Abstract The relationships between soil organic C (SOC) content and soil and site properties, including climate, land use, and other variables, are important for assessing the effects of future land use and climate changes on soil organic C stocks. Data were obtained for 766 soil profiles taken from published soil studies of sites throughout peninsular Spain. Relationships between SOC and mean annual precipitation, mean annual temperature, land use, soil moisture regime, altitude, texture, and slope gradient were statistically analyzed by simple correlation, stepwise multiple regression analysis, and error source estimation. The variables best correlated with SOC were mean annual precipitation and the length of dry summer season. Land use alone explained 33% of the SOC variability. Almost 50% of the SOC variability was explained by all the variables included in the study. Almost 30% of the SOC variability could be attributed to errors in the values assigned to climate variables, SOC measurements, and within‐site SOC variability. If the relationships found between SOC and climate and land use hold in the future, a moderate degree of climate change (10% increase in temperature and 10% decrease in precipitation) could lead to a 15% loss of SOC from soils in the most unfavorable case. The magnitude of the effects of land use changes on SOC content, for example, from cropland to scrubland, may equal that of a climate change.