## 3 Results and Discussion
### Functional Composition of SOM
This occurs when the hydrophobic part of the molecule contacts the water and the hydrophilic group is oriented towards the inorganic species or other organic moieties [7; 36]. It was found that dissimilarities in WDPT and MED results are associated with the absorbance variation detected mainly in two frequency regions. The correlation and regression analysis confirmed that the relative amount of CH groups affects the WDPT or MED value positively. On the other hand, the strongest negative correlation was observed between WR and absorbance within the frequency region from 1190 to 1135 cm\({}^{-1}\). This band was assigned to stretching vibration of the C-O group within structures of alcohols, phenols, anhydrides, ethers, and polysaccharides. Higher relative abundance of C-O groups bound within SOM structures is expected to support a wettable character of soil. By using peak area values associated with these two bands, along with field moisture of topsoil, as the predictors it was possible to explain up to 67 % of WDPT and MED variances. Additionally, 1455-1400 cm\({}^{-1}\) and 1475-1460 cm\({}^{-1}\) peak areas were successfully tested as partial predictors of WDPT\({}_{\text{A}}\) and MED values that led to a further increase in the R\({}^{2}\) value (0.73, Tables S1 and S2). FTIR peak areas may be used in regression analysis alternatively in the form of an A/B ratio, which expresses the relative content of hydrophobic over hydrophilic groups bound in SOM. The \"A\" represents the sum of peak areas calculated in IR bands characteristic of vibrations of hydrophobic groups, while \"B\" expresses the sum of peak areas associated with vibrations of hydrophilic structural units. In this study, the sum of the peak areas calculated within the band Nos. 1, 5, and 7 (Table 1) is represented by A and the sum of the peak areas associated with the five remaining bands is represented by B. The A/B ratio concept was used by [PERSON] et al. [25] who applied additional transformations of the A/B value to achieve good fit with WR data. The peak areas calculated in eight IR bands showed a normal distribution in all cases, unlike the A/B ratio which showed a lognormal distribution. SWR data and FTIR A/B ratios showed r values \(\geq\) 0.7 when plotted on a logarithmic scale (Figure 3). This, however, pertained mainly to simple correlations. In multiple regression, using either A/B values or their logarithms resulted in very similar R\({}^{2}\) values. Apart from A/B ratio, respective peak areas may also be included in regression equations as individual partial predictors. Nevertheless, using the peak areas either as standalone variables or in the form of an A/B ratio had a similar effect on predicted WDPT or MED values, but the general trend in WR change may be visualized more easily when A/B ratio is employed (Figure 4). Selected regression equations, where FTIR peak areas are used as partial predictors of SWR (along with other soil properties), are presented in the Supplementary Material, together with scatter plots displaying observed versus predicted WR data (Tables S1 and S2 and Figures S1 and S2). Figure 4: Two grids showing general trend in MED value change as a result of soil moisture (w\({}_{\rm F}\)), FTIR A/B ratio, and soil pH variation. In each of the two graphs, a fixed pH value is used for the prediction (3.21 in the upper and 4.08 in the lower panel; these represent the lowest and the highest detected pH values among 45 topsoil samples). Used regression equation: MED = \(-\)2.93 Log w\({}_{\rm F}\) + 0.35 (A/B)-1.46 pH + 10.43 (R\({}^{2}\) = 0.71, F = 33.39, \(p\) = 2.51 10\({}^{-21}\), d.f = 41). Figure 3: Scatter plots showing general collinearity between SWR data and FTIR A/B ratio. The term A represents the sum of the peak areas integrated within the band Nos. 1, 5, and 7 (Table 1) and sum of the peak areas associated with 5 remaining bands (Nos. 2–4 and 6–8 in Table 1) are represented by B.