*2.4. Analytical Methods*

The bulk density (*D*B) was determined using the core method [32]. The pH values of the soil samples and biochar materials mixed with DI water (1:2.5 and 1:20 *w*/*v*, respectively) were determined using a Horiba F-74 BW meter [33]. We performed electrical conductivity measurements on saturated paste extracts of the soil samples by using a Horiba F-74 BW meter [34]. The soil particle size distribution was determined using the pipette method [35]. Cation exchange capacity (CEC) was determined using the ammonium acetate method (pH 7.0) [36]. Exchangeable K was extracted using 1 mol L−<sup>1</sup> NH4OAc (1:10 *w*/*v* for the soil samples; 1:20 *w*/*v* for the biochar materials), and the extract was analyzed through atomic absorption spectrometry (Z-2300, Hitachi, Tokyo, Japan). The OC concentration was determined through wet oxidation [37]. Available P was determined using the Bray P-1 extract test [38]. Inorganic N was extracted using 2 M KCl (1:10 *w*/*v*), and the concentrations of NH4 <sup>+</sup>-N and NO3 −-N were determined though steam distillation conducted using MgO and Devarda's alloy [39]. The microscale structure of the biochar materials was characterized through optical microscopy using reflected light, followed by scanning electron microscopy (SEM; Hitachi, S-3000N, Japan). A backscattered electron image representing the mean atomic abundance in a black-and-white image was observed on the surface of the samples coated with Au. The C components of biochar horizons were examined through solid-state CPMAS 13C nuclear magnetic resonance (DSX 400-MHz solid-state NMR, Bruker, Karlsruhe, Germany). Data acquisition was executed under the following conditions: spectrometer frequency, 100.46 MHz; spinning speed, 7000 Hz; contact time, 1 ms; and pulse delay time, 1 s. We determined the total signal intensity and the proportion contributed by each C functional group by integrating the spectra in the chemical-shift region: 0–50 ppm (aliphatic C), 50–110 ppm (O-alkyl-C), 110–165 ppm (aromatic C), and 165–190 ppm (carboxyl C). Methoxyl C contributed a wide shoulder between 50 and 60 ppm within the O-alkyl-C range (Alpha-T, Bruker).
