The impact of land use changes on soil properties has been extensively studied. However, there still appears to be a significant gap in research in this field. Addressing a key gap in previous research, this study uniquely analyzes the impacts of both grassland-to-cropland conversion (barley and wheat) and cropland reversion to grassland over different timeframes (15 and 30 years) and at different soil depths (0-10, 10-20, and 20-30 cm) in a semiarid area of northern Iran. A total of 324 soil samples (9 habitats & times; 3 different depths & times; 12 replicates) were collected and sent to the laboratory. The results indicated that primary grasslands and 30-year-old grasslands with a history of cultivation had the highest mean weight soil particles diameter and percentage of microaggregates. Furthermore, these habitats showed the greatest accumulation of carbon and nitrogen in macroaggregates and micro-aggregates, with the lowest values in 30-year-old grasslands. The land-use change from grassland to cropland has caused structural changes and a long-term decline in soil particles, carbon and nitrogen levels, particulate and dissolved organic matter, mineral forms of nitrogen (ammonium and nitrate), cold and hot water-soluble organic carbon, free and occluded light fractions indices, and soil organic acids (fulvic and humic). With the degradation of primary vegetation in the area and the growth of other vegetation types (especially 30year-old barley and wheat fields), the rate of carbon and nitrogen mineralization in the soil has decreased significantly. The highest values of these parameters were found at greater soil depths, and the intensity of these processes decreased significantly with increasing depth. The study findings emphasize that safeguarding and restoring primary grassland vegetation is vital to maintaining soil structure stability, organic carbon and nitrogen reserves, and the balance of soil biochemical processes in arid ecosystems. They also indicate the negative effects of land use/cover changes on soil fertility and ecosystem capacity.