Organic carbon content under different types of land use and soil in peninsular Spain

The stock and distribution of soil organic C in peninsular Spain were calculated using soil profile descriptions available in the Spanish soil literature. Two soil databases have been compiled, containing data from 2,851 horizons and 1,030 profiles. The soil organic C concentration (SOCC; mass of soil organic C per unit area) has been calculated for each profile, using in most cases a relationship between bulk density (BD) and C content derived from horizon data. The databases have been combined in a geographical information system with a digitized land use map and a soil map to obtain the stocks of soil C under different types of land use and soil, as well as a map of SOCC. The total soil C stock is 3.7 Pg (mean SOCC is 7.59 kg C m(-2)), with an estimated error of 36% due to errors in horizon C content, ED and stoniness. The variability of SOCC is great within land use and soil type data [mean coefficient of variation (100 SD/mean) is about 70% in both cases]. SOCCs are highest in meadows (improved grazinglands) and bushland, and lowest in cultivated areas. Conifer and broadleaved woodlands and pastures show an intermediate range of SOCC. Mean SOCCs under different types of land use differ significantly in many cases, and correlate very well (r=0.94) with the average mean annual precipitation.