*3.1. Properties of the Soil and Biochar Materials*

Table 2 lists the properties of the soil samples and biochar materials. The texture of the studied soil was sandy loam; the soil was determined to have a neutral pH and low OC content. The porosity, bulk density, and particle density were in the normal ranges for the coarse-textured soil samples. The soil used in this study was sourced from an intensively cultivated field with high human input, which may result in high nutrient concentrations. The pH values of WB300 and WB600 (Honduran mahogany wood sawdust pyrolyzed at 300 and 600 ◦C) were 6.5 (neutral) and 10.4 (alkaline), respectively. The OC

content in WB300 was 6.8%, which was higher than that in WB600 (2.0%). By contrast, the total carbon content was 69% in WB300, which was lower than that in WB600 (79.5%). These results indicate that WB600 contained a higher level of inorganic carbon than W300 did. The higher pyrolysis temperature reduced the concentrations of oxygen, nitrogen, ammonium–nitrogen, nitrate–nitrogen, available phosphorus, and exchangeable potassium in the biochar materials. Figure 2 depicts SEM images of both biochar materials. WB300 exhibited coarser pores than WB600 did but had a lower number of pores for the same volume. Because of its more porous structure—signifying a larger surface area—WB600 could have distinct effects on the physicochemical properties of soil and groundwater when compared with WB300.


**Table 2.** Properties of the studied soil and Honduran mahogany (*Swietenia macrophylla*) wood sawdust biochar samples pyrolyzed at 300 ◦C (WB300) and 600 ◦C (WB600).

EC: electrical conductivity; SL: sandy loam; DB: bulk density; DP: particle density; OC: organic carbon; TC: total carbon; H: hydrogen; O: oxygen; N: nitrogen; NH4 <sup>+</sup>-N: ammonium-nitrogen; NO3 −-N: nitrate-nitrogen; Ava. P: available phosphorous; Ex. K: exchangeable potassium; -: Not determined.

**Figure 2.** Scanning electron microscope (SEM) images of Honduran mahogany (*Swietenia macrophylla*) wood sawdust biochar pyrolyzed at (**a**) 300 ◦C and (**b**) 600 ◦C.

Figure 3 illustrates the functional groups of C within the structures of the WB300 and WB600. O-alkyl-C is the major C group in the natural composition of Honduran mahogany. By pyrolyzing at 300 ◦C, the WB300 consisted of more aromatic-C, less O-alkyl-C, more alkyl-C, and more carboxylic-C than the raw wood dust. At 600 ◦C, the pyrolysis process resulted in the predominating aromatic-C in the WB600, and the other C groups became less observable. The results of the physical and chemical properties of the biochars as affected by pyrolysis temperature are consistent with previous studies [7,13,40,41].

**Figure 3.** Solid-state 13C cross-polarization magic-angle spinning nuclear magnetic resonance spectra for Honduran mahogany (*Swietenia macrophylla*) wood sawdust (**a**) and its biochar materials pyrolyzed at 300 ◦C (**b**) and 600 ◦C (**c**).

#### *3.2. Soil Physicochemical Properties*

Table 3 shows the major soil properties under different treatments before and after the 42-days experiment. The pH value of the untreated soil (before mixing with fertilizer) was 6.8, as shown in Table 2. After fertilization, the pH value under control dropped to 6.1. The pH of soil treated with WB300 was ~6.2, whereas WB600 had the highest pH value of 6.5. At day 42, the soil pH values of both biochar treatments were significantly higher than that of the control (pH 4.6), as shown in Table 3. The WB600 treated soil still revealed the highest pH value (5.8) at day 42 among all treatments. Although the soil under WB300 treatment had the similar pH value (6.2) with the control on day 0, it revealed a higher pH than the control at day 42.

The *D*<sup>B</sup> values observed on day 42 for all the treated samples were lower than the initial *D*<sup>B</sup> of 1.20 g cm−<sup>3</sup> (achieved when the soil columns were packed). The control exhibited the highest *D*<sup>B</sup> (1.11 g cm<sup>−</sup>3); the WB600-treated sample had the lowest value (1.05 g cm−3). However, the differences in *D*<sup>B</sup> between the treated samples were not significant (*p* = 0.05) (Table 3). Because biochar typically contains low levels of OC, the SOC levels observed for all treatments were low (0.21%–0.46%). The WB300-treated sample had the highest SOC content levels on both day 0 and day 42 (0.46% and 0.39%, respectively), indicating an SOC content loss of only 0.7% throughout the experiment (Table 3). The SOC levels observed for the WB600-treated sample did not differ significantly from that observed for the control (Table 3).

The NH4 <sup>+</sup>-N concentration did not differ significantly between any of the treated samples on day 0 (Table 3). On day 42, the NH4 <sup>+</sup>-N concentration in all treated samples decreased drastically from approximately 205 mg kg−<sup>1</sup> to less than 6.5% of the initial concentrations. On day 42, the control had the lowest concentration (5.87 mg kg<sup>−</sup>1), and the WB300- and WB600-treated samples had significantly higher concentrations (12.7 mg kg−1). The NO3 −-N and inorganic N concentrations in the treated

samples exhibited similar trends to the NH4 <sup>+</sup>-N concentrations. On day 42, all treated samples exhibited considerably lower NO3 −-N concentrations when compared with the initial concentrations; the NO3 −-N concentrations were high in the samples treated with the two biochar materials, particularly the WB300-treated sample.


**Table 3.** The Soil physicochemical properties on day 0 and day 42 (n = 3).

DB: bulk density; SOC: soil organic carbon; NH4 <sup>+</sup>-N: ammonium–nitrogen; NO3 −-N: nitrate–nitrogen; N: nitrogen; Ava. P: available phosphorous; Ex. K: exchangeable potassium. The values followed by the same superscript letters within a row are not significantly different (*p* > 0.05) between relevant treatments.

On day 0, the Ava. P concentrations did not differ significantly between the three treated soil samples (19.6–19.8 mg kg−1). On day 42, the Ava. P concentration decreased to 4.08, 5.1, and 5.36 mg kg−<sup>1</sup> in the control, WB300-treated, and WB600-treated samples, respectively. On day 0, the Ex. K concentrations in the control, WB300-treated, and WB600-treated samples were 302, 315, and 351 mg kg<sup>−</sup>1, respectively. After the experiment, the Ex. K concentrations increased to 457–488 mg kg−<sup>1</sup> in all treated samples and did not differ significantly between the samples.
