## 4 Discussion
Porosity and surface area remain pivotal controlling factors in determining soil AWC, but additional factors, such as the diverse surface functional groups of BCs (influenced by the feedstock used and pyrolysis temperature), also contribute to enhancing the soil AWC [43]. The study conducted by [PERSON] et al. [36] revealed that the application rate and particle size interaction had a significant impact on the AWC in sandy soil. However, our study showed inconsistent results as we could not confirm the effect of the interaction of these factors on AWC. BC application could increase the AWC by reducing bulk density [47], increasing porosity [3,42], increasing surface area [48], and promoting soil aggregation, especially in coarse-textured soils. All BC treatments resulted in a significant decrease in saturated hydraulic conductivity (\(K_{S}\)) compared to sandy soil, with the greatest \(K\)s decrease observed in treatments with a small-sized BC. It means that the hypothesis that biochar addition will decrease \(K\)s was fulfilled. Our results correspond with the study by [PERSON] et al. [49], who found that switch grass BC application at a 5% _v/v_ rate decreased \(K\)s by about 52% in sandy soil. Also, [PERSON] et al. [45] measured a 54% decrease in \(K\)s after the application of cow manure BC in sandy soil. In our study, \(K\)s in the sandy soil after BC addition was significantly affected by all three factors, which is consistent with [PERSON] et al. [49], who found that \(K\)s was dependent on BC concentration in soil, and with [PERSON] et al. [50], who observed the effect of the BC particle size on \(K\)s in sandy soil with BC. [PERSON] et al. [51] discovered that BCs having larger particle sizes, exceeding 1 mm, caused a more significant reduction in \(K\)s compared to BCs having smaller particles when added to sandy soils. Our results confirmed the significant effect of the interaction between the application rate and particle size on \(K\)s, which is not consistent with the results of the study by [PERSON] et al. [36], which did not confirm the effect of the interaction of the given factors. In coarse-textured sandy soils, decreased \(K\)s is likely to occur when (particularly small particle-sized) BCs fill macropore spaces between soil particles, leading to increased tortuosity and decreased pore diameter [51]. Reduced \(K\)s can also be attributed to the induced water repellency in soils amended with hydrophobic BC [18]. In some cases, the values of \(K\)s in coarse-textured soils may decrease initially due to the disruption of particle structure and clogging of macropores in the early stages. However, after several wetting-drying cycles, \(K\)s may show an increase [13]. All BC treatments resulted in a significant increase in contact angle (CA) compared to sandy soil, with the greater CA increase for the higher BC application rate. It means that the hypothesis that BC addition will increase the CA was fulfilled. Our findings are consistent with those of [PERSON] et al. [18], who observed the greatest CA increase for the highest application rate (10%) of BCs with a contact angle CA = 107\({}^{\circ}\) to sandy soil. According to the results obtained by [PERSON] et al. [43], the SWR depends on BC application rates (the higher the application rates, the higher the SWR), which is consistent with our results. We also observed the effect of BC particle size on the severity of the SWR (measured as the CA), which is not mentioned in previous studies. Our study did not confirm the effect of pyrolysis temperature on the CA in soil with BCs. Contrary to our findings, the study by [PERSON] et al. [43] confirmed the effect of pyrolysis temperature on the SWR of BC-amended soil, and the SWR decreased as the temperature increased. [PERSON] et al. [52] suggested an explanation for this phenomenon. They observed that BCs produced below 500 \({}^{\circ}\)C retained organic functional groups from the feedstock, typically water repellent.