TECHNICAL ARTICLE

Biochar Effects on Soil Aggregate Properties UnderNo-Till Maize

Ataallah Khademalrasoul,1 Muhammad Naveed,1 Goswin Heckrath,1 K.G.I.D. Kumari,1

Lis Wollesen de Jonge,1 Lars Elsgaard,1 Hans-Jörg Vogel,2 and Bo V. Iversen1

Abstract: Soil aggregates are useful indicators of soil structure and sta-bility, and the impact on physical and mechanical aggregate properties iscritical for the sustainable use of organic amendments in agricultural soil.In this work, we evaluated the short-term soil quality effects of apply-ing biochar (0–10 kg m−2), in combination with swine manure (2.1 and4.2 kg m−2), to a no-till maize (Zea mays L.) cropping system on a sandyloam soil in Denmark. Topsoil (0–20 cm) aggregates were analyzed forclay dispersibility, aggregate stability, tensile strength (TS), and specificrupture energy (SRE) using end-over-end shaking, a Yoder-type wet-sieving method, and an unconfined compression test in soil samples col-lected 7 and 19 months after final biochar application. The highest ratesof biochar and swine manure application resulted in the highest aggregatestability and lowest clay dispersibility. Applying both amendments system-atically increased TS and SRE for large aggregates (4–8 and 8–16mm) butnot for small aggregates (1–2 and 2–4 mm). Increased biochar applica-tion also decreased the friability index of soil aggregates. Based on X-rayvisualization, it was found that aggregates containing larger amounts ofbiochar particles had higher TS and SRE probably because of bondingeffects. Based on the improved soil aggregate properties, we suggest thatbiochar can be effective for increasing and sustaining overall soil quality,for example, related to minimizing the soil erosion potential.

Key Words: Biochar, aggregate stability, clay dispersibility, tensilestrength, X-ray CT scanning

(Soil Sci 2014;179: 273–283)

Sustainable use and management of the soil are two of the mainchallenges in modern agriculture. Intensive cultivation causes

a risk of soil organic carbon (SOC) depletion and, to alleviatesuch detrimental effects, carbon-rich organic amendments (suchas straw or animal manure) are often applied to the soil. Biocharis a carbon-rich compound with high porosity (DeLuca et al.,2009) produced by pyrolysis of biomass under oxygen-limitedconditions (Lehmann and Joseph, 2009). It has been suggestedthat adding biochar to agricultural soil could improve soil physicalproperties and crop yields (Biederman and Harpole, 2013) andalso mitigate the atmospheric CO2 increase caused by carbon stor-age (Lehmann, 2007; Sohi et al., 2009). Several studies haveshown that biochar applications have a notable effect on soil prop-erties such as pH and water-holding capacity, whereas fewer stud-ies have focused on aggregate stability (Laird et al., 2010; Singh

1Department of Agroecology, Aarhus University, Tjele, Denmark.2Department of Soil Physics, Helmholtz-Centre for Environmental Research-UFZ, Halle, Germany.Address for correspondence: Ataallah Khademalrasoul, Department of Agro-ecology, Aarhus University, Blichers Allé 20, PO Box 50, DK-8830 Tjele,Denmark; E-mail: ataalah.khademalrasoul@agrsci.dkFinancial Disclosures/Conflicts of Interest: This study was partially fundedby the Ministry of Science and Technology, Iran.Received April 4, 2014.Accepted for publication July 9, 2014.Copyright © 2014 by Lippincott Williams & WilkinsISSN: 0038-075XDOI: 10.1097/SS.0000000000000069

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et al., 2010; Novak et al., 2012; Sun et al., 2013). However,Liu et al. (2012) concluded that effects of biochar on soilaggregates may ultimately depend on inherent soil texture andSOC content.

Soil organic carbon plays a significant role in the formationof stable soil aggregates and, thus, structural stability (Six et al.,2000; Bronick and Lal, 2005; An et al., 2008) because the aggre-gates are composed primarily of mineral particles and organicbinding agents (Tisdall andOades, 1982). The binding agents playa decisive role in bonding the soil particles by their exchangeablesites and thereby influence the morphological and resistive pro-perties of soil aggregates (Blanco-Canqui and Lal, 2004). In ac-cordance, several studies have shown a significant correlationbetween the amount of organic matter in the soil and the soilaggregation process (Abiven et al., 2009; Deurer et al., 2009;Papadopoulos et al., 2009).

Soil aggregate stability refers to the ability of soil aggregatesto resist disruption when external forces are applied (Cosentinoet al., 2006). Aggregates with a higher stability do not easily de-compose and hence promote long-term carbon sequestration andsoil structural stability (Ouyang et al., 2013). Mukherjee and Lal(2013) recently concluded that studies evaluating biochar effectson soil aggregation are scarce. Furthermore, the existing studieshave focused on soil aggregate stability within different soilsand for various types of biochar (Busscher et al., 2011; Novaket al., 2012; Hua et al., 2013; Soinne et al., 2014). In a greenhouseexperiment with biochar made from maize straw, Hua et al.(2013) showed that biochar amendment significantly increasedthe stability of soil aggregates. In contrast, a biochar amendmentbased on pecan shells (Carya illinoinensis) decreased soil aggre-gation as compared with the control (Busscher et al., 2011).In another study, Peng et al. (2011) reported that the applicationof rice-straw biochar to an Ultisol had no effect on aggregate sta-bility. Clearly, those findings are contrasting, thus emphasizingthe importance of quantifying distinct soil and biochar propertiesfor every situation.

Clay dispersibility, defined as the amount of clay that canbe dispersed by water (Calero et al., 2008), can affect the soil ag-gregation and, in turn, soil erodibility. Applying organic amend-ments such as manure can decrease clay dispersibility (Paradeloet al., 2013). Increasing pH, decreasing ionic strength, the pres-ence of monovalent cations (Na+, K+), and increasing water con-tent are major factors that promote clay dispersion (de Jonge andde Jonge, 1999; Soinne et al., 2014). Dispersed clay-sized parti-cles have a potential to travel long distances, carrying pollutantsand nutrients, thus contributing to contamination of groundwaterand eutrophication of receiving waterways (de Jonge et al.,2004a, b; Soinne et al., 2014). Biochar could influence claydispersivity both negatively and positively. Through its influenceon soil pH, ionic strength, zeta potential, and moisture con-tent, biochar has the potential to accelerate clay dispersion insoils. On the other hand, aging biochar forms biochar-mineralcomplexes (Lin et al., 2012) that have the potential to decreaseclay dispersibility by increasing soil structural stability. Despite

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extensive previous studies on clay dispersibility in soils, nostudy has systematically explored the different roles of biocharon clay dispersion. A further issue is that colloid-sized biocharparticles may, by themselves, act as contaminant carriers (Kumariet al., 2014).

Tensile strength (TS) is the force per unit area required tofracture soil aggregates into smaller sizes. Tensile strength is auseful mechanical characteristic for investigating the structuralstability of soil and thereby the resistance of soil aggregatesagainst erosive forces (Dexter and Kroesbergen, 1985; Wattsand Dexter, 1998). Blanco-Canqui and Lal (2007) and Rahimiet al. (2002) showed the positive effects of organic carbon contenton aggregate TS. Blanco-Canqui and Lal (2007) found that ap-plication of an organic amendment (e.g., straw mulching) canincrease the stability and strength of soil aggregates in the 0- to5-cm soil layer. Nevertheless, some studies have reported negativeor no relationship between SOC content and aggregate TS (Wattsand Dexter, 1998; Blanco-Canqui et al., 2005a, b; Abid and Lal,2009). Beare et al. (1994) found that the effects of SOC on aggre-gate TS can be disparate and dependent on soil texture and landmanagement scenarios. Another soil mechanical parameter isthe friability index (FI), which is related to TS and aggregate size(Utomo and Dexter, 1981). Correlations between SOC contentand the FI of soil aggregates have been reported by Rahimi et al.(2002). Guimarães et al. (2009) identified a positive correlationbetween SOC content in the topsoil and FI, whereas the oppositewas found for the subsoil.

X-ray computed tomography (CT) imaging is a noninvasivetechnique based on the attenuation of X-rays and is able to visual-ize intra-aggregate soil pore structure (Wang et al., 2012; DalFerro et al., 2012; Naveed et al., 2014). The use of X-ray CT tech-niques to quantify effects of fertilization (Zhou et al., 2013) andlong-term soil management practices (Kravchenko et al., 2011;Naveed et al., 2014) on aggregate pore heterogeneity and intra-aggregate pore structure is well documented. Naveed et al.(2014) studied aggregate pore structure using X-ray CT in differentlong-term management systems and found significantly higheraggregate porosity, pore connectivity, average pore diameter,and more rounded pores for soil aggregates from cereal cash crop

FIG. 1. Schematic of study field divided into 12 plots receiving differentSwine manure was applied only in 2011. Biochar was applied in 2011, 2

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fields than in aggregates from mixed forage crop and mixed cashcrop fields.

In this study, we tested whether biochar amendment to soilscould be envisaged as a nonstructural best management practice(BMP) that may improve soil mechanical and structural proper-ties. We investigated short-term effects of biochar amendmenton the physical and mechanical properties of soil aggregates spe-cifically addressing:1. The effect of varying amounts of biochar and swine manure ap-

plications on aggregate stability, dispersibility, strength, andpore structure.

2. The commencing aging effect of biochar on aggregate stability,dispersibility, strength, and pore structure.

MATERIALS AND METHODS

Description of Study SiteThe field study was carried out near Kalundborg, Denmark

(55°42′N, 11°18′E), on a sandy loam soil. The site has a temper-ate climate with a 10-year mean annual rainfall (2000–2010) of610mm and amean annual air temperature of 9°C. The study areawas divided into 12 plots (6 � 6 m) receiving different doses ofbiochar varying from 0 to 10 kg m−2 and swine manure in twodoses of 2.1 and 4.2 kg m−2 (Fig. 1). The dry matter of swine ma-nure was ca. 6%. Biochar was applied in 2011 and 2012, whereasswine manure was applied only in 2011. In summary, Plots 1 to4 received 2.1 kg m−2 swine manure and, respectively, 0, 1, 2,and 5 kg m−2 biochar in April 2011; Plots 5 to 8 received4.2 kg m−2 swine manure and, respectively, 0, 1, 2, and 5 kg m−2

biochar in both 2011 and 2012 (i.e., to cumulative biochar ratesof 0, 2, 4, and 10 kg m−2); and Plots 9 to 12 received 2.1 kg m−2

swine manure and, respectively, 0, 1, 2, and 5 kg m−2 biochar inApril 2012. After application, the organic amendments wereharrowed into the topsoil (0–10 cm). The biochar was obtainedfrom Skogens Kol AB (Kilafors, Sweden) and was based onbirch wood pyrolyzed at 500°C to contain 81% total C and0.24% total N. The field site had been managed as no-till since2001 and had been cropped with maize (Zea mays L.) since

amounts of biochar (BC) and swine manure (SM) in 2011 and 2012.012, or both years as indicated by the annual rates.

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2006. More information about the biochar and the experimentalsite can be found in Sun et al. (2014).

Soil Sampling, Measurements, and AnalysisSoil sampling was carried out in October 2012, immediately

after harvest. In each plot, a composite sample (0–20 cm) was ob-tained by mixing soil from six separate sampling points. Soil sam-ples were air-dried at 20°C in the laboratory. Afterward, aggregatestability was measured using a Yoder-type wet-sieving method(Yoder, 1936). This method consists of a sample container and a250-μm sieve connected to an acceleration drive, giving a verticalshake with a frequency of 52 revolutions min−1. The sample con-tainer was placed on a hydraulic platform enabling the sample tobe submerged in water. Ultimately, the mass of stable aggregatesfor each treatment was recorded by weighing the aggregates re-maining on the sieve after sieving.

Clay dispersibility was measured by the method describedby Vendelboe et al. (2012) using 10 g of dry soil with 80 g arti-ficial rainwater (0.012 mmol/L CaCl2, 0.15 mmol/L MgCl2,and 0.121 mmol/L NaCl; pH 7.82; electrical conductivity (EC),2.24 � 10−3 S m−1) added to the soil before shaking. The speedof shaking was 33 revolutions min−1, with each rotation periodlasting 2 min. After shaking, the samplewas left for sedimentationfor 3 h 50 min. The top 50 mm (60 mL) of the suspension, corre-sponding to particles less than 2 μm, was subsequently transferredto a 100-mL glass bottle using a pipette.

For measurements of TS and specific rupture energy (SRE),a nest of sieves was used to acquire air-dried aggregates in foursize fractions including 1 to 2, 2 to 4, 4 to 8, and 8 to 16 mm(Elmholt et al., 2008). After sieving and preparing the soil aggre-gates, the TS was measured using the unconfined compressiontest (ASTM D2166/D2166M-13). For each size fraction in eachstudied plot, 15 soil aggregates were analyzed, and TS was mea-sured for each soil aggregate. The TS (kPa) was calculated usingthe equation of Rogowski et al. (1968):

TS ¼ 0:576F=d2 (1)

where F (N) is the breaking force and d (m) is the mean ag-gregate diameter. The 0.576 value is a constant based on the as-sumption that the aggregates are of spherical form and exhibitplastic behavior (Poisson ratio, 0.5; Dexter, 1975). Based on theassumption that aggregate density is constant, the mean aggregatediameter (d) was estimated using the equation by Dexter andWatts (2000):

d ¼ d0 m=m0ð Þ1=3 (2)

where m is the mass of an individual aggregate, mo is themean mass of a batch of aggregates of the same size class, andd0 is the mean of the openings of the upper and lower sieves forthat size class. Together with the TS measurement, the ruptureenergy of each aggregate was calculated by integrating the areaunder the force (F) versus the displacement curve. The SRE(J kg−1) was achieved by dividing the rupture energy by the aggre-gate mass. The FI was then calculated as the negative slope ofthe log TS versus the log volume of the soil aggregates.

X-ray CT scanning of selected soil aggregates in size classes8 to 16 mm from plots 1, 3, 4, 6, 7, and 8 (cf. Fig. 1) was carriedout with three aggregates per plot. The 18 soil aggregates werescanned using an industrial CT scanner (X-Tek HMX225) atHalle, Germany (Helmholtz Centre for Environmental Research,UFZ). A copper filter was attached in front of the X-ray sourceto alleviate beam hardening. The obtained shadow projections(radiographs) were reconstructed with a Feldkamp cone-beam

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algorithm (Feldkamp et al., 1984) to obtain 16-bit, gray-scale,three-dimensional volumes with a resolution of 30 μm. TheseCT gray-scale volumes were processed with the ImageJ softwarepackage (Rasband, 2011). Only qualitative analysis of soil aggre-gates was carried out using X-ray CT data because it is not possi-ble to distinguish between soil pores and biochar particles in soilaggregates. The SOC content, TS, and SRE of each X-ray CTscanned aggregate were measured. Organic carbon was deter-mined by oxidation of carbon to CO2 using a FLASH 2000 organicelemental analyzer coupled to a thermal conductivity detector(Thermo Fisher Scientific, MA).

Statistical AnalysisLack of true replication at the experimental site hindered a

full statistical analysis, including effects of different rates of swinemanure, years of biochar application, and the interactions of swinemanure and biochar on soil structural properties. Rather, as an ap-proximate approach, significant effects of biochar were tested forindividual years of biochar application (i.e., 2011, 2012, and 2011+2012), as suggested by Sun et al. (2014). A representative soilsample for each treatment was obtained by pooling six individualsoil cores. Working at the (small) scale of individual soil aggre-gates, subsamples of this composite soil sample were consideredto adequately reflect the field-scale variation in physical and me-chanical aggregate properties. Analyses of variance were doneusing the Holm-Sidak test (α = 0.05) in SigmaPlot 11.0 (SystatSoftware, San Jose, CA).

Relationships between measured parameters including ag-gregate stability, clay dispersibility, TS, SRE, FI, and each of thephysical and chemical soil properties were investigated using mul-tiple linear regression models, and the significance of each param-eter was evaluated. Soil properties, including clay, silt, sand, SOC,EC, and pH as independent variables, which did not demonstrate asignificant relationship with measured parameters, were excludedfrom the regression model. The correlation between amounts of bio-char added (i.e., cumulative rates of 0, 1, 2, 4, 5, 10 kg m−2) andmeasured parameters including aggregate stability, clay dis-persibility, and soil strength was also investigated.

To evaluate the quality of the developed multiple linear re-gression models for predicting the measured parameters, statisti-cal performance indicators were calculated based on modelingefficiency (MEF) according to Mayer and Butler (1993):

MEF¼1−∑ yi−y i� �

2=∑ yi−�yi

� �2

(3)

where yi is the individual parameter measurements, ŷi is theindividual parameter predictions, and y-i is the mean of the mea-surement of each parameter.

Further model evaluation used plots of measured versus pre-dicted data to test for significance of the correlation coefficientand for unit slope and zero intercept (simultaneous F test) as ex-plicitly outlined by Haefner (2005).

RESULTS AND DISCUSSION

Soil Texture and CarbonSoil organic carbon varied among the 12 plots from 1.48 to

4.76 g 100 g−1 (Table 1). The SOC increased consistently with ap-plication rates of both biochar and swine manure. The soil textureof studied plots was rather similar with clay, silt, and sand contentswithin the ranges 8 to 12, 23 to 29, and 60 to 69 g 100 g−1, respec-tively. Bulk density varied between 1.09 to 1.37 g cm−3 but wasnot consistently correlated to the rates of biochar application.

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TABLE 1. Soil Physical and Chemical Properties of Studied Plots

Plot ID Clay (<2 μm) Silt (2–50 μm) Sand (50–2,000 μm) SOC† pH EC SOCagg ± SE‡

g 100 g−1 g 100 g−1 g 100 g−1 g 100 g−1 μS/cm g 100 g−1

1 8 23 69 1.59 6.49 43.0 1.39 ± 0.132 10 25 65 1.81 6.64 40.5 nd3 10 26 63 2.19 6.71 45.5 2.17 ± 0.324 11 28 61 2.71 6.34 44.5 2.29 ± 0.555 9 23 69 1.64 6.56 48.0 nd6 9 23 68 2.37 6.70 47.0 1.84 ± 0.147 9 25 66 3.14 6.58 45.0 2.64 ± 0.158 10 24 66 4.76 6.83 47.5 4.64 ± 0.409 9 24 67 1.48 6.69 52.5 nd10 10 27 63 1.74 6.73 43.5 nd11 12 28 60 2.25 6.98 48.0 nd12 12 29 60 3.50 6.73 44.0 nd

Data adopted from Sun et al. (2014) except for values of EC (K.G.I.D. Kumari, unpublished results) and SOCagg.†SOC, soil organic carbon.‡SOCagg, SOC content of soil aggregates. Data are mean and standard error (SE) of three CT-scanned aggregates.

nd, not determined.

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Yet, Plot 8 had the highest biochar dose and the lowest bulk den-sity in accordance with the expected relationship. There was nosystematic difference in soil pH between the individual plots; theoverall mean ± standard error (SE) was 6.66 ± 0.05 (n = 12).

The mean organic carbon content of the scanned indi-vidual soil aggregates (using X-ray CT) varied from 1.39 to 4.64 g100 g−1 (Table 1) and aligned with bulk SOC measurements.

Aggregate Stability and Clay DispersibilityOur results indicate that the stability of soil aggregates in-

creased with increasing doses of biochar (Fig. 2A), which is in ac-cordance with the observations of Brodowski et al. (2006) whofound that black carbon in the soil acts as a binding agent betweenaggregates and increases aggregate stability. Overall, Plot 8,which had the highest application of both biochar and swinemanure, showed the highest degree of aggregate stability, furtherindicating that organic amendments are able to increase soil ag-gregate stability because of the binding of soil particles. Thus,

FIG. 2. Aggregate stability (A) and clay dispersibility (B) of soil aggregatesas mean ± standard error with n = 3; data for clay dispersibility aremean ±each treatment were not significantly different. Swine manure was appliat 4.2 kg m−2 for Plots 5 to 8.

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Deurer et al. (2009) and Papadopoulos et al. (2009) showed thatadding organic materials to soil can potentially increase soil ag-gregation because of the activity of binding agents, for example,exopolysaccharides obtained from turnover of the organic amend-ments. Tsai et al. (2012) suggested that the aromatic nature of bio-char may function as a bond between soil particles. In the presentstudy, a comparison between the plots established in 2011 and2012 indicated an aging effect of the applied biochar. For the plotsreceiving biochar in 2012, there were no strong differences be-tween individual plots in terms of aggregate stability, whereas sig-nificant differences were seen in aggregate stability for the plotsreceiving increasing doses of biochar in 2011 (Fig. 2A). Such pos-sible aging effects could be caused by changes in surface func-tional groups of the applied biochar, resulting in different soilphysical properties during the 2-year period.

Figure 2B shows the clay dispersibility of the soils from eachplot. In the plots receiving biochar in 2012 (Plots 9–12), claydispersibility increased with biochar application compared withthe reference plot. However, the effect was significant only at

sampled fromdifferent plots. Data for aggregate stability are shownstandard error with n = 4. Barsmarked by the same letters withined in 2011 at a rate of 2.1 kg m−2 for Plots 1 to 4 and 9 to 12 and

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FIG. 3. Measured versus predicted aggregate stability (A) and clay dispersibility (B) using multiple linear regression models. Calculated MEFand 1:1 lines are also shown. MEF, modeling efficiency.

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the highest application dose (Plot 12). The trend of increasing claydispersibility in the biochar-amended plots aligned with a higherpH and lower EC (Table 1) in accordance with previous studies(de Jonge and de Jonge, 1999; Soinne et al., 2014). However, insoils treated with biochar in 2011, we found no significant differ-ences in clay dispersibility between reference and biochar-amended plots. This might suggest an effect of biochar aging,but because a similar trend was found for plots receiving biocharin both 2011 and 2012, this effect was inconclusive. Conceivableaging effects could be related to oxidative processes, which willincrease the prevalence of acidic functional groups and the cationexchange capacity, resulting in an increase in structural stabilitythrough cation bridge formation, eventually leading to a loweringof the ionic strength. Thus, the stabilizing effect of biochar on thesoil aggregate structure and the decrease in liming effect counter-act the ionic strength effect on clay dispersibility. The lowest claydispersibility values were observed in plots receiving biochar inboth 2011 and 2012 and the double dose of swine manure (Plots5–8). For these plots, therewas no significant effect of biochar ap-plication compared with the corresponding reference plot (Plot 5).This might reflect the importance of higher-dose swine manureapplications for the formation of a stable structure, which counter-balances the effect of fresh biochar addition on clay dispersibility.However, the present data were merely indicative rather than con-clusive on these possible relationships.

Multiple linear regression analyses (Fig. 3A; Table 2)showed that aggregate stability can be adequately expressed as afunction of SOC, clay content, and amount of biochar application(R2 = 0.98, P < 0.024). On the other hand, clay dispersibility(Fig. 3B; Table 2) was found to be poorly expressed even withfive variable model parameters that included SOC, clay content,silt, EC, and pH (R2 = 0.37, P = 0.001). Modeling efficiencies

TABLE 2. Multiple Linear Regression Models Predicting the Aggreg

Parameter Constant Biochar Amount SOC†

Aggregate stability 34.8 0.46 −6.8Clay dispersibility −8.4 — −0.10

†Soil organic carbon (including biochar).‡EC, electrical conductivity.

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of aggregate stability and clay dispersibility were 69.4% and31.6%, respectively.

Tensile Strength, Specific Rupture Energy, andFriability Index

No systematic trends in TS and SRE were seen for the 1- to2-mm and 2- to 4-mm aggregate sizes with an increase in the ap-plication rate of biochar (Fig. 4A, B). However there was a ten-dency for a more systematic increase in TS and SRE for thelarger 4- to 8-mm and 8- to 16-mm aggregate sizes with an in-crease in the amount of applied biochar (Fig. 4C, D). Thus, addingbiochar in combination with swine manure to the soil seemed tobe able to change the physical and mechanical properties of thesoil for the larger size fraction of aggregates. The difference be-tween small-sized aggregates and large-sized aggregates couldprobably be related to the size fractions of applied biochar. Forlarger-sized aggregates, there was the possibility that soil particlescould bond with biochar particles, whereas there was no bindingbetween smaller aggregates and biochar particles. Our resultsare comparable to those of recent studies that showed the positiveeffects of SOC content on aggregate TS (Blanco-Canqui and Lal,2007; Rahimi et al., 2002).

Multiple linear regression analyses revealed that TS and SREcould be expressed as a function of SOC (including biochar),clay, and silt contents (Table 3). For TS and SRE for 4- to 8-mmaggregates, the modeling efficiencies were 63.4% and 49.7%, re-spectively (Fig. 5A, B). For the larger aggregates (8–16 mm),the modeling efficiencies of TS and SRE were 95.1% and91.6%, respectively (Fig. 5C, D), which imply that the developedmodels adequately predicted the TS and SRE for the larger soilaggregates as compared with smaller soil aggregates.

ate Stability and Clay Dispersibility

Clay Silt EC‡ pH R2 P

−0.55 — — — 0.98 0.0240.21 0.072 0.062 1.89 0.37 0.001

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FIG. 4. (A) Tensile strength of 1- to 2-mm and 2- to 4-mm soil aggregates, (B) specific rupture energy of 1- to 2-mm and 2- to 4-mmsoil aggregates, (C) tensile strength of 4- to 8-mm and 8- to 16-mm soil aggregates, and (D) specific rupture energy of 4- to 8-mm and8- to 16-mm soil aggregates sampled from studied plots. Data are shown as mean ± standard error of 15 aggregates. Bars marked by thesame letters within each treatment and aggregate class were not significantly different. For treatments related to Plot ID, see Fig. 2.

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With an increase in the applied amount of biochar, the FI ofsoil aggregates decreased for all application years (Fig. 6A). Thelowest FI was observed for Plot 12, which received biochar in2012 at a relatively high dose of 5 kg m−2 (and 2.1 kg m−2 ofswine manure in 2011). The change in the FI with an increase inthe amount of biochar is probably related to the aggregating ef-fects of biochar. In comparison, Guimarães et al. (2009) found thatSOC is able to change the FI of soil aggregates because of aggre-gating effects. The change in the FI might also be related to thequality of the SOC. Multiple linear regression analysis showedthat the FI of soil aggregates could reasonably be predicted as a

TABLE 3. Multiple Linear Regression Models Predicting Aggregate TSoil Aggregates in Different Size Fractions

Parameter Fraction, mm Constant

Tensile strength 4–8 −111.4Specific rupture energy 4–8 −11.6Tensile strength 8–16 −150.8Specific rupture energy 8–16 −18.6Friability index All fractions 1.0

†Soil organic carbon (including biochar).

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function of SOC, clay, and silt content (R2 = 0.79, P = 0.004;Table 3). The modeling efficiency of the FI was 79.3% (Fig. 6B).

Visualization of Aggregates Using X-ray CTScanning to Characterize the Biochar Effects

From the visual evaluation of the three-dimensional X-rayCT scans, it is not possible to automatically distinguish the soilpores from the biochar particles because of quite similar CT num-bers (Fig. 7). Indeed, no report has so far been able to documentthe ability to differentiate soil pores from biochar particles in the

ensile Strength, Specific Rupture Energy, and Friability Index of

SOC† Clay Silt R2 P

9.75 25.2 0.09 0.63 0.0371.04 0.6 0.38 0.50 0.1228.72 11.4 3.04 0.95 <0.0012.05 0.1 1.08 0.92 <0.001

−0.03 0.03 −0.41 0.79 0.004

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FIG. 5. Measured versus predicted (A) tensile strength for 4- to 8-mm aggregates, (B) specific rupture energy for 4- to 8-mm aggregates,(C) tensile strength for 8- to 16-mm aggregates, and (D) specific rupture energy for 8- to 16-mm aggregates using multiple linearregression models. Modeling efficiency (MEF) and 1:1 lines are also shown.

Soil Science • Volume 179, Number 6, June 2014 Biochar Effects on Soil Aggregate Properties

soil matrix. However, a qualitative comparison of aggregates fromplots with and without biochar application can reveal some infor-mation because the black spots on aggregates from plots withoutbiochar application in general originate solely from soil pores,whereas black spots on aggregates from the biochar-treated plotsare a combination of both soil pores and biochar particles. The in-spection of the gray-scale images (Fig. 7) indicated that aggregates

FIG. 6. A, Friability index of soil aggregates from the studied plots and (Bmultiple linear regression models. Modeling efficiency (MEF) and 1:1 lin

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from biochar-amended plots were different from aggregates inplots receiving no biochar, and that biochar application dose hadan effect on the prevalence of biochar particles in the soil aggre-gate structure (Fig. 7). Biochar particles in the inner part of soilaggregates were visible to various extents in the aggregates takenfrom the plots receiving biochar. Biochar particles are clearly vis-ible in the aggregates from Plot 8, which received the largest

) measured versus predicted friability index of soil aggregates usinges are also shown.

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FIG. 7. X-ray CT gray-scale images of soil aggregates sampled from selected plots. Three handpicked soil aggregates were scanned from theselected plots 1, 3, 4, 6, 7, and 8 (cf. Fig. 1).

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FIG. 8. A, Tensile strength and (B) specific rupture energy for CT-scanned aggregates plotted as a function of SOC. Aggregates (n = 3) werepicked from the selected plots 1, 3, 4, 6, 7, and 8 (cf. Fig. 1).

Soil Science • Volume 179, Number 6, June 2014 Biochar Effects on Soil Aggregate Properties

amount of biochar. However, such presence of biochar in aggre-gates was less clear for Plots 3 and 4 (Fig. 7) where biochar wasapplied in 2011. This might indicate that biochar, to some extent,disintegrated after the application in 2011.

The measurements of SOC on the CT-scanned aggregatesshowed that the increase in SOC resulting from biochar and swinemanure applications increased the TS of soil aggregates (Fig. 8A).There was also a positive correlation (R2 = 0.60) between SOCand SRE of soil aggregates, and Plot 8 with the highest SOC con-tent showed the highest SRE (Fig. 8B). Similar results have previ-ously been reported (Blanco-Canqui and Lal, 2007). The increasein TS and SRE of biochar-amended soil aggregates might be re-lated to aggregating effects of biochar particles.

Our results indicate that the application of biochar has a pos-itive effect on the soil aggregation process, thereby increasingaggregate stability and soil strength. Such results indicate a poten-tial role of biochar to be used as an organic amendment to alleviatethe risk of detrimental processes such as soil erosion and asso-ciated side effects. However, further studies directly targeting theeffects of biochar on soil erosion are needed to substantiatethis hypothesis.

CONCLUSIONSThe present results showed that plots receiving the highest

application of biochar and swine manure had the highest aggre-gate stability and lowest clay dispersibility. Multiple linear regres-sions between parameters suggested that aggregate stability was afunction of organic carbon, clay content, and amount of appliedbiochar, whereas clay dispersibility was poorly predicted evenwhen using a model including SOC, clay content, silt, EC, andpH as variables. The increase in biochar and swine manure appli-cation led to an increase in TS and SRE for aggregates in the sizeclasses 4 to 8 and 8 to 16 mm; whereas, for 1- to 2-mm and 2- to4-mm aggregates, there was no clear systematic effect on TS andSRE. Moreover, the increase in biochar application led to a de-crease in the FI of soil aggregates.When using X-ray CT scanningon individual aggregates, the positive relationship between SOCand TS and SRE was substantiated. This method also indicateda cumulative effect of biochar by allowing the detection of biocharparticles as bonding agents in the plots receiving the highest doseof biochar. Generally, the biochar application was able to increasethe aggregate stability, TS, and SRE of larger aggregates (4–8 mmand 8–16 mm) and decrease the general FI of soil aggregates.These results on the effects on soil physical and mechanical

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properties indicate that biochar applications may lead to generalimprovements of aggregate stability, which, on a larger scale,could have an effect on minimizing the soil erosion potential.Yet, such effects still need to be specifically addressed.

ACKNOWLEDGMENTSThe authors thank Stig T. Rasmussen and Jørgen Munksgaard

Nielsen for technical assistance, and Michael Meyer for accessto the Kalundborg field site.

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