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Soil organic carbon ( SOC) plays an essential role in the sustainability of natural and agricultural systems. The identification of sensitive SOC fractions can be crucial for an understanding of SOC dynamics and stabilization. The objective of this study was to assess the effect of long-term no-tillage ( NT) on SOC content and its distribution between particulate organic matter ( POM) and mineral-associated organic matter (Min) fractions in five different cereal production areas of Aragon (north-east Spain). The study was conducted under on-farm conditions where pairs of adjacent fields under NT and conventional tillage ( CT) were compared. An undisturbed soil nearby under native vegetation (NAT) was included. The results indicate that SOC was significantly affected by tillage in the first 5 cm with the greatest concentrations found in NT (1.5-43% more than in CT). Below 40 cm, SOC under NT decreased (20-40%) to values similar or less than those under CT. However, the stratification ratio ( SR) never reached the threshold value of 2. The POM-C fraction, disproportionate to its small contribution to total SOC (10-30%), was greatly affected by soil management. The pronounced stratification in this fraction ( SR>2 in NT) and its usefulness for differentiating the study sites in terms of response to NT make POM-C a good indicator of changes in soil management under the study conditions. Results from this on-farm study indicate that NT can be recommended as an alternative strategy to increase organic carbon at the soil surface in the cereal production areas of Aragon and in other analogous areas. [ABSTRACT FROM AUTHOR]