.83 52.64 54.55 58.47

18.91 14.84 10.39 8.12 8.77 17.07 10.09 9.72 8.64 8.00 17.72 10.16 8.30 7.19 7.15

0.3115 0.2671 0.3786 0.3177 0.1608 0.3543 0.3550 0.3559 0.3412 0.1587 0.3869 0.3153 0.3563 0.2771 0.1905

1.26 1.23 1.19 1.21 1.26 1.24 1.19 1.17 1.20 1.25 1.28 1.20 1.19 1.22 1.24

99.3 99.7 99.7 99.8 99.9 99.3 99.8 99.8 99.8 99.9 99.4 99.7 99.7 99.8 99.9

57.39 63.80 64.80 67.45 69.47 55.78 56.95 58.14 61.13 66.28 52.83 55.95 56.20 57.68 63.38

19.25 16.54 13.31 10.15 9.94 17.56 11.09 12.17 10.08 8.93 18.00 10.75 9.93 7.97 7.56

0.3225 0.3790 0.3004 0.2330 0.1757 0.4171 0.5302 0.3091 0.1859 0.2194 0.3978 0.6354 0.4084 0.2369 0.2988

1.24 1.21 1.20 1.23 1.24 1.23 1.15 1.18 1.24 1.23 1.26 1.16 1.18 1.22 1.22

99.3 99.6 99.6 99.8 99.9 99.4 99.5 99.6 99.7 99.9 99.4 99.5 99.5 99.7 99.9

Note: L1 refers to the surface layer (0–7 cm), L2 is the subsurface layer (7–15 cm), and L3 is the bottom plough layer (15–30 cm).

respectively, before and after freezing-thawing, whereas those obtained with the CK treatment were only 1.85%, 2.83% and 1.54%, respectively. An obvious change in θs was found between different treatments during the thawing period. Compared with the CK treatment, the treatments with increasing carbon amounts increased the θs in L1 by 11.17%, 12.91%, 17.53% and 21.06%, respectively, whereas an opposite (downward) trend was obtained for θr. The RCs between the carbon treatments and the CK treatment were −14.11%, −30.88%, −47.29% and −48.39%, respectively. If the difference between θs and θr is the water storage capacity of the soil, the water storage in the different soil layers under the different treatments is proportional to the amount of carbon applied. This result agrees with that shown in Fig. 2(f), but this strategy should not be used to judge the water-holding properties of soil. The effect of biochar on the water-holding performance of soil should be analysed by considering the soil water characteristic parameters, and no significant change in the parameters α and n was obtained between the treatments with biochar and the CK treatment.

significantly affected by biochar before and after freezing-thawing, as demonstrated by an obtained RC > 0 for the CK treatment before and after freezing-thawing. As the carbon content increased, the RC value gradually decreased until RC < 0; the RC value obtained for the B120 treatment before and after freezing-thawing was decreased by 60.44% compared with that found with the CK treatment. The AC also showed a similar trend, but the B60 treatment had a more significant effect on the AC. We selected four typical parameters detailed in Table 3 to compare the RCs for the carbon treatments with those of the CK treatment in the thawing period, and the results are shown in Fig. 3. The application of carbon increased the FC compared with that found with the CK treatment (Fig. 3(a)), but the B90 and B120 treatments decreased this value by 3.50% and 5.66% compared with that found with the B60 treatment. Compared with that obtained with the CK treatment, the soil PWP decreased significantly after the application of carbon (Fig. 3(b)). Specifically, the amount of applied carbon decreased the PWP in the different soil layers by 3.45%, 7.32%, 21.58% and 25.30%. The change in the FC and PWP is reflected in the AWC (Fig. 3(c)). Although the FC is not proportional to the amount of carbon applied, as shown in Fig. 3(a), this did not affect the finding that AWC was strongly affected by the amount of carbon applied. Increases in the carbon content increased the AWC by an average of 64.37% in the different soil layers. As shown in Fig. 3(d), the application of a high amount of carbon (B90 and B120 treatments) had a negative effect on the RFC compared with that obtained with the CK treatment.

3.2. Change in soil water characteristic parameters The soil water characteristic parameters obtained for same treatment before and after freezing-thawing are shown in Table 3. Compared with those of the CK soil, the application of biochar significantly changed the soil hydraulic characteristic parameters before and after freezing-thawing, and the significance of the effects on different parameters in different soil layers was different. The greatest RCs in the FC, PWP and AWC before and after freezing-thawing were obtained with the B60 treatment (RCs = 16.15%, 14.53% and 19.35%, respectively). The effect of the B60 treatment on the soil FC, PWP and AWC, particularly in L2, was greater than that of the other treatments. The AWC of soil is a water resource that is available to plants and can well reflect the water-holding performance of soil (Ma and Zhang, 2016). The RCs in the AWC before and after freezing-thawing were higher after the application of carbon than in the CK treatment (RC = 6.93%). The CK treatment decreased the RFC before and after freezing-thawing, whereas the opposite effects were obtained with the carbon treatments. Statistical analyses showed that biochar had no significant effect on the FC, but a significant difference in the RFC was obtained in all the treatments. The different carbon treatments also exerted different effects on soil physical indexes before and after freezing-thawing, as shown in Table 3. Biochar changed the Sinf, but the degree of change was not significantly related to the amount of carbon applied. In addition, Pmac was

3.3. Change in the soil pore size distribution To more intuitively show the effects of the same treatment on soil pores before and after freezing-thawing, the statistical results are shown in Fig. 4(a)–(c). The change in the soil pore size was affected by the dual effects of seasonal freezing-thawing and biochar application. If RC > 0, freezing-thawing increased the proportion of the corresponding aperture. A longitudinal analysis showed that a larger RC value indicated that the carbon content could promote an increase in the corresponding pore diameter before and after freezing-thawing. Thus, based on the change found in the CK treatment, seasonal freezing-thawing had different effects on the change in the proportion of pores with an extremely micro pore size (< 0.3 μm) in the different soil layers, and the proportion of medium pores (≥30–75 μm) increased in L3. The carbon treatments induced different changes in the proportion of extremely micro pores (< 0.3 μm) in the different soil layers under the dual effects of biochar application and freezingthawing, and no significant difference was observed. As shown in 117

118

0.321 ± 0.017d 0.342 ± 0.018 ac 0.348 ± 0.027a 0.337 ± 0.019bc 0.330 ± 0.03bd 0.311 ± 0.018d 0.326 ± 0.016ab 0.332 ± 0.024a 0.322 ± 0.022bc 0.318 ± 0.021 cd 0.281 ± 0.018d 0.294 ± 0.019ab 0.298 ± 0.017a 0.289 ± 0.021bc 0.286 ± 0.021 cd

0.5364 ± 0.015a 0.5117 ± 0.0079b 0.4877 ± 0.0018c 0.4689 ± 0.002d 0.4285 ± 0.0009e

0.5509 ± 0.0126a 0.5226 ± 0.0117b 0.4901 ± 0.0125c 0.4660 ± 0.014c 0.4291 ± 0.0172d 0.5426 ± 0.0153a 0.5047 ± 0.0108b 0.4874 ± 0.0155c 0.4706 ± 0.0159d 0.4291 ± 0.0137e 0.5157 ± 0.0127a 0.5076 ± 0.0128b 0.4857 ± 0.017c 0.4700 ± 0.0132d 0.4272 ± 0.0173e 0.5278 0.5487 0.5449 0.5124 0.4757

± ± ± ± ±

0.0105b 0.0096a 0.0114a 0.0083c 0.0083d

0.5137 ± 0.012ab 0.5399 ± 0.0091a 0.5340 ± 0.023a 0.5023 ± 0.0118bc 0.4789 ± 0.0245c 0.5387 ± 0.0168b 0.5620 ± 0.0118a 0.5607 ± 0.0265a 0.5226 ± 0.0171c 0.4839 ± 0.0126d 0.5310 ± 0.0131b 0.5442 ± 0.0135a 0.5399 ± 0.0105a 0.5123 ± 0.0155c 0.4644 ± 0.0168d

F

T

F

−0.053 −0.057 −0.056 −0.065 −0.080

−0.056 −0.062 −0.062 −0.072 −0.088 −0.051 −0.055 −0.054 −0.061 −0.079 −0.054 −0.052 −0.053 −0.062 −0.073

0.304 ± 0.017 ns 0.320 ± 0.02 0.326 ± 0.021 0.316 ± 0.02 0.311 ± 0.019 Sinf

0.291 ± 0.019 ns 0.285 ± 0.022 0.281 ± 0.022 0.275 ± 0.019 0.271 ± 0.018

0.313 ± 0.017a 0.312 ± 0.019a 0.308 ± 0.021ab 0.297 ± 0.021bc 0.291 ± 0.026c 0.293 ± 0.018a 0.283 ± 0.016b 0.280 ± 0.021bc 0.277 ± 0.019bc 0.274 ± 0.019c 0.267 ± 0.017a 0.259 ± 0.017b 0.253 ± 0.02bc 0.251 ± 0.017c 0.248 ± 0.021c

T

± ± ± ± ±

0.017a 0.015b 0.018b 0.012c 0.009c

−0.053 −0.051 −0.057 −0.072 −0.079

−0.054 −0.061 −0.064 −0.078 −0.085 −0.052 −0.045 −0.053 −0.072 −0.079 −0.052 −0.048 −0.053 −0.067 −0.074

0.222 0.197 0.186 0.160 0.155

0.241 ± 0.018a 0.215 ± 0.017b 0.208 ± 0.023b 0.176 ± 0.023c 0.166 ± 0.026d 0.223 ± 0.018a 0.196 ± 0.018b 0.186 ± 0.02c 0.159 ± 0.017d 0.156 ± 0.021d 0.201 ± 0.015a 0.179 ± 0.017b 0.165 ± 0.023c 0.146 ± 0.019d 0.144 ± 0.02d

F

F

T

PWP (cm3·cm−3)

FC (cm3·cm−3)

RFC

CK B30 B60 B90 B120

CK B30 B60 B90 B120 CK B30 B60 B90 B120 CK B30 B60 B90 B120

Treatments (t·hm−2)

0.231 ± 0.015a 0.222 ± 0.012ab 0.214 ± 0.012b 0.181 ± 0.01c 0.172 ± 0.008c

0.249 ± 0.016a 0.237 ± 0.019b 0.228 ± 0.018b 0.193 ± 0.019c 0.182 ± 0.021d 0.230 ± 0.017a 0.222 ± 0.027b 0.214 ± 0.009c 0.180 ± 0.018d 0.170 ± 0.021e 0.213 ± 0.02a 0.208 ± 0.02a 0.199 ± 0.019b 0.169 ± 0.02c 0.164 ± 0.02c

F

0.097 ± 0.0078c 0.100 ± 0.0074c 0.107 ± 0.0116bc 0.119 ± 0.0145ab 0.130 ± 0.0138a

0.101 ± 0.0008b 0.105 ± 0.0015b 0.119 ± 0.0000ab 0.132 ± 0.0005a 0.144 ± 0.0013a 0.105 ± 0.0007c 0.107 ± 0.0004c 0.111 ± 0.001c 0.126 ± 0.001b 0.135 ± 0.0003a 0.087 ± 0.0007c 0.090 ± 0.001c 0.091 ± 0.0031c 0.099 ± 0.0016b 0.111 ± 0.0004a

Pmac (cm3·cm−3)

T

T

F

± ± ± ± ±

0.0023c 0.0071b 0.0049b 0.0075a 0.0086a

0.130 ± 0.0102a 0.109 ± 0.0047b 0.103 ± 0.0066bc 0.100 ± 0.0072bc 0.095 ± 0.0078c

0.141 ± 0.0028 ns 0.114 ± 0.0082 0.111 ± 0.009 0.109 ± 0.0034 0.105 ± 0.0016 0.133 ± 0.0045a 0.110 ± 0.0008b 0.102 ± 0.0073c 0.098 ± 0.0000 cd 0.093 ± 0.005d 0.117 ± 0.0008a 0.103 ± 0.0007b 0.095 ± 0.0001c 0.091 ± 0.0008 cd 0.086 ± 0.0022d

0.0691 0.0878 0.0942 0.1148 0.1160

0.0714 ± 0.0077c 0.0972 ± 0.0094b 0.0999 ± 0.0091b 0.1215 ± 0.0092a 0.1252 ± 0.0122a 0.0700 ± 0.0083d 0.0864 ± 0.0072c 0.0947 ± 0.0105b 0.1187 ± 0.0097a 0.1184 ± 0.0087a 0.0659 ± 0.0088d 0.0799 ± 0.0082c 0.0879 ± 0.0092b 0.1043 ± 0.0078a 0.1045 ± 0.01d

AWC (cm3·cm−3)

± ± ± ± ±

F

0.0058d 0.0089c 0.0095b 0.0105a 0.0123a

0.251 0.271 0.295 0.312 0.362

± ± ± ± ±

0.0032e 0.0146d 0.0214c 0.0237b 0.0225a

0.255 ± 0.0016d 0.285 ± 0.0008c 0.321 ± 0.0006b 0.341 ± 0.0004b 0.388 ± 0.0008a 0.247 ± 0.003e 0.278 ± 0.001d 0.295 ± 0.0007c 0.312 ± 0.0019b 0.365 ± 0.0008a 0.251 ± 0.0006d 0.251 ± 0.0008d 0.268 ± 0.0014c 0.283 ± 0.0008b 0.333 ± 0.0013a

AC (cm3·cm−3)

0.0739 0.0981 0.1124 0.1354 0.1394

0.0723 ± 0.0085d 0.1047 ± 0.008b 0.1205 ± 0.0155bc 0.1434 ± 0.0091 ac 0.1478 ± 0.0163a 0.0817 ± 0.0087d 0.1041 ± 0.0068c 0.1176 ± 0.0163b 0.1422 ± 0.0113a 0.1483 ± 0.0101a 0.0677 ± 0.0083d 0.0855 ± 0.0091c 0.0991 ± 0.0078b 0.1206 ± 0.0098a 0.1221 ± 0.0103a

T

Note: The results are presented as the means ± S.D. FC refers to the field capacity, PWP is the permanent wilting point, AWC is the available water content, RFC refers to the relative field capacity, Sinf is the slope at the inflection point of the SWRC, Pmac is the macroporosity, AC refers to the air capacity, F indicates the pre-freezing period, and T indicates the thawing period. Lowercase letters indicate significant differences in the parameters of different treatments at a given depth (p < 0.05); there were no significant differences in the parameters with the same letter; ns indicates a non-significant difference.

Means for treatments

L3

L2

L1

Layer

Means for treatments

L3

L2

L1

Layer

Table 3 Soil water characteristic parameters. Q. Fu et al.

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Fig. 3. Relative changes between the biochar treatments and the CK treatment during the thawing period. Note: (a) field capacity; (b) permanent wilting point; (c) available water content; (d) relative field capacity. Lowercase letters indicate significant differences in the relative changes of the parameters between the different treatments at a given depth (p < 0.05); there were no significant differences in the parameters with the same letter.

Fig. 4(a)–(c), the application of biochar combined with freezingthawing promoted increases in the proportions of micro pores (≥0.3–5 μm) and voids (> 100 μm) in soil and inhibited the changes in small pores (≥5–30 μm), medium pores (≥30–75 μm) and large pores (≥75–100 μm) in soil. A higher amount of applied carbon was associated with a more significant change. As shown in Fig. 4(d), the TP under the same treatment significantly increased before and after freezing-thawing, and the different carbon treatments increased the TP by 1.22%, 2.69%, 3.96% and 4.98% compared with that found with the CK treatment (RC = 0.37%). The amount of applied carbon exerted an extremely significant effect on the TP in L1. The above results show that the effect of the application of biochar combined with freezing-thawing on the soil pore size was complex and variable. The soil pore size distributions obtained with the different treatments after freezing-thawing are shown in Fig. 5. Biochar application changed the soil pore size distribution and significantly reduced the proportion of extremely micro pores (< 0.3 μm) in different soil layers. Specifically, the application of increasing amounts of biochar decreased the proportion of extremely micro pores (< 0.3 μm) by 9.25%, 17.07%, 23.60% and 32.53% in L3 compared with the proportion found with the CK treatment. The application of biochar decreased the ratio of large pores (≥75–100 μm) in L1 but increased the ratio of pores of this size in L2 and L3 because large pores correspond to less soil suction, whereas the carbon content in L1 and the waterholding action of biochar resulted in less drainage at low suction (the displacements obtained with CK treatment and the application of

increasing amounts of carbon were 0.0339 g, 0.0337 g, 0.0302 g, 0.0308 g and 0.0226 g, respectively). However, water holding had no effect on the proportion of surface soil voids, and the proportion of other pore segments increased with increases in the carbon content. As shown in Fig. 5, a higher amount of applied carbon led to a higher TP. 3.4. Change in soil saturated hydraulic conductivity Because the Ksat of L2 and L3 was extremely unstable during the prefreezing period, only the change in the median Ksat in surface soil was analysed before and after freezing-thawing, as shown in Fig. 6. As discovered in the CK treatment, the Ksat during the thawing period was 2.12% higher than that before the freezing period. The addition of biochar made this difference more obvious, but this change was not directly proportional to the amount of carbon applied. No significant difference in the Ksat was found between the carbon treatments. Fig. 7 shows the changes in the soil Ksat between the different treatments during the melting period. The application of an increasing amount of biochar yielded lower median Ksat values in the different soil layers, and the difference between the treatments was extremely significant. In the carbon-soil mixing layer, the median Ksat decreased by 3.89%, 8.12%, 13.02% and 18.14% with increasing carbon application, respectively. However, the differences in the Ksat between the soil layers tended to slightly increase with increases in the amount of carbon applied. The slope of the variation in the Ksat of different soil layers was on the order of CK > B60 > B30 > B90 > B120. 119

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Fig. 4. Relative change in the pore size distribution and total porosity before and after freezing-thawing. Note: Lowercase letters indicate significant (p < 0.05) differences in the pore size distribution and total porosity at a given depth among treatments with different biochar amounts; parameters with the same letter exhibit no significant differences.

4. Discussion

for this decrease was that the soil water in the large pore of the carbonsoil mixture was first frozen during the early period of freezing, with the decrease in temperature, and the unfrozen water in the shallow layer began to migrate to the freezing front, resulting in the decrease of water content in the middle pore diameter section, so the proportion of aperture decreases. Freezing-thawing rearranged the soil particles such that the pore size distribution during the thawing period was completely different from that in the early stage of freezing. Fu et al. (2017) studied the change in the soil moisture content under different covering conditions during the freeze-thaw period and found that the main reason for the reduction in the water content in shallow soil under natural conditions was the migration of soil moisture to a deep-freezing area. Tang et al. (2012) discovered that the microstructure of argillaceous clay changed during the freeze-thaw process, especially the adjustment of soil pores. During the thaw period, different treatments were affected only by biochar, and the soil extremely micro pore (< 0.3 μm) size was significantly decreased (Fig. 5). Increases in the biochar content resulted in average reduction rates of 16.44%, 16.18% and 17.56% in the different soil layers, and the other proportion of other-sized pores increased. Previous researchers have reported that different types of biochar with different contents increase the proportion of small pores and reduce the proportion of large pores. For example, VillagraMendoza and Horn (2018) applied 2.5% and 5% mango charcoal to sandy loam and observed an increase in narrow pores (between −6 and

4.1. Pore size distribution and total porosity The pore size distribution and TP of soil are considered to play an important role in soil water-solute transport, fertility and crop root extension (Sun et al., 2015). By applying biochar before and after freezing-thawing, it was found that the proportion of pores diameters (small pore size, medium pore size and large pore size) in the middle section of soil was decreased under the influence of double action, and the proportions of pores diameters at both ends were increased. It has been hypothesized that seasonal freezing-thawing changes not only the soil structure but also the structure of biochar itself. Lu et al. (2018) found that the first freeze-thaw cycle exerted the most significant effect on the structure and pore size distribution of soil. Liu et al. (2018) found by studying the effect of freezing-thawing cycles on the particle size of biochar under undrained conditions that freezing-thawing reduced the median pore size of biochar. Based on this conclusion and combined with this experiment, which was significantly affected by biochar before and after freezing-thawing, the pore size of the middle section of the same treatment was reduced. If all intermediate soil pore sizes are regarded as medium porosity (≥5–100 μm), the proportion of medium-porosity soil in the carbon-soil mixtures with increasing amounts of applied carbon decreased by an average of 6.57%, 12.37%, 15.04% and 20.29%, respectively, as shown in Fig. 4. The main reason 120

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Fig. 5. Pore size distribution and total porosity change during the thawing period. Note: Lowercase letters indicate significant differences in the total porosity at a given depth (p < 0.05) between different biochar applications; the parameters with the same letter do not show a significant difference.

−30 kPa drainage pores) and mesopores (between −30 and −1500 KPa drainage pores) in soil. In addition, Amoakwah et al. (2017) applied 10 t·ha−1 and 20 t·ha−1 corncob biochar and found a significant increase in the fine pore size (< 0.3 μm) compared with that found with the CK treatment. However, Głąb et al. (2016) applied miscanthus and winter wheat straw biochar consisting of particles with sizes in the range of 0–500 μm and found that the proportion of < 0.5 μm soil pores decreased and that the volume of 0.5–500 μm soil pores increased. These results are very similar to our results. In our study, the decrease in extremely micro pores (< 0.3 μm) observed in our study with the different treatments was attributed to the redistribution of soil moisture and snowmelt runoff after freezing-thawing, which resulted in an increase in the pore water content in biochar, the formation of larger aggregates through the combining with fine soil particles, and thereby a reduction in the proportion of extremely micro pores in soil. Yuan et al. (2018) found that biochar significantly increased the content of > 0.25 mm water-stable macroaggregates in soil and that the content of these macroaggregates was proportional to the applied amount of biochar. From a macro point of view, the TP of the different treatments was proportional to the biochar content (Fig. 5), which is consistent with the conclusions reached by Głąb et al. (2016). Liu et al. (2015) found that freezing-thawing increased the soil TP, which was consistent with our results for the CK treatment before and after freezing-thawing (Fig. 4(d)), but increases in biochar content more

significantly changed the TP. We thus concluded that the application of biochar under freeze-thaw conditions changes the bulk density of the surrounding soil. 4.2. Soil water retention Soil moisture retention has notable effects on crop growth and soil microbial activities. The increase in θ(h) in the soil under set matrix suction after biochar application is one of the easily recognized effects of water retention. Our results show that before and after freezingthawing, whether the same treatment SWRCs change or soil hydraulic characteristic parameters, the water retention effect during the thawing period was higher than that during the pre-freezing period. Moreover, the slope of the SWRC increased with increases in the amount of biochar applied, resulting in gradual increases in the difference in θ(h) between the initial suction and the final suction. This finding indicates that the effect on soil water retention in cold and dry climates depends on the soil porosity and amount of biochar. Liu et al. (2016) also showed similar results; by comparing the SWRCs between the CK treatment and the biochar treatment (16 t·ha−1), these researchers found that biochar increased the soil water-holding capacity. Amoakwah et al. (2017) believed that the increase in soil water retention obtained with the application of biochar is due to an increase in small pores in soil. However, Romualdo et al. (2018) applied four doses 121

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Fig. 7. Change in the saturated hydraulic conductivity during the thawing period. Note: The red marks represent the median saturated hydraulic conductivities. Lowercase letters indicate significant differences in the saturated hydraulic conductivity at a given depth (p < 0.05) between the different carbon treatments; there were no significant differences in the parameters with the same letter. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. Relative change in the saturated hydraulic conductivity of the surface soil before and after freezing-thawing. Note: The figure shows the relative changes in the median saturated hydraulic conductivity during the pre-freezing and thawing periods. Lowercase letters indicate significant differences in the relative changes in the median saturated hydraulic conductivity at a given depth (p < 0.05) between the different biochar treatments; there were no significant differences in the parameters with the same letter.

(0.5487 and 0.5449, respectively), but the values obtained with these treatments were still far below the optimal equilibrium range (0.6), which indicates that although the application of biochar improves the water-air balance of frozen-thawed soils, this balance is not completely restored. More precisely, the improvement degree of biochar on the structure of frozen-thawed soil has not completely restored the waterair balance condition in soil. However, the application of a high carbon amount (B90 and B120 treatments) results in a lower RFC compared with that found with the CK treatment, resulting in the excessive aeration of frozen-thawed soil. Castellini et al. (2015) also found that the excessive application of biochar could cause excessive aeration of soil, resulting in an insufficient level of moisture in farmland soil. In addition, a lower RFC value might limit the development of silicate microorganisms (Reynolds et al., 2009). Therefore, it was recommended that an excessive amount of biochar should not be applied to avoid damaging the physical and chemical properties of soil. About the remediation of the soil moisture balance through the application of biochar should be observed over a long period. Jeffery et al. (2015) and Gray et al. (2014) believe that biochar exhibits hydrophobicity. However, based on the changes in the AWC shown in Table 3 and Fig. 3(c), we can see that this has not had any negative effect. The different treatments increased the AWC from 0.0793 cm3·cm–3 to 0.0981–0.1394 cm3·cm–3. We believe that the higher the amount of carbon applied, the higher the amount of water was stored between the pores of the biochar and the soil, and the lower was the corresponding water content (PWP) of the carbon-soil mixture (100 cm3) at −15 bar suction. The V-G fitting results also show that the higher the amount of carbon applied, the lower was the θr value that was fitted, which increased the ability of the soil to accommodate the water available to plants and thereby enhanced their drought resistance. This result is consistent with those of previous studies. Lima et al. (2018) found that the PWP reduce with increases in the biochar content, which aids the storage of an increasing amount of water at the end of the soil moisture characteristic curve. Liu et al. (2017) believed that the pore size of biochar will affect the soil water-holding capacity. Our results also confirms the hypothesis proposed by Liu et al. (2018), who indicated that biochar might increase the AWC after freezingthawing. The results revealed the value of θS(%) in Table 2 was slightly

of biochar and found that the soil water retention was significantly improved with increases in the biochar dose. But, other researchers reported that the use of biochar does not improve the retention of soil water in situ; Jeffery et al. (2015) applied 10 t·hm−2 biochar produced at two different temperatures and 1, 5, 20, and 50 t·hm−2 biochar produced at 400 °C to sandy soil in two independent field experiments, it was found that biochar had no significant effect on soil water retention; Hardie et al. (2014) applied 47 mg·hm−2 Arabic gum green waste biochar to soil and found no effect on the soil moisture content after 30 months. This finding is quite different from our results, which might be attributed to the effects of freezing-thawing and biochar application, which changed the pore size distribution of the soil and increased the TP. These effects led to corresponding increases in water storage in the soil pores, which increased the soil water retention before and after freezing-thawing. The use of the pore size distribution to infer changes in soil moisture caused by biochar application is already common in soil science. Amoakwah et al. (2017) attributed the increase in water retention to an increase in the proportion of fine pores, and Wang et al. (2015) also believed that the application of biochar increased the soil porosity and consequently increased the soil waterholding capacity. In contrast, a comparison of different amounts of applied carbon during the thawing period (Fig. 2(f)) revealed that in the mid-high suction range, corresponding to the soil had a small pore size (≥ 5–30 μm), micro pore size (≥ 0.3–5 μm) and extremely micro pore size (< 0.3 μm). The water content corresponding to the mid-high suction section was exactly equal to the soil AWC, and the AWC of plants is mainly found in the pores with a diameter of < 30 μm. The comparison of the different treatments during the thawing period revealed that the largest FC was obtained with the application of 60 t·ha−1 biochar, but a lower FC was obtained with a dosage > 60 t·ha−1. From an agricultural perspective, the use of 60 t·ha−1 biochar would be optimal for agricultural production. An RFC of 0.6–0.7 was the best equilibrium range between the soil liquid and gas phases (Reynolds et al., 2009). As shown in Table 3 and Fig. 3, compared with the CK treatment, the B30 and B60 treatments improved the RFC 122

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higher, and we analysed the main reason for this finding. First, we used conventional agricultural tillage machine to mix biochar with soil during its application, which disturbs the soil structure. In addition, the liquid moisture stored in soil during the freezing-thawing period is frozen into ice, which increases the soil porosity. At the same time, due to the high water absorption capacity of biochar, with increases in the amount of applied increase the θS(%). Wu and Liu (2018) applied 25, 50, 75 and 100 t·hm−2 biochar and found that increases in the saturated water content of 11.16%–29.01%.

found that the Ksat increased under dry and wet cycling conditions, which indicates that the effect of biochar on the soil Ksat depends not only on the soil structure and texture but also on regional environmental conditions. Therefore, the mechanism underlying the physical response of Ksat to the application of biochar to black loam in cold and arid regions should be studied over a long period.

4.3. Soil hydraulic conductivity

The results of this study show that the effect of biochar application on soil hydraulic characteristics in cold and arid regions is affected by the effects of seasonal freezing-thawing and the biochar content. The following results were found: (1) Under the dual influence of freezing-thawing and biochar, the formation of moderately sized pores (≥ 5–100 µm) in soil was inhibited, and the ratio of extremely micro pore size (< 0.3 µm) decreased after the different biochar treatments. Pores with a size of < 30 µm play an important role in the soil AWC. From a macro perspective, the same treatment resulted in an increase in the soil TP before and after freezing-thawing, and the TP was significantly affected by the biochar content. (2) Biochar application improved the soil water retention capacity before and after freezing-thawing, and in particular, the B60 treatment resulted in significant changes in soil water characteristics parameters in the subsurface. The results showed that the water retention effect improved with increases in the amount of carbon applied between different treatments, which positively improved the prevention of drought in spring. Biochar mainly affects the water retention in carbonsoil mixtures in cold and arid regions in two ways. First, the combination of freezing-thawing and biochar changed the soil pore size distribution and porosity to increase the water retention in soil, and second, biochar itself can retain water in its internal pores, thereby directly increasing the soil water moisture. (3) Biochar application increased the change in the soil Ksat before and after freezing-thawing. However, the Ksat of different soil layers decreased with increases in carbon application due to the blockage of soil pores by the biochar fine particles. The change in the Ksat of different soil layers was on the order of CK > B60 > B30 > B90 > B120. Biochar might indirectly inhibit the evaporation effect of soil moisture in spring. (4) Biochar significantly increases the soil FC and AWC. Although the B30 and B60 treatments increased the RFC compared with that found with the CK treatment, the obtained RFC values did not reach the lowest critical value of RFC (0.6). Excessive carbon application might lead to excessive aeration in frozen-thawed soils. Therefore, it is necessary to control the amount of applied biochar to prevent the excess biochar-induced degradation of the physical and chemical properties of soil during the freeze-thaw period. From the perspective of spring sowing, the results suggest that 60 t·hm−2 is a suitable application dosage of biochar in cold and arid regions. In summary, this study confirms that biochar is a good soil conditioner and that the appropriate use of biochar can improve the functioning of carbon-soil mixtures and help alleviate soil water loss under cold and drought conditions. Considering improve of biochar addition on soil hydraulic properties to improve soil drought resistance and prevent erosion due to freezing and thawing, it is necessary to continuously observe the effects of the interactions of biochar and soil on crop yield and physicochemical properties.

5. Conclusions

In essence, changes in the Ksat are due to changes in soil structure. Chamberlain and Gow (1979) found through experiments that freezingthawing would increase the hydraulic conductivity, and Tang and Yan (2015) found that freeze-thawing changes the pore diameter and increases the Ksat. It can be seen from these results that freezing-thawing can promote the Ksat. As shown in Fig. 6, the soil Ksat increased by 7.17%, 6.92%, 6.30% and 5.66%, respectively, under the dual effects of freeze-thaw and increasing amounts of biochar (the value obtained with CK treatment was only 2.12%). This finding is different from the hypothesis proposed by Liu et al. (2018) that freeze-thawing reduces the hydraulic conductivity of a carbon-soil mixture. As shown in Fig. 4, the soil voids (> 100 μm) and TP increased before and after freezingthawing; therefore, the RC in the Ksat resulting from the same treatment before and after freezing-thawing was higher than that in the CK treatment. The relative change of Ksat before and after freezing-thawing decreased with increases in the biochar content mainly because the migration of liquid water in soil during freezing-thawing causes some fine particles of biochar to enter the soil pores. Slightly block the pore size that increases with freezing-thawing, leading to the relative change of Ksat before and after freezing-thawing has a tendency to decrease. Although there was no significant difference in the relative change of Ksat between the carbon treatment before and after freezing-thawing. However during the melting period, the trend is more obvious with the increase of carbon application. During the thawing period, the median soil Ksat between the different treatments was observed (Fig. 7). The Ksat of each soil layer was significantly reduced by the different biochar treatments. Obia et al. (2017) confirmed that the reduction in Ksat obtained with different amounts of applied biochar should not be attributed to the water repellency of biochar. There was no indication in the preceding description that biochars are water repellent, thereby excluding the possibility of water repellency. A linear relationship between the Ksat median and the carbon application rate was observed (in L1: y = − 0.0917x + 1.9512, in L2: y = − 0.0833x + 1.7447, and in L3: y = − 0.05x + 1.4429). Although biochar increased the soil pore size and TP between different treatments (Fig. 5), the biochar fine particles blocked the pores in the soil, resulting in a significant decrease in the Ksat. This finding is consistent with the test results reported by Amoakwah et al. (2017). Soil pore size blockage may also lead to a reverse relationship between soil evaporation and applied amount, but the specific response relationship needs to be studied. At present, no consistent conclusion regarding the Ksat of biochar-treated soil has not been described. Although some researchers have shown that the Ksat increases with increases in the amount of applied biochar (Castellini et al., 2015; Wang et al., 2015; Wong et al., 2018), some researchers have found that biochar application decreases the Ksat (Laird et al.,2010; Jeffery et al., 2015; Saitua et al., 2017) and even has no effect on the Ksat (Zhao et al., 2015; Mendoza et al., 2018; Obia et al., 2017). Wang et al. (2015) showed that the increase in Ksat caused by biochar application was related to high soil porosity, and Wong et al. (2018) also found that the Ksat increased with thechanges in the soil structure through improvements in clay soil by biochar application. However, Zhao et al. (2015) showed that biochar reduced the Ksat of silty loam. Karolina et al. (2018) applied biochar to soils with different textures and

Acknowledgements We acknowledge that this research was supported by the National Natural Science Foundation of China (51679039) and the National Science Fund for Distinguished Young Scholars (51825901). 123

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