Effects of mapped variation in soil conditions on estimates of soil carbon and nitrogen stocks for South America

Organic carbon and total nitrogen stocks for South America are computed using four 1:5,000,000 scale soil data sets of different spatial resolution. These are the 60′ by 60′ resolution Zobler soil data file, the 30′ by 30′ resolution World Inventory of Soil Emission Potentials (WISE) database, a 5′ by 5′ raster version of the Digital Soil Map of the World (DSMW), and a vector-based Soil and Terrain Database (SOTER). Estimates for total, regional stocks of organic carbon (SOC) range from 146.1 Pg C (Zobler) to 159.7 Pg C (SOTER) for the first 1 m of soil. More pronounced differences are observed among the four data sets when values for SOC stocks are compared at the level of the major soil grouping and their component soil units. In the case of Ferralsols, for example, the estimates for SOC stocks are 31.1 Pg C (SOTER), 41.4 Pg C (DSMW), 41.6 Pg C (WISE), and 58.3 Pg C (Zobler), respectively. In the case of Xanthic Ferralsols (Fx), the SOC stocks estimated with the Zobler and DSMW sets differ by about 14 Pg C (≈157%). By and large, the observed differences are due to varying areal estimates for the various soil units for the data sets under consideration, and the selection of soil profile data used. This study illustrates (a) the importance of the aggregation procedure used to account for the mapped spatial variation in soil conditions and (b) the need for updating the soil geographic and attribute data for the world in SOTER, so as to provide modelers with up-to-date soil data for studying interactions of human and natural systems at the regional and continental level within the context of global change.