## 4 Discussion  
Since soils and the responsible soil forming factors are unique to the spatial domain under consideration, objective comparisons with studies conducted elsewhere are likely to fail. These difficulties are enhanced by the fact that the available environmental covariates also strongly depend on administrative units. This is especially the case for soil and geological maps. However, despite these limitations, the underlying principles of SOC storage in soils are, of course, transferable to other regions, and thus, some similarities can be observed. A strong influence of terrain attributes associated with catchment area on SOC was already found by the pioneers in the field of soil attribute prediction through digital terrain analysis [18]. Similar findings have also often been reported in more recent studies. The _TWI_ (topographic wetness index) was one of the best predictors for mapping soil organic matter in Croatia [38]. A high regression coefficient of _TWI_ was also found for mapping mineral soil SOC within the pre-alpine lowlands [91]. Concerning terrain parameters, the most similar results to those of this study were reported by [PERSON] et al. [20], who found \(A_{C}\), \(RSP(...)\), \(LS\) (LS factor from the Revised Universal Soil Loss Equation), and _TWI_ as variables with the highest importances for mapping SOC with the topsoil layer on Barro Colorado Island. Slightly different rankings of variable importance were given by [PERSON] [16]: For the prediction of SOC stored in A horizons within the Italian Alps, elevation, \(\hat{S}\), and \(\Psi_{S}\) (sky-view factor) had the highest relative influence, whereas _TWI_ and \(LS\) were ranked third and fifth. The findings of [PERSON] [101], that for the prediction of topsoil SOC stocks in Japanese cedar plantations with multiple linear regression analysis only \(\beta\) and \(\Psi_{S}\) were selected by stepwise variable selection, while _TWI_ and _north_ (northness derived from local aspect) had no significant influence, seem even more contradictory to the variable importances observed here. Beyond that, there are also landscapes where topography appears to be behind other soil forming factors in general. In addition to studies that focused on soil properties and land-use effects [45], in some cases, the influence of terrain on mineral soil SOC was low, even where it was explicitly tested [19; 21; 102]. Increased influences of landcover classes and tree species compositions were generally observed when SOC within topsoil or forest floor layers was predicted [21; 91; 102]. This is in good agreement with the superior importance of \(p_{\text{deciduous}}\) in the CART model for the prediction of relative depth gradients. In studies where data on the depth of the groundwater table were available, it was naturally one of the main factors that controlled SOC at the respective sites [45]. The fairly high influence of \(z_{\text{CaCO}_{3}}\) in our regression models must be interpreted in relation to the often-observed influences of mapping units [19; 21; 102], since this covariate seems to be a property unique to local forest site mapping. The achieved model performance is in a range comparable to that of other studies dealing with the prediction of SOC for numerous depth layers [32; 36; 37]. With low values \begin{table}
\begin{tabular}{l c c c c c c c|c} \hline \hline  & & \multicolumn{4}{c|}{**Carbon Storage in NFSI Depth Increments (10\({}^{9}\) kg)**} & \multicolumn{2}{c}{**Sums (10\({}^{9}\) kg)**} \\  & & **Floor** & **0–5 cm** & **5–10 cm** & **10–30 cm** & **30–60 cm** & **60–90 cm** & **0–90 cm** & **Solum** \\ \hline Hydro. & (1302 km\({}^{2}\)) & 6.073 & 3.576 & 2.087 & 4.084 & 2.321 & 1.075 & 13.143 & 19.216 \\ Terr. & (9139 km\({}^{2}\)) & 26.373 & 13.921 & 7.971 & 15.197 & 8.668 & 4.078 & 49.834 & 76.207 \\ \hline Total & (10,441 km\({}^{2}\)) & 32.447 & 17.497 & 10.058 & 19.281 & 10.988 & 5.153 & 62.977 & 95.424 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Carbon storage in forest soils of Brandenburg at terrestrial sites (Terr.)