Erosion as a selective filter of soil organic matter quality: quantifying degradation impacts on chernozem quality using the Soil Quality Index (SQI) and Structural Equation Modelling (SEM)

This study investigates whether water erosion in Chernozems acts as a selective filter of soil organic matter, preferentially affecting humus quality rather than only its quantity. Specifically, we test the hypothesis that erosion reduces the stability of humic substances (Humic acid / fulvic acid ratio) and that this qualitative shift is the primary driver of soil quality decline as expressed by the Soil Quality Index (SQI). Soil samples ( n = 60) were collected along erosion transects in South Moravia (Czech Republic), covering eroded and depositional slope positions. A comprehensive set of physical, chemical, and biochemical properties was analysed, including soil organic carbon (SOC) content, total nitrogen, enzyme activities, and humic substances (HA, FA). A Soil Quality Index (SQI) was constructed using linear scoring functions and expert-based weighting of six indicators. Erosion intensity was quantified using the Universal Soil Loss Equation (USLE) model. Differences between slope positions were evaluated using Linear Mixed Models (LMM), while causal relationships among erosion, soil properties, and SQI were analysed using Piecewise Structural Equation Modelling (SEM). Erosion significantly reduced the HA/FA ratio ( β = −0.46, p < 0.001), confirming its role as a selective filter of humic substances quality. The HA/FA ratio emerged as the dominant predictor of SQI ( β = 0.77, p < 0.001), explaining most of its variability ( R² = 0.99). In contrast, soil organic carbon (SOC) had a weaker direct effect (β = 0.33). Chemical indicators such as Ca²⁺ and pH responded strongly to erosion through carbonate exposure but showed no direct influence on SQI. Structural modelling revealed that 95% of the negative impact of erosion on soil quality is mediated indirectly through qualitative changes in humic substances, highlighting the importance of internal soil transformations over simple material loss. Water erosion in Chernozems primarily drives soil degradation through qualitative changes in humic substances composition rather than quantitative SOC loss. The HA/FA ratio is a key mediator of soil quality decline and should be integrated into soil monitoring frameworks. Standard assessments based solely on SOC may substantially underestimate degradation. Incorporating soil organic matter quality indicators into agro-environmental policy is essential for accurately evaluating and managing soil degradation in intensively cultivated regions.