Pedotransfer functions for forest soils with low bulk density and high organic carbon content: Insights from the Eastern Alps.

Pedotransfer functions (PTFs) are widely used to estimate soil hydraulic properties from easily measurable soil attributes and are essential in hydrological modelling and soil evaluation. However, most established PTFs are derived from datasets dominated by agricultural soils or mineral forest soils with moderate to high bulk density (>0.9 g cm^-3). As a result, forest soils characterized by low bulk density and elevated organic carbon (Corg) contents are insufficiently represented, despite their abundance in mountainous and temperate forest regions. The hydraulic characteristics of such soils, intermediate between mineral and organic soils, differ markedly and therefore the application of existing PTFs fails to adequately represent the hydraulic complexity of these soils.

In the present study, we developed and evaluated new PTFs specifically tailored to forest soils with low bulk density (0.1-0.9 g cm^-3) also including organic horizons and litter layers (Corg >20%). The analysis is based on an extensive dataset of undisturbed soil samples collected from forest sites across the Eastern Alps from different depth intervals. Soil water retention curves were determined over a wide suction range using the evaporation method (HYPROP) combined with dew-point-method (WP4C). The dataset was further extended with a comparable, recently published dataset for soil hydraulic properties of forest soils from a global compilation to enable independent validation.

In a first step we investigated the dependence of derived Mualem van Genuchten parameters of the selected soil horizons on soil texture, bulk density, and Corg across different bulk-density classes. Results show a clear shift in controlling factors below a bulk density threshold of 0.9 g cm⁻³. While soil texture remains the dominant predictor in higher bulk-density classes, its influence diminishes at lower bulk densities. In contrast, bulk density and Corg content increasingly control the shape of the water retention curve, indicating that structural and organic-matter-related effects outweigh textural controls in these soils.

Based on these findings, we derived PTFs for forest soils grouped in bulk density classes. Validation against an independent dataset demonstrates that the proposed PTFs significantly reduce root mean squared errors compared to established PTFs developed for mineral soils, forest soils with higher bulk density, or pure organic horizons. Improvements are particularly pronounced in the wet and intermediate suction ranges that are most relevant for plant available water.

Our results highlight the need for specialized PTFs for forest soils with low bulk density and high Corg content. The new PTFs contribute to reducing the knowledge gap in soil hydraulic properties within the complex transition space between mineral soils and purely organic soils (Corg > 20%) and support improved representation in hydrological and ecosystem models.