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Soil organic matter is the most important factor controlling aggregation processes in many soils, but studies on clayey soils have reported contradictory results. This study explored the formation of water stable aggregates in a Pellic-vertisol [FAO (1998). World Reference Base for Soil Resource. World Soil Resource Reports vol. 84. Rome.] or fine to very-fine Udic Haplusterts [Soil Survey Staff. (1999). Soil taxonomy: A basic system of soil classification for making and interpreting soil surveys. USDA-SCS Agric. Handb. vol. 436. U.S. Gov. Print. Office, Washington, D.C.] dominated by smectitic clays, from a semi-arid region in Northeastern Mexico. Changes in aggregation were studied in relation to the addition of maize residues and application of wetting and drying cycles under controlled laboratory weathering conditions during 105 days of incubation. Large water stable macroaggregate (>2.00 mm) formation was controlled by the addition of organic residues and application of one wetting and drying event after organic amendment increased large water stable macroaggregate formation by 26% compared to constantly wet soil. Despite the reduction on the amount of large water stable macroaggregates (>2.00 mm) with repeated wetting and drying events the amended treatment under wetting and drying showed 136% and 70% more large water stable macroaggregates than the constantly wet soil after 58 and 105 days of incubation, respectively, indicating a higher degree of stability. Conversely to large macroaggregates (>2.00 mm), formation of small water stable macroaggregates (0.25-2.00 mm) was not enhanced by the organic addition, but was affected by wetting and drying and time. Unamended soil under wetting and drying showed an increase of 20% in the amount of small water stable macroaggregates (0.25-2.00 mm) compared to amended samples after one wetting and drying cycle. In addition, the control treatment showed a similar increase after 58 days of incubation. which could probably indicate the influence of other mechanisms such as "ageing" in the formation of small macroaggregates. Results of this study indicated a more complex interrelation among factors involved in the formation of macroaggregates in Vertisols compared to rigid soils. These results provided much needed information on the differences in the relative influence of organic residues addition and wetting and drying on the formation and stabilization of macroaggregates in smectitic Vertisols. Further research is necessary to identify the mechanisms, nature of cementing agents and influence of different organic matter fractions involved in water stable aggregate formation. (C) 2009 Elsevier B.V. All rights reserved.