Introduction

Biochar is a kind of porous material rich in carbon, which is produced by pyrolysis of biomass in a high temperature (e.g., 300–800 °C) and oxygen-free or low-oxygen environment (Lei and Zhang 2013; Wong et al. 2016; Adhikari et al. 2019). Biochar has been proved to be a good amendment for landfill cover material and agricultural soil in recent years (Wu et al. 2017; Su et al. 2019; Wong et al. 2016; Zhang et al. 2018; Chen et al. 2019b; Chen and Ng 2013). There are many materials for producing biochar, such as wood chips, peanut shells, crop straws, and bamboo stalks (Fang et al. 2014; Garg et al. 2019; Chen et al. 2018; Senbayram et al. 2019; Shang et al. 2013). Adding biochar to soil can change the physical and hydrological properties of soil, such as pore characteristics (Kuncoro et al. 2014), water retention capacity (Wong et al. 2017; Lei and Zhang 2013; Liu et al. 2016), and water/air permeability (Gopal et al. 2019; Chen et al. 2015, 2019a; Garg et al. 2019). Biochar-amended soil (BAS) also has good performance in harmful gas/chemical adsorption (e.g., hydrogen sulfide; Shang et al. 2013; Xu et al. 2014; and nitrate; Iberahim et al. 2018; Tan et al. 2018), mitigation of greenhouse gas emission and reduction of nitrate leaching in agricultural soil (e.g., methane, Sun et al. 2019; and NO3, Sanford et al. 2019), and methane oxidation capacity enhancement in landfill covers (Reddy et al. 2014; Yargicoglu and Reddy 2017).

Gas transports through soil mainly through diffusion and advection, which are driven by gas concentration difference and gas pressure difference, respectively. Diffusion is a very important way for gas flow in soils. For example, oxygen, which is essential for methane oxidation in landfill covers and root respiration in shallow soil layers, enters the soil mainly through diffusion from the atmosphere (Feng et al. 2019; Chen et al. 2019c; Lipiec et al. 2012; Wall and Heiskanen 2009). Although the influence of biochar on gas advection in soils was investigated (Garg et al. 2019; Wong et al. 2016), little attention has been paid on gas diffusion in BAS.

It has been recognized that there are many factors affecting gas diffusion coefficient (DP) of soil, including soil air content (SAC; mm3air mm−3soil), degree of compaction (DOC), and micro-structure of soil (e.g., pore path tortuosity, pore size). (Kuncoro et al. 2014; Moldrup et al. 2000b; Resurreccion et al. 2008b; Schjønning 1989). When SAC is lower than a certain value (e.g., ≤ 10%), soil pores could be almost filled by water, which impedes gas diffusion (Resurreccion et al. 2008a). Some studies reported that under the same SAC, the DP of soil would increase with DOC due to reduced water blockage effects under extreme compaction (Hamamoto et al. 2011; Millington and Quirk 1961). However, the opposite trend was found by Kuncoro et al. (2014), who attributed the reduction of Dp with the increase of DOC to the more tortuous pore path and the more vulnerable compaction of macro-pore in soil. Evidently, the influence of DOC on gas diffusion coefficient of soil is still controversial.

In this experimental study, the effect of biochar content (BC; gbiochar g−1soil) on DP of the soil was investigated under different controlled DOCs and SACs. The DP of BAS was measured by a two-chamber diffusion apparatus. To interpret the measurements, the micro-structure characteristics of BAS were investigated by scanning electron microscopy (SEM).

Materials and methods

Properties of soil and biochar

The biochar used in this study was made from fir wood chips, which was collected from wood processing plants. The biochar was produced in an oxygen-free environment at 400–800 °C (according to manufacturer). The test soil was collected from the Hongmiaoling Waste Sanitary Landfill Site in Fuzhou City, Fujian Province, China (N 26° 10′ 11″, S 119° 18′ 13″), and was classified as silty sand (ASTM D2487 2017).

The soil and the biochar were air-dried in room temperature of 25 °C ± 1 °C, and then sieved by a 2-mm sieve and a 0.425-mm sieve, respectively. Subsequently, the soil and biochar were stored in sealed plastic bags. The initial gravimetric water content (GWC) of the soil and biochar were measured by continuous drying for 24 h in an oven with controlled temperature of 105 °C. The specific gravities of the soil and biochar were measured according to the ASTM D854 (2014). The particle size distributions of the soil and biochar were measured according to the ASTM D422 (2007). The liquid limit, plastic limit, and plasticity index of the soil were measured according to the ASTM D4318 (2017). BAS compaction curves were measured according to the ASTM D698 (2010) to obtain the optimum moisture content (%, w/w) and the maximum dry density (g/cm3). Some basic properties of the biochar and soil were given in Table 1.

Table 1 Basic properties of soil and biochar
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Specimen preparation

Four different contents of the air-dried biochar (0%, 5%, 10%, and 15% by dry mass) were mixed with the air-dried soil thoroughly. Then, water was added to the mixture of soil and biochar to the target GWC, which was determined based on the target DOC (85%, 90%, and 95%) and SAC (5%, 10%, and 15%). Then, the moist BAS mixture was sieved through a 2-mm sieve and was subsequently stored in sealed plastic bags for at least 24 h for moisture equilibrium. Before compaction of the soil sample, the inner wall of the sample cutting ring (internal diameter of 79.8 mm, wall thickness of 1.8 mm, and height of 20 mm) was pasted with a thin layer of Vaseline to avoid any preferential flow along the interface between the soil sample and the inner wall of the cutting ring (Allaire et al. 2008). The dry density of the mixture was determined by the measured compaction curves according to the target BC and DOC. The mixture of the soil and biochar was compacted to form a cylindrical soil sample with a diameter of 79.8 mm and a height of 20 mm. After the soil sample preparation, the test was carried out immediately to prevent the formation of a moisture gradient in the soil sample (Boon et al. 2013).

Measurement apparatus and test procedure

The DP was measured by a two-chamber diffusion system device (Fig. 1) (Schjønning et al. 2013). The device was made of polymethyl methacrylate (PMMA). It consisted of an active chamber, a passive chamber, and a soil sample chamber in between. The inner diameter of the active chamber and passive chamber were both 80 mm, while the soil sample chamber had an inner diameter of 90 mm. All the three chambers had the same wall thickness of 10 mm. The connections between different chambers were sealed by a silicon gasket (3-mm thick). Capillary tube (internal diameter of 4 mm), sampling ports, and intake ports were installed in the active and passive chambers. Water droplets were added to the capillary tube to keep the gas pressure inside the device equal to the atmospheric pressure during the test. A soil sample contained inside the sample cutting ring was placed in the soil sample chamber. Two O-rings were installed at the gap between the outermost of the cutting ring and the inner wall of the soil sample chamber to prevent any gas preferential flow through the gap. A perforated PMMA plate (5-mm thick) with uniform distributed 4-mm diameter holes was installed at each end of the soil sample to prevent any potential soil collapse. A thin plastic membrane (0.1-mm thick) was installed between the passive chamber and soil sample chamber to block any gas exchange between the passive and active chambers through the soil before the test. The test was carried out in room with controlled temperature of 25 °C ± 1 °C.

Fig. 1
figure 1

Schematic of the two-chamber apparatus for measuring gas diffusion coefficient of biochar-amended soil

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After installing the soil sample in the device, a certain concentration of gas (O2) was injected into the active chamber through the intake port, while pure N2 was pumped in to the passive chamber until negligible O2 could be measured (e.g., < 0.1%) (Bonroy et al. 2011). A 5-mL air tight syringe was adopted for taking 2-mL gas samplings. The volumetric concentration of O2 (C0) of each gas sampling was measured by a gas chromatograph (GC112A; Shanghai INESA Scientific Instrument Co., Ltd. in Shanghai, China; measurement accuracy of 0.1% vol). The diffusion test commented after pulling out the thin plastic membrane. The test duration was recorded by using a stopwatch. The initial gas concentration in the active chamber was measured after test begun for 15 min. This aimed to avoid (i) the potential change of gas pressure inside the device caused by the change of chamber volume at the moment of pulling out the thin plastic membrane, and (ii) the associated non-diffusive gas flow through the soil sample (Schjønning et al. 2013). Then, a 10-min interval was adopted for the subsequent gas samplings. Five gas samplings were taken for each test.

Measurement of DP

During the test, the device was completely sealed, and the pressure inside the device was always equal to the atmospheric pressure. Therefore, the gas diffusion in the chamber satisfied Fick’s second law (Dane and Topp 2002). The DP in the soil sample can be calculated from the temporal variation of oxygen concentration inside the active chamber by the following formula (Glauz and Rolston 1989):

$$ \ln \left\{\frac{B\left({\alpha}_1\right)}{A\left({\alpha}_1\right)}\left[\frac{1}{2\left(1+ L\varepsilon /H\right)}-\frac{C_{\mathrm{g}}}{C_0}\right]\right\}=-\frac{D_{\mathrm{p}}{\alpha}_1^2}{L^2\varepsilon }t $$
(1)

where DP is the gas diffusion coefficient of BAS (mm3soil air mm−1soil s−1), t is the diffusion time (s), ε is the soil air content (mm3air mm−3soil), L is half the height of the soil sample (10 mm), H is the height of the active/passive chamber (100 mm), Cg is the volumetric concentration of O2 within the active chamber at time t (mL/mL); \( {\alpha}_1^2 \)is defined by the following equation when the sizes of the active and passive chambers are the same (Glauz and Rolston 1989):

$$ {\alpha}_1^2=\frac{1}{\beta }-\frac{1}{3{\beta}^2}+\frac{4}{45{\beta}^3}+\frac{16}{945{\beta}^4}+.\dots $$
(2)
$$ \beta =\frac{H}{L\varepsilon} $$
(3)
$$ A\left({\alpha}_1\right)=-\frac{1}{\beta^2}-{\alpha}_1^2 $$
(4)
$$ B\left({\alpha}_1\right)={\alpha}_1^4\beta +{\alpha}_1^2\left(\frac{2}{\beta }+1\right)+\frac{1}{\beta^3}+\frac{1}{\beta^2} $$
(5)

In the present study, the calculated value of β was larger than 67, so only the first three terms on the right side of Eq. (2) were adopted for calculating \( {\alpha}_1^2 \)as the rest terms were negligible.

The time plot of the measured oxygen concentrations was best fit using Eq. (1). Accordingly, the slope (i.e.,\( \eta =-\frac{D_{\mathrm{p}}{\alpha}_1^2}{L^2\varepsilon } \)) of Eq. (1) could be determined. Thereafter, DP was determined as follows:

$$ {D}_{\mathrm{p}}=-\frac{L^2\varepsilon \eta}{\alpha_1^2} $$
(6)

Afterwards, the porosity, SAC, and degree of saturation (DS) were calculated according to the measured soil GWC and particle density. Three replicated tests were conducted to measure DP of each soil sample amended with a given BC under a controlled DOC and SAC. In total, 108 soil samples were prepared and tested.

Micro-structure analysis by SEM

Two groups of test soil samples with the following target variables were selected for micro-structure analysis, i.e., (a) 10% BC, 10% SAC, and 85% DOC; and (b) 10% BC, 10% SAC, and 95% DOC. After measuring the DP, a soil specimen with volume about 0.5 cm3 was collected from the soil sample inside the cutting ring, dried at 45 °C for 48 h, sprayed with a thin layer of gold, and then investigated by scanning electron microscopy (Nova Nano SEM 230; FEI Czech Republic s.r.o. Co., Ltd. in Czech; energy resolution 132 eV).

Statistical interpretation

One-way analysis of variance (ANOVA) and subsequent significant difference tests were conducted to analyze the measurements (Ng et al. 2018). The significance level of all ANOVA analyses was 0.05, corresponding to 95% confidence interval. Statistical analysis was performed using the statistical toolbox provided by Matlab (Matlab 2014). The standard deviation was calculated for all the data measured in repeated experiments, as shown in Table 2.

Table 2 The soil air content (SAC), gravimetric water content (GWC), degree of saturation (DS), gas diffusion coefficient (DP), and porosity of each soil sample
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Results and discussion

Effects of biochar on soil compaction curve

Figure 2 shows the compaction curves of soils with different BCs. After the addition of biochar, the maximum dry density decreased, which was consistent with the reported findings in the literature (Garg et al. 2019; Reddy et al. 2014). It was due to that the particle density of the biochar is smaller than the soil used in the tests (Table 1). On the contrary, a larger optimum moisture content could be observed after adding biochar. It could be probably attributed to that the porosity of the biochar was larger than soil particles (Guo et al. 2014). Compared with the compaction curves of BAS to those of Garg et al. (2019), a lower decrease in the maximum dry density and a larger increase in the optimum moisture content of the soil samples were observed in the present study (Fig. 2). It was likely because the biochar adopted in Garg et al. (2019) had a lower particle density (0.8 g/cm3) than that in the present study (2.56 g/cm3, Table 1).

Fig. 2
figure 2

Compaction curves for soils amended with different biochar contents (BCs)

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Effects of BC on DP at different DOCs

Figure 3 shows a time series plot of oxygen concentration (Cg) for a set of repeat tests of soil samples with 0% BC, 10% SAC, and 85% DOC. Satisfactory fit with goodness of fit (R2) larger than 95% could be obtained by using Eq. (1) through the least square method. Accordingly, DP could be calculated using Eq. (6) and were summarized in Table. 2.

Fig. 3
figure 3

The measured and fitted temporal variations of oxygen concentration in the active chamber for a set of replicated tests

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Figure 4 shows the relationship between DP and SAC for soils amended with different BCs. In all test groups, DP increased as SAC increased under a given DOC, which was consistent with the existing findings in the literature (Moldrup et al. 2000a; Hamamoto et al. 2011). It was because there were more gas-filled pores and the connectivity of these pores was improved under larger SAC. Under SAC of 5%, similar DP could be observed for soil samples with different DOCs. It was likely due to the blockage of gas flow by water, as the DS corresponding to SAC of 5% was about 85% (Table 2), at/above which the gas phase generally became discontinuous (Qiu 2016). As SAC further increased to 15%, the DP of soil samples with 85% DOC was significantly larger than those under larger DOC (90–95%), regardless of BC (p < 0.05). It was because that there were more relative amounts of large pores (pore diameter d > 30 μm) (Kuncoro et al. 2014) in soil samples with lower DOC, as shown in the two SEM images of soil samples with the same BC (10%), SAC (10%) but different DOCs of 95% and 85%, respectively (Fig. 5 a and b). It can be seen that the pore size of soil sample with low compactness (Fig. 5a) was larger than that with high compactness (Fig. 5b), since the large-size pores in soil would be preferentially compressed as DOC increased (Berisso et al. 2012; Kuncoro et al. 2014). Therefore, soil samples with 85% DOC might have more connected large-size pores at larger SAC, resulting in more available space, shorter flow paths, and reduced tortuosity of pores for gas diffusion and hence larger DP (Allaire et al. 2008; Moldrup et al. 2000b).

Fig. 4
figure 4

Effects of soil air content (SAC) on gas diffusion coefficient (DP) for soils amended with different biochar contents (BC) under different degree of compaction (DOC). a 0% BC. b 5% BC. c 10% BC. d 15% BC

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Fig. 5
figure 5

Soil aggregates morphology at 500X magnification for soils with different BC, SAC, and DOC. a 10% BC, 10% SAC, 85% DOC. b 10% BC, 10% SAC, 95% DOC

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Figure 6 shows the effects of BC on DP of soils under different DOCs and SACs. When SAC was below 10%, BC had negligible effects on DP (p > 0.05) under a given DOC and SAC. It was likely due to that most of the pores in the BAS were filled by water, leading to a discontinuous gas phase. As the diffusion of gas in water is 3–4 orders of magnitude lower than that in the air, SAC dominated DP under low SAC (Resurreccion et al. 2008b). As SAC further increased to 15%, the addition of biochar generally promoted the gas diffusion in soil, and the effect depended on DOC. For example, when the DOC was about 85%, the DP increased as BC increased to 5% and remained substantially unchanged thereafter, while DP generally increased with BC under larger DOC (≥ 90%). This indicated that the relatively large-size pores in soils with DOC of 85% (Fig. 5a) dominated gas diffusion, while the addition of biochar might increase the pore connectivity under high DOC (≥ 90%). A direct engineering implication of this finding is that biochar addition could be adopted to compromise the contradictory requirements of high DOC (e.g., ≥ 90%, leading to a reduced soil aeration) and high soil aeration (i.e., large DP) in sloping landfill cover and man-made slope, where a high DOC is necessary for stability concern but a good aeration is needed for vegetation growth.

Fig. 6
figure 6

Effects of biochar content on DP of soils with different DOC and SAC. SAC values are presented in mean ± standard deviation

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Conclusions

Effects of BC (0%, 5%, 10%, and 15%) on gas DP of soil were measured by a two-chamber apparatus under different DOC (85%, 90%, and 95%) and SAC (5%, 10% and 15%). It was found that when SAC was less than about 5%, DP was relatively low and controlled by SAC, regardless of DOC and BC. This was likely due to blockage of pore volume by water, resulting in less available space for gas flow and poor air-filled pore connectivity. On the contrary, when the SAC was relatively high (≥ 15%), soil with DOC of 85% had the largest DP for DOC ranging from 85 to 95% under a given SAC. No noticeable difference could be observed for DP under DOC of 90% and 95%. It was because soil samples with DOC of 85% had relatively larger pore size than those with higher DOC.

The influence of BC on DP depended on SAC and DOC. When SAC was less than 10%, the influence of BC on DP could be neglected. As SAC increased to about 15%, the DP of soil samples with high DOC (≥ 90%) generally increased with BC, while DP increased with BC up to 5% and then remained basically unchanged under DOC of 85%. This indicated that gas diffusion was affected not only by available gas-filled pores but also by characteristics of pores (e.g., pore-size distribution, pore connectivity). Further effort is needed to investigate the effects of pore characteristic on DP.