Visual Evaluation of Soil Structure (VESS) in temperate forest soils: pH mediates the effects of texture and organic carbon on soil structure and hydraulic functioning

Abstract. Soil structure is an important determinant of ecosystem functioning in forest soils, because it influences water flow, aeration, root growth, and biological activity. The Visual Evaluation of Soil Structure (VESS) is a rapid field method to assess soil structural quality in agricultural systems, but its applicability to temperate forest soils remains largely unknown. We evaluated the applicability and interpretation of VESS across 27 temperate forest sites in Switzerland that span a wide range of soil textures, pH values, and soil organic carbon (SOC) contents. At each site, VESS assessments were combined with measurements of saturated hydraulic conductivity (K_sat), bulk density, soil texture, SOC content, pH and the SOC:clay ratio. A single property alone was not sufficient to explain the variation in VESS scores. However, strong relationships emerged when soils were grouped into three aggregation regimes defined by pH, clay and sand content (R² = 0.65). In acidic coarse-textured soils, VESS scores improved with increasing pH and silt content, controlled by aggregate stabilization by Fe/Al-oxyhydroxides. In acidic soils with more clay (19–43 %), better VESS scores were associated with increasing SOC content but decreasing SOC:clay ratios, indicating the limiting effect of clay surfaces saturated with organic matter. In contrast, in near-neutral soils VESS scores improved as SOC:clay ratios increased. This trend is in agreement with findings from non-forested lands where the pH is typically in the neutral range as well. The relationship between VESS and K_sat was likewise dependent on aggregation regimes defined by pH and texture. Since saturated conductivity is very sensitive to the formation of large pore networks and not only aggregate formation, cluster-specific analyses identified four hydraulic regimes. In three of the four identified clusters, better VESS scores were associated with higher K_sat. Only under strongly acidic conditions did this relationship reverse. This indicates a decoupling between aggregate-scale structural quality and the preferential flow pathways governing saturated water flow. The classification of four regimes allowed to express K_sat as function of VESS score and one basic soil property with coefficient of determination R² = 0.89. We conclude that VESS is a useful tool for assessing soil structural quality in temperate forest soils across a broad range of textural conditions. However, its interpretation depends strongly on the aggregation regimes that are controlled by pH, governing both the visual expression of soil structure and its relationship to hydraulic functioning.