Interactive effects of landscape position and soil diversity drive the spatial variability of soil organic carbon concentration in subalpine soils of Switzerland

Bence Dienes , Orly Mendoza, Kristina Bright, Gino Licini, Meret Aeppli 

Subalpine soils store a significant amount of soil organic carbon (SOC), yet the factors driving its landscape-scale variability remain poorly constrained. Although topography, soil type, soil texture, and moisture are recognised as key drivers of SOC concentration, their interactive effects in subalpine environments remain largely unexplored. In particular, the extent to which soil type shapes landscape-scale SOC patterns remains unclear.
In this study, we assessed SOC concentration, soil type, soil moisture conditions and texture, elemental composition across 100 plots in two subalpine catchments in the Swiss Alps. Using factor analysis, we generated three standardised continuous predictors from our dataset, namely, mineral content, sedimentary rock influence and texture index. We investigated the joint influence of these continuous predictors on SOC variability at the landscape scale in a linear mixed-effects model with topography (plain vs. slope areas), soil depth and soil type as additional categorical predictors.
We found that soil type diversity was greater on the plain than on the slope, reflecting a greater hydrological heterogeneity. Plains supported a broad spectrum of soils, ranging from organic-rich Histosols to mineral-rich Fluvisols, while slopes were dominated by Cambisols. Although plains exhibited higher moisture and SOC concentration, and slopes were characterised by higher clay fractions and lithogenous elements, topographic position alone was insufficient to explain the observed spatial distribution of SOC.
Our linear mixed-effect model identified mineral content as the strongest predictor of SOC concentration, followed by sedimentary rock influence and the texture index. SOC concentration decreased as mineral content and sedimentary rock influence increased, and as the texture index shifted towards a higher proportion of clay-sized particles. Crucially, while mineral content integrates much of the variability described by the categorical variables included in the model, soil type diversity effectively captures the range of mineral content that drives SOC concentration across our sampling locations and soil depth. Our findings suggest that relying strictly on topographic positions overlooks important nuances in soil property distribution and SOC concentration. We therefore suggest that soil type provides a more integrative predictor of landscape-scale SOC variability in subalpine environments than topographic position alone.