Soil organic carbon contents in arid and semiarid regions of peninsular Spain

Information about original soil organic carbon stocks and their controlling factors is needed in order to better estimate the potential of carbon sequestration in Mediterranean regions. About 1315 surface horizons located in peninsular Spain were studied to determine soil organic carbon (SOC) content and its relation with climate and other variables such as land use, altitude, UTM coordinates, slope gradient and soil texture. A simple climatic factor, the ratio of annual mean precipitation to annual mean temperature, was used to distinguish dry from humid areas. Climate and land use were the main factors controlling SOC content. The highest SOC content values corresponded to scrubland (5.2%) and the lowest to cultivated soils (1.1%). Climate variables alone explained 35% of SOC variability, their influence being greater in the humid than in dry regions. In general, soil and site variables played a minor role, but the influence of these variables oil SOC in the driest areas is worthy of note, while no correlation was found in humid region,;.