Document Type : Research Article
Authors
Soil and Water Research Department, Khorasan-Razavi Agricultural and Natural Resources Research and Education Center, Agricultural Research, Education and Extension Organization (AREEO), Mashhad, Iran
Abstract
Introduction
Soil is a fundamental natural resource that underpins ecosystem functioning, agricultural productivity, water regulation, and biogeochemical cycling. Vertical heterogeneity of soil properties along the profile reflects the combined influence of biological inputs, pedogenic processes, and land management. Soil depth strongly controls the distribution of organic matter, nutrients, soluble salts, and textural components, thereby affecting root development, nutrient uptake, and crop resilience under environmental stresses such as drought.
Most previous studies relied primarily on classical statistical tests and comparisons of mean values between layers. While useful for detecting statistically significant differences, these approaches provide limited information on the magnitude, direction, and functional relevance of depth-related changes. In highly variable and non-normally distributed soil systems, reliance on p-values alone can lead to overinterpretation of minor differences or underestimation of ecologically important gradients.
Recent advances emphasize effect size metrics and uncertainty estimation to quantify soil variability more robustly. Non-parametric indices, such as the rank-biserial effect size, allow depth-driven changes to be quantified independent of sample size and distributional assumptions. This study focuses on providing a comprehensive depth-oriented assessment of soil physicochemical properties using effect size–based approaches and classification into depth stability and management-relevant categories.
Materials and Methods
Soil samples were collected from agricultural fields of Razavi Khorasan Province of Iran at two depths: 0–30 cm (surface) and 30–60 cm (subsurface), using a paired design to minimize spatial heterogeneity. A wide range of physicochemical properties was analyzed, including organic carbon, available P and K, exchangeable cations, EC, pH, carbonate parameters, and selected micronutrients.
Paired non-parametric tests were applied for each variable, and the rank-biserial effect size (r_rb) was calculated to quantify the magnitude and direction of depth-related changes. Bootstrap resampling was used to estimate 95% confidence intervals. Based on effect size magnitude and confidence intervals, variables were classified into stable, quasi-stable, and depth-sensitive categories, which were further translated into practical management-oriented classes.
Results and Discussion
Depth-sensitive properties included organic carbon, available phosphorus, potassium, and manganese, all exhibiting large effect sizes and pronounced accumulation in the surface layer. The enrichment of organic carbon reflects concentrated biological inputs, crop residue return, and intensified microbial activity in the topsoil, whereas its sharp decline with depth indicates restricted vertical transfer and limited subsoil biological functioning. Phosphorus accumulation in the surface layer is consistent with its low mobility and strong fixation in calcareous soils, underscoring the importance of depth-aware phosphorus management strategies to enhance root access in deeper horizons. The marked differences in potassium likely result from the combined effects of crop uptake, mineral weathering, and partial leaching, highlighting the necessity for root-zone–oriented monitoring and fertilization practices. Among micronutrients, manganese showed the greatest depth sensitivity, reflecting its responsiveness to redox dynamics and pH variations, and indicating potential risks of localized deficiency or toxicity within the soil profile.
Copper and bicarbonate demonstrated moderate effect sizes, suggesting measurable but less pronounced depth-related variation. In contrast, clay, silt, pH, TNV, Zn, and Ca exhibited small effect sizes, indicating relative structural stability across depths. Sodium and chloride remained largely unchanged, implying that salinity distribution is relatively uniform within the studied profile and not strongly stratified under current conditions.
The classification of variables into depth-stability and management-priority categories provides a practical framework for translating statistical outputs into agronomic decision-making. Overall, the findings reveal that reliance on surface soil analysis alone can lead to incomplete or potentially misleading interpretations of nutrient status. Depth-aware management, including precision placement technologies and fertilizer banding systems, can improve nutrient use efficiency, enhance subsoil fertility, and reduce environmental losses, thereby contributing to more resilient soil–plant systems under semi-arid conditions.
Conclusion
By integrating paired depth-wise comparisons, PERMANOVA, and nonparametric effect size metrics, this study provides a robust framework for assessing vertical soil heterogeneity beyond conventional p-value–based approaches. The results showed pronounced and practically meaningful depth-related differences for key chemical properties, particularly soil organic carbon, available phosphorus, potassium, and manganese, while physical properties and basic indicators such as pH, clay, silt, and calcium carbonate exhibited limited variation. These patterns reflect the concentration of biological activity and management inputs in surface layers and the relative chemical depletion of deeper horizons, which constrains root development, nutrient uptake, and drought resilience in agricultural systems of Razavi Khorasan Province. Overall, the findings highlight that soil fertility assessments based solely on surface sampling can be misleading and underscore the need for targeted, depth-oriented soil management to improve nutrient use efficiency and enhance the sustainability of soil–plant systems, particularly under water-limited conditions.
Acknowledgements
The authors would like to express their sincere appreciation to the Soil and Water Research Department of the Khorasan Razavi Agricultural and Natural Resources Research and Education Center for their support in conducting this study. The authors also gratefully acknowledge the valuable cooperation and technical assistance provided by the laboratory staff.
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