Effects of Biochar on Properties of Tropical Sandy Soils Under Organic Agriculture

This study evaluated the influences of biochar made from local agricultural wastes on sandy soils in farmer fields where biochar has been used as a soil amendment for more than three years. The major objective of this study was to gain insight into the effects of long-term biochar application on properties of sandy soil. Unamended soil properties were compared to biochar-amended soils properties using the paired samples t-test (p < 0.05). The statistical results of the study indicated that cation exchange capacity, exchangeable potassium, available phosphorus, field capacity, plant available water, water-stable aggregate size fractions (> 1 and < 0.25 mm), median aggregate size and aggregate stability were significantly different at p < 0.05. Clearly, biochar present for 3 or more years can improve soil physicochemical properties. We conclude that sandy soil properties, especially soil physical properties, are very strongly affected by biochar application combined with conservative soil management. Biochars affect both physical and biological mechanisms of soil aggregate formation because the biochar particle sizes influence the arrangement of clay on biochar and biochar grains provide a favorable microbial habitat and food source for fungi creating microorganism-biochar-soil associations which enhance water-stable aggregates and water holding capacity.


Introduction
Sandy soils are widely distributed across the world covering approximately 4,990,200,000 ha, accounting for 31% of the total land area (Huang & Hartemink, 2020). As world population is increasing rapidly, sandy soils are being increasingly used for food production. Physical properties of sandy soils (e.g., bulk density, porosity, aggregation) vary considerably because of the size and organization of the grains, type of clay, natural processes such as biological activities or human activities and soil management (Bruand et al., 2005). Sandy soils usually have a single grained structure and poor physical properties which result in a low water-holding capacity (Andry et al., 2009;Huang & Hartemink, 2020). Their properties restrict plant growth and local economic development (Zhou et al., 2016) as they have a coarse texture, little or no composite soil structure and low values of cation exchange capacity, base saturation, organic carbon concentration and fertility (Huang & Hartemink, 2020).
The carbon content of tropical soils especially sandy soils declines substantially under intensive agriculture (Mekuria & Noble, 2013). Land use intensity and soil management strongly affect soil organic carbon content and soil aggregate stability (Spohn & Giani, 2011). Maintaining the long-term aggregate stability of soil by applying fresh organic residues is difficult in high temperature tropical regions because of the rapid decomposition of fresh organic residues. These are the conditions where more stable biochar can provide a soil management solution (Hseu et al., 2014).
Biochar is the product of pyrolysis of organic materials in the absence of oxygen and at high temperature. Biochar can be used for increasing sustainable soil carbon as it contains stable carbon which remains sequestered for a long period of time. Recent studies have shown that biochar not only increases soil carbon, it also improves soil structure and soil functions for sandy soils. It also supplies plant nutrients because biochar contains several essential elements (Abujabhah et al., 2016;Sorrenti & Toselli, 2016). When biomass is heated in kilns, water in plant cells evaporates and mineral compounds are precipitated within the relic plant structure. The nutrients added in biochar result in higher plant nutrient availability (Lehmann et al., 2003). For these reasons, biochar has been considered as a suitable soil amendment for improving soil physicochemical properties, of degraded and sandy soils in subtropical and tropical regions (Chan et al., 2007;Hseu et al., 2014).
Many studies report that biochar improved both chemical (e.g., pH, cation exchange capacity) and physical soil properties (e.g., bulk density, water holding capacity) of soils (Glaser et al., 2002;Lehmann, 2007;Jeffery et al., 2011;Hseu et al., 2014;Yadav et al., 2018). Baiamonte et al. (2015) has indicated that the application of biochar to soil improves soil structure, increases porosity, decreases bulk density, and enhances aggregation and water retention in acidic, degraded and sandy soils (Jeffery et al., 2011;Crane-Droesch et al., 2013). Some farmers have used biochar as a soil amendment for sandy soils; however, the beneficial effects of long-term field scale biochar application are quite poorly understood, especially effects on soil physical properties such as soil aggregation (Blanco-Canqui, 2017). Additional studies are needed to evaluate the long-term effects of biochar on the physical, chemical, and biological properties of sandy soils (Huang & Hartemink, 2020). In particular the role of biochar in sustainable organic agriculture has not been clearly established. To obtain a comprehensive evaluation of biochar effects on soil properties, long-term studies (more than 1 year) should be conducted. In this study we investigated the long-term (> 3 years) effects of biochar application on aggregation in sandy soils and changes in other soil properties for sites under organic agricultural management in Thailand.

Soil and Biochar
Soil samples at depths of 0-20 cm were collected from four farmer fields in Thailand (Table 1) where various biochars had been used as soil amendments for more than three years (Table 2). Farmers at each location practice organic agriculture with no use of chemical fertilizers or ameliorants. At Chainat and Chonburi, vegetables were grown with minimal cultivation. Chainat's vegetable gardener uses raised beds whereas at other locations the dominant land use is growing fruit and other trees such as para rubber (Table 1). Green manure and other agricultural wastes were returned to the soils. Unamended soil (control) and biochar-amended soil were collected at the same location. Two types of soil samples were collected including disturbed and undisturbed soil samples. Disturbed soil samples were homogenized, air dried and crushed to pass through a screen (2 mm-opening) for chemical analysis. Additionally, undisturbed samples were collected for micromorphology (kubiena box) and physical (core sampler) analysis. The eight biochars used on these plots had been prepared by farmers using local materials and technology (carbonization temp.ca. 400 o C). Biochar types and rates applied to soils are shown in Table 2 with total of about 100 ton per ha being applied over periods up to 10 years.

Chemical Properties of Soils and Biochars
Soil samples were analysed according to methods used by Burt (2004). Soil pH and electrical conductivity (EC) of soils and biochar were measured in 1:1 and 1:5 soil:water extracts, respectively (Tables 3 and 7). Organic matter in soils was the organic carbon determined by the Walkley and Black wet oxidation method whereas total organic carbon was measured by combustion (Table 7). Total nitrogen in soils was determined by the Kjeldahl method. Cation exchange capacity of soils was measured by saturating the exchange sites with 1 M NH 4 OAc at pH 7.0 and displacing NH 4 + with 10% acidified NaCl. Available phosphorus in soils was determined by the Bray II method.
Biochar pH and electrical conductivity (EC) were measured using a 1:5 biochar:MilliQ (MQ) water extraction with shaking end-over-end for 12 hr. The ash content of biochars was determined by dry combustion in a ventilated muffle furnace at 600 o C overnight (Table 3). Total element concentrations in biochar were determined by digesting biochar ash in 10% HCl with analysis of the solution by ICP-OES (Table 4). The total water soluble elements in biochar were determined by extracting the biochar with MQ water (0.3 g biochar per 10 mL MQ water) and shaking end-over-end for 12 h ( Table 5). The extracts were analysed by ICP-OES. The proportion of elements soluble in water is listed in Table 6.  Note. nd = non detected.

Physical Properties
Water-stable aggregates were fractionated according to Kemper and Rosenau (1986). Two core samples from control and treated soils were oven dried (105 o C, 48 h) and used to calculate bulk densities (Table 8). Particle size distribution was determined by the pipette method. Soil water retention was determined using a pressure plate apparatus at suctions equivalent to field capacity (FC) 1/3 atm and permanent wilting point (PWP) 15 atm. Saturated hydraulic conductivity (Ksat) measurements were obtained using a constant head method.   Note. FC = Field capacity; PWP = Permanent wilting point, PAW = Plant available water.

Micromorphology, SEM and EDS
Kubiena boxes were used to collect undisturbed blocks of unamended and biochar-amended soils to make thin sections after impregnation with resin. Soil micromorphology was studied on thin sections of resin impregnated soils using a Leitz polarizing microscope. Thin sections of soils, original and recovered biochars from soils were used for scanning electron microscopy using a Carl Zeiss SIGMA VP fitted with an Energy Dispersive Spectrometer (EDS) for elemental analysis.

Image Analysis
Thin sections of the Ap-horizon of soil from each location were examined. To determine the nature of soil aggregates a Leitz polarizing microscope with plane polarized light (PPL) was used to produce optical micrographs ( Figure 4a). The optical micrographs were adjusted using the Photoshop program prior to image analysis. Raw optical micrographs were converted to grayscale mode and TIFF file format with 1600 × 1200 pixels (Figure 4b) then the aggregate boundaries were identified. An image analysis program (ImageJ 1.52a) was used to measure the size of soil aggregates seen in optical micrographs (Cox & Budhu, 2008). Grayscale mode images were used for automatic variable threshold (binary contrast enhancement) and the location, size and shape of biochar particles was determined by normal thresholding (Figure 4c). Soil aggregates were analysed by ImageJ following these steps: Image > Adjust > Threshold > Analyze > Analyze particles > Outline > Clear results. Aggregate size was obtained by measurement of major and minor axes of aggregates.

XRD
Compounds (minerals) in ground biochars (size < 0.05 mm) were identified by X-ray diffraction analysis using an XPert 3 powder diffractometer with Ni filter (CuKα, 45 kV, 40 mA) and Pixel detector. Powders were scanned from 5 to 70 o 2θ, using a step size of 0.015 o 2θ and a scan speed of 0.02 o 2θ s -1 .

Statistical Analysis
Paired samples for means t-tests in Microsoft Excel 2010 were used to compare the means of soil properties of unamended and biochar amendment treatments which were performed to determine effects of biochar on soil properties. Significant differences are given with a significance level of p < 0.05.

Biochar Characteristics
Biochar pH was neutral to alkaline (6.8 to 10.2) with low to moderate electrical conductivity values (0.03 to 4.0 mS m -1 ). Ash contents were low (3 to 17%); except for rice husk biochar (33%) which contains much amorphous silica (Table 3, Figure 3). Mineral phases in biochar depended on the type of biomass. Calcite, sylvite and struvite were present in some biochars as has been reported by other studies (Prakongkep et al., 2015). Variations in mineral phases were related to the variable solubility of nutrient elements in biochar (Table 6 and Figure 1). All biochars have a low aluminum concentration because Al is not a vital element for plant tissue (Table 4); therefore, the presence of Al is probably due to contamination by soil clay minerals. The Ca concentration in sweet acacia wood biochar (13,646 mg kg -1 ) is higher than for other biochars (Table 4). Biochar made from the various woods contains high amounts of calcium because calcium is commonly a major element in wood (Fromm, 2010); however, the Ca in biochar is mostly in calcite and poorly soluble in water (Tables 5 and 6); consequently, it will dissolve slowly in soil. Copper, iron, manganese and sodium concentrations are quite low in A paired t-test (p < 0.05) was used to determine the mean scores between unamended and biochar-amended soils at each location. The results of the test are presented in Table 10 which indicate that the mean score for cation exchange capacity; exchangeable potassium and available phosphorus showed a significant difference between unamended and biochar-amended soils at p < 0.05. This observation is as same as reported by several researchers who have confirmed the beneficial effect of biochar on the soil cation exchange capacity and available phosphorous and potassium (Laird et al., 2010;Yuan et al., 2011;Fellet et al., 2011;Wang et al., 2014;Gondek et al., 2019). The higher exchangeable potassium and available phosphorus values of biochar-amended soils are due to the total potassium and phosphorus contents of biochars and the high proportion of water-soluble potassium and phosphorus in biochar (Table 6).

Soil Physical Properties
All studied soils are sandy with a low water holding capacity (Table 8). Biochar application increases soil water holding capacity at FC by 1.3-1.7%. Plant available water content of biochar-amended soils increased (by 0.8 to 2.4%) ( Table 8). Biochar application slightly reduced bulk density because biochar is less dense than soil minerals and also because biochar improves soil structure. Biochar application did not systematically affect saturated hydraulic conductivity (Table 8).
Paired differences of physical properties of unamended and biochar-amended soils are shown in Table 11. Field capacity, plant available water, water-stable aggregate-size fractions (> 2, 2-1, 0.25-0.1 and < 0.1 mm), median jas.ccsenet.org Vol. 13, No. 1; aggregate size and aggregate stability were significantly different according to paired t-test at p < 0.05. Special attention has been paid to evaluate the effects of biochar on the formation and protection of soil aggregates. Soil aggregate stability is a key indicator of soil structure. Biochar application increased the average size of aggregates and increased wet aggregate stability by 6 to 25% (Table 9). Biochar enhances soil aggregation by providing organic binding agents and stimulates growth of fungi that produce abundant hyphae that bind to other soil particles (Blanco-Canqui, 2017).

Soil-Biochar-Microbe Associations
Crystalline minerals present in biochar contain important nutrients including phosphorus, potassium and calcium (Figures 1 and 2). These minerals are a food source for soil microorganisms making biochar a focus for microbial growth in soil (Lehmann et al., 2011). Biochar provides an excellent habitat for fungi and bacteria because of its porous structure that protects microorganisms and retains water in pores (Figure 2c) (Steinbeiss et al., 2009). Biochar grains become the nuclei of aggregates (Figures 3 to 4). Optical micrographs under plane polarized light demonstrated that the unamended sandy soil (control) consists mostly of single quartz grains including runiquartz (Figure 3a) whereas biochar-amended sandy soil also contains biochar grains surrounded by soil particles forming soil aggregates (Figure 3b). Biochar is mostly retained in aggregates by the action of fungal hyphae (Figure 4). Microorganisms proliferate in biochar as they extract phosphorus, potassium, calcium and other nutrients with fungi hyphae extending into the soil and holding biochar and soil particles together creating soil aggregates (Figure 4). Busscher et al. (2010Busscher et al. ( , 2011 found that biochar did not affect aggregation in a coarse-textured soil possibly due to the low OM and clay contents of their soil. Several authors have reported that coarse-textured soils (e.g., sand to sandy loam) with low SOM contents need to be co-amended with biochar and organic residues to promote soil aggregation (Busscher et al., 2010(Busscher et al., , 2011Awad et al., 2013;Khademalrasoul et al., 2014). In this study, biochars were capable of improving soil aggregation in coarse-textured soils with quite low organic matter contents despite farmers following organic agriculture methods. There is a close positive relationship between aggregate size and aggregate stability ( Figure 5). Clearly soil physical properties are strongly affected by the capacity of biochar to enhance the extent and quality of soil aggregation.

Biochar Characteristics
The highly porous nature of biochar results from retention of the cell wall structure of the biomass feedstock (Yadav et al., 2018). These studied biochars contain varying concentrations of plant elements (Tables 4 to 6) in the forms of crystalline and non-crystalline minerals (Figures 1 and 2). The fertilizer efficiency of plant nutrients in biochar is highly dependent on the type of feedstock. Based on elemental concentrations and the solubility of elements in biochar, durian shell biochar can be used as a K fertilizer. Wood biochars can be used as a liming agent. Clearly, biochars from the different feedstocks had different properties.

Soil Chemical Characteristics
Biochar application may improve the soil fertility status of coarse textured soils, especially soil OC, CEC, available P, exchangeable K, Ca, Mg (Chan et al., 2007;Sukartono et al., 2011) as biochar contains available nutrients (Sohi et al., 2009). In our case, CEC, exchangeable K and available P of biochar-amended soils were higher than for unamended soils whereas adding biochar into sandy soils did not affect pH, EC, OM, TOC, total N and exchangeable Ca, Mg and Na.
Biochar application resulted in no significant change of soil pH as was also observed by Adekiya et al., 2020 who recognized that there were no significant differences in soil pH values for additions of 20 and 30 t ha -1 biochar. It is probable that calcium in biochars of this study was contained mostly in calcite which dissolves slowly in water although other studies have concluded that soil pH increased after biochar was mixed into the soil (Lehmann et al., 2003;Rodriguez et al., 2009;Major et al., 2010;Masulili et al., 2010).
The increase in extractable cations in biochar-amended soil improved soil fertility especially for potassium. Our results illustrate that biochar significantly increased CEC, exchangeable K and available P (P < 0.05) but the differences between total carbon and organic matter contents of biochar-amended soils and unamended soil were not significant. Biochar-amended soils have significantly higher CEC than unamended soils due to the presence of cation exchange sites on the surface of biochar and its large surface area (Adekiya et al., 2020). CEC of unamended and biochar-amended soils were less than 10 cmol kg -1 corresponding to low amounts of organic carbon and clay in the soil. The higher exchangeable potassium and available phosphorus values of biochar-amended soils are due to the crystalline and non-crystalline minerals on the biochar surface and inside the biochar structure. These minerals dissolve and release nutrients to soil solution. Exchangeable K significantly increased with biochar application to soil so evidently biochar can be a source of potassium in soil fertility management (Ghorbani & Amirahmadi, 2018). The P availability of the soil increased after applying biochar due to the high P concentration in some biochars.

Soil Physical Properties
The results revealed that biochar application could improve sandy soil quality and significantly affect soil physical properties. Biochar improved field capacity, plant available water, water-stable aggregate size fractions (> 1 mm and < 0.25 mm) but no significant difference was observed in bulk density and hydraulic conductivity which was similar to results of Laird et al. (2010). Numerous studies have indicated that biochar application could reduce bulk density (Chan & Xu, 2009;Downie et al., 2009, Jien & Wang, 2013Hseu et al., 2014;Barus, 2016;Głąb et al., 2016;Blanco-Canqui, 2017). For this study, the bulk density of biochar-amend soil is slightly lower than unamended soils but the difference was not significant which was similarly reported by Abrishamkesh et al. (2015). Although, biochar is a porous material with low density (Yadav et al., 2018), this study has shown that biochar application did not affect bulk density and hydraulic conductivity. Probably these biochar application rates were not large enough to reduce bulk density and hydraulic conductivity (Mukherjee & Lal, 2013;Abujabhah et al., 2016). The biochar application rates in this study range from 10 to 20 t ha -1 each year corresponding to the suggestion of a minimal rate of 10 t ha -1 (Dariah et al., 2010); however, Adekiya et al. (2020) found that an application rate of 30 t ha -1 biochar was required to reduce the bulk density of a tropical sandy loam Alfisols. Even larger amounts of biochar may be needed to make significant changes to some soil physical properties (Blanco-Canqui, 2017).
Soil physical properties were dependent not only on the biochar application rate but also on its particle size (Głąb et al., 2016). Biochar acts as a nucleus for soil aggregation. The average biochar particle size in this study is less than 2 mm. The micromorphological observations of the biochar-amended soils illustrate the rearrangement of soil particles onto biochar surface with consequent soil aggregate formation. Applying biochar into sandy soils created more macroaggregates. Our study showed that the percentage of water-stable aggregate in size fractions (> 2 and 2-1 mm) increased whereas the percentage of water-stable aggregates in the size fraction (< 0.25 mm mm) decreased. Similarly, Sun and Lu (2014) reported that application of biochar significantly enhanced the formation of 5-2 and 0.25-0.5 mm macroaggregates in a clayey soil relative to the control treatment, while the < 0.25-cm microaggregate fraction decreased with biochar additions. Burns (2014) carried out a similar study on a very sandy soil in South Western Australia where addition of 100 t ha -1 of wheat straw biochar increased the abundance of water stable aggregates from 50 to 61% of the soil over 3 years.
Regarding increases of field capacity and plant available water content in this study, the increases in median aggregate size of macroaggregates might be the cause. Larger aggregates retain more water than smaller aggregates because of intraaggregate pores (Liu et al., 2012). Biochar application directly increased the water holding capacity due to the inner surface area of the biochar and indirectly increased the water holding capacity by facilitating the formation of soil aggregates and macropores (Lei & Zhang, 2013). Similar conclusions hold for the effect of biochar application on the water content at field capacity, permanent wilting point and plant available water content in sandy soils (Glaser et al., 2002;Downie et al., 2009;Laird et al., 2010;Bass et al., 2016;Barus, 2016). Tryon (1948) reported no significant effect of biochar application on plant available water of a loamy soil but there was a significant increase for a sandy soil. Bass et al. (2016) demonstrated that biochar application of 10 t ha -1 significantly increased soil water content compared to control. Application of biochar to sandy soil at 10 t ha -1 increased water use efficiency by 6 percent (Yadav et al., 2018). Increasing plant available water of biochar amended soils indicates that biochar application to agricultural land could contribute to the reduction of irrigation frequency (Blanco-Canqui, 2017).

Soil-Biochar-Microbe Associations
The microstructures of the biochar-amended soils indicate that soil particles especially clay are adsorbed on the biochar surface. The soil particles rearrange when the surfaces of clay and biochar interact forming microaggregates and combining with other soil-biochar complexes to form macroaggregates. Macroaggregate formation was an important factor in maintaining soil porosity and was a result of the increase in microbial activity in and adjacent to biochar grains (Barus, 2016).
Biochar application affects soil fauna and microorganism activities (Lehmann et al., 2011). Biochar properties may enhance soil microbial communities and create microenvironments that encourage microbial colonization as it is a porous material which is a good habitat for microbes (Sohi et al., 2009). Soils amended with biochar experience increases in soil microbes, earthworms, or other soil fauna particularly fungi promotion (Steinbeiss et al., 2009). Biochar acts as a refuge for soil microorganisms and adheres to soil particles creating soil aggregates. Soil aggregation is an important mechanism for stabilization of soil organic matter (Spohn & Giani, 2011). Soil particles are bound together into microaggregates due to persistent binding agents. These microaggregates, in turn, build macroaggregates due to transient and temporary organic binding agents like polysaccharides, roots, and fungal hyphae (Tisdall & Oades, 1982). Biochar is a food source and habitat for fungi that extend hypha out of biochar particles and which act as an organic binding agent.

Conclusions
Biochar was added by farmers to sandy soils with the goal of improving soil properties. The results indicated that applying large amounts of biochar to sandy soils for more than 3 years significantly increased some plant nutrients, cation exchange capacity, aggregation, soil aggregate stability and available water content. Applying biochar could redistribute soil particles increasing soil aggregate stability and biochar grains also are a potential habitat and a food source for microorganisms enhancing the microbial abundance in soil. In particular, biochar application increased the number of water-stable aggregates consisting of soil-biochar-microbe associations. Fungal hyphae, feeding in biochar, can act as a binding agent for soil particles. Fungal hyphae proliferate from biochar grains throughout the soil with a dense mesh of hyphae providing strength to soil aggregates. Fungi also exude organic compounds that will adhere to soil particles providing additional strength (Chenu & Stotzky, 2002). The presence of biochar within soil aggregates can physically protect biochar particles in aggregates from decomposers. Consequently, aggregate formation due to the presence of biochar particles could enhance the storage of organic carbon in soils. Increasing soil aggregate stability improved field capacity and plant available water content. Clearly, biochar application can improve soil physicochemical properties. Biochar should gain acceptance as a long-term soil amendment for tropical sandy soils.