Influence of soil density on gas permeability and water retention in soils amended with in-house produced biochar

Ankit Grg,He Hung,Weiling Ci,Nrl Gngdhr Reddy,b,Peinn Chen,Yifn Hn,Viroon Kmhoom,Shubhm Gurv,d,Hong-Hu Zhu

a Department of Civil and Environmental Engineering,Guangdong Engineering Center for Structure Safety and Health Monitoring,Shantou University,Shantou,China

b Department of Civil Engineering,Kakatiya Institute of Technology and Science,Warangal,India

c Department of Civil Engineering,King Mongkut’s Institute of Technology Ladkrabang,Bangkok,Thailand

d Department of Biology Science and Biology Technology,Indian Institute of Technology Guwahati,Guwahati,India

e School of Earth Sciences and Engineering,Nanjing University,Nanjing,210023,China

Keywords:Biochar Degree of compaction (DOC)Gas permeability Soil water retention Wetting-drying cycle

ABSTRACT Biochar has been used as an environment-friendly enhancer to improve the hydraulic properties (e.g.suction and water retention) of soil.However,variations in densities alter the properties of the soil-biochar mix.Such density variations are observed in agriculture (loosely compacted) and engineering(densely compacted) applications.The influence of biochar amendment on gas permeability of soil has been barely investigated,especially for soil with different densities.The major objective of this study is to investigate the water retention capacity,and gas permeability of biochar-amended soil (BAS) with different biochar contents under varying degree of compaction (DOC) conditions.In-house produced novel biochar was mixed with the soil at different amendment rates (i.e.biochar contents of 0%,5% and 10%).All BAS samples were compacted at three DOCs (65%,80% and 95%) in polyvinyl chloride (PVC)tubes.Each soil column was subjected to drying-wetting cycles,during which soil suction,water content,and gas permeability were measured.A simplified theoretical framework for estimating the void ratio of BAS was proposed.The experimental results reveal that the addition of biochar significantly decreased gas permeability kg as compared with that of bare soil (BS).However,the addition of 5%biochar is found to be optimum in decreasing kg with an increase of DOC(i.e.kg,65%>kg,80%>kg,95%)at a relatively low suction range (<200 kPa) because both biochar and compaction treatment reduce the connected pores.

1.Introduction

Recent studies have engrossed in converting the agricultural and industrial wastes into biochar and using them to improve fertility and engineering properties of soil (Lehmann and Joseph,2015;Colantoni et al.,2016;Bordoloi et al.,2018;Gao and DeLuca,2018;Kavitha et al.,2018).Addition of biochar to the soil can also improve its water retention and hence reduce the consumption of water in agricultural applications.The soils are usually compacted at around 65% degree of compaction (DOC) (i.e.65% of the maximum dry unit weight) (Garg and Ng,2015) and thus are more permeable and help to transport air and nutrients (Bruun et al.,2014).

On the other hand,previous literature has shown that amendment of biochar can improve the water retention of a dense soil(i.e.over 90%DOC)(Wong et al.,2017;Bordoloi et al.,2018;Garg et al.,2019).Such high densities are often required in geotechnical infrastructures,such as man-made slopes,compacted embankments,and landfill covers (Wickramarachchi et al.,2011;Jien and Wang,2013;Ni et al.,2018).The soil used in landfills is more impermeable than those in other engineering structures for preventing the infiltration of leachate into the waste (Mateus et al.,2012) and reducing the emission of greenhouse gases(CO2and methane)into the atmosphere(Mohareb et al.,2011).Overall,gas permeability is an important property of soil (Ni and Ng,2019;Yang et al.,2019),which is related to void space or DOC(Joseph et al.,2019).Usually,the application of biochar to a loose soil may increase its gas permeability due to the porous structure of biochar,which results in the decrease in bulk density of biochar-amended soil (BAS) and increase in total porosity(Sun et al.,2013).

Fig.1.(a)Compaction curves of BS and BAS;(b)FTIR analysis for biochar;(c)FE-SEM images for biochar;and(d)Thermogravimetric analysis of water hyacinth in the current study.

On the contrary,Garg et al.(2019) found that the addition of biochar can reduce gas permeability of compacted soils by 50%-65%.Moreover,organic particles of biochar enhance the bonding between large particles (Liu et al.,2012).There is a lack of systematic study on effects of DOC (i.e.dense and loose soils) on gas permeability and water retention properties of BAS.This study aims at providing an improved insight into this issue and clarifying soilbiochar-water interactions at different DOCs for agriculture and engineering applications.

In addition to improving soil properties,biochar production and its usage in agriculture and geotechnical applications can help to manage waste.Biochar can be produced from various wastes and feedstocks,such as pig manure,wood,poultry litter,and crop residues(Kumar et al.,2019).Water hyacinth is an invasive weed that is spread throughout the world(Patel,2012),especially in southern China,Thailand,and India,where the governments have been spending a huge amount of money on its control(Gao and Li,2004;Lu et al.,2007;Zhang et al.,2015;Yan et al.,2017).Water hyacinth is rich in cellulose,making it a favorable material for biochar production (Masto et al.,2013;Bordoloi et al.,2018).Therefore,it is important to explore the possible use of biochar obtained from water hyacinth in engineered and agricultural soils.The usage of water hyacinth can help to create micro-industries and hence increase income in rural areas (Bordoloi et al.,2015),where such invasive weeds are abundantly found.

The major objective of this study is to investigate the effect of biochar addition and DOC on the soil-water characteristic curve(SWCC)and gas permeability.For this purpose,in-house produced biochar was amended to the soil at 0%,5%and 10%(by weight)and compacted in fabricated columns under three DOCs(65%,80%and 95%).All soil columns were subjected to a 49-d monitoring period,consisting of 42-d drying and 7-d wetting.The soil suction,moisture content,and gas permeability were measured during the testing period.Furthermore,a theoretical model was developed to comprehend the mechanism of soil-biochar interaction at different DOCs (i.e.void ratio).The outcome of this study helps to understand soil-biochar-water interactions at varying DOC for engineering applications.

2.Material and methods

2.1.In-house biochar production and preparation of soil-biochar composites

Biochar used in the present study was produced from water hyacinth collected from a local pond in Shantou,China.The collected water hyacinth was preliminarily treated by cutting roots and leaves,and stems were retained for production of biochar.Subsequently,the treated water hyacinth was air-dried to remove the available free water for efficient pyrolysis.The water hyacinth biochar was produced in a pyrolysis furnace at 600°C under zerooxygen supply.Thermogravimetric analysis (TGA) of dried water hyacinth is shown in Fig.1d,which yields around 20% of the product at 600°C pyrolysis.Water hyacinth (mainly contains cellulose) breaks down when heated at 240°C-350°C,while higher temperature(600°C) results in higher specific surface area and porosity of biochar(Liu et al.,2015).To understand the physical and chemical properties of biochar,field emission scanning electron microscopy (FE-SEM) and Fourier transform infra-red (FTIR)tests were conducted.The SEM images and FTIR analysis of a novel biochar produced are shown in Fig.1.While SEM images suggest a very porous surface with numerous nano-pores and very high specific surface area (Bordoloi et al.,2018),FTIR analysis indicates three major surface functional groups (i.e.-OH,-COOH and -CO,as shown in Fig.1b),which are hydrophilic.In the current study,biochar particles passing through 0.425 mm sieve were considered for experimentation.

The soil used in this study was collected from Shantou University,China.American Society for Testing and Materials(ASTM)standards were used to determine the geotechnical properties of the soil,as summarized in Table 1.Table 1 shows that the soil is dominated by coarse sand,and the major particle sizes are in the range of 1.18-2.36 mm and 2.36-4.75 mm(29.7%and 50%,respectively).The soil can be categorized as SP (poorly graded sand) according to the Unified Soil Classification System (USCS) (ASTM D2487-17,2017).The compaction curves of bare soil (BS) and BAS with different biochar contents are presented in Fig.1a.The maximum dry unit weight (MDUW) and optimum moisture content (OMC) of soil are found to be 16.9 kN/m3and 18.8%,respectively.With an increase in biochar up to 5% and 10%,MDUW decreases by 1.8% and 2.4%,respectively.On the other hand,OMC increases by 3.7% and 5.3%with an increase in biochar by 5% and 10%,respectively.This is because the biochar particles are much lighter(i.e.very low specific gravity) as compared to that of soil.Thus,MDUW is expected to decrease when biochar particles replace the soil in a given volume(Bordoloi et al.,2018;Ni et al.,2020).The OMC increases due to high water adsorption properties of biochar(Reddy et al.,2015).

2.2.Test plan

All the experimental works were conducted in the greenhouse(Fig.2c) established at Shantou University,China.The tests were divided into three different series,including one series of tests on BS samples and two sets on BAS with biochar contents of 5% and 10%(by weight).Three DOCs(65%,80%and 95%)were used for each series of tests.All the soil columns were labeled as S-X-Z,where S-X represents the series with different biochar contents (0%,5% and 10%) and Z represents the compaction state (i.e.DOCs of 65%,80%and 95%).Thus,in the current study,a total of nine soil columns were monitored.The biochar percentages were considered basedon the studies of Reddy et al.(2015),Wong et al.(2017) and Bordoloi et al.(2018).All of the nine soil samples were compacted in a custom-made polyvinyl chloride(PVC)column with a diameter of 300 mm and a height of 250 mm (Fig.2b).

Table 1Basic characteristics of soil and BAS.

2.3.Test procedures

The compaction procedure was adopted from the studies of Li et al.(2016) and Huang et al.(2020).The compacted soil columns are shown in Fig.2a and b.Holes were drilled at the bottom of the PVC column to allow water to drain without any loss of soil particles.Each sample was dry-mixed using a mechanical mixing device in three batches with a predetermined proportion of biochar (by weight).The composite with requisite water content was then compacted into three layers with the preset DOC using static compaction.Distilled water was used in the experimental study to avoid any error due to salinity in the suction measurement.It should be noted that the circumference of soil column was sealed using clay slurry to minimize any gas leakage along the soil-column interface (Zhang and Rothfuchs,2008).

After preparation of BAS,all the columns were placed in the greenhouse.To achieve the initial condition (i.e.suction close to 10 kPa),a sprayer device was used to irrigate each column thoroughly,and they were left overnight for water equilibrium.Then the soil columns were monitored for a 42-d drying and a 7-d wetting period,respectively.The experimental system used for monitoring the soil columns is shown in Fig.2a-c.An MPS-6 suction sensor (10-100,000 kPa) and an EC-5 volumetric water content sensor(Decagon Devices Inc,2016)were inserted 100 mm into the soil from the top.The two sensors were placed 100 mm apart to minimize the interaction,and connected to a data logger for minoring suction and water content,respectively (Huang et al.,2020).A digital environmental sensor was used to record temperature,relative humidity and evaporation,as shown in Fig.2d.The average relative humidity of 70.4%,temperature of 16.9°C-26.1°C,and evaporation rate of 0.56-1.02 mm/d were observed during the monitoring period.

Fig.2b shows the details of the gas permeability determining apparatus.Gas cylinder,flow meter,soil column,and pressure sensor were connected successively using rubber hosepipes.The gas cylinder was used to supply CO2gas.Flow meter was used to measure the flow rate of CO2flow,which was provided by the gas cylinder to pass through the soil column.Gas flow rate could be detected from 0 to 20 mL/min with a sensitivity of 0.1 mL/min.The digital pressure sensor (with a sensitivity of 1 Pa) was installed to measure the gas pressure(P)in the lower chamber of soil column.During the measurement of gas permeability,CO2was supplied at a steady flow rate (q).

The difference of gas pressure between the bottom and top of the soil column can be represented by gage pressure at the bottom,which is measured by a pressure sensor.The difference of gas pressure(Δp)is the driving head for flow permeating.Based on the Darcy’s law,the gas permeability (kg) of soil column can be determined by the following equation (Damkjaer and Korsbech,1992;Garg et al.,2019;Ni and Ng,2019):

where A is the cross-sectional area of the soil column,L is the length of the soil column,and μ is the absolute viscosity of CO2gas flow(14.8 × 10-6N s/m2).The measurement of gas permeability was fulfilled by recording instantaneous measurements of gas pressure and flow rate.

Fig.2.Test column in the greenhouse at Shantou University,China:(a)Original diagram of soil column measurement system;(b)Schematic diagram of soil column measurement system;(c) Greenhouse interior;and (d) Climate conditions.

3.Simplified theoretical framework for estimating void ratio of soil-biochar composites

3.1.Determination of biochar content for a BAS sample

In the past decade,use of biochar as a sustainable material has increased for geo-environmental and agricultural engineering applications.It is essential to propose an effective method to measure biochar content for practical applications.Based on the classical soil mechanics,three assumptions for soil-biochar composites were considered:

(1) Biochar particles are considered as rigid body which is the same as soil particles in the BAS.

(2) Particles of biochar and soil are mixed uniformly.

(3) Calculation of void ratio only concerns physical relationship and neglects volume change of solid particles due to chemical interactions.

The specific gravity of soil,biochar and soil-biochar mixture is expressed as Gss,Gsband Gsm,respectively.

For soil-biochar mixture,the following physical relationships exist:

where α and β are the soil and biochar contents in the soil-biochar mixture,respectively.

The biochar content and the specific gravity of soil-biochar mixture can be respectively derived as

3.2.Determining void ratio of soil-biochar composite

Based on the basic relationship in soil mechanics,the void ratio of BS is

where w is the gravimetric water content,γwis the unit weight of water (9.81 kN/m3),and γ is the unit weight of soil.

The void ratio of soil-biochar composite can be written as

Subtracting Eq.(5) from Eq.(6) yields

Substituting Eq.(6) into Eq.(7),we have

Thus,the following equation can be obtained:

Substituting Eq.(4) into Eq.(9),we have

Therefore,the void ratio of BAS can be expressed as a function of biochar content,specific gravities of soil and biochar,and void ratio of BS:

Table 2Theoretical calculations of the void ratio.

To study the behavior of soil-biochar composite and the application of biochar in the field,it is practical to measure the above four parameters,so that the real void ratio of BAS can be determined by such a simple method.

3.3.Verification and application of new simplified equations

The specific gravities(Gs)of soil and biochar were measured to be 2.59 and 2.26,respectively.Eq.(3)can be verified for Gsof BAS.Thus,Gsof BAS can be calculated as follows:

(1) For mixture with 5%biochar content,Gs=0.05×2.26+(1-0.05) × 2.59=2.573;and

(2) For mixture with 10%biochar content,Gs=0.1×2.26+(1-0.1) × 2.59=2.557.

The specific gravity of BAS measured in the laboratory is given in Table 1.Compared with theoretical calculations,the errors are found to be about 0.23%,which might be contributed to uneven mixing and measuring errors.Thus,the proposed method is acceptable with the least error.

Fig.1a shows the compaction curves of BS and BAS.The MDUW of the BAS decreases with an increase in biochar content,which is mainly due to the lighter specific gravity of biochar.The void ratio of each soil column was computed using Eq.(11)and is summarized in Table 2.As seen from the table,the change in the void ratio of BAS can be well illustrated according to biochar percentage.With the increase in DOC,the void ratio of the soil sample decreases.The high biochar content changed the void ratio of soils.This explains why the BAS holds high water retention at lower DOC at the same biochar content (Wong et al.,2017).

4.Results and discussion

4.1.Variation of suction and water content for samples

Suction and water content are the two vital variables in SWCC.Hence these parameters were monitored for a period of 49 d.Fig.3a shows the variation of suction with respect to time for BAS at different DOCs.From the 1st to the 41st day,the suction of all the columns increases in drying period,and from the 41st to the 48th day,the suction of each column declines for wetting period as expected in soil water retention properties.Fig.3b shows the variation of volumetric water content for all columns with variation of time.The trend of volumetric water content is the same as suction measurements and can be used for plotting SWCCs in drying and wetting.Fluctuations in environmental condition occurred in the greenhouse at around the 25th,30th,and 35th days because of humidity changes due to rainy weather (see Fig.2d) and its consequence on suction and water content can be observed.

Among nine suction curves,the columns with higher DOC and higher biochar content (i.e.S-10-95,S-5-95 and S-10-80) achieve relatively higher suction compared to the columns with lower DOC and lower biochar content(S-0-65,S-0-80 and S-5-65).The results also show that the columns with lower DOC and higher biochar can retain more water among all the samples,as shown in Fig.3a.Samples S-10-65 and S-10-80 retain the highest water content and S-5-95 has the lowest water content.Under wetting period,BAS samples are able to retain more water under given suction.This is in agreement with the result of Abel et al.(2013)who studied the use of biochar on soil water retention capacity of sandy soil.This is mainly due to the increases in pore size distribution and pore volume of the BAS.

Fig.3.Variations of (a) soil suction and (b) soil water content (Feddes et al.,1978).

Fig.4.Comparisons based on compaction state and biochar content for SWCCs.

4.2.Variation of SWCC model

Fig.4 shows the comparison of SWCCs of nine samples with varying DOCs and biochar contents.The selected soil is a poorly graded sand,and it is easy to drain water out after saturation and results in low air entry potential.It can be seen from Fig.4 that the suction in all columns remains nearly constant at a minimal value(about 10 kPa) at the beginning of the drying-wetting cycle.Air entry potential of the ceramic of sensor is around 9 kPa (Decagon Devices Inc.,2016).This will limit measurement range to around 9 kPa(Garg et al.,2019).

On the one hand,Fig.4 compares the difference in DOC for each biochar content.For BS,samples with 65% and 80% DOCs share almost the same SWCC,but the curve of the sample S-0-95 moves up.It means that under the same water content,the suction of S-0-95 is higher than those of the other two samples.For 5% BAS samples,the curves under 95%DOC nearly lie on that of the samples under 65% DOC,while the curves of the samples under 80% DOC have higher suction values than the other two series.For 10% BAS,the curve of the sample S-10-65 is slightly higher than that of S-10-80.The 10% BAS sample under 95% DOC has relative low suction values at the same water content.It should be noted,however,that the sample S-10-65 cannot simply be assumed to have the best water retention capacity.The sample void ratio shown in Table 2 should be fully considered.On the other hand,from Fig.4,the sequence of curves (i.e.95% >80% >65%) becomes the new sequence(i.e.65%>80%>95%)due to the biochar addition.Fig.4 compares the volumetric water content of samples with different biochar contents for each DOC.Generally,the curves are in the order of 10%BAS>5%BAS>BS,which suggests the suction values of 10%BAS>5%BAS>BS for the same water content.The 5%biochar addition improved the volumetric water content to some extent as compared to BS.The 10%biochar content shows higher volumetric water content.For the same suction,the water content of 10%BAS samples is significantly higher than that of the other two samples.

For moderately compacted soils(80%DOC),peak suction values of samples with 5%and 10%biochar amendment rates are 1745 kPa and 2755 kPa,respectively.These values are significantly higher as compared to that of BS(553 kPa).However,soils under 95%DOC do not follow a similar trend to those of soils under 65%and 80%DOCs.For soils under 95% DOC,the peak suction of 5% BAS is 3265 kPa,which is higher than that of 10% BAS (i.e.2953 kPa) and BS (i.e.2425 kPa).

4.3.Variation of gas permeability

Gas permeability is related to the SWCC and reflects pore structure characteristics.Thus,the relationship of gas permeability-suction is plotted,as shown in Fig.5.Comparisons among samples with different DOCs and biochar contents are illustrated in Fig.5.Generally,due to high porosity,the gas permeability of sample under 65%DOC is higher than those of soils under 80%and 95%DOCs.However,in high suction range(i.e.about>1000 kPa),the gas permeability of samples under 95% DOC is higher than those under 80% and 65% DOCs.This phenomenon occurred in each sample treated with different biochar contents.Furthermore,the slope gradient of gas permeability-suction curves changes according to DOC,i.e.slope of 95% >slope of 65% >slope of 80%.The slopes of these curves are related to the pore structure characteristics(Brooks and Corey,1964;Huang et al.,2020),which means that biochar addition changes the innerstructure of soil.In Fig.5,for each DOC,gas permeability shows magnitude order of BS>10%BAS>5%BAS.Moreover,the slope of gas permeability trends to have a similar relationship.In addition,as shown in Fig.5,Garg et al.(2019) found a similar trend,suggesting reduction of gas permeability with addition of biochar.It should be noted that their study was conducted for only one soil density(80%DOC).However,both the current and previous studies confirm the potential role of biochar in reducing gas permeability.

Fig.5.Comparisons of gas permeability varying with suction based on compaction state and biochar content.

4.4.Discussion on the effects of DOC and biochar addition

As shown in Table 2,calculations with the proposed equations(Eq.(4)and(7))indicate that the reduced DOC and biochar addition can increase the void ratio of samples theoretically.On the one hand,higher DOC leads to lower void ratio.The addition of biochar increases the void ratio,but 5% BAS shows higher void ratio than 10%BAS.The DOC changes the void ratio due to externally applied energy without changing the soil particle size distribution.In contrast,biochar addition changes the internal pore size redistribution as the biochar particles change the particle size distribution of soil.Similarly,Lipiec et al.(2012) suggested that the soil void ratio is lower at higher DOC and high for loose or moderately compacted soils.Bodman and Constantin (1965) also concluded that compaction efforts could be attributed to the fact that loose soil has large voids which are likely to retain more water.The loss of water from larger pores of loose soil may not be captured effectively by suction measuring probe (as compared to small pores)because of low tension (i.e.capillary suction) produced by water in large pores.The study by Garg et al.(2015) suggested that this is likely because of the larger number of smaller pore sizes (i.e.intrabiochar pore size and biochar-soil pore size) for 10% BAS.Smaller pore sizes lead to higher tension.

The interaction mechanism among the particles in this study can be considered as soil-soil (i.e.BS),biochar-biochar (i.e.pure biochar particles),and soil-biochar(i.e.composite)interaction.BS samples only have soil-soil interaction,while BAS samples have three kinds of interactions.Therefore,the void ratios from three types of interaction mechanisms are also various,i.e.es-s,eb-b,and es-bare the void ratios of soil-soil,biochar-biochar,and soilbiochar particles,respectively.BS samples have only es-s,and 5%BAS samples have the three types of voids,and 10% BAS samples have eb-b.This can be attributed to the similarity between the soilsoil pore size and soil-biochar pore size under 80%DOC and porous structure of biochar.Thus,the properties of biochar start to dominate over that of soil as the amendment rate increases.

Total suction (i.e.water potential) consists of three kinds of effects,i.e.capillary effects,short-range adsorption effects,and osmotic effects (Lu and Likos,2004).Capillary effects,due to airwater interface,are controlled by pore structures.Short-range adsorption effects are from electrical and van der Waals force fields with solid-liquid interaction.Osmotic effects originate from dissolved solutes in pore water,soil mineral surfaces,and exchangeable cations.Thus,we have

where Ψtis the total suction;Ψmis the matric suction,which contains capillary effects and short-range adsorption effects;and Ψois the osmotic suction,resulting from dissolved solutes in the pore water(e.g.exchangeable ions from soil mineral or biochars).In the current study,BAS samples have extra osmotic suction due to the introduction of biochar,which consists of exchangeable ions(see Fig.1b).Oxhydryl groups,ether groups,and carboxyl groups of biochars in pore water can enhance water retaining ability,osmotic suction,as well as electrical fields to improve short-range adsorption effects (Huang et al.,2020).

4.4.1.SWCC models

Suction and water content curves in Fig.3 are similar to theoretical calculations shown in Table 2.It can be observed that low DOC (65%) state provides more pore space for holding water.Biochar has a great water retention ability due to the porous structure and large specific surface.In SWCC models (Fig.4),80% DOC can reduce primary pore structure in soil samples and slightly increase the matric suction due to increased capillary effects.In comparison,95% DOC reduces the internal pore structure,and thus capillary effects are enhanced with narrower pore size.Therefore,for BS,the curve of the sample S-0-80 almost cover that of the sample S-0-65,and the sample S-0-95 has larger suction than the other two samples at the same water content.In Fig.4,the order of the curves should be BS >10% BAS>5% BAS according to comparison of calculated void ratios listed in Table 2.This suggests that BS with the lowest void ratio should have higher suction and 10%BAS with the highest void ratio should have lower suction at the same water content according to the capillary effects.However,osmotic and matric suctions from short-range adsorption effects due to biochar presence increase with an increase in biochar content.Hence,as shown in Fig.4,the order (10% BAS >BS >5% BAS) might occur because the osmotic suction and short-range adsorption effects arise the suctions of 5%BAS and 10%BAS,especially 10%BAS.Fig.4 shows that the capillary effects are enhanced due to compaction variation.Different parts in the suction increase at different rates with varying compaction degree.The sample S-X-65 shows increasing suction as the biochar content increases,the increasing rate of which is less than those of S-X-80 and S-X-95.The sequence of curves (i.e.95% >80% >65%) turns into the new one (i.e.65%>80%>95%)due to biochar addition.The study of Berisso et al.(2012)also shows that the increasing rate of suction for moderately and highly compacted soils(80%and 95%DOCs)is higher than that for loose soils (65% DOC).This is because of a relatively smaller number of large pores (containing bulk water) in the soils,which takes less time to dry up,and the drying affects the water held in capillaries earlier in the compacted soils.

4.4.2.Gas permeability

Intrinsic permeability K is defined as K=Cd2,where C is a dimensionless constant related to the geometry of soil pores,and d is the pore diameter.Accordingly,the gas permeability can be also defined as kg=(ρg/μ)K (Brooks and Corey,1964).Therefore,pore size and pore structure characteristics play a vital role in the gas permeability.As for external factors,the SWCC governs the behaviors of gas permeability.The flow of pore air in unsaturated soil is governed by the total potential (absolute pressure) of the air phase.

As kgis directly correlated with pore size distribution (Ball,1981),it decreases as density increases.Thus,in this study,the increase of DOC apparently decreases the void pore size,resulting in a reduction of gas permeability.At high-suction range,samples under 95%DOC increase the gas permeability until the DOC is less than 65%.This is because of the high head gradient due to high suction (Rouf et al.,2016;Garg et al.,2019;Gopal et al.,2019).Biochar rich in surface hydrophilic groups(Fig.1b)can improve the water retention of sandy soils(Hardie et al.,2014).This means that the water content in the pore increases and hence slows down the gas transport in pore path (Garg et al.,2019).Therefore,the gas permeability in BS is generally higher than that in BAS (Fig.5).Comparison of the gas permeability (Fig.5) perfectly meets the assumption of Eq.(12).With increasing biochar content,as indicated in Table 2,the void ratio changes following the sequence of 5%BAS >10% BAS >BS,which results in various saturation states(relative saturation degree ranking:BS >10% BAS >5% BAS) and arises corresponding capillary effects (i.e.enhanced suction).Therefore,the gas permeability has a trend of BS >10% BAS >5%BAS,as shown in Fig.5,which conforms to the theory proposed by Brooks and Corey (1964).The slope of gas permeability-suction curve changes due to internal structure reorganized by DOC and biochar addition.In 5% BAS samples,biochar-biochar and soilbiochar particles replace some parts of the soil-soil particles,and smaller biochar particles make pore path narrow.As for the 10%BAS sample,the percentage of biochar-biochar and soil-biochar particles increases,especially the biochar-biochar particles,when increasing the dosage of biochars.Therefore,more soilbiochar and biochar-biochar interactions widen the pore path in some way compared to 5% BAS samples.Garg et al.(2019) investigated the influence of biochar on air permeability in unsaturated soils and also found that biochar reduced the air permeability.However,the variations in gas permeability caused by biochar content in these two works are different.Garg et al.(2019)pointed out that the gas permeability decreased by up to 50%and 65%for 5%BAS and 10% BAS,respectively.On the contrary,the results of the current study illustrate that the gas permeability shows a magnitude order of BS>10%BAS>5%BAS for each DOC(see Fig.5).The discrepancy could be attributed to the different gradations of soils used in the two studies.The former study uses SC (sand clay mixture,with around 81% sand and 19% clay) while the current study uses SP (poorly graded sand,see Table 1).Different soil gradations will probably influence the extent of variations in biochar affecting soil properties (Razzaghi et al.,2020) including the gas permeability,A systematic review by Razzaghi et al.(2020) found that effects of biochar can vary significantly depending on the type of soil.Biochar is able to improve water retention of coarser soil more effectively than that of fine soil.

Fig.6.Effects of biochar content and compacted state on gas permeability under different suctions:(a) 100 kPa,(b) 1000 kPa,and (c) 2000 kPa.

4.5.Prospect of biochar in agriculture and engineering applications

Fig.6 shows the influence of biochar content and compacted state on gas permeability under three different suctions (low(100 kPa),moderate(1000 kPa),and high(2000 kPa)suctions).Gas permeability,as obtained from Fig.5,is further normalized(0 ≤kg,n≤1) to interpret the effects of biochar content and compacted state on the gas permeability of samples.As shown in Fig.6a,the gas permeability of 65%DOC samples is the highest at suctions near to 100 kPa.The gas permeability in 95% DOC samples at suction near to 100 kPa is almost negligible.In agriculture,air exchange is one of the most important parameters that directly impact plant growth and crop production(Tang et al.,2011).Thus,S-10-65 can be recommended for use in agriculture land.The effective soil gas exchange(high gas permeability)is higher even at higher water content (i.e.at field capacity).On the contrary,in engineering applications such as landfills,less permeable soil is often desirable to minimize greenhouse gas emissions (Mohareb et al.,2011).From Fig.6a-c,80% DOC soils have lower gas permeability than other samples over a wide suction range(100 kPa,1000 kPa and 2000 kPa).It should be noted that 95%DOC soils have the lowest gas permeability under low suction(100 kPa).However,the gas permeability of soils under 95% DOC increases rapidly with an increase in suction(suffering drought).Considering that the soils used in engineering applications are often subjected to heavy rain (which causes low suction) or prolonged drought(which causes high suction),80%of DOC can be recommended for use in engineering applications.Besides,as shown in Fig.6,soils with 5% biochar have lower gas permeability than other soil samples.Thus,the sample S-5-80 is strongly suggested for engineering applications.It should be noted that these are preliminary recommendations based on the soil and climate conditions given.Any optimal content of biochar will also depend on plant type and soil type(Razzaghi et al.,2020).Further systematic studies are needed to evaluate effects of biochar on shear strength considering simultaneous influence on plant growth in long term (Ni et al.,2020;Razzaghi et al.,2020).

The current study involves field tests over a 49-d period to explain the effects of biological content and compaction on soil gas permeability.Previous studies have shown that under the longterm effect,biochar can improve the nitrogen-use efficiency in agricultural soil(Xie et al.,2020),reduce greenhouse gas emissions from agriculture (Qin et al.,2016),and play a positive role in the availability of phosphorus (P) and potassium (K) (Farkas et al.,2020).Besides,researchers have pointed out that biochar produced by pyrolysis has stable properties(Liu et al.,2015).It can be seen from this study that the influence of biochar on gas permeability is mainly due to its change in soil pore structure.Biochar particles could migrate along with water transfer in soil under long-term effect and hence,pore structure change.Aggregation in long term can also influence permeability,which has been rarely investigated (Jien and Wang,2013).The gas permeability of BAS under long-term effects is a potential field that is worth exploring.

5.Conclusions

In this paper,the experimental results and proposed equations show that DOC and biochar content reduced the gas permeability of the soil.However,kgwas minimum for 5%BAS and maximum for BS at all DOC states.Soil amended with 10%biochar content under 95%DOC is found to have the highest water retention ability.Biochar addition changes the intra-pore structure among soil and can be used to adjust the properties of soils to meet engineering needs.This study suggests 10% and 5% amendment rates as the optimum for agricultural(65%DOC)and engineering(80%DOC)applications,respectively.

Declaration of competing interest

The authors wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

Acknowledgments

The authors would like to acknowledge the National Natural Science Foundation of China (Grant No.41907252),Shantou University Scientific Research Fund (Grant No.NTF17007),and International Collaborative Research Fund provided by King Mongkut’s Institute of Technology Ladkrabang,Thailand.