Quantitative Relationships between Soil Organic Matter and Physicochemical Properties in a Semi-Arid Region

Background and Objectives: Soil organic matter is a critical component of productive soils. It influences a wide range of physical, chemical, and biological attributes and processes, including the formation and stabilization of soil aggregates, nutrient cycling, water retention, disease suppression, and cation exchange capacity. The relationships between soil organic matter (SOM) and soil physicochemical properties are profound and multifaceted. One of the main research gaps is the lack of comprehensive, integrated, and quantitative studies that simultaneously investigate the relationships between a wide range of soil physical and chemical properties and SOM under semi-arid climatic conditions. Most previous studies have focused on one or two properties and do not provide a holistic understanding of this complex system. Therefore, the primary objective of this study was to examine the relationships between SOM and selected soil physical and chemical properties, including particle size distribution, structural attributes, salinity, sodium content, and fertility. 
Materials and Methods: This study was conducted in the semi-arid region of Zanjan. Sixty-eight sampling points were selected from different land uses using available soil maps to examine the relationships between SOM and various physical and chemical soil properties. Standard physical and chemical laboratory methods were used to measure particle size distribution, aggregate stability, bulk density, electrical conductivity, pH, organic matter content, and cation exchange capacity. Descriptive statistics, Pearson correlation, linear regression, analysis of variance (ANOVA), and discriminant analysis were employed to analyze the relationships between SOM and soil properties, and variables showing significant correlations with SOM were further analyzed using ANOVA and discriminant analysis to investigate differences among SOM classes (<1%, 1–2%, 2–3%, and >3%). All statistical analyses, data visualizations, and mean comparison procedures were carried out using Microsoft Excel (2016) and Python (version 3.x).
Results: The results showed that cation exchange capacity (R² = 0.31, p < 0.01), bulk density (R² = 0.28, p < 0.01), wet aggregate mean weight diameter (MWDwet) (R² = 0.25, p < 0.01), wet aggregate geometric mean diameter (GMDwet) (R² = 0.24, p < 0.01), and dry aggregate geometric mean diameter (GMDdry) (R² = 0.22, p < 0.01) exhibited stronger relationships with SOM than the other soil properties examined. One-way ANOVA performed for four SOM classes (<1%, 1–2%, 2–3%, and >3%) revealed substantial differences among the groups in terms of soil structural properties and cation exchange capacity. Specifically, significant differences in GMDwet were observed between the <1% SOM group and the 2–3% and >3% SOM groups, as well as between the 1–2% and >3% SOM groups. For MWDwet, significant differences were detected between the <1% SOM group and the 2–3% and >3% SOM groups, and between the 1–2% SOM group and the 2–3% and >3% SOM groups (p < 0.000001, F = 12.23, and p < 0.000001, F = 12.17, respectively). Furthermore, discriminant analysis indicated that soil physical properties were more strongly associated with SOM than soil chemical properties. 
Conclusion: This study demonstrated that SOM is one of the key factors influencing soil quality, exerting significant effects on both soil physical and chemical properties. The results of correlation, linear regression, and one-way analyses of variance indicated that CEC increased with increasing SOM content. Among all measured soil properties, CEC exhibited the strongest relationship with SOM and showed a significant increase across higher SOM levels. Strong relationships were also observed between SOM and soil physical properties. In particular, increasing SOM was associated with significant improvements in soil aggregate stability, total porosity, and soil moisture content, while bulk density decreased significantly. Furthermore, discriminant analysis revealed that the influence of SOM was more strongly reflected in soil physical properties than in chemical properties. In both stages of the discriminant analysis, structural soil attributes exhibited the highest discriminant coefficients among the SOM groups (<1%, 1–2%, 2–3%, and >3%), highlighting the close association between SOM and soil structural characteristics. Following structural attributes, cation exchange capacity ranked as the second most important discriminating variable.