In dryland fanning systems under a Mediterranean climate, soil quality and productivity can be enhanced by increasing the content of soil organic carbon (SOC) through alternative soil management systems. Some fractions of C are directly involved in increasing total SOC and therefore in enhancing any benefits in terms of soil properties. This study compares the viability of no-till farming (NT) with conventional (traditional) tillage (TT) for improving SOC levels. The influence of management practices was investigated for different fractions of C (particulate OC, active OC, humic acids, fulvic acids) and C[O.sub.2] emissions in clayey soils in the south of Spain. The experiment was conducted over three farming seasons (2006-07, 2007-08 and 2008-09) covering a crop rotation of peas (Pisum sativum L.), wheat (Triticum aestivum L.) and sunflowers (Helianthus annuus L.). The NT system improved the levels of the different fractions of C in the surface soil and reduced the amount of C[O.sub.2] released into the atmosphere compared with the TT system. Generally, the relationship between C[O.sub.2] and SOC content was greater in soils under NT for the farming seasons sampled. Additional keywords: conventional tillage, C[O.sub.2] emissions, no tillage, particulate organic carbon, soil organic carbon.