REGULAR ARTICLE

Does biochar influence soil physical properties and soil wateravailability?

Marcus Hardie & Brent Clothier & Sally Bound &

Garth Oliver & Dugald Close

Received: 27 August 2013 /Accepted: 14 November 2013 /Published online: 8 December 2013# Springer Science+Business Media Dordrecht 2013

AbstractAims This study aims to (i) determine the effects ofincorporating 47 Mg ha−1 acacia green waste biocharon soil physical properties and water relations, and (ii) toexplore the different mechanisms by which biocharinfluences soil porosity.Methods The pore size distribution of the biochar wasdetermined by scanning electron microscope and mercuryporosimetry. Soil physical properties and water relationswere determined by in situ tension infiltrometers, desorp-tion and evaporative flux on intact cores, pressure cham-ber analysis at −1,500 kPa, and wet aggregate sieving.Results Thirty months after incorporation, biochar ap-plication had no significant effect on soil moisture con-tent, drainable porosity between –1.0 and −10 kPa, fieldcapacity, plant available water capacity, the vanGenuchten soil water retention parameters, aggregatestability, nor the permanent wilting point. However,the biochar-amended soil had significantly higher near-saturated hydraulic conductivity, soil water content at−0.1 kPa, and significantly lower bulk density than theunamended control. Differences were attributed to the

formation of large macropores (>1,200 μm) resultingfrom greater earthworm burrowing in the biochar-amended soil.Conclusion We found no evidence to suggest applica-tion of biochar influenced soil porosity by either directpore contribution, creation of accommodation pores, orimproved aggregate stability.

Keywords Plant available soil water (PAWC) . In situ .

Soil amendment . Apple . Soil water retention

Introduction

Biochar is a predominantly stable, recalcitrant organiccarbon compound created by pyrolysis of biomass attemperatures between 300 and 1,000 °C under low or nooxygen conditions (Jeffery et al. 2011; Krull 2011;Verheijen et al. 2010). The use of biochar as a soiladditive has been proposed as a means of mitigatingclimate change through long-term sequestration of car-bon whilst simultaneously improving soil properties andfunctions (Jeffery et al. 2011; Kookana et al. 2011;Verheijen et al. 2010). Biochar is highly porous, thusits application to soil is considered to improve a range ofsoil physical properties including total porosity, pore-size distribution, soil density, soil moisture content,water holding capacity or plant available water content(PAWC), and infiltration or hydraulic conductivity(Atkinson et al. 2010; Major et al. 2009; Sohi et al.2009b, 2010; Zwieten et al. 2012). However, there islittle peer-reviewed evidence that demonstrates that

Plant Soil (2014) 376:347–361DOI 10.1007/s11104-013-1980-x

Responsible Editor: Simon Jeffery.

M. Hardie (*) : S. Bound :G. Oliver :D. ClosePerennial Horticulture Centre, Tasmanian Institute ofAgriculture, University of Tasmania,Private Bag 98, Hobart, Tasmania 7001, Australiae-mail: marcus.hardie@utas.edu.au

B. ClothierPlant and Food Research, Food Industry Science Centre,Bachelor Road, PO Box 11-600, Palmerston North 4442,New Zealand

biochar application significantly improves the physicalproperties of in situ agricultural soils (Atkinson et al.2010; Shackley and Sohi 2010; Sohi et al. 2009a).Furthermore, the mechanisms or processes by whichbiochar may influence soil pore size distribution havenot been clearly established or demonstrated (Verheijenet al. 2010).

The specific mechanisms by which biochar influ-ences water retention, macro-aggregation, and soil sta-bility are poorly understood (Sohi et al. 2009b). Wepropose that biochar application may influence soilporosity and thus soil water retention via three mecha-nisms (1) direct pore contribution from pores within thebiochar, (2) creation of packing or accommodationpores between biochar and the surrounding soil aggre-gates, and (3) through improved persistence of soil poresdue to increased aggregate stability.

A number of researchers have suggested that due tothe highly porous nature of biochar, its application tosoil may improve soil physical properties through directcontribution of new pores (Atkinson et al. 2010; Downieet al. 2009; Major et al. 2009; Sohi et al. 2010; Verheijenet al. 2010). Despite the apparent link between biocharporosity and soil porosity, surprisingly few studies havereported the pore size distribution of the biochar used forsoil amendment. Biochar pore size is known to varyover several orders of magnitude depending on feed-stock and pyrolysis temperature (Thies and Rillig 2009).Major et al. (2009) suggested that 95 % of pores withinmost biochars are less than 0.002 μm diameter; howev-er, biochars have also been shown to contain a largedegree of macroporosity in the 1 to 10 μm range(Downie et al. 2009; van Zwieten et al. 2010).Verheijen et al. (2010) proposed that direct pore contri-bution from biochar potentially increased water storagebetween −10,000 and −1,000,000 kPa and thus poten-tially increased the number of pores between 0.03 and0.0003 μm diameter in the amended soil. However,most plants are not able to extract soil water from poressmaller than 0.2 μm (below the permanent wilting point−1,500 kPa) or utilise the transient water passingthrough pores greater than 30 μm diameter (above fieldcapacity −10 kPa). Therefore according to Verheijenet al. (2010) whilst biochar amended soils may havehigher total porosity or lower bulk density, the PAWC orwater holding capacity would have remained unchanged.

Few studies have considered that biochar applicationto soil may create accommodation pores between thebiochar particles and the soil aggregates. The size and

proportion of accommodation pores is potentially influ-enced by the size of the soil aggregates, the size of thebiochar particles, and the degree of compaction or set-tling following incorporation. Evidence for the creationof accommodation pores following biochar applicationis limited. Jones et al. (2010) reported that application of40 and 80 Mg ha−1 of green waste biochar to bauxiteprocessing residue coarse sand significantly decreasedmacroporosity (pore diameters >29 μm) whilst signifi-cantly increased mesoporosity (pore diameters between0.2 and 0.29 μm). Increased mesoporosity was attribut-ed to the biochar partly filling large voids between thecoarse sand particles. Evidence from pot trials alsosuggest that short-term changes in pore-size distributionfollowing biochar application may result from aggregatesettling and thus changes to accommodation pores(Eastman 2011; Novak et al. 2012). However, the extentto which biochar application influences resettling ofdisturbed soils has not been specifically investigated.

Verheijen et al. (2010) suggested that biochar appli-cation may improve aggregate stability and thus soilporosity. Given that biochar incorporation requires cul-tivation and tillage, improved aggregate stability inbiochar-amended soils may act to maintain the porescreated during incorporation. However, there is little andoften conflicting evidence to suggest biochar improvesaggregate stability. Liu et al. (2012) reported that theapplication of between 8 and 16 g kg−1 of sawdustbiochar significantly increased the aggregate stabilityin only 3 out of 12 soil type–biochar combinations.Whilst Busscher et al. (2010), Peng et al. (2011), andEastman (2011) reported that biochar had no significanteffect on aggregate stability.

A number of studies indicate that, at least for somesoils, various biochars when applied at sufficiently highrates may improve soil physical properties (Chan et al.2007; Chen et al. 2011; Kameyama et al. 2012;Mukherjee and Lal 2013; Novak et al. 2012; Streubelet al. 2011). However these studies, and many of thestudies which are cited in review documents (Atkinsonet al. 2010; Cox et al. 2012; Krull 2011; Mukherjee andLal 2013; Sohi et al. 2009b, 2010; Verheijen et al. 2010),are of questionable relevance to agriculture, as mosthave been conducted (1) on ancient anthropogenic soilsrather than current soils (Ayodele et al. 2009; Glaseret al. 2002, 2004), (2) on non-agricultural soils(Belyaeva and Haynes 2012; Jones et al. 2010; Uzomaet al. 2011), (3) using charcoals rather than biochars(Ayodele et al. 2009; Tryon 1948), (4) at impractically

348 Plant Soil (2014) 376:347–361

high rates of application for agriculture >40 Mg ha−1

(Gaskin et al. 2007; Jones et al. 2010), or (5) usingrepacked rather than in situ soils (Belyaeva andHaynes 2012; Chan et al. 2007; Dugan et al. 2010;Kameyama et al. 2012; Laird et al. 2010; Liu et al.2012; Novak et al. 2012; Novak and Watts 2013;Streubel et al. 2011; Tryon 1948; Uzoma et al. 2011;van Zwieten et al. 2010). Of particular concern is the useof sieved repacked soils for the study of soil physicalcharacteristics. In sieved repacked soil or pot trials, soilstructure, pore architecture, and pore size distribution,and thus values of field capacity, PAWC, infiltration,hydraulic conductivity, and drainable porosity are anartefact of the sieving and the repacking process thatbear little resemblance to in situ soil properties.

Currently, there are surprisingly few studies whichdemonstrate that biochar application significantly im-proves the physical properties of agricultural soils.Further research is required to evaluate the potentialfor biochar as an in situ soil amendment. To be relevantto agriculture, these studies need to be conducted in situin agricultural production systems employing biocharscontaining a large proportion of pores within the PAWCpore size range (0.2–30 μm).

This study was conducted to determine the effects ofbiochar application on the soil physical properties of anorchard soil 31 months after biochar application. BetweenMay 2012 andApril 2013, an intensive field sampling andmeasurement campaign was conducted to (1) determinethe effects of incorporating 47Mg ha−1 acacia greenwastebiochar on soil physical properties including hydraulicconductivity, PAWC, and aggregate stability in a produc-tive apple orchard and (2) explore the mechanisms bywhich biochar influences soil size and arrangement.

Methods

Site characteristics

The trial was established in November 2009 atMountain River in the Huon Valley, Tasmania (42°57′2.91″S, 147°5′52.13″E) during replanting of an existingapple orchard. Soils were classified according to Isbell(2002) as a Bleached Mottled Grey Kurosol (texture-contrast) or Planosol (IUSS Working Group WRB2006) developed on Permian Mudstone with a minorcontribution from Jurassic dolerite colluvium. The soilprofile was described and classified according to

McDonald et al. (1990), with chemical analysis con-ducted by CSBP laboratories, Western Australia. TheA1 horizon (0–38 cm depth) into which the biochar wasapplied consisted of a dark brown–black sandy loam,with 2–50 mm polyhedral aggregates, common smallfine stones, CEC of 35.15 cmol kg−1, pHcacl2 of 5.7, andorganic carbon at 2.42 % (Walkley and Black 1934).Particle size consisted of 10.39 % clay, 72.81 % sand,and 16.80 % silt (2–20μm). Climate data from a Bureauof Meteorology station located 7 km from the siteshowed the mean annual rainfall was 744 mm, meanmaximum and minimum temperatures were 17.1 and5.8 °C, and mean annual sunshine of 5.5 h per day.

Trial design

The site was levelled and re-mounded 1 week after theremoval of the old trees. The trial design consisted of arandomised complete block with four treatments andfive replicates, trees were blocked on position withinthe tree-row. Each replicate was 3.18 m long and 1 mwide and contained three trees. The four treatments wereuntreated control, biochar, compost, and biochar+com-post. The biochar was sourced from Pacific Pyrolysis,Somersby, NSW (Australia). The feedstock consisted ofacacia whole tree green waste which underwent pyrol-ysis in a continuous flow kiln at temperatures up to550 °C for 30–40 min. The biochar treatments wereapplied on the 2nd of November 2009, with each repli-cate receiving 15 kg of biochar, equivalent to 5 kg pertree space or 47 Mg ha−1. The biochar was spreadevenly by raking across the mound and was incorporat-ed to approximately 10 cm depth. Each block received15 kg of biochar whichwas equivalent to 5 kg per tree or47 Mg ha−1. The orchard was replanted with ‘Naga-FuNo 2 Fuji’ trees on M26 rootstock with a ‘Royal Gala’interstem. Tree spacing within the row was 1.06 m, and4.5 m between rows. The compost and mixed biochar–compost treatments were not sampled in this study andthus are not reported. All sampling and measurementswere conducted from the biochar and control treatmentsin replicates 2, 4, and 5 as to avoid disturbing perma-nently installed moisture probes and flux meters inreplicate 1.

Biochar porosity

Six transverse images of a biochar particle were obtain-ed at 300 times magnification using a Hitachi SU-70

Plant Soil (2014) 376:347–361 349

field emission scanning electron microscope, 3.0 kVaccelerating voltage, 40.0 mm working distance, andSE(M)=mix of upper and lower SE detector. The scan-ning electron microscopy (SEM) images were manuallycorrected in Photoshop CS3 to remove obvious debrisand darken pores, which contained either foreign mate-rial or pore sidewalls. Pore count, total porosity, andmaximum, minimum and modal pore size were deter-mined in Image J (Schneider et al. 2012) using thegij_Pore Analysis plugin (Impoco et al. 2006).

Mercury intrusion porosimetry was conducted onfour 300–400 mg batches of biochar using amicrometrics Autopore Iv 9500, with stepped intrusionat filling pressures of 10.41 kPa. Average pore diameterwas calculated using the Washburn model:

D ¼ 4V.A

where D is the average pore diameter (in nanometre), Vis the total intrusion volume (in millilitre per gram), andA is the total pore surface area (in square metre pergram). Bulk density was calculated from the mass ofbiochar and the total intrusion volume. Apparent densityof the biochar skeleton was calculated from the sum ofthe volume of the solid (non-intruded) material. Porositywas determined as the volume of pores divided by thesample volume. The characteristic length of the poreswas calculated from the Washburn equation from thepressure at which percolation through the porous mediafirst occurred. Tortuosity was calculated as the ratiolength of the path described by the pore space lengthto the length of the shortest path across a porous mass(Webb 2001).

Amount of applied biochar

The mass of biochar was determined from the 560 and249 cm3 cores obtained for determination of drainablemacroporosity and soil water retention by the evapora-tive flux method. Biochar was recovered by dis-aggregating the soil cores in a 30-L container thensieving the floated material to extract the >250 μmbiochar fraction. The floated material was dried at105 °C for 24 h. Foreign material including roots andparticulate organic matter was manually removed withtweezers before determining the oven-dried mass ofrecovered biochar. The size distribution of the biocharprior to soil incorporation was determined by dry siev-ing in triplicate for 3 min using a stack of 4,000, 2,000,

1,000, 350, 250, and 125 μm sieves. The mean weightdiameter of the biochar was 3.84 mm. The averageproportion of biochar less than 250 μm was 1.67 %(SD 0.10 %) of the total biochar mass. The effect ofreplicate location and core sample size on biochar masswas investigated by univariate ANOVA in SPSS V 20.

Soil bulk density and porosity

Soil bulk density was determined by the intact coremethod (Cresswell and Hamilton 2002) in May 2012.From each of the three control and biochar-amendedreplicates, three 50×80 mm cores, plus three 75×100 mm cores, and three 60×61 mm cores were obtain-ed from which the bulk density was determined.Gravimetric moisture content was determined by dryingthe entire core at 105 °C for 24 h. Total porosity wascalculated from bulk density assuming a particle densityof 2.65 g cm−3 and 98 % saturation. The direct effect ofbiochar porosity on soil density was determined bycalculating the soil density without the porosity contrib-uted by the biochar. The volume of biochar was calcu-lated from the biochar density (0.51 g cm−3) determinedby mercury porosimetry and the mass of recoveredbiochar in each sample in which the volume and massof recovered biochar were removed from the originaldry soil mass and soil volume. This gave the density ofthe soil as would have occurred without the direct con-tribution of pores from the biochar. The effect of biocharapplication on bulk density and total porosity was in-vestigated by univariate ANOVA in SPSS V 20.

Soil moisture

Soil moisture was measured in triplicate 10 cm from thecentre tree in each replicate every 2 weeks betweenJuly 2010 and April 2013 using an ICT InternationalPty Ltd TDR based Moisture Probe Meter MPM-160-Bwith a 6-cm-long probe. The effect of biochar applica-tion on bimonthly soil moisture was investigated usingunivariate ANOVA in SPSS. Differences in soil mois-ture content between treatments were demonstrated bycalculating the cumulative soil moisture content overtime from the bimonthly soil moisture sampling.

Drainable porosity and field capacity

The drainable porosity was determined by desorptionusing ceramic suction plates at 0.0, −0.1, −1.0, −3.0, and

350 Plant Soil (2014) 376:347–361

−10.0 kPa (field capacity) according to Cresswell(2002) and Reynolds and Topp (2008). Three replicate100×75 mm intact cores were obtained from each of thecontrol and biochar replicates when the soil profile wasmoist but below field capacity. The cores were incre-mentally brought to saturation in a 0.01 M CaCl2 solu-tion over a period of 4–5 days prior to analysis, and thenpicked to expose open pore faces before being imbed-ded onto the suction table with diatomaceous earth. Thecores were allowed to equilibrate at each matric poten-tial over a period of 5–12 days until the outflow ceased.The effect of biochar application on drainable porosityand field capacity was determined by univariateANOVA in SPSS V 20.

Soil water retention

The soil water release curve was determined by theevaporative flux method according to the proceduredescribed by Wendroth et al. (1993) and Peters andDurner (2008) using the HYPROP apparatus andtensioVIEW software (UMS 2013). The soil water re-tention curve was fitted using both the van Genuchten–Mualem equation (van Genuchten 1980) and the bimod-al vanGenuchten–Mualem equation (Durner 1994), withand without the soil moisture content at −1,500 kPawhich was predetermined by the pressure chamber anal-ysis. The lowest RMSE (0.0015) and highest absoluteAkaike Information Criterion (−2,399) (Akaike 1974)indicated that the best model fit was achieved for thebimodal van Genuchten–Mualem model (Durner 1994)without the supplementary −1,500 kPa data, in which

Se hð Þ ¼X2

j¼1ω j 1þ α j hj j� �n j� �1=n j−1

Se ¼ θ − θrð Þ.

θs− θrð Þ

where Se is the effective saturation, h is the matricpotential, j is an index of the parameters of each vanGenuchten functions, ωj is the weight of both partialfunctions, alpha (α) is an empirical parameter related toair entry, θ is the soil moisture content, θs is the saturatedor field saturated soil volumetric water content, θr is theresidual soil volumetric water content, and n is a dimen-sionless empirical constant. As the data pairs wereunique to each soil core, the volumetric soil moisturecontent was calculated from the bimodal vanGenuchten–Mualem equation at matric potentials of 0,

−10, −20, −30,and −50 kPa within the measurementrange of the evaporative flux approach, and by extrap-olation to −100, −300, −1,000, and −1,500 kPa for eachsoil core. Treatment and plot effects were thus able to beinvestigated by univariate ANOVA for each of the bi-modal van Genuchten–Mualem soil parameters and thepredetermined matric potentials.

The PAWC was calculated as the water-filled porespace between field capacity, said to exist at −10 kPaand the permanent wilting point (PWP) at −1,500 kPa(Brady and Weil 2010; James 1988; Marshall andHolmes 1988). The pore size distribution was estimatedfrom the soil water characteristic according to theYoung–Laplace equation which assumes that the poresare perfectly cylindrical, uniform, and equally drained.The Young–Laplace equation is approximated by

D ¼ 30.Ψm

where D is the pore diameter (in micrometre) and Ψm isthe absolute value of matric potential (in metre), suchthat PAWC corresponded to water stored within poresbetween 30 μm (field capacity) and 0.2 μm (permanentwilting point) diameter.

Permanent wilting point

The PWP was determined by pressure chamber analysisat −1,500 kPa using air-dried <2 mm soil from thecontrol and biochar treatments. Due to the likelihoodthat sieving <2 mm would result in the removal ofbiochar, the biochar application was replicated by arti-ficially adding 2, 5, 10, and 20 % by mass of biochar tothe control sample. The effect of biochar application rateon microporosity (θ at −1,500 kPa) was determined byone-way ANOVA.

Infiltration and hydraulic conductivity

Infiltration rate and unsaturated hydraulic conductivitywere determined by tension infiltration (or discpermeameter) following a procedure similar to Lin andMcInnes (1995), in which the upper 1–2 cm soil wasremoved and the soil surface cleaned with compressedair to prevent interference to flow from surface crustingand foreign particles. The tension infiltrometers wereconnected to the soil surface by a 3-mm deep <250 μmwashed sand pad. The apparent steady-state infiltrationrates were measured sequentially from three devices for

Plant Soil (2014) 376:347–361 351

each plot at five supply potentials (ψ) of −0.95, −0.55,−0.35, −0.15, and −0.05 kPa. The soil moisture prior toinfiltration was determined from five 60 mm×60 mmintact cores per replicate according to the proceduredescribed by Cresswell and Hamilton (2002). The un-saturated hydraulic conductivity was calculated accord-ing to the procedure developed by Ankeny et al. (1991)and Reynolds and Elrick (1991), and presented inMcKenzie et al. (2002) in which;

Kx;y ¼ Gdαx;yqx� �.

r 1þ Gdαx;yπr� �

qx.qy

� �P

K Ψð Þ ¼ Kx;yexp αx;yΨ� �

αx;y ¼ ln qx

.qy

� �.Ψ x−Ψ y

� �P ¼ Ψ x

.Ψx−Ψ y

� �

where Kx,y is the average hydraulic conductivity for datapairs, K(Ψ) is the unsaturated hydraulic conductivity (inmillimetre per hour), αx,y is the soil structure parameter,P is a shape parameter, r is the radius of the disk (incentimetre), Ψx,y are the supply potentials (incentimetre), qx,y is the steady state infiltration rate (incubic centimetre per minute), Gd is a shape parameter=0.25, Ψ equals (Ψx+Ψy)/2), and x,y represent measure-ments at sequentially less negative supply potentials.The flow weighted mean pore diameter was determinedaccording to Philip (1985)) in which;

FWMPD ¼ 7:4 lnK2=K1

ψ2−ψ1

��

Here, FWMPD is the flow weighted mean pore di-ameter (in millimetre), and K1 and K2 are the first andsecond hydraulic conductivities (in millimetre per hour)at ψ1 and ψ2, where ψ1 and ψ2 are the first and secondsupply potentials (in millimetre). Infiltration and unsat-urated hydraulic conductivity data were log transformedprior to analysis by univariate ANOVA in SPSS.

Aggregate stability

Aggregates were sampled from 0 to 3 cm depth usingthree shovel loads per treatment. Aggregates weretransported in open trays to reduce compaction anddeformation. Aggregates were air-dried at ambient tem-peratures for 3 days before being sieved to obtain the1–2-mm fraction. The 1–2-mm fraction was oven-driedat 40 °C for 24 h to ensure consistent starting moisturefor all treatments. Air-dried 1–2-mm aggregates wereimmersed in water for 1 min on a 250-μm sieve then

mechanically raised and lowered 3.7 cm in tap water(250 μS cm−1) for 10 min. Retained aggregates andcoarse fraction (stones, roots, and biochar) were collect-ed then dried at 105 °C for 24 h. The mass of theretained coarse fraction was determined by dispersingthe stable aggregates in a 5 % w/v hexametaphosphatewith horizontal shaking for 18 h before re-sieving torecover the >250 μm course fragments. The mass of theretained coarse fraction was determined by oven-dryingat 105 °C for 24 h. Aggregate stability was calculated as;

AS1−2mm>250 μm ¼ R>250 μm–SB>250 μm

.Sair � 1−θð Þð Þ

−SB>250 μm

where AS1–2mm>250 μm is the aggregate stability of the1–2-mm fraction measured as the proportion of totalaggregates (minus the resilient stone, root, and biocharfraction) retained on a 250-μm sieve (in gram per gram);R>250 μm is the oven-dried retained persistent aggregatesand coarse fraction greater than 250 μm (in gram);SB>250 μm is the oven-dried resilient stone, root, andbiochar component greater than 250 μm (in gram); Sairis the air-dried (40 °C) mass of aggregates includingresilient fraction prior to sieving; and θ is the gravimetricmoisture content of the air-dried 1–2-mm fraction (ingram per gram). The effect of biochar application onaggregate stability was investigated by univariateANOVA in SPSS v20.

Results

Biochar porosity

SEM demonstrated that the average pore size from thesix SEM images of a single biochar particle (measuredalong the longest pore axis) ranged from 0.844 μm (sd±0.13 μm) to 235 μm (sd±123 μm), with the mean poresize ranging from 13.09 μm (sd±20.03 μm) to 7.08 μm(sd±6.98μm) for the six images, depending on pore andmeasurement orientation. Pores were highly ellipticalsuggesting that the original porous structure of the woodfeedstock had become distorted during pyrolysis. Poresize distribution was highly skewed with 95 % of poresbeing less than 14.43–79.15 μm (range due to poreelongation) (Fig. 1).

Mercury porosimetry revealed that the average min-imum pore diameter was approximately 0.1 μm, inwhich 95 % of all pores were less than 22 μm diameter.

352 Plant Soil (2014) 376:347–361

The average median pore diameter can be considered torange from 0.4–13 μm. The characteristic length ofpores averaged 44 μm which was also reflected in theaverage tourtuosity value of 6.6 indicating pores were4.2–8.0 longer than they were wide (Table 1).Differences in biochar porosity determined by SEMversus the mercury porosimetry were considered minor,and largely due to detection of asymmetrical pore prop-erties by SEM.

Biochar recovery

The mean biochar recovery was 3.39 g 100 cm−3 (SD1.46 g 100 cm−3); however, themass of recovered biochar(>250 μm) varied between individual samples from 1.06to 6.75 g 100 cm−3. Differences in amount of recoveredbiochar between replicates and the two soil sample vol-umes were not significant. Analysis of the 100-mm-diameter soil cores demonstrated that the volume of therecovered biochar was 6.53 % of the total soil volume.

Soil bulk density, total porosity, and saturated watercontent

Biochar application significantly reduced the soil bulkdensity in all replicates and for all three core samplesizes (F=59.226, P=0.015) (Fig. 3a). Consequently,biochar amendment resulted in significantly higher total

porosity. A significantly higher saturated water contentwas also observed in the biochar treatment measuredusing 50×80 mm and 75×100 mm cores (F=27.215,P=0.031), and significantly higher soil moisture contentat −0.1 kPa occurred in the biochar treatment measuredusing the 75×100 mm cores during desorption(F=34.584, P<0.028) (Fig. 2). Within the biochartreatment, a significant linear relationship existedbetween amount of applied biochar (>250 μm) andbulk density (F=37.231, P=0.0001) for both the50×80 mm and 75×100 mm cores (Fig. 3b).

The lower bulk density of the biochar-amended soildid not result from direct pore contribution from thebiochar itself, as the bulk density of the biochar excludedtreatment (effects of biochar porosity had been removedfrom the soil volume) was significantly lower than theunamended control (F=320.26, P=0.0001) (Fig. 3a).

Soil moisture

Biochar application had no significant effect on soilmoisture content (Fig. 4a) or cumulative soil moisturebetween July 2010 and May 2013 (Fig. 4b).

Drainable porosity and field capacity

Biochar application had no significant effect on thedrainable porosity between −1.0 and −10 kPa, nor on

0 50 100 150 200 250 300

Pro

port

ion

of to

tal p

ores

(%

)

0

5

10

15

20

25(e)

(d)(c)

(b)(a)

Pore size µm

Fig. 1 An example of a scanning electron microscopy (SEM)image of biochar and pore size analysis. a Original SEM imageat ×300 magnification; b corrected SEM image to remove foreignparticles and pore side walls; c inverse binary image, black is pore

wall, white is pore space; d greyscale classification of pores sizeusing the Image J gij_Pore Analysis plugin. e Example of the poresize frequency distribution (pore size of longest pore axis)

Plant Soil (2014) 376:347–361 353

the field capacity at −10 kPa. However, biochar appli-cation significantly increased the saturated soil moisturecontent (F=132.878, P=0.007), and soil moisture con-tent at −0.1 kPa (F=32.639, P=0.029). This supportsthe previous finding that application of biochar signifi-cantly reduced the soil bulk density and increased thetotal porosity, presumably due to the creation or preser-vation of large pores (>300 μm) in the surrounding soil(Fig. 5).

Soil-water retention and plant available water

Determination of the soil-water retention function by theevaporative flux method revealed considerable variabil-ity within treatments and between replicates. Biocharapplication was found to have no significant effect on(1) the bimodal van Genuchten–Mualem soil waterparameters (α1,2, θs, θr, n1,2, ω), (2) the measured equil-ibration potentials between −10 and −50 kPa, (3) theextrapolated equilibration potentials between −100 and

−1,500 kPa, (4) the PAWC between −10 and−1,500 kPa, or (5) PWP at −1,500 kPa (Fig. 6). Therewas no significant relationship between the mass ofrecovered biochar >250 μm and soil moisture contentfor any of the eight equilibration steps. Similarly, therewas no significant linear relationship between the massof recovered biochar >250 μm and any of the bimodalvan Genuchten–Mualem soil water parameters (α1,2, θs,θr, n1,2, ω2).

Infiltration and hydraulic conductivity

Overall, biochar amendment had no significant effect oninfiltration or the unsaturated hydraulic conductivity.However, the biochar amended soil had significantlyhigher infiltration at −0.15 and −0.05 kPa, and signifi-cantly higher unsaturated hydraulic conductivity at−0.25 and −0.10 kPa (Fig. 7a). At all other supplypotentials, biochar application had no significant effecton unsaturated hydraulic conductivity or infiltration

Table 1 Biochar properties determined by mercury porosimetry

Sample Mean porediameter (4V/A)

Median porediameter (volume)

Bulkdensity

Skeletaldensity

Porosity Characteristiclength

Tourtuosity 95 % porediameter <

μm μm g cm−3 g cm−3 % μm μm

A 13.02 14.23 0.51 0.64 20.34 41.94 8.01 32.93

B 0.92 3.46 0.50 1.13 55.65 50.09 4.16 7.75

C 2.17 11.37 0.47 0.68 31.47 45.05 7.05 45.31

D 0.44 3.102 0.51 0.90 42.82 38.26 7.07 1.05

Mean 4.14 8.04 0.50 0.84 37.57 43.84 6.57 21.76

±SD 5.97 5.62 0.02 0.22 15.15 5.01 1.67 20.85

Fig. 2 The effect of biochar application on total porosity, saturat-ed moisture content, volumetric soil moisture content at −0.1 kPa(desorption), and van Genuchten parameter θs. All comparisons

between the control and biochar treatments were significant, otherthan for the van Genuchten parameter θs. Error bars indicate +1standard deviation

354 Plant Soil (2014) 376:347–361

(Fig. 7b). Biochar application had no significant effecton the calculated flow weighted mean pore diameter.

Microporosity and the permanent wilting point

Biochar application had no significant effect on the per-manent wilting point at −1,500 kPa and thus did notincrease the number of pores smaller than 0.2 μm diam-eter. Addition of 5 % by weight of biochar to the controltreatment had no significant effect on soil moisture con-tent at −1,500 kPa. However, application of 20 %wtbiochar significantly (F=16.106, P=0.0001) increasedthe soil moisture content at −1,500 kPa (Fig. 8a).

Aggregate stability

Overall, biochar application had no significant effect(F=0.021, P=0.90) on the stability of the 1–2-mmaggregates. At replicates 4 and 5, biochar application

significantly decreased the aggregate stability, whilst atreplicate 1 biochar application significantly increasedaggregate stability (Fig. 8b).

Discussion

The pore size of the acacia greenwaste biochar used inthis study ranged from 0.1 to 235 μm with a medianpores size between 0.4 and 13 μm. The similarity be-tween the pore size distribution of the acacia biochar andthe PAWC pore size range of soil (0.2–30 μm) sug-gested that application of the acacia biochar shouldincrease soil PAWC, and to a lesser extent increasedrainable porosity through direct pore contribution.However, the application of 47 Mg ha−1 acacia greenwaste biochar had no significant effect on drainableporosity (−1.0 to −10 kPa), field capacity (−10 kPa),PAWC between −10 and −1,500 kPa, nor the PWP

Soi

l bul

k de

nsity

(g

cm-1

)

0.90

1.00

1.10

1.20

1.30

1.40

BiocharControl Biochar excluded Recovered Biochar (g per 100 cm3)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.00.90

1.00

1.10

1.20

1.30

1.40

50 x 80 mm cores

75 x 100 mm cores

y = -0.036 x + 1.24

R2 = 0.662, P<0.001

(a) (b)

Fig. 3 a Effect of biochar on soil bulk density. The ‘biocharexcluded’ refers to the biochar treatments in which the effects ofthe biochar density have been removed from the calculation of soil

density. Error bars indicate +1 standard deviation. b Linear re-gression between amount of applied (recovered) biochar(>250 μm) and soil bulk density

(a) Mean soil moisture

Date

06/10 12/10 06/11 12/11 06/12 12/12 06/13

Soi

l moi

stur

e co

nten

t (%

vol

)

0

10

20

30

40

50

BiocharControl

(b) Cumulative soil moisture

Date

06/10 12/10 06/11 12/11 06/12 12/12 06/13

Cum

ulat

ive

soil

moi

stur

e co

nten

t (%

vol

)

0

500

1000

1500

2000

ControlBiochar

Fig. 4 Effect of biochar application on soil moisture content 0–6 cm depth (% vol): a bimonthly mean soil moisture content, and bcumulative soil moisture. Error bars represent ±1 standard deviation

Plant Soil (2014) 376:347–361 355

(−1,500 kPa). Other researchers have also reported thatbiochar application failed to improve soil water reten-tion of in situ soils. Major et al. (2012) found thatapplication of 20 Mg ha−1 biochar had no significanteffect on soil water retention or drainage of a clay soil.Gaskin et al. (2007) reported that application of biocharat 11 and 22 Mg ha−1 had no significant effect on waterholding capacity between −20 and −100 kPa. In theirreview of the literature, Mukherjee and Lal (2013) sug-gested that the PAWC response to biochar application isboth soil and biochar specific. This is supported bystudies such as Streubel et al. (2011) who found thatonly 25 of 60 soil-type biochar application rate combi-nations resulted in significantly higher water holdingcapacity. Despite most biochars containing a high pro-portion of micropores (Downie et al. 2009; Kookana2010; Major et al. 2009) including the acacia biocharused in this study, very few investigations have reportedthe effects of biochar application on soil microporosity.

Our study demonstrated that application of biochar at47 Mg ha−1 had no significant effect on the PWP or thevan Genuchten residual water content parameter (θr).Similar findings have been reported by Eastman(2011)) and Laird et al. (2010) who also found thatapplication of biochar at rates up to 20 Mg ha−1 hadno significant effect on water retention at −1,500 kPa.Uzoma et al. (2011) reported biochar application had nosignificant effect on θr.

Application of acacia green waste biochar at47 Mg ha−1 significantly reduced soil bulk density andthus increased total porosity and saturated water content.Reduced bulk density of biochar-amended soil has alsobeen reported by Chen et al. (2011) who found applica-tion of 2.3 and 4.5 Mg ha−1 biochar decreased bulkdensity by 4.5 and 6.0 %, respectively. Zhang et al.(2010) reported application of wheat straw biochar de-creased the bulk density of a rice paddy soil at40 Mg ha−1 but not at 10 Mg ha−1. Major et al. (2012)

Matric Potential (-kPa)

0.0 2.0 4.0 6.0 8.0 10.0Vol

umet

ric s

oil m

oist

ure

(cm

3 cm

-3)

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

BiocharControl

Maximum equavalent pore diameter (um)

Sat. 3000 300 100 30Pro

port

ion

of to

tal p

oros

ity (

cm3 c

m-3

)

0.5

0.6

0.7

0.8

0.9

1.0

1.1

BiocharControl

(a) (b)

Fig. 5 Effect of biochar application on drainable porosity a soil water retention function (0–10 kPa), b drainable porosity expressed as theproportion of total porosity versus maximum equivalent pore diameter

Maximum equivalent pore diameter (um)30 15 10 6 3 1 0.3 0.2P

ropo

rtio

n of

tota

l por

osity

cm

3 cm

-3)

0.0

0.2

0.4

0.6

0.8

1.0

BiocharControl

Matric potential (-kPa)

0 200 400 600 800 1000 1200 1400 1600Vol

umet

ric s

oil m

oist

ure

(cm

3 cm

-3)

0.0

0.1

0.2

0.3

0.4

0.5

BiocharControl

(a) (b)

Fig. 6 The effect of biochar application on a soil water characteristic, and b proportion of total porosity determined by evaporative flux.Note, values more negative than −100 kPa or pores less than 3 μm were determined by extrapolation of the bimodal van Genuchten curve

356 Plant Soil (2014) 376:347–361

reported application of 20Mg ha−1 biochar significantlyreduced the density of a heavy clay soil at 0–15 cmdepth, but not at the soil surface or at 0.15–0.30m depth.

In our study, the reduced bulk density in the biochar-amended soil did not result from the internal porosity ofthe biochar, as the bulk density of the biochar-amendedtreatment in which the internal porosity has been re-moved from the calculation of bulk density was signif-icant ly lower than the unamended control .Consequently, the reduction in bulk density or increasein total porosity must have occurred in the soil surround-ing the biochar via mechanisms other than direct porecontribution. It is also unlikely that the increased totalporosity resulted from the creation of accommodationpores as biochar application had no significant effect onthe soil water retention below −0.1 kPa. Nor is it

probable that the increased total porosity resulted fromthe biochar protecting pores from clogging by aggregatebreakdown as application of biochar had no significanteffect on aggregate stability.

Several lines of evidence suggest the increase in totalporosity in the biochar-amended soil resulted from thecreation of large macropores in the soil surrounding thebiochar particles. Drainable porosity of the biochar treat-ment was significantly higher than the control at−0.1 kPa but not −1.0 kPa. This suggests the biocharapplication resulted in the formation of largemacropores of at least 3,000 μm diameter, but notsmaller than 300 μm diameter. Likewise, the unsaturat-ed hydraulic conductivity was significantly higher in thebiochar treatments than the control at −0.25 kPa, but notat −0.45 kPa. This suggests biochar application resulted

Supply Potential (kPa)

-1.00 -0.75 -0.50 -0.25 0.00

Infil

trat

ion

& H

ydra

ulic

Con

duct

ivity

(m

m h

r-1)

0

1

10

100

BiocharControl

Supply Potential (kPa)-1.00 -0.75 -0.50 -0.25 0.00

BiocharControl

(a) Infiltration (b) Hydraulic Conductivity (c) FWMPD

Supply Potential (kPa)-0.45 -0.25 -0.01

FW

MP

D (

mm

)

0.0

0.2

0.4

0.6

0.8

1.0

BiocharControl

Fig. 7 Effect of biochar application on a infiltration rate (in millimetre per hour); b unsaturated hydraulic conductivity (in millimetre perhour); c flow weighted mean pore diameter (in millimetre). Note error bars indicate ±1 standard deviation

Control Biochar 3.4 % 5 % 10 % 20 %

Soi

l moi

stur

e co

nten

t at -

1500

kP

a

6

8

10

12

Agg

rega

te S

tabi

liy (

g g-

1 )

0.0

0.2

0.4

0.6

0.8

1.0

ControlBiochar

(a) Permanent Wilting Point (PWP) (b) Aggregate Stability

Replicate 2 Replicate 4 Replicate 5

Fig. 8 Effect of biochar application on a soil moisture content(water filled porosity) at −1,500 kPa; the biochar treatment hadapproximately 3.4 %wt biochar. +5, +10, and +20 % refer to the

amount of biochar (by weight) added to the control. b Aggregatestability, presented as the proportion of 1–2 mm water stableaggregates >250 μm. Error bars represent +1 standard deviation

Plant Soil (2014) 376:347–361 357

in the formation of macropores at least 1,200 μm diam-eter, but not smaller than 660 μm diameter. This findingcontrasts to other studies conducted on in situ agricul-tural soils which have reported that biochar applicationhad no significant effect on saturated hydraulic conduc-tivity (Eastman 2011; Major et al. 2012). The formationof these large macropores was attributed to a fourth andpreviously unreported mechanism by which biocharmay influence soil porosity: that is, increased inverte-brate burrowing. At the time of sampling, earthwormnumbers were visibly higher in the biochar-amendedsoil than the untreated control. Consequently, the in-creased number of large macropores (>1,200 μm), andthus increased total porosity, saturated water content andnear saturated hydraulic conductivity (≥−0.25 kPa) ofthe biochar-amended soil was attributed to increasedearthworm burrowing. Few studies have investigatedthe effect of biochar application on invertebrates(Lehmann et al. 2011). In their review, Weyers andSpokas (2011) concluded that biochar may have short-term negative impacts on earthworm population densityand total biomass. However, there was little evidence tosuggest biochar had any long-term effects on earthwormdensity or total biomass. In a behavioural experiment,van Zwieten et al. (2010) showed that earthworms pre-ferred a biochar-amended Ferrosol but had no preferencefor biochar in a Calcarosol. Gomez-Eyles et al. (2011)however found that biochar application in a contaminatedsoils resulted in the loss of earthworm mass and condi-tion, whilst Busch et al. (2012) demonstrated biochar hadno effect on earthworm avoidance in a contaminated soil.Earthworm response to biochar application appears todepend on biochar type, soil type, and time.

Results demonstrate that substantial within- andbetween-replicate variation existed in the amount ofrecovered biochar, and physical soil properties such asbulk density, hydraulic conductivity, soil water reten-tion, and aggregate stability. Although variation in theamount of recovered biochar (1.06–6.75 g 100 cm−3)may have influenced measured values, this effect wasonly apparent for bulk density (Fig. 2b). Furthermore,data indicates that soil hydraulic properties such asdrainable porosity, hydraulic conductivity, and the soilwater retention function varied due to the high spatialvariation in soil pore size and pore arrangement at thesite, not biochar amendment. Orchard soils are expectedto have a higher degree of pore space variation thanarable soils as they are relatively undisturbed, allowingdevelopment of soil structure by processes such as

freeze–thawing, bioturbation, microbiological activity,and shrink-swelling (Hillel 1998). Consequently, in con-trast to pot trials in which natural soil structure isdestroyed and homogenised, in situ studies of orchardsoils require treatment effects to be substantially greaterthan pot trials in order to yield a statistically significantchange in soil physical properties. In this study, appli-cation of 47 Mg ha−1, acacia biochar had no significanteffect on a range of soil hydraulic properties due in partto the high natural variation in soil physical properties.Consequently in order to produce a statistically signifi-cant effect, biochar would need to be applied at rates inexcess of 50 Mg ha−1, which is both physically andeconomically prohibitive in commercial orchards.

Conclusion

We proposed three mechanisms by which biochar ap-plication might increase soil porosity. They were (1)direct pore contribution from the pores within the bio-char, (2) creation of packing or accommodation pores,and (3) improved aggregate stability. Mercuryporosimetry and SEM analysis demonstrated that theacacia biochar used in this study contained pores be-tween approximately 0.1 and 240 μm in diameter, with95 % of all pores being less than 22 μm. Consequently,application of the acacia biochar was expected to in-crease plant available water through direct pore contri-bution by increasing the proportion of pores betweenfield capacity (30 μm) and the permanent wilting point(0.2 μm), and to a lesser extent macroporosity (porediameters >75 μm). However, application of47 Mg ha−1 of acacia green waste biochar had nosignificant effect on the following: drainable porosity(–1.0 and −10 kPa), field capacity, PAWC, PWP, the vanGenuchten soil water retention parameters (α1,2, θs, θr,n1,2, ω), or soil moisture content. We found no evidenceto suggest that biochar application directly influencedsoil porosity through either direct pore contribution, thecreation of accommodation pores, or increased aggre-gate stability as we had speculated. However, thebiochar-amended soil had significantly higher near-saturated hydraulic conductivity (−0.25 and−0.10 kPa), total porosity, and soil water retention at−0.1 kPa resulting from the presence of largemacropores (> ∼1,200 μm). These large macroporeswere attributed to increased earthworm burrowing,based on unrecorded observations of earthworm

358 Plant Soil (2014) 376:347–361

presence during the experiments. More research is re-quired to verify this hypothesis.

Our study demonstrated that despite use of a biochardominated by pores within the PAWC range, applicationat 47 Mg ha−1 to a loamy sand soil within an appleproduction system had no significant effect on soil wateravailability or soil moisture content. Lack of a significantdifference in all soil physical properties between the con-trol and biochar treatments resulted in part from the highspatial variation in pore size and architecture at the site.

Acknowledgments This project was conducted as part of thenational apple and pear industry Productivity Irrigation Pests andSoils flagship program and was funded by Horticulture AustraliaLimited using the apple and pear industry levy, voluntary contri-bution from the New Zealand Institute for Plant and Food Re-search, and matched funds from the Australian Government. Wethank Justin Direen for assistance with trial establishment andBenedicte Patin, Steve Patterson, Jocelyn Parry-Jones, and AnnaWrobel-Tobiszewska for assistance with field work. Assistancewith SEM and mercury porosimetry was gratefully received fromDario Arrua and Jocelyn Parry-Jones. Thanks to Drs CarolineMohammed and Alieta Eyles for valuable comments on an earlierdraft of the manuscript. This work was conducted whilst the firstauthor was seconded from the Department of Primary Industries,Parks, Water and Environment.

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