effect of biochar application rate on soil physical and hydraulic properties of a sandy loam

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This article was downloaded by: [University of Arizona] On: 05 July 2014, At: 04:29 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Archives of Agronomy and Soil Science Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gags20 Effect of biochar application rate on soil physical and hydraulic properties of a sandy loam Leonard Githinji a a Agricultural and Environmental Sciences, Tuskegee University, Tuskegee 36088, AL, USA Accepted author version posted online: 02 Jul 2013.Published online: 16 Aug 2013. To cite this article: Leonard Githinji (2014) Effect of biochar application rate on soil physical and hydraulic properties of a sandy loam, Archives of Agronomy and Soil Science, 60:4, 457-470, DOI: 10.1080/03650340.2013.821698 To link to this article: http://dx.doi.org/10.1080/03650340.2013.821698 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms- and-conditions

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This article was downloaded by: [University of Arizona]On: 05 July 2014, At: 04:29Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Archives of Agronomy and Soil SciencePublication details, including instructions for authors andsubscription information:http://www.tandfonline.com/loi/gags20

Effect of biochar application rate onsoil physical and hydraulic propertiesof a sandy loamLeonard Githinjiaa Agricultural and Environmental Sciences, Tuskegee University,Tuskegee 36088, AL, USAAccepted author version posted online: 02 Jul 2013.Publishedonline: 16 Aug 2013.

To cite this article: Leonard Githinji (2014) Effect of biochar application rate on soil physical andhydraulic properties of a sandy loam, Archives of Agronomy and Soil Science, 60:4, 457-470, DOI:10.1080/03650340.2013.821698

To link to this article: http://dx.doi.org/10.1080/03650340.2013.821698

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the“Content”) contained in the publications on our platform. However, Taylor & Francis,our agents, and our licensors make no representations or warranties whatsoever as tothe accuracy, completeness, or suitability for any purpose of the Content. Any opinionsand views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Contentshould not be relied upon and should be independently verified with primary sourcesof information. Taylor and Francis shall not be liable for any losses, actions, claims,proceedings, demands, costs, expenses, damages, and other liabilities whatsoever orhowsoever caused arising directly or indirectly in connection with, in relation to or arisingout of the use of the Content.

This article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Effect of biochar application rate on soil physical and hydraulicproperties of a sandy loam

Leonard Githinji*

Agricultural and Environmental Sciences, Tuskegee University, Tuskegee 36088, AL, USA

(Received 17 May 2013; final version received 27 June 2013)

Biochar is used as a soil amendment for improving soil quality and enhancing carbonsequestration. In this study, a loamy sand soil was amended at different rates (0%,25%, 50%, 75%, and 100% v/v) of biochar, and its physical and hydraulic propertieswere analyzed, including particle density, bulk density, porosity, infiltration, saturatedhydraulic conductivity, and volumetric water content. The wilting rate of tomato(Solanum lycopersicum) grown in soil amended with various levels of biochar wasevaluated on a scale of 0–10. Statistical analyses were conducted using linear regres-sion. The results showed that bulk density decreased linearly (R2 = 0.997) from 1.325to 0.363 g cm−3 while the particle density decreased (R2 = 0.915) from 2.65 to1.60 g cm−3 with increased biochar amendment, with porosity increasing(R2 = 0.994) from 0.500 to 0.773 cm3 cm−3. The mean volumetric water contentranged from 3.90 to 14.00 cm3 cm−3, while the wilting rate of tomato ranged from 4.67to 9.50, respectively, for the non-amended soil and 100% biochar-amended soil. Theseresults strongly suggest positive improvement of soil physical and hydraulic propertiesfollowing addition of biochar amendment.

Keywords: biochar amendment; soil quality; soil hydraulic properties; soil physicalproperties

Introduction

Biochar is a carbon-rich material produced from pyrolysis. Recently, there has been muchinterest in its use as a soil amendment to improve and maintain soil fertility and toenhance soil carbon sequestration (Glaser et al. 2002; Lehmann et al. 2003). The latter canbe attributed to the relative stable nature, hence the long turnover time of biochar in soiland its relevance as a solution to climate change (Lehmann et al. 2006). Application ofbiochar to infertile soils has been shown to provide longer-lasting improvements in soilfertility (Glaser et al. 2002; Lehmann et al. 2003; Steiner et al. 2007). Since biochar iscomposed primarily of single and condensed ring aromatic carbon (Lehmann 2007), it hasboth a high surface area per unit mass and a high charge density, contributing to a highercapacity to adsorb cations compared with soil organic matter (Sombroek et al. 2003;Liang et al. 2006). Application of biochar to soils is not a new concept (Mann 2005); ithas been used in the Amazon basin leading to formation of anthropogenic dark earth soils,referred to as Terra Preta (Glaser et al. 2001). These soils occur in distinct small to largesites of between 20 and 350 ha, with a total area of approximately 50,000 ha in centralAmazon (Glaser et al. 2001). Terra Preta contain large amounts of charred materials mostlikely added by pre-Columbian farmers who are reported to have practiced a form of slash

*Email: [email protected]

Archives of Agronomy and Soil Science, 2014Vol. 60, No. 4, 457–470, http://dx.doi.org/10.1080/03650340.2013.821698

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and char agriculture (Glaser et al. 2001) along with disposal of charcoal remains fromhearths (Glaser et al. 2002). According to Glaser et al. (2001), Terra Preta are high inavailable plant nutrients, pH, cation exchange capacity (CEC), and water and nutrientholding capacity, which supports higher levels of agriculture compared with adjacenthighly weathered and acidic soils. In these soils, the biochar acts as a soil conditioner,improving soil physical properties and nutrient use efficiency, thereby increasing plantgrowth. After more than five centuries since the cessation of the practices that createdTerra Preta, the soils are still highly valued for agricultural and horticultural use in theAmazon basin (Glaser et al. 2001).

Biochars can be produced from a range of organic materials and under differentconditions resulting in products of varying morphological, chemical, and physical proper-ties and are therefore of different soil amendment values (Baldock & Smernik 2002;Nguyen et al. 2004; Guerrero et al. 2005; Lehmann & Rondon 2006). Thus, biochars fromplant materials are often low in nutrient content, particularly N, compared with otherorganic fertilizers (Lehmann et al. 2003; Chan et al. 2007). Additionally, Chan et al.(2007) reported a lack of positive plant response when green waste biochar was applied atup to 100 metric ton ha−1 and attributed this to the low N availability of the plant-derivedbiochar. Due to the generally higher nutrient content of animal wastes than plant wastes(Shinogi 2004), biochars produced from animal origins may have higher nutrient content.The benefit achieved by adding amendments to soil is partly determined by the amend-ment type as well as the property of the soil. For instance, previous researchers workingwith sandy soils reported high N leaching after addition of fertilizer or manure (Trindaleet al. 1997; Ritter et al. 1998; Zotarelli et al. 2007) and this could be attributed to lownutrient holding capacity of sandy soil, especially those with low organic matter. Biocharproperties also differ based on specific pyrolysis conditions, that is, final pyrolysistemperature or peak temperature, rate of charring or ramp rate, and duration of charringtime (Mukherjee & Lal 2013). Soil function and quality depend on physical, chemical,and biological properties, and various studies have focused on impact of biochar applica-tion on these properties. Whereas biochar is widely considered as a soil amendment, thefocus of the past studies has been mainly limited to the nutrient status of the amended soilincluding CEC, pH, nutrient content, and carbon sequestration of the amended soil(Mukherjee & Lal 2013). Researchers have reported beneficial impacts of biochar appli-cation including carbon sequestration (Glaser et al. 2002), improved microbial communityor soil biota (Lehmann et al. 2011), increased nutrient status (Glaser et al. 2002; Haefeleet al. 2011), and reduced greenhouse gas emissions (Fowles 2007; Lehmann 2007).Biochar incorporation in the soil as an amendment or in combination with biomassfeedstock can help combat global climate change by displacing fossil fuel use, bysequestering carbon in stable pools, and by reducing emissions of nitrous oxide(Rondon et al. 2005; Yanai et al. 2007). Additionally in the soil, biochar provides ahabitat for soil organisms but is not readily consumed by soil microbes. Hence, most ofthe applied biochar can remain in the soil for several hundreds to thousands of years(Pessenda et al. 2001; Schmidt et al. 2002). According to Glaser et al. (2001), biochardoes not disturb carbon–nitrogen balance in the long term, but holds and makes water andnutrients available to plants. When used as a soil amendment along with organic andinorganic fertilizers, biochar has been reported to significantly improve soil tilth, produc-tivity, and nutrient retention and availability to plants (Glaser et al. 2002).

Recently, researchers have focused on the impact of biochar application on selectedsoil physical and hydraulic properties of various soils. Asai et al. (2009) reportedimproved saturated hydraulic conductivity of a top soil in Laos; Novak et al. (2009)

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reported an increase in water retention of a loamy sand; Brockhoff et al. (2010) reportedan increase in water retention but a decrease in saturated hydraulic conductivity for sand-based root zones; Busscher et al. (2010) reported a decrease in soil penetration resistance,but the impact on soil aggregation, infiltration, and water-holding capacity showed mixedresults; Haefele et al. (2011) reported a decrease in topsoil bulk density for an irrigatedlowland site and a rainfed upland, but no effect on a rainfed lowland site; and Novak et al.(2012) reported enhanced moisture storage capacity of Ultisols and Aridisols. Thesestudies focused on a narrow range of physical and hydraulic properties and some of thestudies reported mixed findings. Hence, there is a need to focus on the effects of biocharon a wide range of soil physical and hydraulic properties (Hammes & Schmidt 2009;Atkinson et al. 2010). Mukherjee and Lal (2013) reported that there is a knowledge gap asto how biochar alters soil physical properties.

In this study, a loamy sand (fine-loamy, kaolinitic, thermic Typic Kandiudult) wasamended with different levels (0%, 25%, 50%, 75%, and 100% v/v) of biochar and itsphysical and hydraulic properties were analyzed, including particle density, bulk density,porosity, infiltration, saturated hydraulic conductivity, and volumetric water content. Thewilting rate of Amelia tomato (Solanum lycopersicum) grown in soil amended withvarious levels of biochar-amended soil was also evaluated.

Materials and methods

The biochar used in the experiment was obtained from The University of Georgia and hadbeen produced from peanut hulls (Arachis hypogaea) using the slow pyrolysis method at500°C for 1 h. The properties of the biochar used for this study are shown in Table 1. Inthe laboratory, the biochar was passed through a 2-mm sieve to correspond with theUnited States Department of Agriculture (USDA) textural limit for soil. Various labora-tory procedures were used to determine the physical and hydraulic properties of soil(loamy sand) mixed with various levels (0%, 25%, 50%, 75%, and 100% v/v) of biochar.Each air-dried sample was packed in a standard brass core, with successive amounts ofabout 5 cm3 of material added gradually with frequent stirring to avoid layering. Thecylinders were tapped gently until they were completely full. All samples were packed intriplicate; hence, for all the properties under consideration, the values were obtained fromthe average of triplicates.

The physical properties determined were particle size distribution, bulk density,particle density, and porosity. Soil textural analysis was determined using theBouyoucos hydrometer method (Gee & Bauder 1979), which involves dispersing soilparticles and determining the amount of sand, silt, and clay in the soil sample by ahydrometer based on Stokes’ Law. Both chemical and mechanical dispersion of the soilparticles were conducted before textural analysis. For the chemical dispersion, 5 ml of 1Nsodium hexametaphosphate solution was added to each sample while mechanical disper-sion was achieved by placing the samples in a mechanical shaker for 24 h at roomtemperature. Following the mechanical dispersion, the soil solutions were transferred into

Table 1. Properties of the peanut hulls biochar used for this study (Novak et al. 2009).

Elemental composition (%, oven-dry wt. basis)

Surface area (m2 g−1) pH Ash C H O N P S

1.22 8.6 9.3 81.8 2.9 3.3 2.7 0.26 0.10

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a 1000-ml sedimentation cylinder and deionized water was added to bring the volume to1000 ml. To begin the sedimentation process, the cylinder was agitated by manuallyshaking the cylinder back and forth for a minimum of 1 min and using a plunger toremove any particles that adhered to the surface of the sedimentation cylinder. Afteragitation, the cylinder was placed upright on a laboratory bench and a timer simulta-neously started. Readings were then taken at elapsed times of 40 s and 2 h, using a 152Hhydrometer (H-B Instrument Company, Collegeville, Pennsylvania, USA). The tempera-ture of the suspension liquid was recorded simultaneously with hydrometer readings tocorrect for its effect on density and viscosity. To calibrate the hydrometer, a blank readingwas taken in a solution containing 100 ml of 1N sodium hexametaphosphate solution and900 ml deionized water, but without soil. Hence, the particle sizes were determined byusing the following equations:

%Sand ð2� 0:05mmÞ ¼ ðOD soil wtÞ � ðcorr: 40 sec readingÞOD soil wt:

� 100% (1)

%Clay ð< 0:002mmÞ ¼ corr: 2 h reading

OD soil wt:� 100% (2)

%Silt ð0:05� 0:002mmÞ ¼ 100%� %sandþ%clayð Þ (3)

For the bulk density determination, core samples were placed in the oven at 105°C for24 h to obtain the oven-dry mass. The volume of the samples was calculated from thevolume of the core with a Vernier caliper used to measure the height and internal diameter.Hence, the bulk density was determined by calculating the ratio of the oven-dry mass tovolume of the sample (g cm−3). Particle density was determined using the pycnometermethod (Flint & Flint 2002), which is based on the determination of the volume of aknown mass of soil by liquid displacement. All the samples were first moistened using anaspirator bottle and placed in zip-lock bags and allowed to sit overnight to equilibrate.This was done to overcome any hydrophobicity due to addition of biochar that containsorganic constituents. Total porosity was calculated from particle and bulk density by usingthe following equation:

f ¼ 1� ρbρp

!(4)

where ρb is the bulk density (g cm−3), ρp is the particle density (g cm−3), and f is the totalporosity (cm3 cm−3).

The hydraulic properties determined were cumulative infiltration (I), saturated hydrau-lic conductivity (k), and volumetric water content. Cumulative infiltration and hydraulicconductivity were determined using a 32.7-cm length by 4.5-cm diameter Decagoninfiltrometer (Decagon Devices Inc., Pullman, WA, USA), and applying the methodproposed by Zhang (1997) that involves measuring cumulative infiltration against timeand fitting the results in the following equation:

I ¼ C1t þ C2

ffiffit

p(5)

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where C1 (m s−1) and C2 (m s−½) are curve fitting parameters; C1 is related to hydraulicconductivity, and C2 is related to soil sorptivity. The hydraulic conductivity of the soil (k)is then computed by using the following equation:

k ¼ C1

A(6)

where A is a value relating to the van Genuchten parameters for a given soil type to thesuction rate and radius of the infiltrometer disk.

The matric head values were determined using tensiometers equipped with vacuumgauges (Soil Moisture Inc., Santa Barbara, CA, USA). Before the installation, tensi-ometers were tested for leaks using the method proposed by Puckett and Dane (1981)that involves filling them with water and subjecting them to negative pressure whilechecking for signs of bubbling. After confirming the absence of leaks, tensiometers wereinstalled in each plant pot at a depth of 15 cm. The tensiometer gauge readings wererecorded daily and converted to matric head values using the method proposed byKoorevaar et al. (1983), using the following equation:

hm ¼ hguage þ Δz1 þ Δz2 (7)

where hm is the matric head (cm), hgauge is the pressure head of the water in the gauge, Δz1is the height of the gauge above the soil, and Δz2 is the depth of the ceramic cup from thesoil surface. The volumetric water content was determined using the time domainreflectometry (TDR) method. Thus, 15-cm long TDR probes (Soil Moisture Inc., SantaBarbara, CA, USA) were inserted into the soil on a daily basis and volumetric watercontent values were read and recorded.

To estimate the plant-available water, a bioassay was determined to determine plantavailable water. This was done using Amelia tomato (Solanum lycopersicum) wherewilting rate was determined for tomatoes grown on various levels of biochar-amendedsoil. The tomato transplants were established in the greenhouse (35/20°C day/night) for2 months on each of the biochar–soil mixtures packed in plastic pots (20 cm dia-meter × 25 cm depth). Before the drying experiment, the tomatoes were well wateredand fertilized with a soluble fertilizer (20–20–20, N–P–K) at a rate of 70 kg N ha−1. Afterthe tomatoes were well established, they were watered up to saturation, thereafter, droughtstress was imposed and the leaves monitored daily for signs of wilting. The leaf qualityrating (LQR) was hence evaluated on a scale of 0–10, with 10 representing no wilting; theaverage time to wilting for the tomato plants was 9 days.

Results and discussions

The results of the particle size fractions for the soil amended at varying rates of biochar(Table 2) showed that the sand fraction (0.05 mm < particle size < 2 mm) ranged from88% for the soil without biochar amendment to 92% for 100% biochar rate. It wasapparent that the percentage of particle size in the sand fraction increased with biocharapplication rate. The percentage of silt fraction (0.002 mm < particle size < 0.050 mm)ranged from 6% for the non-amended soil to 0% for 100% biochar application rate, whilethe clay fraction (particle size < 0.002 mm) ranged from 6% for non-amended soil to 8%for 100% biochar application rate. The USDA textural class was determined as ‘sand’ for

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all the biochar application rates. These results suggest that the additional of biochar as asoil amendment modified the soil texture slightly from ‘loamy sand’ to ‘sand’, due to thepresence of many larger particles in the range of sand in the biochar. However, the slightincrease in clay-sized particles upon increasing the biochar application rates suggestsincreased surface area that could potentially affect the physiochemical activity of the soil.

The particle density values ranged from 2.62 g cm−3 for the non-amended soil, to2.43 g cm−3 for 25% application rate, 2.37 g cm−3 for 50% application rate, 2.09 g cm−3

for 75% application rate, and 1.60 g cm−3 for 100% application rate of biochar (Figure 1).The regression equation for particle density as a function of biochar amendment rate wasdetermined as shown in the following equation:

Particle density ¼ �0:244� %biocharð Þ þ 2:961 (8)

It is clear that from the trend and the regression equation that increasing the rate ofbiochar amendment significantly (R2 = 0.915; p < 0.01) decreases the particle density.This is due to differences in physical and mineralogical properties of biochar and soilmaterial. Although the particle density of a soil is not always affected by particle size orarrangement of particles, the mineralogical component or type of solid particles presenthas a major impact on particle density. In this case, it is apparent that although most ofbiochar particles are of a size similar to that of sand, they have a lower particle densitycompared to that of sand. For the non-amended soil, it is apparent that the particle densityis very close to that of quartz and this can be explained by the soil textural class which isloamy sand which according to the USDA textural triangle has at least 70% sand. Theparticle density of quartz is approximately 2.70 g cm−3. Addition of biochar involves

Table 2. Particle size fractions of the soil at varying biochar application rates.

Biochar application rate (%) Sand (%) Silt (%) Clay (%)

0 88.0 6.0 6.025 89.3 3.2 7.550 90.2 2.1 7.775 91.3 0.7 8.0100 92.0 0.0 8.0

3.00 y = �0.244x + 2.961R2 = 0.915

2.50

2.00

1.50

Par�

cle

dens

ity (g

cm

–3)

1.00

0.50

0.000% biochar 25% biochar 50% biochar 75% biochar 100% biochar

Figure 1. Particle density (g cm−3) of the soil at varying biochar application rates.

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introducing particles that contain substantially higher organic matter that has lowerdensity (usually less than 1.0 g cm−3) compared to sand (quartz) particles, hence thedecrease in particle density on a unit volume basis.

Just like particle density, bulk density decreased with the rate of biochar amendment(Figure 2). The highest bulk density was 1.33 g cm−3 for the soil without biocharamendment, decreasing to 1.09 g cm−3 for 25% rate, 0.89 g cm−3 for 50% rate,0.61 g cm−3 for 75% rate, and 0.36 g cm−3 for 100% rate of biochar application. Thebulk density is related to the rate of biochar amendment by the following equation:

Bulk density ¼ �0:240� %biocharð Þ þ 1:576 (9)

The regression equation reveals a significant (R2 = 0.997; p < 0.01) decrease in bulkdensity as the rate of biochar amendment is increased. Since bulk density is a measure ofthe relative mass of a solid relative to the bulk volume the solid occupies, including thevoid spaces, it follows that the greater is the portion occupied by the pores, the lower isthe bulk density of a solid. The upper limit of the bulk density would be a situation wherethere are no pores and this limit will approach that of particle density of a solid. As aporous medium, the soil always has some pores, hence the bulk density is always muchlower compared with the particle density. The advantage of a lower bulk density valuerelative to the particle density is that it accounts for an increase in pore space whichenhances the potential for soil aeration and increased water content in soil, hence a goodindicator of soil quality. However, as reported by Githinji et al. (2011), bulk density aloneis not considered to be an adequate indicator of a successful amendment–soil mixture.

Unlike the results of particle density and bulk density, porosity increased withincreased rate of biochar application (Figure 3). For the non-amended soil, porosity was0.50 cm3 cm−3, increasing to 0.55, 0.61, 0.69, and 0.78 cm3 cm−3, respectively, for 25%,50%, 75%, and 100% rates of biochar application. The linear increase in porosity as afunction of biochar application rate is represented by the following:

Porosity ¼ 0:07� %biocharð Þ þ 0:42 (10)

These results showed a significant (R2 = 0.994; p < 0.01) increase in porosity as afunction of biochar application rate. Soil porosity includes macropores that allow forthe movement of air and drainage of water and micropores that exhibit attractive forces

1.60

y = –0.240x + 1.576

R2 = 0.9971.40

1.20

1.00

0.80

0.60

Bu

lk d

en

sit

y (

g c

m–

3)

0.40

0.20

0.00

0% biochar 25% biochar 50% biochar 75% biochar 100% biochar

Figure 2. Bulk density (g cm−3) of the soil at varying biochar application rates.

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strong enough to hold water in the pore. Hence, macropores improve soil aeration whichis important for root health as well as facilitating the growth of beneficial microorganismsin the soil (Brady & Weil 2002), while micropores improve the soil water holdingcapacity. Additionally, many important soil processes take place in soil pores. It isimperative to enhance soil porosity as poor porosity often facilitates erosion of soils asrainwater washes particles off the surface rather than penetrating into the soil.

Infiltration is a measure of the amount of water that penetrates the ground surface andis an important process that determines how much water gets to plant roots as well andhow much runoff takes place. Hence, knowledge of cumulative infiltration is importantfor efficient soil and water management. The results of cumulative infiltration revealed adecreasing trend with increasing rate of biochar amendment application (Figure 4).Although the initial decrease in cumulative infiltration was not marked at 25% applicationrate, subsequent decreases in response to successive additions of biochar were veryprominent. The regression equation describing the rate of decrease of cumulative infiltra-tion as a function of biochar amendment rate is given by the following:

Cumulative infiltration ¼ �3:379� %biocharð Þ þ 18:09 (11)

The decrease in cumulative infiltration as a function of biochar amendment was signifi-cant (R2 = 0.952; p < 0.01) and this could be due to the hydrophobic nature of organicmatter present in biochar amendment. According to Leelamanie and Karube (2009), waterrepellency is associated with the content and the composition of soil organic matter.Organic matter can exhibit hydrophobic or hydrophilic properties and water repellencydepends on the contents and the dominant factor. It is apparent that the biochar used inthis study exhibited hydrophobic properties, hence decreasing the rate of infiltration as theamount of biochar was increased in the soil.

Hydraulic conductivity (k) measures the ease with which water can move through asoil, subject to a hydraulic gradient and is essential in infiltration-related applications suchas irrigation and drainage management (Wu et al. 1999; Radcliffe & Rasmussen 2002).Saturated hydraulic conductivity (ks) is the conductivity measured while the soil issaturated. The results of ks showed a similar pattern to cumulative infiltration describedearlier (Figure 5). Thus, there was a linear decrease in ks with increasing rate of biocharamendment. The ks values were 0.49, 0.31, 0.23, 0.20, and 0.18 cm min−1, respectively,for the non-amended soil, 25% rate, 50% rate, 75% rate, and 100% rate of biochar

0% biochar

0.0

0.1

0.2

0.3

Po

ro

sit

y (

cm

3 c

m–

3)

0.4

0.5

0.6

0.7

0.8

0.9

25% biochar 50% biochar 75% biochar 100% biochar

y = 0.070x + 0.420

R2 = 0.994

Figure 3. Porosity (cm3 cm−3) of the soil at varying biochar application rates.

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application. The regression equation describing the rate of decrease of ks as a function ofbiochar amendment rate is given by the following:

ks ¼ �0:060� %biocharð Þ þ 0:456 (12)

The decrease in ks was significant (R2 = 0.840; p < 0.01) as a function of biocharamendment and just like the results of the cumulative infiltration; this was likely due tothe hydrophobicity of the organic matter present in biochar amendment.

Matric head is a measure of adsorptive forces of water onto the soil particle andcapillary forces maintained within the soil pore. Hence, energy must be expended byplants to overcome these adsorptive and capillary forces in the soil. Thus, matric head isalways a negative quantity in unsaturated material (Young & Sisson 2002). The matrichead versus volumetric water content for the soil amended at varying biochar rates areshown in Figure 6, which is an inverse relationship where the volumetric water contentdecreases with increase in matric head. The matric head values close to zero correspondwith the volumetric water at saturation. The results showed that at saturation, the

0% biochar 25% biochar 50% biochar 75% biochar 100% biochar

16.0

14.0

12.0

Cu

mu

lativ

e i

nfil

tra

tio

n (

cm

)10.0

8.0

6.0

4.0

2.0

0.0

y = –3.379x + 18.09

R2 = 0.952

Figure 4. Cumulative infiltration (cm) of the soil amended at varying biochar rates.

0% biochar 25% biochar 50% biochar 75% biochar 100% biochar

0.05

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0.00

y = –0.060x + 0.456

R2 = 0.840

Figure 5. Saturated hydraulic conductivity (cm s−1) of the soil amended at varying biochar rates.

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Figure6.

Matrichead

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volumetricwater

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thesoilam

endedat

varyingbiocharrates.

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volumetric water content was 80 cm3 cm−3 for 100% and 80% biochar amendment rates.The values decreased to 62 and 58 cm3 cm−3 for 100% and 80% biochar amendment rates,respectively, while this was 50 cm3 cm−3 for the non-amended soil. It is clear from theseresults that increasing biochar application rate increased the volumetric water content atevery matric head level. A plot of matric head against time showed a decrease in matrichead values as time elapsed (Figure 7), attributed to the fact that water was not replenishedduring the drying experiment. The results revealed a rapid decrease in matric headvalues during the first 5 days, but a lower rate of decrease after that period. This impliesthat during the first 5 days, a plant would expend less energy to extract water, but energydemand would increase thereafter as water became less readily available. These resultsalso revealed higher matric head (less negative) values as the rate of biochar amendmentwas increased. Hence, the higher is the rate of biochar amendment, the lesser is the energyexpended by plants for water uptake, and therefore less plant stress (less wilting). Theresults of the volumetric water content over time for the soil amended at varying biocharrates showed a similar pattern to the matric head trends (Figure 8). Thus, there was a rapid

00 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

–20

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H2O

)

–80

–100100% biochar

50% biochar + 50% soil

75% biochar + 25% soil

25% biochar + 75% soil

Days

100% soil–120

Figure 7. Matric head (cm) of the soil at varying biochar application rates.

00

10

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60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Vol

umet

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tent

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100% biochar

50% biochar + 50% soil

75% biochar + 25% soil

25% biochar + 75% soil

Days

0% biochar

Figure 8. Volumetric water content (cm3 cm−3) of the soil at varying biochar application rates.

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decline in the volumetric water content during the first 5 days of time elapsed, but verygradual decline after that period. It is clear that the processes of evapotranspiration anddrainage were rapid when water was more readily available during the first 5 days. Due toreduced amount of volumetric water content after 5 days, evapotranspiration processescontinued at a much reduced rate. Also, plants tend to conserve water by reducing the rateof transpiration whenever the available water decreases.

The results revealed that the higher was the rate of biochar amendment, the higher wasthe volumetric water content. The results of the LQR are shown in Figure 9. The linearincrease in LQR as a function of biochar application rate is represented by the followingequation:

LQR ¼ 1:087� %biocharð Þ þ 3:354 (13)

The results showed a significant (R2 = 0.768; p < 0.01) increase in LQR as a function ofbiochar application rate.

Conclusion

Results from this study showed that biochar amendment had a positive impact on thephysical and hydraulic properties of soil. In particular, biochar decreased the bulk densityof the soil leading to increased total soil porosity. Depending on pore size distribution,total porosity may increase soil aeration, and/or increase volumetric water content andwater retention. This would lead to enhanced plant growth as roots access sufficientmoisture and oxygen. This is supported by the results of the bioassy that revealedincreased quality rating of tomato plants with increased biochar amendment in the soil.Although the results of cumulative infiltration showed reduced infiltration as the biocharamendment rate was increased, this can be avoided by mitigation strategies (e.g. minimumirrigation) to ensure that the soil is not completely dry to cause water repulsion byhydrophobic forces (Githinji et al. 2011).

10.00

y = 1.087x + 3.354R2 = 0.768

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7.00

6.00

5.00

Leaf

qua

lity

ratin

g (–

)

4.00

3.00

2.00

1.00

0.000% biochar 25% biochar 50% biochar 75% biochar 100% biochar

Figure 9. Leaf quality rating (–) of tomatoes at varying biochar application rates.

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