Topographical influences on soil properties in boreal forests

Topography is a major factor controlling both hydrological and soil processes at the landscape scale. This is well-known qualitatively in that topography, along with parent material, climate, biota, and time, is one of the fundamental soil forming factors. The topographic influence is also apparent in the soil catena concept. Digital elevation models (DEMs) and topographic indices calculated from these DEMs allow the relationship between topography and soil characteristics to be measured quantitatively as well. In this study we use data from the Swedish National Forest Soil Inventory, which is a long-term inventory of permanent sample plots from the Swedish National Forest Inventory. It includes a description of soil types and soil horizons as well as sampling of organic and mineral soil horizons for subsequent chemical analyses. We focused on Podzols and Histosols, which provided 4000 sample plots distributed over almost all of Sweden. Plot locations were determined accurately by GPS, which allowed the overlaying of plot data and the DEM. Topographic indices such as the topographic wetness index, TWI (ln(a/tanβ)), were computed from gridded digital elevation data for all sample plots. We found several significant correlations between topographic indices and soil properties. The thickness of the organic layer increased with TWI and the thickness of the leached E-horizon increased with upslope area. Soil pH in the organic layer increased with TWI, while the C–N ratio decreased. Soil pH in the organic layer was also found to be higher for south facing slopes than for north facing slopes. The ratio between the divalent base cation (Ca and Mg) and the monovalent base cation (K and Na) concentrations in the O-horizon increased with TWI. These correlations confirmed the importance of topography on soil properties, although there was considerable scatter, which could be attributed to heterogeneity in the large data set