Does biochar influence soil physical properties and soil ... · Does biochar influence soil...

PAPER OF THE MONTH Tasmanian Institute of Agriculture

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Monday, March 24, 2014

Does biochar influence soil physical properties and soil water availability?

Marcus HardieA, Brent ClothierB, Sally BoundA, Garth OliverA, Dugald CloseA

A Perennial Horticulture Centre, Tasmanian Institute of Agriculture, University of Tasmania, Private Bag 98, Hobart, Tasmania 7001, Australia. [email protected]

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

Abstract

Aims:

To determine (i) the effects of incorporating 47 Mg ha-1 acacia green waste biochar on soil physical properties and (ii) water relations, and to explore the different mechanisms by which biochar influences soil porosity.

Methods:

The pore size distribution of the biochar was determined by scanning electron microscope and mercury porosimetry. Soil physical properties and water relations were determined by in situ tension infiltrometers, desorption and evaporative flux on intact cores, pressure chamber analysis at -1500 kPa, and wet aggregate sieving.

Results:

Thirty months after incorporation, biochar application had no significant effect on soil moisture content, drainable porosity between –1.0 kPa and -10 kPa, field capacity, plant available water capacity (PAWC), the van Genuchten soil water retention parameters, aggregate stability, nor the permanent wilting point (PWP). 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 the unamended control. Differences were attributed to the formation of large macropores (>1200 μm) resulting from greater earthworm burrowing in the biochar amended soil.

Conclusion:

We found no evidence to suggest application of biochar influenced soil porosity by either direct pore contribution, creation of accommodation pores, or improved aggregate stability.

Key Words

Plant available soil water (PAWC), in situ, soil amendment, apple, soil water retention

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Introduction

Biochar is a predominantly stable, recalcitrant organic carbon compound created by pyrolysis of biomass at temperatures between 300 and 1000 °C under low or no oxygen conditions (Jeffery et al. 2011; Krull 2011; Verheijen et al. 2010). The use of biochar as a soil additive has been proposed as a means of mitigating climate change through long term sequestration of carbon whilst simultaneously improving soil properties and functions (Jeffery et al. 2011; Kookana et al. 2011; Verheijen et al. 2010). Biochar is highly porous, thus its application to soil is considered to improve a range of soil 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. 2010; Sohi et al. 2009b; Zwieten et al. 2012). However there is little peer-reviewed evidence that demonstrates that biochar application significantly improves the physical properties of in situ agricultural soils (Atkinson et al. 2010; Shackley and Sohi 2010; Sohi et al. 2009a). Furthermore the mechanisms or processes by which biochar may influence soil pore size distribution have not been clearly established or demonstrated (Verheijen et al. 2010).

Evidence that biochar application improves the physical properties of in situ agricultural soils is limited and frequently inconsistent between biochar types, rates of application and soil types. For example, Major et al. (2012) found incorporation of 20 Mg ha-1 biochar had no significant effect on soil moisture retention, saturated hydraulic conductivity or the bulk density of the surface soil, however significant differences existed at 0.15 m depth. Gaskin et al. (2007) found that application of pine-chip biochar at 11 and 22 Mg ha-

–1

1 had no significant effect on the soil moisture holding capacity of a loamy sand. However, at 88 Mg ha significant differences existed. Asai et al. (2009) showed that wood-residue biochar applied at 4 Mg ha-1 and 8 Mg ha-1 had no significant effect on saturated hydraulic conductivity. However, at 16 Mg ha-1 significant differences existed at one of two sites.

The specific mechanisms by which biochar influences water retention, macro-aggregation and soil stability are poorly understood (Sohi et al. 2009b). We propose that biochar application may influence soil porosity and thus soil water retention via three mechanisms (i) direct pore contribution from pores within the biochar, (ii) creation of packing or accommodation pores between biochar and the surrounding soil aggregates, and (iii) through improved persistence of soil pores due to increased aggregate stability.

A number of researchers have suggested that due to the highly porous nature of biochar, its application to soil may improve soil physical properties through direct contribution of new pores (Atkinson et al. 2010; Downie et al. 2009; Major et al. 2009; Sohi et al. 2010; Verheijen et al. 2010). Despite the apparent link between biochar porosity and soil porosity, surprisingly few studies have reported the pore size distribution of the biochar used for soil amendment. Biochar pore size is known to vary over several orders of magnitude depending on feedstock and pyrolysis temperature (Thies and Rillig 2009). Major et al. (2009) suggested that 95 % of pores within most biochars are less than 0.002 μm diameter, however biochars have also been shown to contain a large degree 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 contribution from biochar potentially increased water storage between -10 000 and -1 000 000 kPa and thus potentially increased the number of pores between 0.03 - 0.0003 μm diameter in the amended soil. However most plants are not able to extract soil water from pores smaller than 0.2 μm (below the permanent wilting point -1500 kPa) or utilise the transient water passing through pores greater than 30 μm diameter (above field capacity -10 kPa). Therefore according to Verheijen et al. (2010) while biochar amended soils may have higher total porosity or lower bulk density, the PAWC or water holding capacity which have remain unchanged.



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Few studies have considered that biochar application to soil may create accommodation pores between the biochar particles and the soil aggregates. The size and proportion of accommodation pores is potentially influenced by the size of the soil aggregates, the size of the biochar particles, and the degree of compaction or settling following incorporation. Evidence for the creation of accommodation pores following biochar

application is limited. Jones et al. (2010) reported that application of 40 and 80 Mg ha-1 of green waste biochar to bauxite processing residue coarse sand significantly decreased macroporosity (pore diameters >29 μm) whilst significantly increasing mesoporosity (pore diameters between 0.2 - 0.29 μm). Increased mesoporosity was attributed to the biochar partly filling large voids between the coarse sand particles. Evidence from pot trials suggests that short-term changes in pore-size distribution following biochar application may result from aggregate settling and thus changes to the accommodation pores (Eastman 2011; Novak et al. 2012). However the extent to which biochar application influences resettling of disturbed soils has not been specifically investigated.

Verheijen et al. (2010) suggested that biochar application may improve aggregate stability and thus soil porosity. Given that biochar incorporation requires cultivation and tillage, improved aggregate stability in biochar amended soils may act to maintain the pores created during incorporation. However, there is little and often conflicting evidence to suggest biochar improves aggregate stability. Liu et al. (2012) reported that the application of between 8 to 16 g kg-1 of sawdust biochar significantly increased the aggregate stability in only three out of twelve soil type - biochar combinations. While Busscher et al. (2010), Peng et al. (2011) and Eastman (2011) reported biochar had no significant effect on aggregate stability.

A number of studies indicate, that at least for some soils, various biochars when applied at sufficiently high rates 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; Streubel et al. 2011). However these studies, and many of the studies which are cited in review documents (Atkinson et al. 2010; Cox et al. 2012; Krull 2011; Mukherjee and Lal 2013; Sohi et al. 2010; Sohi et al. 2009b; Verheijen et al. 2010) are of questionable relevance to agriculture, as most have been conducted: (i) on ancient anthropogenic soils rather than current soils (Ayodele et al. 2009; Glaser et al. 2004; Glaser et al. 2002), (ii) on non-agricultural soils (Belyaeva and Haynes 2012; Jones et al. 2010; Uzoma et al. 2011), (iii) using charcoals rather than biochars (Ayodele et al.

2009; Tryon 1948), (iv) at impractically high rates of application for agriculture >40 Mg ha-1 (Gaskin et al. 2007; Jones et al. 2010), or (v) using repacked rather than in situ soils (Belyaeva and Haynes 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 use of sieved repacked soils for the study of soil physical characteristics. In sieved repacked soil or pot trials soil structure, pore architecture, and pore size distribution, and thus values of field capacity, PAWC, infiltration, hydraulic conductivity and drainable porosity are an artefact of the sieving and the repacking process that bear little resemblance to in situ soil properties.

Currently there are surprisingly few studies which demonstrate that biochar application significantly improves the physical properties of agricultural soils. Further research is required to evaluate the potential for biochar as an in situ soil amendment. To be relevant to agriculture, these studies need to be conducted in situ in agricultural production systems employing biochars containing a large proportion of pores within the PAWC pore size range (0.2 μm - 30 μm).

This study was conducted to determine the effects of biochar application on the soil physical properties of an orchard soil 31 months after biochar application. Between May 2012 and April 2013 an intensive field sampling and measurement campaign was conducted to (i) determine the effects of incorporating 47 Mg

ha-1 acacia green waste biochar on soil physical properties including hydraulic conductivity, PAWC, and



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aggregate stability in a productive apple orchard and (ii) explore the mechanisms by which biochar influences soil size and arrangement.

Methods

Site characteristics

The trial was established in November 2009 at Mountain River in the Huon Valley, Tasmania (42°57’2.91”S, 147°5’52.13”E) during replanting of an existing apple orchard. Soils were classified according to Isbell (2002) as a Bleached Mottled Grey Kurosol (texture-contrast) or Planosol (IUSS Working Group WRB 2006) developed on Permian Mudstone with a minor contribution from Jurassic dolerite colluvium. The soil profile was described and classified according to McDonald et al. (1990), with chemical analysis conducted by CSBP laboratories, Western Australia. The A1 horizon (0 - 38 cm depth) into which the biochar was applied consisted of a dark brown –black sandy loam, with 2 mm – 50 mm polyhedral aggregates, common

small fine stones, CEC of 35.15 cmol kg-1, pHcacl2 of 5.7, and organic carbon at 2.42 % (Walkley and Black 1934). Particle size consisted of 10.39 % clay, 72.81 % sand and 16.80 % silt (2-20 μm). Climate data from a Bureau of Meteorology station located 7km from the site showed the mean annual rainfall was 744 mm, mean maximum and minimum temperatures were 17.1 °C and 5.8 °C, and mean annual sunshine of 5.5 hours per day.

Trial design

The site was levelled and re-mounded one week after the removal of the old trees. The trial design consisted of a randomised complete block with four treatments and five replicates, trees were blocked on position within the tree-row. Each replicate was 3.18 meters long and 1 meter wide and contained three trees. The four treatments were: untreated control, biochar, compost, biochar + compost. The biochar was sourced from Pacific Pyrolysis, Somersby, NSW (Australia). The feedstock consisted of acacia whole tree green waste which underwent pyrolysis in a continuous flow kiln at temperatures up to 550 °C for 30 - 40 minutes.

The biochar treatments were applied on the 2nd November 2009, with each replicate receiving 15 kg of biochar, equivalent to 5 kg per tree space or 47 Mg ha-1. The biochar was spread evenly by raking across the mound and was incorporated to approximately 10 cm depth. The orchard was replanted with ‘Naga-Fu No 2 Fuji’ trees on M26 rootstock with a ‘Royal Gala’ interstem. Tree spacing within the row was 1.06 m, and 4.5 m between rows. The compost and mixed biochar-compost treatments were not sampled in this study and thus are not reported. All sampling and measurements were conducted from the biochar and control treatments in replicates 2, 4, and 5 as to avoid disturbing permanently installed moisture probes and flux meters in replicate 1.

Biochar porosity

Six transverse images of a biochar particle were obtained at 300 times magnification using a Hitachi SU-70 field emission scanning electron microscope, 3.0kV accelerating voltage, 40.0 mm working distance, SE(M) = mix of upper and lower SE detector. The SEM images were manually corrected in Photoshop CS3 to remove obvious debris and darken pores, which contained either foreign material or pore sidewalls. Pore count, total porosity, maximum, minimum and modal pore size were determined in Image J (Schneider et al. 2012) using the gij_Pore Analysis plugin (Impoco et al. 2006).

Mercury intrusion porosimetry was conducted on four 300 -400 mg batches of biochar using a micrometrics Autopore Iv 9500, with stepped intrusion at filling pressures of 10.41 kPa. Average pore diameter was calculated using the Washburn model:

D = 4V / A



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where D is the average pore diameter (nm), V is the total intrusion volume (mL g-1), and A is the total pore

surface area (m2 g-1). Bulk density was calculated from the mass of biochar and the total intrusion volume. Apparent density of the biochar skeleton was calculated from the sum of the volume of the solid (non- intruded) material. Porosity was determined as the volume of pores divided by the sample volume. The characteristic length of the pores was calculated from the Washburn equation from the pressure at which percolation through the porous media first occurred. Tortuosity was calculated as the ratio length of the path described by the pore space length to 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 cm3 and 249 cm3 cores obtained for determination of drainable macroporosity and soil water retention by the evaporative flux method. Biochar was recovered by dis-aggregating the soil cores in a 30 L container then sieving the floated material to extract the >250 μm biochar fraction. The floated material was dried at 105 °C for 24 hours. Foreign material including roots and particulate organic matter was manually removed with tweezers before determining the oven dried mass of recovered biochar. The size distribution of the biochar prior to soil incorporation was determined by dry sieving in triplicate for 3 minutes using a stack of 4000, 2000, 1000, 350, 250, and 125 μm sieves. The mean weight diameter of the biochar was 3.84 mm. The average proportion of biochar less than 250 μm was 1.67 % (SD 0.10%) of the total biochar mass. The effect of replicate location and core sample size on biochar mass was investigated by univariate ANOVA in SPSS V 20.

Soil bulk density and porosity

Soil bulk density was determined by the intact core method (Cresswell and Hamilton 2002) in May 2012. From each of the three control and biochar amended replicates, three 50 x 80 mm cores, plus three 75 x 100 mm cores, and three 60 x 61 mm cores were obtained from which the bulk density was determined. Gravimetric moisture content was determined by drying the entire core at 105 °C for 24 hours. Total

porosity was calculated from bulk density assuming a particle density of 2.65 g cm-3 and 98 % saturation. The direct effect of biochar porosity on soil density was determined by calculating the soil density without the porosity contributed by the biochar. The volume of biochar was calculated from the biochar density

cm-3) determined by mercury porosimetry and the mass of recovered biochar in each sample in which the volume and mass of recovered biochar were removed from the original dry soil mass and soil volume. This gave the density of the soil as it would have occurred without the direct pore contribution of the biochar. The effect of biochar application on bulk density and total porosity was investigated by univariate ANOVA in SPSS V 20.

Soil moisture

Soil moisture was measured in triplicate 10 cm from the centre tree in each replicate every two weeks between July 2010 and April 2013 using an ICT International Pty Ltd TDR based Moisture Probe Meter MPM-160-B with a 6 cm long probe. The effect of biochar application on bi-monthly soil moisture was investigated using univariate ANOVA in SPSS. Differences in soil moisture content between treatments were demonstrated by calculating the cumulative soil moisture content over time from the bi-monthly soil moisture sampling.

Drainable porosity and field capacity

The drainable porosity was determined by desorption using ceramic suction plates at 0.0 , -0.1, -1.0, -3.0 and -10.0 kPa (field capacity) according to Cresswell (2002) and Reynolds and Topp (2008). Three replicate 100 x 75 mm intact cores were obtained from each of the control and biochar replicates when the soil



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profile was moist but below field capacity. The cores were incrementally brought to saturation in a 0.01 M CaCl2 solution over a period of 4 - 5 days prior to analysis, and then picked to expose open pore faces before being imbedded onto the suction table with diatomaceous earth. The cores were allowed to equilibrate at each matric potential over a period of 5 -12 days until the outflow ceased. The effect of biochar application on drainable porosity and field capacity was determined by univariate ANOVA in SPSS V 20.

Soil water retention

The soil water release curve was determined by the evaporative flux method according to the procedure described by Wendroth et al. (1993) and Peters and Durner (2008) using the HYPROP apparatus and tensioVIEW software (UMS 2013). The soil water retention curve was fitted using both the van Genuchten- Mualem equation (van Genuchten 1980) and the bimodal van Genuchten-Mualem equation (Durner 1994), with and without the soil moisture content at -1500 kPa which was predetermined by the pressure chamber analysis. The lowest RMSE (0.0015) and highest absolute Akaike Information Criterion (-2399) (Akaike 1974) indicated the best model fit was achieved for the bimodal van Genuchten-Mualem model (Durner 1994) without the supplementary -1500 kPa data, in which

( ) ∑ ( ( | |) )

( ) ( ) where Se is the effective saturation, h is the matric potential, j is an index of the parameters of each van Genuchten functions, ωj is the weight of both partial functions, alpha (α) is an empirical parameter related to air entry, θ is the soil moisture content, θs is the saturated or field saturated soil volumetric water content, θr is the residual soil volumetric water content, and n is a dimensionless empirical constant. As the data pairs were unique to each soil core, the volumetric soil moisture content was calculated from the bimodal van Genuchten-Mualem equation at matric potentials of 0, -10, -20, -30, -50 kPa within the measurement range of the evaporative flux approach, and by extrapolation to -100, -300 ,-1000, and -1500 kPa for each soil core. Treatment and plot effects were thus able to be investigated by univariate ANOVA for each of the bimodal van Genuchten-Mualem soil parameters and the predetermined matric potentials.

The plant available water content (PAWC) was calculated as the water filled pore space between field capacity (FC), said to exist at -10 kPa and the permanent wilting point (PWP) at -1500 kPa (Brady and Weil 2010; James 1988; Marshall and Holmes 1988). The pore size distribution was estimated from the soil water characteristic according to the Young–Laplace equation which assumes the pores are perfectly cylindrical, uniform and equally drained. The Young - Laplace equation is approximated by

D = 30 / Ψm

where D is the pore diameter (μm), and Ψm is the absolute value of matric potential (m), such that PAWC corresponded to water stored within pores between 30 μm (field capacity) and 0.2 μm (permanent wilting point) diameter.

Permanent wilting point

The permanent wilting point (PWP) was determined by pressure chamber analysis at -1500 kPa using air dried < 2 mm soil from the control and biochar treatments. Due to the likelihood that sieving < 2 mm would result in the removal of biochar, the biochar application was replicated by artificially adding 2 %, 5 %, 10 % and 20 % by mass of biochar to the control sample. The effect of biochar application rate on microporosity (θ at -1500 kPa) was determined by one-way ANOVA.



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Infiltration and hydraulic conductivity

Infiltration rate and unsaturated hydraulic conductivity were determined by tension infiltration (or disk permeameter) following a procedure similar to Lin and McInnes (1995), in which the upper 1-2 cm soil was removed and the soil surface cleaned with compressed air to prevent interference to flow from surface crusting and foreign particles. The tension infiltrometers were connected to the soil surface by a 3 mm deep <250 μm washed sand pad. The apparent steady-state infiltration rates were measured sequentially from 3 devices for each plot at five supply potentials (ψ) of -0.95, -0.55, -0.35, -0.15, and -0.05 kPa. The soil moisture prior to infiltration was determined from five 60 mm x 60 mm intact cores per plot according to the procedure described by Cresswell and Hamilton (2002). The unsaturated hydraulic conductivity was calculated according to the procedure developed by Ankeny et al. (1991), Reynolds and Elrick (1991), and presented in McKenzie et al. (2002) in which;

Kx,y = (Gd αx,y qx) / r(1 + Gd αx,y πr) (qx / qy)P

K(Ψ) =Kx,y exp (αx,y Ψ)

αx,y = ln (qx / qy) / (Ψ x- Ψ y)

P = Ψ x / (Ψ x- Ψ y)

where, Kx,y is the average hydraulic conductivity for data pairs, K(Ψ) is the unsaturated hydraulic

conductivity (mm hr-1), αx,y is the soil structure parameter, P is a shape parameter, r is the radius of the disk

(cm), Ψx,y are the supply potentials (cm), qx,y is the steady state infiltration rate (cm3 min-1), Gd is a shape parameter = 0.25, Ψ equals (Ψx+ Ψy)/2), and x,y represent measurements at sequentially less negative supply potentials. The flow weighted mean pore diameter was determined according to Philip (1985) in which;

[

⁄ ]

Here, FWMPD is the flow weighted mean pore diameter (mm), and K1 and K2 are the first and second hydraulic conductivities (mm hr-1) at ψ1 and ψ2, where ψ1 and ψ2 are the first and second supply potentials (mm). Infiltration and unsaturated hydraulic conductivity data were log transformed prior to analysis by univariate ANOVA in SPSS.

Aggregate stability

Aggregates were sampled from 0 - 3 cm depth using three shovel loads per treatment. Aggregates were transported in open trays to reduce compaction and deformation. Aggregates were air-dried at ambient temperatures for 3 days before being sieved to obtain the 1-2 mm fraction. The 1-2 mm fraction was oven dried at 40 °C for 24 hours to ensure consistent starting moistures for all treatments. Air dried 1-2 mm aggregates were immersed in water for one minute on a 250 μm sieve then mechanically raised and

lowered 3.7 cm in tap water (250 μS cm-1) for 10 minutes. Retained aggregates and coarse fraction (stones, roots and biochar) were collected then dried at 105 °C for 24 hours. The mass of the retained coarse fraction was determined by dispersing the stable aggregates in a 5 % w/v hexametaphosphate with horizontal shaking for 18 hours before re-sieving to recover the > 250 μm course fragments. The mass of the retained coarse fraction was determined by oven drying at 105 °C for 24 hours. Aggregate stability was calculated as;

AS1-2mm > 250μm = R>250 μm – SB>250 μm / (Sair x (1-θ))-SB>250 μm



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where AS1-2mm > 250μm is the aggregate stability of the 1-2 mm fraction measured as the proportion of total

aggregates (minus the resilient stone, root and biochar fraction) retained on a 250 μm sieve (g g-1), R>250 μm

is the oven dried retained persistent aggregates and coarse fraction greater than 250 μm (g), SB>250 μm is the oven dried resilient stone, root and biochar component greater than 250 μm (g), Sair is the air dried (40 °C) mass of aggregates including resilient fraction prior to sieving, θ is the gravimetric moisture content of the

air dried 1-2 mm fraction (g g-1). The effect of biochar application on aggregate stability was investigated by univariate ANOVA in SPSS v20.

Results

Biochar porosity

Scanning Electron Microscopy (SEM) demonstrated the average pore size from the six SEM images of a single biochar particle (measured along the longest pore axis) ranged from 0.844 μm (sd ± 0.13 µm) to 235 μm (sd ± 123 µm), with the mean pore size ranging from 13.09 μm (sd ± 20.03 μm) to 7.08 μm (sd ± 6.98 μm) for the six images, depending on pore and measurement orientation. Pores were highly elliptical suggesting the original porous structure of the wood feedstock had become distorted during pyrolysis. Pore size distribution was highly skewed with 95 % of pores for the six images being less than 14.43 μm - 79.15 μm (range due to pore elongation), (.

).

Figure 1 An example of a Scanning Electron Microscopy (SEM) image of biochar and pore size analysis. (a) Original SEM image at 300 x magnification, (b) corrected SEM image to remove foreign particles and pore side walls, (c) inverse binary image, black is pore wall, white is pore space, (d) greyscale classification of pores size using the Image J gij_Pore Analysis plugin (e) Example of the pore size frequency distribution (pore size of longest pore axis).

Mercury porosimetry revealed that the average minimum pore diameter was approximately 0.1 μm, in which 95 % of all pores were less than 22 μm diameter. The average median pore diameter ranged 0.5 from approximately -0.4 - 13 um between samples. The characteristic length of pores averaged 44 µm which was also reflected in the average tourtuosity value of 6.6 indicating pores were 4.2 - 8.0 longer than they were



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wide (Error! Reference source not found.). Differences in biochar porosity determined by SEM versus the mercury porosimetry were considered minor, and largely due to detection of asymmetrical pore properties by SEM.

Table 1 Biochar properties determined by mercury porosimetry

Sample

Mean pore diameter

(4V/A)

Median pore

diameter (Volume)

Bulk density

Skeletal density

Porosity

Characteristic length

Tourtuosity

95% pore diameters <

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

Biochar recovery

The mean biochar recovery was 3.39 g 100 cm-3 (SD 1.46 g 100 cm-3), however the mass of recovered

biochar (>250 μm) varied between individual samples from 1.06 g per 100 cm-3 to 6.75 g per 100 cm-3. Differences in amount of recovered biochar between replicates and the two soil sample volumes were not significant. Analysis of the 100 mm diameter soil cores demonstrated that the volume of the recovered biochar was 6.53 % of the total soil volume.

Soil bulk density, total porosity, and saturated water content

Biochar application significantly reduced the soil bulk density in all replicates and for all three core sample sizes (F= 59.226, P= 0.015) (Error! Reference source not found.a). Consequently, biochar amendment resulted in significantly higher total porosity. A significantly higher saturated water content was also observed in the biochar treatment measured using 50 x 80 mm and 75 x 100 mm cores (F= 27.215, P= 0.031), and significantly higher soil moisture content at -0.1 kPa occurred in the biochar treatment measured using the 75 x 100 mm cores during desorption (F= 34.584, P< 0.028), (Error! Reference source not found.). Within the biochar treatment, a significant linear relationship existed between amount of applied biochar (>250 μm) and bulk density (F= 37.231, P=0.0001) for both the 50 x 80 mm and 75 x 100 mm cores (Error! Reference source not found.b).



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So

il b

ulk

de

nsity (

g c

m-1

) P

oro

sity,

Mois

ture

co

nte

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s (

cm

3 cm

-3)

0.65

0.60

0.55

0.50

0.45

0.40

0.35

a b i ii x xi

ns ns

Total Porosity - Biochar - all cores

Total Porosity - Control - all cores

Saturated moisture - Biochar - all cores

Saturated moisture - Control - all cores

Desoprtion -0.1 kPa - Biochar - 100 mm cores

Desoprtion -0.1 kPa - Control - 100 mm cores

van Genuchten s - Biochar - 80 mm cores

van Genuchten s - Control - 80 mm cores

Figure 2 The effect of biochar application on total porosity, saturated 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, other than for the van Genuchten parameter θs. Error bars indicate + 1 standard deviation.

The lower bulk density of the biochar amended soil did not result from direct pore contribution from the biochar itself, as the bulk density of the biochar excluded treatment (effects of biochar porosity had been removed from the soil volume) was significantly lower than the unamended control (F= 320.26, P= 0.0001), (Error! Reference source not found.a).

1.40 1.40 (a) (b)

1.30

1.20

1.30

1.20

50 x 80 mm cores

75 x 100 mm cores

1.10 1.10

1.00

1.00

y = -0.036 x + 1.24

R2

= 0.662, P<0.001

0.90 Biochar

Control

0.90 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Biochar excluded Recovered Biochar (g per 100 cm3)

Figure 3 (a) Effect of biochar on soil bulk density. The ‘biochar excluded’ refers to the biochar treatments in which the effects of the biochar density have been removed from the calculation of soil density. Error bars indicate + 1 standard deviation. (b) Linear regression between amount of applied (recovered) biochar (>250 μm) and soil bulk density.

Soil Moisture

Biochar application had no significant effect on soil moisture content (Error! Reference source not found.a) or cumulative soil moisture between July 2010 and May 2013 (Error! Reference source not found.b).



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-3

So

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(% v

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(cm

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(% v

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3

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Pro

po

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tal p

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sity (

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(a) Mean soil moisture

50

2000

(b) Cumulative soil moisture

40

1500

30

1000

20

10 Biochar

Control

0

(a)

500

0

Control

Biochar

(b)

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

Date

Date

Figure 4 Effect of biochar application on soil moisture content 0 - 6 cm depth (% vol.), (a) bi-monthly mean soil moisture content, and (b) cumulative soil moisture. Error bars represent ±1 standard deviation.

Drainable porosity and field capacity

Biochar application had no significant effect on the drainable porosity between -1.0 kPa and -10 kPa, nor on the field capacity at -10 kPa. However biochar application significantly increased the saturated soil moisture content (F = 132.878, P = 0.007), and soil moisture content at – 0.1 kPa (F = 32.639, P = 0.029). This supports the previous finding that application of biochar application significantly reduced the soil bulk density and increased the total porosity, presumably due to the creation or preservation of large pores (> 300 μm) in the surrounding soil.

0.60

0.55

0.50

0.45

0.40

0.35

0.30

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0.25

(a) (b) 0.5

0.0 2.0 4.0 6.0 8.0 10.0

Matric Potential (-kPa)

Sat. 3000 300 100 30

Maximum equavalent pore diameter (um)

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

Soil-water retention and plant available water

Determination of the soil-water retention function by the evaporative flux method revealed considerable variability within treatments and between replicates. Biochar application was found to have no significant effect on (i) the bimodal van Genuchten –Mualem soil water parameters (α1,2, θs, θr, n1,2, ω), (ii) the measured equilibration potentials between -10 and -50 kPa, (iii) the extrapolated equilibration potentials between -100 and -1500 kPa, (iv) the PAWC between -10 kPa and -1500 kPa, or (v) PWP at -1500 kPa (Error!



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Volu

metr

ic s

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(cm

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Reference source not found.). There was no significant relationship between the mass of recovered biochar >250 μm and soil moisture content at any of the eight equilibration steps. Similarly, there was no significant linear relationship between the mass of recovered biochar >250 μm and any of the bimodal van Genuchten – Mualem soil water parameters (α1,2, θs, θr, n1,2, ω2).

0.5 1.0

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0.2

(a) (b) 0.0

0 200 400 600 800 1000 1200 1400 1600

30 15 10 6 3 1 0.3 0.2

Matric potential (-kPa)

Maximum equivalent pore diameter (um)

Figure 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.

Microporosity and the Permanent Wilting Point

Biochar application had no significant effect on the permanent wilting point at -1500 kPa and thus did not

increase the number of pores smaller than 0.2 μm diameter. Addition of 5 % by weight of biochar to the

control treatment had no significant effect on soil moisture content at -1500 kPa. However application of

20 % wt biochar significantly (F = 16.106, P = 0.0001) increased the soil moisture content at -1500 kPa (

a).

Infiltration and Hydraulic conductivity.

Overall, biochar amendment had no significant effect on infiltration or the unsaturated hydraulic conductivity. However, the biochar amended soil had significantly higher infiltration at -0.15 kPa and -0.05 kPa, and significantly higher unsaturated hydraulic conductivity at -0.25 kPa and -0.10 kPa (Error! Reference source not found.a). At all other supply potentials biochar application had no significant effect on unsaturated hydraulic conductivity or infiltration (Error! Reference source not found.b). Biochar application had no significant effect on the calculated flow weighted mean pore diameter.



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So

il m

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co

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at

-15

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a

Infiltra

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Hyd

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Co

nd

uctivity (

mm

hr-1

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Ag

gre

ga

te S

tab

iliy (

g g

-1)

FW

MP

D (

mm

)

100

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

1.0

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Control

Biochar

Control

0.8

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Control

10

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1

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0

-1.00 -0.75 -0.50 -0.25 0.00

Supply Potential (kPa)

-1.00 -0.75 -0.50 -0.25 0.00


0.0 -0.45 -0.25 -0.01


Figure 7 Effect of biochar application on (a) infiltration rate (mm hr-1

), (b) unsaturated hydraulic conductivity (mm hr-1

), (c) flow weighted mean pore diameter (mm). Note error bars indicate ± 1 standard deviation.

Aggregate stability

Overall, biochar application had no significant effect (F= 0.021, p =0.90) on the stability of the 1 - 2 mm aggregates. At replicates 4 and 5, biochar application significantly decreased the aggregate stability, whilst at replicate 1 biochar application significantly increased aggregate stability (

b).

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

1.0

12

0.8

Control

Biochar

10 0.6

0.4

8

0.2

6

Control Biochar 3.4 % 5 % 10 % 20 %

0.0

Replicate 2 Replicate 4 Replicate 5

Figure 8 Effect of biochar application on (a) soil moisture content (water filled porosity) at -1500 kPa, the biochar treatment had approximately 3.4 % wt biochar. +5%, +10% and +20% refer to the amount of biochar (by weight) added to the control, (b) aggregate stability, presented as the proportion of 1-2 mm water stable aggregates > 250 μm. Error bars represent + 1 standard deviation.

Discussion

The pore size of the acacia greenwaste biochar used in this study ranged from 0.1 μm to 235 μm with a median pores size between 0.4 μm to 13 μm. The similarity between the pore size distribution of the acacia biochar and the PAWC pore size range of soil (0.2 μm - 30 μm) suggested that application of the acacia biochar should increase soil PAWC, and to a lesser extent the drainable porosity through direct pore



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contribution. However, the application of 47 Mg ha-1 acacia green waste biochar had no significant effect on drainable porosity (-1.0 to -10 kPa), field capacity (-10 kPa), PAWC between -10 kPa and -1500 kPa, nor the PWP (-1500 kPa). Other researchers have also reported that biochar application failed to improve soil

water retention of in situ soils. Major et al. (2012) found that application of 20 Mg ha-1 biochar had no significant effect on soil water retention or drainage of a clay soil. Gaskin et al. (2007) reported that

–1

application of biochar at 11 and 22 Mg ha had no significant effect on in situ water holding capacity between -20 kPa and – 100 kPa. In their review of the literature, Mukherjee and Lal (2013) suggested that the PAWC response to biochar application is both soil and biochar specific. This is supported by studies such as Streubel et al. (2011) who found that only 25 of 60 soil type-biochar application rate combinations resulted in significantly higher water holding capacity. Despite most biochars containing a high proportion of micropores (Downie et al. 2009; Kookana 2010; Major et al. 2009) including the acacia biochar used in this study, very few investigations have reported the effects of biochar application on soil microporosity.

Our study demonstrated that application of biochar at 47 Mg ha-1 had no significant effect on the PWP or the van Genuchten residual water content parameter (θr). Similar findings have been reported by Eastman

(2011) and Laird et al. (2010) who also found application of biochar at rates up to 20 Mg ha-1 had no significant effect on water retention at -1500 kPa. Uzoma et al. (2011) reported biochar application had no significant effect on θr.

Application of acacia green waste biochar at 47 Mg ha-1 significantly reduced soil bulk density and thus increased total porosity and saturated water content. Reduced bulk density of biochar amended soil has

also been reported by Chen et al. (2011) who found application of 2.3 Mg ha-1 and 4.5 Mg ha-1 biochar decreased bulk density by 4.5 and 6.0 % respectively. Zhang et al. (2010) reported application of wheat

straw biochar decreased the bulk density of a rice paddy soil at 40 Mg ha-1 but not at 10 Mg ha-1. Major et

al. (2012) reported application of 20 Mg ha-1 biochar significantly reduced the density of a heavy clay soil at 0-15 cm depth, but not at the soil surface or at 0.15 – 0.30 m depth.

In our study, the reduced bulk density in the biochar amended soil did not result from the internal porosity of the biochar, as the bulk density of the biochar amended treatment in which the internal porosity has been removed from the calculation of bulk density was significantly lower than the unamended control. Consequently the reduction in bulk density or increase in total porosity must have occurred in the soil surrounding the biochar via mechanisms other than direct pore contribution. It is also unlikely that the increased total porosity resulted from the creation of accommodation pores as the biochar application had no significant effect on the soil water retention curve below -0.1 kPa. Nor is it probable that the increased total porosity resulted from the biochar protecting pores created during the incorporation as application of biochar had no significant effect on aggregate stability.

Several lines of evidence suggest the increase in total porosity in the biochar amended soil resulted from the creation of large macropores in the soil surrounding the biochar particles. Drainable porosity of the biochar treatment was significantly higher than the control at -0.1 kPa but not -1.0 kPa. This suggests the biochar application resulted in the formation of large macropores of at least 3000 µm diameter, but not smaller than 300 µm diameter. Likewise the unsaturated hydraulic conductivity was significantly higher in the biochar treatments than the control at -0.25 kPa, but not at -0.45 kPa. This suggests biochar application resulted in the formation of macropores at least 1200 µm diameter, but not smaller than 660 µm diameter. This finding contrasts to other studies conducted on in situ agricultural soils which have reported that biochar application had no significant effect on saturated hydraulic conductivity (Eastman 2011; Major et al. 2012). The formation of these large macropores was attributed to a fourth and previously unreported mechanism by which biochar may influence soil porosity: that is increased invertebrate burrowing. At the time of sampling earthworm numbers were visibly higher in the biochar amended soil than the untreated



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control. Consequently the increased number of large macropores (>1200 µm), and thus increased total porosity, saturated water content and near saturated hydraulic conductivity (≥-0.25 kPa) of the biochar amended soil was attributed to increased earthworm burrowing. Few studies have investigated the effect of biochar application on invertebrates (Lehmann et al. 2011). In their review, Weyers and Spokas (2011) concluded biochar may have short-term negative impacts on earthworm population density and total biomass. However there was little evidence to suggest biochar had any long-term effects on earthworm density or total biomass. In a behavioural experiment van Zwieten et al. (2010) showed earthworms preferred a biochar amended Ferrosol but had no preference for biochar in a Calcarosol. Gomez-Eyles et al. (2011) however found that biochar application in a contaminated soils resulted in the loss of earthworm mass and condition, while Busch et al. (2012) demonstrated biochar had no effect on earthworm avoidance in a contaminated soil. Earthworm response to biochar application appears to depend on biochar type, soil type, and time.

Results demonstrate that substantial within and between replicate variation existed in the amount of recovered biochar, and physical soil properties such as bulk density, hydraulic conductivity, soil water retention and aggregate stability. Although variation in the amount of recovered biochar (1.06 - 6.75 g per

100 cm-3) may have influenced measured values, this effect was only apparent for bulk density (Error! Reference source not found.b). Furthermore, data indicates that soil hydraulic properties such as drainable porosity, hydraulic conductivity, and the soil water retention function varied due to the high spatial variation in soil pore size and pore arrangement at the site. Orchard soils are expected to have a higher degree of pore space variation than arable soils, as they are relatively undisturbed allowing development of soil structure by processes such as freeze – thawing, bioturbation, microbiological activity, and shrink-swelling (Hillel 1998). Consequently, in contrast to pot trials in which natural soil structure is destroyed and homogenised, in situ studies of orchard soils require treatment effects to be greater than pot trials in order to yield a statistically significant change in soil physical attributes. In this study application of 47 t/ha acacia biochar had no significant effect on a range of soil hydraulic properties due in part to the high natural variation in soil physical properties. Consequently in order to produce a statistically significant

effect, biochar would need to be applied at rates in excess of 50 Mg ha-1, which is both physically and economically prohibitive in commercial orchards.

Conclusion

We proposed three mechanisms by which biochar application might increase soil porosity. They were (i) direct pore contribution from the pores within the biochar, (ii) creation of packing or accommodation pores, and (iii) improved aggregate stability. Mercury porosimetry and SEM analysis demonstrated that the acacia biochar used in this study contained pores between approximately 0.1 μm and 240 μm in diameter, with 95 % of all pores being less than 22 μm. Consequently, application of the acacia biochar was expected to increase plant available water through direct pore contribution by increasing the proportion of pores between field capacity (30 μm) and the permanent wilting point (0.2 μm), and to a lesser extent

macroporosity (pore diameters > 75 μm). However, application of 47 Mg ha-1 of acacia green waste biochar had no significant effect on; drainable porosity (–1.0 kPa and -10 kPa), field capacity, PAWC, PWP, the van Genuchten soil water retention parameters (α1,2, θs, θr, n1,2, ω), nor soil moisture content. We found no evidence to suggest biochar application directly influenced soil porosity through either direct pore contribution, the creation of accommodation pores or increased aggregate stability as we had speculated. However, the biochar amended soil had significantly higher near saturated hydraulic conductivity (-0.25 kPa and -0.10 kPa), total porosity, and soil water retention at -0.1 kPa resulting from the presence of large macropores (> ~1200 μm). These large macropores were attributed to increased earthworm burrowing,



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based on unrecorded observations of earthworm presence during the experiments. More research is required to verify this hypothesis.

Our study demonstrated that despite use of a biochar dominated by pores within the PAWC range,

application at 47 Mg ha-1 to a loamy sand soil within an apple production system had no significant effect on soil water availability or soil moisture content. Lack of a significant difference in all soil physical properties between the control and biochar treatments resulted in part from the high spatial variation in pore size and architecture at the site.

Acknowledgements

This project was conducted as part of the national apple and pear industry Productivity Irrigation Pests and Soils (PIPS) flagship program and was funded by Horticulture Australia Limited using the apple and pear industry levy, voluntary contribution from the New Zealand Institute for Plant and Food Research and matched funds from the Australian Government. We thank Justin Direen for assistance with trial establishment and Benedicte Patin, Steve Patterson, Jocelyn Parry - Jones and Anna Wrobel-Tobiszewska for assistance with field work. Assistance with SEM and mercury porosimetry was gratefully received from Dario Arrua and Jocelyn Parry – Jones. Thanks to Drs Caroline Mohammed and Alieta Eyles for valuable comments on an earlier draft of the manuscript. This work was conducted while the first author was seconded from the Department of Primary Industries, Parks, Water and Environment (DPIPWE).



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