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The replacement of natural vegetation by crop systems directly impacts the soil organic matter fractions. The objective of this study was to evaluate the total organic carbon (TOC) and nitrogen (TN) contents in different fractions of the soil organic matter (SOM) of an Oxisol of the Brazilian semiarid region under different irrigated crops and different soil management systems. Seven treatments were evaluated, which consisted of two soil management systems (no-till and conventional tillage) and three crops (maize, sunflower and sorghum), using as reference the soil under a native forest (NF). The summer crops preceded common bean crops in the autumn-winter. The total organic carbon content, total nitrogen, carbon content in humic substances and their constituents (fulvic acids, humic acids and humin) and labile, non-labile and water-soluble carbon contents were evaluated two years and three months after the experiment implementation to determine the carbon lability (L) lability index (LI), partitioning index (CPI) and management index (CMI). The greatest carbon, nitrogen and organic matter contents in the soil surface layer (0.00-0.05 m) were found in crops under no-till system (NTS), especially maize. The crops under NTS presented greater carbon content in humic substances than the conventional tillage system (CTS) ones in the layer 0.05-0.10 m. The crops under NTS presented greater sustainability in the Brazilian semiarid region compared with those under CTS, as shown by their higher CMI in the soil surface layer.