## 2 Materials and Methods
### Experimental Design  
Sandy soil was sieved to a fraction with a particle size \(\leq\) 2 mm. Both BCs were grounded with a hammer mill and sieved to a fraction with 125 \(\upmu\)m and 2 mm particle sizes, so two different BC sizes were studied: \(<\)125 \(\upmu\)m and 125 \(\upmu\)m-2 mm. Also, two different application rates were examined in our research: 20 Mg ha\({}^{-1}\) and 40 Mg ha\({}^{-1}\). Eight soil-BC mixture treatments and one control without BC were prepared in steel cylinders with a volume of 100 cm\({}^{3}\) and 5 replicates for each treatment were prepared in laboratory conditions. Hydrophysical measurements were provided one week after soil-BC mixture preparation. The porosity \(P\) of the soil and soil-BC mixtures was calculated based on the Equation (1) [26]:  
\[P=\frac{\rho s-\rho d}{\rho s}\times 100 \tag{1}\]  
where \(P\) is soil/soil-BC mixture porosity (%), \(\rho s\) is soil/soil-BC mixtures particle density (g cm\({}^{-3}\)) measured by the pycnometric method [27], and \(\rho d\) is the bulk density of the soil/soil-BC mixtures (g cm\({}^{-3}\)) determined based on Equation (2) [28]:  
\[\rho_{d}=\frac{ms}{V} \tag{2}\]  
where \(ms\) is the weight of the soil/soil-BC mixture samples dried at 105 \({}^{\circ}\)C (g) and \(V\) is the steel cylinder volume (cm\({}^{3}\)). Five values were analyzed for each treatment. The available water content for plants (AWC) is defined as the difference between the field capacity (FC) determined at pF = 2 (10 kPa) and the wilting point (WP) determined at pF = 4.18 (1500 kPa). The above-mentioned hydrolimits were determined from the soil water retention curves, estimated on five samples for each treatment. The soil water retention curves were measured with the pressure plate apparatus (Soil Moisture Equipment Corp., Santa Barbara, CA, USA) according to ISO 11274 [29]. A total of 9 measurement points were used at pressure potentials of 0, 6, 10, 30, 56, 100, 300, 480, and 1500 kPa. Based on measured data, the soil water retention curve values were fitted using the unimodal van Genuchten [30] model as follows:  
\[\theta=\theta_{r}+(\theta_{S}-\theta_{r})\big{[}1+(\alpha|h|)^{n}\big{]}^{-m} \tag{3}\]  
where \(\theta\), \(\theta_{r}\), and \(\theta_{S}\) (cm\({}^{3}\) cm\({}^{-3}\)) are the actual, residual, and saturated volumetric water content, respectively, \(h\) (cm) is matric potential, \(m\) and \(n\) (\(-\)) are water retention shape parameters, and \(\alpha\) (cm\({}^{-1}\)) is the parameter describing the matric potential of air entry. The saturated hydraulic conductivity \(K_{S}\) (cm h\({}^{-1}\)) was measured based on the modified falling-head method [31]. For the calculation, Equation (4) was used:  
\[K_{s}=\frac{L}{t}ln\frac{h_{2}}{h_{1}} \tag{4}\]  
where \(L\) is the soil/soil-BC mixture core height (cm), \(h_{2}\) is the initial water level (cm), \(h_{1}\) is the water level at the end of the measurement (cm), and \(t\) is the time for the water level to decrease from \(h_{2}\) to \(h_{1}\) (h). Twelve measurements were made for each treatment. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Pylysis** & **pH (H\({}_{2}\)O)** & **C** & **H** & **N** & **WDFT** & **CA** \\
**(\({}^{\circ}\)C)** & **(-)** & **(\%)** & **(\%)** & **(\%)** & **(\%)** & **(s)** & **(\({}^{\circ}\))** \\ \hline
300 & 9.14 & 82.2 & 2.74 & 0.86 & 12,609 & 115.56 \\
520 & 7.93 & 83.1 & 2.21 & 1.19 & 12,613 & 128.30 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters of used biochars (C—carbon content, H—hydrogen content, N—nitrogen content, WDPT—water drop penetration time, CA—contact angle). The sessile drop method, used for contact angle (CA) estimation, involved placing a water drop on the soil sample's surface and analyzing the CA by reviewing the image recordings taken with the optical goniometer OCA 11 (DataPhysics Instruments GmbH, Filderstadt, Germany). The procedure was described by [PERSON] et al. [32].