Conservation tillage by increasing soil organic carbon can improve soil structure, especially in arid and semi-arid regions. In this study, the effects of three tillage systems (conventional (CT), reduced (RT) and no-till (NT)) in a wheat-maize crop rotation on soil organic carbon content (SOC) and some physical properties of a clay loam soil were investigated. The field experiment was set up in a randomized complete block design with three replications in the third year in northeastern Iran. A total of disturbed and undisturbed soil samples were collected from 0-15 and 15-30 cm depths and analyzed to quantify SOC content, pore size distribution, hydraulic properties and aggregate stability. SOC was higher in NT than in the RT and CT treatments, due to the use of cover crops (clover) and the retention of crop residues in this system. The highest water-stable and macro-aggregate content were also obtained in the NT system. The highest water contents (for most of the matric suctions) were found in the treatments CT and RT, which also had a high total soil porosity. It was found that tillage in CT and RT loosened the soil and thus increased the total porosity. The highest air-entry value was found in the CT treatment at 15-30 cm depth, which contained the most micro-aggregates. However, the maximum n-value (shape parameter of the water retention curve) was obtained in the treatment with the largest pores, i.e. NT at a depth of 15-30 cm. X-ray images of the pore space and pore function data (dmean, dmode, and dmedian) showed that the largest soil pores were found in the NT treatment, probably due to the high SOC in the NT system. The results of this study showed that the effects of tillage systems on soil physical properties are strongly dependent on SOC. Based on the results of this study, the use of a no-till system with crop rotation that includes cover crops and crop residue retention is recommended to increase soil organic matter and thereby improve soil structural stability in arid and semi-arid regions.

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