The scarce information about soil carbon (C) storage in water-stressed ecosystems limits our understanding on soil contribution to the C cycle in arid zones. This study, performed in arid NW Mexico: (i) estimated soil capacity to store organic C (SOC) and inorganic C (SIC) linked to topography; (ii) determined the spatial distribution of regional SOC stocks; and (iii) analysed if different land-uses affect the C:N ratio. Contents of SOC from different landforms and land uses varied from <0.2 to -45 Mg ha(-1); shallow mountain soils and alluvial plains showed the minimum and maximum values, respectively, underlying the topography relevance to soil depth. The SIC varied from <0.05 to-32 Mg ha(-1), highlighting inorganic C storage. With SOC content data and environmental databases, this study achieved the regional spatial distribution of SOC stocks (0-30 cm) by predicting modelling. The average SOC stock (0-30 cm) was estimated in 25 +/- 9.9 Mg ha(-1) for the region. Changes in land-use from natural conditions apparently reduced N content, varying C:N ratio from 7.90 to 13.68 and reflecting the importance of land management. The results confirmed the value of arid soils to store C and their contribution to global C and N cycles.