## 3 Results  
### Mapping Total Solum SOC  
The best covariates for predicting solum SOC, which were determined by stepwise regression, are given in Table 3. The signs of \(RSP(s)\) (relative slope position) and \(s_{\text{DN}}(s)\) (horizontal distance to drainage network) show that SOC increases at low relative slope positions and at toe-slope positions in close proximity to the drainage line. In terms of soil forming processes, this indicates lateral inputs of organic matter due to flooding and depositions by overland flow from higher slope positions. Similar explanations can be given for the increase of SOC with the steepness of catchment slopes \(\beta_{\text{C}}\) and the decrease with the stream power index \(SPI\). Erosion within the catchments causes a corresponding deposition at the sites, while at high erosive potential (\(SPI\)), the SOC is lowered by topsoil losses or loss of surface inputs (litter). In addition to these covariates related to erosion by overland flow, the terrain parameter \(east\times\beta\) (product of local easness and slope) can be considered as an indicator for the redistribution of organic matter by the prevailing westward winds. An indicator for the vertical translocation of SOC within the soil profile can be seen in the winter precipitation \(P_{\text{win}}\). Especially during winter season, when the soil is wet, rainfall strengthens podzolization (e.g., ([PERSON] et al., 2018; [PERSON] et al., 2018)), and thus enhances SOC sequestration in deeper soil layers. Covariates focusing on in situ soil development are \(z_{\text{CaCO}_{3}}\) (depth of calcareous horizon), \(CF_{0..90}\) (proportions of coarse fragments) and \(\hat{S}\) (direct solar beam irradiance). At first glance, the positive correlation coefficient of \(z_{\text{CaCO}_{3}}\) appears contradictory to the in situeffects of calcium. Actually, calcium bridges organic matter and clay minerals and stabilises aggregates [86]. However, in acidic soils without near-surface or even any calcareous layers within the solum (e.g. \(z_{\text{CaCO_{3}}}>3.0\) m), soil respiration is often hindered due to the low activity of soil microbial communities under acid conditions. Of course, acid conditions induced by high \(z_{\text{CaCO_{3}}}\) may also enhance podzolization processes. On the contrary, soil respiration usually increases with increasing soil temperatures [87,88] under intense direct insolation \(\hat{S}\). The reduction of SOC stocks at high rock contents \(CF_{0...90}\) indicates that the corresponding loss of soil volume is not balanced by higher concentrations of SOC in the remaining fine-earth fraction (see [89]). The land-use effects on SOC are captured by the proportions of deciduous trees \(P_{\text{deciduous}}\). The negative regression coefficient implies the well-known risk of litter loss by wind blowing for leaf litter. On the other hand, this might also signal that SOC stored in the generally thicker forest floor layers at coniferous sites is not transferred to mineral soil at deciduous sites to the same extent. The raised SOC storage at juvenile glacial deposits (seriesI) appears as a result of the higher ecosystem productivity in these nutrient-rich soils. The increase of SOC stocks in the presence of high groundwater tables (\(z_{\text{GW}}\leq 300\) cm) is in line with expectations. Carbon mineralisation is more and more inhibited due to the development of anaerobic conditions with rising groundwater levels at these sites. To describe the nonlinear response to groundwater levels \(z_{\text{GW}}\) properly, the best fit was found for the logarithmic transformation. Larger catchments also result in higher SOC stocks. In addition to the mean depth of groundwater, a large catchment area \(A_{\text{C}}\) indicates stable, less-fluctuating groundwater levels, thus causing higher ecosystem productivity at low water tables and reduced mineralisation (especially at high water tables). The effect of \(z_{\text{CaCO_{3}}}\) is similar to that of the terrestrial sites. Acidic conditions inhibit soil microbial activity.