Biochar Effects on Soil Aggregate Properties Under ...

TECHNICAL ARTICLE

Biochar Effects on Soil Aggregate Properties Under No-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 stability, and the impact on physical and mechanical aggregate properties is critical for the sustainable use of organic amendments in agricultural soil. In this work, we evaluated the short-term soil quality effects of applying biochar (0–10 kg m−2), in combination with swine manure (2.1 and 4.2 kg m−2), to a no-till maize (Zea mays L.) cropping system on a sandy loam soil in Denmark. Topsoil (0–20 cm) aggregates were analyzed for clay dispersibility, aggregate stability, tensile strength (TS), and specific rupture energy (SRE) using end-over-end shaking, a Yoder-type wetsieving method, and an unconfined compression test in soil samples collected 7 and 19 months after final biochar application. The highest rates of biochar and swine manure application resulted in the highest aggregate stability and lowest clay dispersibility. Applying both amendments systematically increased TS and SRE for large aggregates (4–8 and 8–16 mm) but not for small aggregates (1–2 and 2–4 mm). Increased biochar application also decreased the friability index of soil aggregates. Based on X-ray visualization, it was found that aggregates containing larger amounts of biochar particles had higher TS and SRE probably because of bonding effects. Based on the improved soil aggregate properties, we suggest that biochar 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, tensile strength, X-ray CT scanning (Soil Sci 2014;179: 273–283)

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ustainable use and management of the soil are two of the main challenges in modern agriculture. Intensive cultivation causes a risk of soil organic carbon (SOC) depletion and, to alleviate such detrimental effects, carbon-rich organic amendments (such as straw or animal manure) are often applied to the soil. Biochar is a carbon-rich compound with high porosity (DeLuca et al., 2009) produced by pyrolysis of biomass under oxygen-limited conditions (Lehmann and Joseph, 2009). It has been suggested that adding biochar to agricultural soil could improve soil physical properties and crop yields (Biederman and Harpole, 2013) and also mitigate the atmospheric CO2 increase caused by carbon storage (Lehmann, 2007; Sohi et al., 2009). Several studies have shown that biochar applications have a notable effect on soil properties such as pH and water-holding capacity, whereas fewer studies have focused on aggregate stability (Laird et al., 2010; Singh

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Department of Agroecology, Aarhus University, Tjele, Denmark. Department of Soil Physics, Helmholtz-Centre for Environmental ResearchUFZ, Halle, Germany. Address for correspondence: Ataallah Khademalrasoul, Department of Agroecology, Aarhus University, Blichers Allé 20, PO Box 50, DK-8830 Tjele, Denmark; E-mail: [email protected] Financial Disclosures/Conflicts of Interest: This study was partially funded by the Ministry of Science and Technology, Iran. Received April 4, 2014. Accepted for publication July 9, 2014. Copyright © 2014 by Lippincott Williams & Wilkins ISSN: 0038-075X DOI: 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 soil aggregates may ultimately depend on inherent soil texture and SOC content. Soil organic carbon plays a significant role in the formation of stable soil aggregates and, thus, structural stability (Six et al., 2000; Bronick and Lal, 2005; An et al., 2008) because the aggregates are composed primarily of mineral particles and organic binding agents (Tisdall and Oades, 1982). The binding agents play a decisive role in bonding the soil particles by their exchangeable sites and thereby influence the morphological and resistive properties of soil aggregates (Blanco-Canqui and Lal, 2004). In accordance, several studies have shown a significant correlation between the amount of organic matter in the soil and the soil aggregation process (Abiven et al., 2009; Deurer et al., 2009; Papadopoulos et al., 2009). Soil aggregate stability refers to the ability of soil aggregates to resist disruption when external forces are applied (Cosentino et al., 2006). Aggregates with a higher stability do not easily decompose and hence promote long-term carbon sequestration and soil structural stability (Ouyang et al., 2013). Mukherjee and Lal (2013) recently concluded that studies evaluating biochar effects on soil aggregation are scarce. Furthermore, the existing studies have focused on soil aggregate stability within different soils and for various types of biochar (Busscher et al., 2011; Novak et al., 2012; Hua et al., 2013; Soinne et al., 2014). In a greenhouse experiment with biochar made from maize straw, Hua et al. (2013) showed that biochar amendment significantly increased the stability of soil aggregates. In contrast, a biochar amendment based on pecan shells (Carya illinoinensis) decreased soil aggregation as compared with the control (Busscher et al., 2011). In another study, Peng et al. (2011) reported that the application of rice-straw biochar to an Ultisol had no effect on aggregate stability. Clearly, those findings are contrasting, thus emphasizing the importance of quantifying distinct soil and biochar properties for every situation. Clay dispersibility, defined as the amount of clay that can be dispersed by water (Calero et al., 2008), can affect the soil aggregation and, in turn, soil erodibility. Applying organic amendments such as manure can decrease clay dispersibility (Paradelo et al., 2013). Increasing pH, decreasing ionic strength, the presence of monovalent cations (Na+, K+), and increasing water content are major factors that promote clay dispersion (de Jonge and de Jonge, 1999; Soinne et al., 2014). Dispersed clay-sized particles have a potential to travel long distances, carrying pollutants and nutrients, thus contributing to contamination of groundwater and eutrophication of receiving waterways (de Jonge et al., 2004a, b; Soinne et al., 2014). Biochar could influence clay dispersivity both negatively and positively. Through its influence on soil pH, ionic strength, zeta potential, and moisture content, biochar has the potential to accelerate clay dispersion in soils. On the other hand, aging biochar forms biochar-mineral complexes (Lin et al., 2012) that have the potential to decrease clay dispersibility by increasing soil structural stability. Despite www.soilsci.com

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extensive previous studies on clay dispersibility in soils, no study has systematically explored the different roles of biochar on clay dispersion. A further issue is that colloid-sized biochar particles may, by themselves, act as contaminant carriers (Kumari et al., 2014). Tensile strength (TS) is the force per unit area required to fracture soil aggregates into smaller sizes. Tensile strength is a useful mechanical characteristic for investigating the structural stability of soil and thereby the resistance of soil aggregates against erosive forces (Dexter and Kroesbergen, 1985; Watts and Dexter, 1998). Blanco-Canqui and Lal (2007) and Rahimi et al. (2002) showed the positive effects of organic carbon content on aggregate TS. Blanco-Canqui and Lal (2007) found that application of an organic amendment (e.g., straw mulching) can increase the stability and strength of soil aggregates in the 0- to 5-cm soil layer. Nevertheless, some studies have reported negative or no relationship between SOC content and aggregate TS (Watts and Dexter, 1998; Blanco-Canqui et al., 2005a, b; Abid and Lal, 2009). Beare et al. (1994) found that the effects of SOC on aggregate TS can be disparate and dependent on soil texture and land management scenarios. Another soil mechanical parameter is the friability index (FI), which is related to TS and aggregate size (Utomo and Dexter, 1981). Correlations between SOC content and the FI of soil aggregates have been reported by Rahimi et al. (2002). Guimarães et al. (2009) identified a positive correlation between SOC content in the topsoil and FI, whereas the opposite was found for the subsoil. X-ray computed tomography (CT) imaging is a noninvasive technique based on the attenuation of X-rays and is able to visualize intra-aggregate soil pore structure (Wang et al., 2012; Dal Ferro et al., 2012; Naveed et al., 2014). The use of X-ray CT techniques to quantify effects of fertilization (Zhou et al., 2013) and long-term soil management practices (Kravchenko et al., 2011; Naveed et al., 2014) on aggregate pore heterogeneity and intraaggregate pore structure is well documented. Naveed et al. (2014) studied aggregate pore structure using X-ray CT in different long-term management systems and found significantly higher aggregate porosity, pore connectivity, average pore diameter, and more rounded pores for soil aggregates from cereal cash crop


fields than in aggregates from mixed forage crop and mixed cash crop fields. In this study, we tested whether biochar amendment to soils could be envisaged as a nonstructural best management practice (BMP) that may improve soil mechanical and structural properties. We investigated short-term effects of biochar amendment on the physical and mechanical properties of soil aggregates specifically addressing: 1. The effect of varying amounts of biochar and swine manure applications on aggregate stability, dispersibility, strength, and pore structure. 2. The commencing aging effect of biochar on aggregate stability, dispersibility, strength, and pore structure.

MATERIALS AND METHODS Description of Study Site The field study was carried out near Kalundborg, Denmark (55°42′N, 11°18′E), on a sandy loam soil. The site has a temperate climate with a 10-year mean annual rainfall (2000–2010) of 610 mm and a mean annual air temperature of 9°C. The study area was divided into 12 plots (6 6 m) receiving different doses of biochar varying from 0 to 10 kg m−2 and swine manure in two doses of 2.1 and 4.2 kg m−2 (Fig. 1). The dry matter of swine manure was ca. 6%. Biochar was applied in 2011 and 2012, whereas swine manure was applied only in 2011. In summary, Plots 1 to 4 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 received 4.2 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 rates of 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 in April 2012. After application, the organic amendments were harrowed into the topsoil (0–10 cm). The biochar was obtained from Skogens Kol AB (Kilafors, Sweden) and was based on birch wood pyrolyzed at 500°C to contain 81% total C and 0.24% total N. The field site had been managed as no-till since 2001 and had been cropped with maize (Zea mays L.) since

FIG. 1. Schematic of study field divided into 12 plots receiving different amounts of biochar (BC) and swine manure (SM) in 2011 and 2012. Swine manure was applied only in 2011. Biochar was applied in 2011, 2012, or both years as indicated by the annual rates.

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Biochar Effects on Soil Aggregate Properties

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

Soil Sampling, Measurements, and Analysis Soil sampling was carried out in October 2012, immediately after harvest. In each plot, a composite sample (0–20 cm) was obtained by mixing soil from six separate sampling points. Soil samples were air-dried at 20°C in the laboratory. Afterward, aggregate stability was measured using a Yoder-type wet-sieving method (Yoder, 1936). This method consists of a sample container and a 250-μm sieve connected to an acceleration drive, giving a vertical shake with a frequency of 52 revolutions min−1. The sample container was placed on a hydraulic platform enabling the sample to be submerged in water. Ultimately, the mass of stable aggregates for each treatment was recorded by weighing the aggregates remaining on the sieve after sieving. Clay dispersibility was measured by the method described by Vendelboe et al. (2012) using 10 g of dry soil with 80 g artificial 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 speed of shaking was 33 revolutions min−1, with each rotation period lasting 2 min. After shaking, the sample was left for sedimentation for 3 h 50 min. The top 50 mm (60 mL) of the suspension, corresponding to particles less than 2 μm, was subsequently transferred to 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 four size 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 aggregates, the TS was measured using the unconfined compression test (ASTM D2166/D2166M-13). For each size fraction in each studied plot, 15 soil aggregates were analyzed, and TS was measured for each soil aggregate. The TS (kPa) was calculated using the equation of Rogowski et al. (1968): TS ¼ 0:576F=d 2

(1)

where F (N) is the breaking force and d (m) is the mean aggregate diameter. The 0.576 value is a constant based on the assumption that the aggregates are of spherical form and exhibit plastic behavior (Poisson ratio, 0.5; Dexter, 1975). Based on the assumption that aggregate density is constant, the mean aggregate diameter (d) was estimated using the equation by Dexter and Watts (2000): d ¼ d0 ðm=m0 Þ1=3

(2)

where m is the mass of an individual aggregate, mo is the mean mass of a batch of aggregates of the same size class, and d0 is the mean of the openings of the upper and lower sieves for that size class. Together with the TS measurement, the rupture energy of each aggregate was calculated by integrating the area under the force (F) versus the displacement curve. The SRE (J kg−1) was achieved by dividing the rupture energy by the aggregate mass. The FI was then calculated as the negative slope of the log TS versus the log volume of the soil aggregates. X-ray CT scanning of selected soil aggregates in size classes 8 to 16 mm from plots 1, 3, 4, 6, 7, and 8 (cf. Fig. 1) was carried out with three aggregates per plot. The 18 soil aggregates were scanned using an industrial CT scanner (X-Tek HMX225) at Halle, Germany (Helmholtz Centre for Environmental Research, UFZ). A copper filter was attached in front of the X-ray source to alleviate beam hardening. The obtained shadow projections (radiographs) were reconstructed with a Feldkamp cone-beam © 2014 Lippincott Williams & Wilkins

algorithm (Feldkamp et al., 1984) to obtain 16-bit, gray-scale, three-dimensional volumes with a resolution of 30 μm. These CT gray-scale volumes were processed with the ImageJ software package (Rasband, 2011). Only qualitative analysis of soil aggregates was carried out using X-ray CT data because it is not possible to distinguish between soil pores and biochar particles in soil aggregates. The SOC content, TS, and SRE of each X-ray CT scanned aggregate were measured. Organic carbon was determined by oxidation of carbon to CO2 using a FLASH 2000 organic elemental analyzer coupled to a thermal conductivity detector (Thermo Fisher Scientific, MA).

Statistical Analysis Lack of true replication at the experimental site hindered a full statistical analysis, including effects of different rates of swine manure, years of biochar application, and the interactions of swine manure and biochar on soil structural properties. Rather, as an approximate approach, significant effects of biochar were tested for individual years of biochar application (i.e., 2011, 2012, and 2011 +2012), as suggested by Sun et al. (2014). A representative soil sample for each treatment was obtained by pooling six individual soil cores. Working at the (small) scale of individual soil aggregates, subsamples of this composite soil sample were considered to adequately reflect the field-scale variation in physical and mechanical aggregate properties. Analyses of variance were done using the Holm-Sidak test (α = 0.05) in SigmaPlot 11.0 (Systat Software, San Jose, CA). Relationships between measured parameters including aggregate stability, clay dispersibility, TS, SRE, FI, and each of the physical and chemical soil properties were investigated using multiple linear regression models, and the significance of each parameter was evaluated. Soil properties, including clay, silt, sand, SOC, EC, and pH as independent variables, which did not demonstrate a significant relationship with measured parameters, were excluded from the regression model. The correlation between amounts of biochar added (i.e., cumulative rates of 0, 1, 2, 4, 5, 10 kg m−2) and measured parameters including aggregate stability, clay dispersibility, and soil strength was also investigated. To evaluate the quality of the developed multiple linear regression models for predicting the measured parameters, statistical performance indicators were calculated based on modeling efficiency (MEF) according to Mayer and Butler (1993): 2 2 (3) MEF¼1−∑ yi −y^ i =∑ yi − yi where yi is the individual parameter measurements, ŷi is the individual parameter predictions, and y-i is the mean of the measurement of each parameter. Further model evaluation used plots of measured versus predicted data to test for significance of the correlation coefficient and for unit slope and zero intercept (simultaneous F test) as explicitly outlined by Haefner (2005).

RESULTS AND DISCUSSION Soil Texture and Carbon Soil organic carbon varied among the 12 plots from 1.48 to 4.76 g 100 g−1 (Table 1). The SOC increased consistently with application rates of both biochar and swine manure. The soil texture of studied plots was rather similar with clay, silt, and sand contents within the ranges 8 to 12, 23 to 29, and 60 to 69 g 100 g−1, respectively. Bulk density varied between 1.09 to 1.37 g cm−3 but was not consistently correlated to the rates of biochar application. www.soilsci.com


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TABLE 1. Soil Physical and Chemical Properties of Studied Plots Plot ID 1 2 3 4 5 6 7 8 9 10 11 12

Clay (