Introduction

A wide range of factors influence the movement of salts in soils. Among these, cropping systems are key physical determinants governing salt transport processes. Soil structure plays a pivotal role in controlling water flow and solute transport by affecting pore-size distribution, pore tortuosity and connectivity, as well as the soil hydraulic conductivity. Tillage management is another critical factor that significantly affects the leaching of chemicals and contaminants in agricultural systems. Different types of vegetation, particularly cover crops, influence solute transport through multiple mechanisms, including increasing soil organic matter, enhancing nutrient availability for subsequent crops, improving soil physical properties, reducing soil erosion, and suppressing weed growth. The present study was conducted to evaluate the effects of different tillage practices, cover crops, and cropping patterns on chloride transport, which was employed as a conservative tracer to represent the movement of salts and contaminants in soil.

Materials and Methods

The experiment was conducted at the research farm of Bu-Ali Sina University, Hamedan, Iran. A factorial split-plot arrangement within a randomized complete block design (RCBD) with three replications was employed. Tillage system and cover cropping were assigned to main plots, while cropping pattern was allocated to subplots. Tillage was evaluated at two levels: conventional tillage and minimum tillage. Cover cropping was also considered at two levels: presence of grass pea (Lathyrus sativus L.) and absence of cover crop. Cropping pattern consisted of three treatments: (i) summer squash (Cucurbita pepo), (ii) sole green bean (Phaseolus vulgaris L.), and (iii) additive intercropping with a 50% increase in green bean density combined with summer squash (Cucurbita pepo).

Prior to the initiation of the experiment, the field had been managed under a rotation of grass pea (as a cover crop) and maize for three consecutive years; the present study was implemented in the fourth year. The cover crop was sown in March. Before reaching full maturity, it was either retained on the soil surface or incorporated into the soil using moldboard plowing, depending on the tillage treatment. The main crops (summer squash and green bean) and their intercropping treatment were established simultaneously in June. Both species are characterized by taproot systems. At the end of the growing season, undisturbed soil samples were collected from the 0–25 cm depth. Sampling was performed using 36 cylindrical columns made of galvanized iron, with an inner diameter of 16 cm and a height of 32 cm. Care was taken to preserve the soil structure during sampling. A potassium chloride (KCl) solution with a constant concentration of 0.01 M was applied to the suface of the soil columns under the steady-state saturated flow conditions. The imposed flow rate corresponded to the saturated hydraulic conductivity of each treatment. Leaching was continued until approximately four pore volumes had passed through the columns. Effluent samples were collected at intervals of 0.2 pore volumes, and chloride concentration was determined for each fraction. Breakthrough curves (BTCs) were constructed by plotting the relative concentration (C/C₀) against the number of pore volumes for chloride. These curves were used to characterize solute transport behavior under different treatments. All data were analyzed using SAS software (version 9.1). Mean comparisons were performed using Duncan’s multiple range test at the 5% significance level.

Results and Discussion

Duncan’s multiple range test revealed that tillage system, cover cropping, and cropping pattern exerted significant effects on the saturated hydraulic conductivity (Kₛ) of undisturbed soil columns. The minimum tillage–cover crop (Lathyrus)–50% intercropping treatment exhibited the highest Kₛ among all treatments. This increase was statistically significant compared with both the minimum tillage–no cover crop– summer squash monoculture and the conventional tillage–no cover crop– summer squash monoculture treatments. The enhancement in saturated hydraulic conductivity can be primarily attributed to improvements in soil structural attributes, including aggregate stability and pore continuity. Moreover, tillage practice, cover cropping, and cropping pattern significantly influenced chloride transport behavior in the soil profile. Under saturated flow conditions, breakthrough curve analysis indicated the earliest chloride breakthrough for the minimum tillage–cover crop–50% intercropping treatment, followed by the conventional tillage–cover crop–50% intercropping treatment. In subsequent ranks, the minimum tillage–no cover crop–50% intercropping and conventional tillage–no cover crop–50% intercropping treatments also demonstrated relatively early breakthrough. This pattern can be ascribed to the beneficial role of 50% intercropping in enhancing soil organic matter content, which in turn promotes aggregate stabilization and the development of a more continuous and functionally effective pore network, thereby facilitating advective solute transport under saturated conditions.

Conclusion

The findings of this study demonstrated that tillage practices, cover cropping, and cropping patterns exert significant influences on chloride transport under saturated soil conditions. Among the evaluated treatments, the combination of minimum tillage with grass pea (Lathyrus sativus L.) as a cover crop and an intercropping system was identified as the most effective management strategy. This treatment enhanced the connectivity of macropores, thereby facilitating preferential flow pathways and resulting in a greater mass of chloride leached compared to other treatments. The results obtained after four years of implementing the experimental treatments further indicated that integrated management involving conservation tillage and grass pea cover cropping provided the most favorable conditions for improving soil structural quality. Based on these findings, the long-term adoption of integrated conservation practices, particularly the combined use of cover crops and reduced tillage systems, is strongly recommended for sustainable soil management and improved soil structural stability in agricultural systems.