Introduction

Conservation agriculture is widely promoted as a strategy for improving soil structure and water-use efficiency in degraded cropping systems. In northern Iran, intensive conventional tillage combined with crop-residue removal has contributed to structural degradation and reduced soil water storage. Assessing how no-tillage and reduced-tillage systems affect the soil water retention curve (SWRC), pore-size distribution (PSD), and soil physical quality can support sustainable soil management. Therefore, this study evaluated the effects of conventional tillage, reduced tillage, and no-tillage on soil water retention, pore-size distribution, functional pore classes, and physical quality in agricultural soils of Bandar-e Gaz County, Golestan Province, Iran.

Materials and Methods

The study was conducted in agricultural lands of Bandar-e Gaz County in northern Iran. Three management systems representing different levels of soil disturbance were evaluated: conventional tillage, reduced tillage (semi-conservation), and no-tillage (conservation agriculture). Soil was sampled at depths of 0–15, 15–30, and 30–45 cm.

The experiment was arranged as a split-plot based on a randomized complete block design with three replications. Tillage management systems were assigned to the main plots and soil depths to the subplots. Disturbed and undisturbed soil samples were collected after crop harvest and analyzed for pH, electrical conductivity (EC), organic matter (OM), bulk density, total porosity, and mean weight diameter of soil aggregates (MWD) using standard procedures.

Soil water retention was determined at absolute matric potentials of 0, 300, 500, 1000, 2000, and 15000 cm of water column. FC and PWP were obtained directly from the measured water contents at 300 and 15000 cm, respectively, and AW was calculated as their difference. The measured retention data were fitted with the van Genuchten model using RETC software. Pore-size distribution was derived from the fitted curves, and pores were classified by size and function. Functional classes included transmission pores (> 50 μm), storage pores (0.5–50 μm), and residual pores (< 0.5 μm). The soil physical quality index (S-index) was calculated from the slope of the fitted retention curve at its inflection point. Statistical analyses were conducted using SAS. Effects were evaluated at the 1% and 5% probability levels, and means were compared using the LSD test at the 5% level.

Results and Discussion

Tillage management significantly affected organic matter content, EC, MWD, FC, AW, pore-size distribution, and functional pore classes. In contrast, soil pH, bulk density, and total porosity were not significantly affected, indicating that changes in pore organization and function were more important than changes in total pore volume.

Reduced tillage produced the highest organic matter content in the upper soil layers and increased MWD relative to conventional tillage. These results suggest that reduced soil disturbance combined with residue retention promoted aggregate formation and structural stability.

The SWRCs clearly differentiated the management systems. At 0–15 and 15–30 cm, reduced tillage generally retained more water across a wide range of matric suctions, whereas conventional tillage showed the lowest retention capacity. At 30–45 cm, no-tillage exhibited the highest water retention. These shifts reflected changes in soil structure and in the proportion of water-retaining pores.

FC and AW followed similar patterns. Their highest values in the upper layers occurred under reduced tillage, reaching 0.1776 and 0.1390 cm³ cm⁻³, respectively, whereas conventional tillage generally produced the lowest values. At 30–45 cm, no-tillage had the greatest FC and AW, indicating improved water storage associated with reduced disturbance and greater pore continuity. PWP was not significantly affected by tillage management, soil depth, or their interaction.

Pore-size distribution differed significantly among management × depth combinations. Conservation-oriented systems increased the proportion of pores within the water-retention domain, particularly medium-sized pores. At 0–15 cm, reduced tillage had the greatest proportion of storage pores, approximately 69%, whereas conventional tillage had the greatest proportion of transmission pores, approximately 57%. The larger storage-pore fraction under reduced tillage was consistent with its higher FC and AW. Overall, the agreement among water-retention characteristics, pore-size distribution, and functional pore classes indicates that management-induced improvements in hydraulic behavior were mainly governed by changes in pore organization and functionality rather than by total porosity.



A close agreement was observed among soil water retention characteristics, pore-size distribution, and functional pore classes. This consistency suggests that management-induced improvements in soil hydraulic properties were primarily governed by modifications in pore organization and functionality rather than changes in total porosity. The results therefore highlight the importance of evaluating pore quality in addition to pore quantity when assessing soil physical condition.

The S-index ranged from approximately 0.012 to 0.027 and remained below the critical threshold of 0.035 proposed by Dexter (2004) for desirable soil physical quality. The effects of tillage management, soil depth, and their interaction on the S-index were not statistically significant. Thus, the observed numerical trends should not be interpreted as evidence of the superiority of any management system. The persistence of low S-index values suggests that four years of conservation management was insufficient to produce a statistically detectable improvement in this integrated physical-quality indicator, even though several water-retention and pore-function characteristics responded significantly.

Conclusion

Conservation-oriented management improved soil hydraulic behavior primarily by modifying pore organization, pore-size distribution, and water-retention capacity. Reduced tillage provided the most favorable conditions in the surface and subsurface layers by increasing OM, MWD, storage-pore proportion, FC, and AW. At the deepest layer, no-tillage showed greater water retention, FC, and AW, indicating that some benefits of conservation management may become more evident with depth and time.

The results further showed that soil-quality responses were more strongly associated with pore functionality than with total porosity or bulk density. Although several hydraulic properties improved, S-index values remained below the optimum threshold and did not differ significantly among treatments. Longer implementation of conservation agriculture, together with residue retention and strategies that increase organic matter, may therefore be required to achieve measurable improvements in overall soil physical quality and long-term agricultural sustainability.