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Aggregation in many soils in semi-arid land is affected by their high carbonate contents. The presence of lithogenic and/or primary carbonates can also influence the role of soil organic matter (SOM) in aggregation. The role of carbonates and SOM in aggregation was evaluated by comparing the grain-size distribution in two carbonate-rich soils (15% and 30% carbonates) under conventional tillage after different disaggregating treatments. We also compared the effect of no-tillage and conventional tillage on the role of these two aggregating agents in the soil with 30% of carbonates. Soil samples were treated as four different ways: shaking with water (control), adding hydrochloric acid (HCl) to remove carbonates, adding hydrogen peroxide (H(2)O(2)) to remove organic matter, and consecutive removal of carbonates and organic matter (HCl H(2)O(2)), and then analyzed by laser diffraction grain-sizing. The results showed that different contributions of carbonates and SOM to aggregate formation and stability depended not only on their natural proportion, but also on the soil type, as expressed by the major role of carbonates in aggregation in the 15% carbonate-rich soil, with a greater SOC-to-SIC (soil organic C to soil inorganic C) ratio than the 30% carbonate-rich soil. The increased organic matter stocks under no-tillage could moderate the role of carbonates in aggregation in a given soil, which meant that no-tillage could affect the organic and the inorganic C cycles in the soil. In conclusion, the relative role of carbonates and SOM in aggregation could alter the aggregates hierarchy in carbonate-rich soils.