The impact of different tillage systems on the structural stability of the soil as quantified by high energy moisture characteristics has not been studied to date in the semi-arid region of northeastern Iran. Therefore, in this research, the effects of conventional, reduced, and no-tillage treatments under wheat-corn crop rotation on the organic carbon content, soil organic carbon stocks and microbial biomass carbon and also the aggregate stability of a clay loam soil (Aridisol, in USDA soil classification) were evaluated in northeastern Iran. Measurements were made at two depths (0-15 and 15-30 cm) three years after the establishment of experiment. Aggregate stability was determined using the high energy moisture characteristics, mean weight diameter and the percentage of water-stable aggregates. The high energy moisture characteristics stability indices (i.e. structural index, volume of drainable pores, and slope at the inflection point of high energy moisture characteristics) were higher for the slow (on average 0.053 hPa–1, 0.4 g g–1, and 0.022 hPa–1 for structural index, volume of drainable pores and slope at the inflection point, respectively) than the fast wetting rate (on average 0.014 hPa –1, 0.2 g g –1, and 0.012 hPa –1 for structural index, volume of drainable pores and slope at the inflection point, respectively). The modal suction was higher for the fast (18.61 hPa in average) than for the slow wetting rate (7.63 hPa in average). Organic carbon, soil organic carbon stocks and microbial biomass carbon were higher for no-tillage as compared with the reduced and conventional treatments, due to the use of a cover crop (clover) and because crop residues were retained in this system. The treatments contained greater organic carbon, soil organic carbon stocks and microbial biomass carbon (no-tillage at both depths and reduced at a depth of 0-15 cm) showed mainly high stability ratios and indices, regardless of the wetting rate. The values of modal suction were lower in the treatments which contained greater organic carbon, soil organic carbon stocks, and microbial biomass carbon, which results in a stable soil structure. The no-tillage system, which produced the maximum organic carbon and soil organic carbon stock among the tillage systems, showed the highest amounts of mean weight diameter (0.15 mm), and water-stable aggregates (92%). Our findings revealed that the high energy moisture characteristics method was highly sensitive to macroaggregate stability in water.

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