Sample Design for National Carbon Stock Assessment.

Global and regional models of carbon dynamics and climate change require accurate, spatially explicit estimates of soil carbon stocks. Furthermore, international agreements on mitigation should be based on reasonable estimation of aggregate soil carbon stocks on regional and national scales. The Rapid Assessment of Soil Carbon (RaCA) was developed to collect an unbiased estimate of soil organic (SOC) and inorganic carbon stocks (SCS) across the conterminous US. A secondary objective was to evaluate differences in carbon stocks associated with various ecosystems and land uses. We developed an algorithm that groups soil series by expected SOC stocks (total mass of carbon to a depth of 1m). The objective of this algorithm was to group soils that are likely to have similar SOC stocks and that will respond similarly to changes in land use on a major land recourse area regional office (MO) wide basis. Data about all soils currently mapped by the National Cooperative Soil Survey in the CONUS was gathered by linking map components to official series descriptions (OSDs). Scores were assigned to each class in the OSD fields of soil order, soil suborder, soil great group, soil subgroup, family particle size class, soil moisture regime, soil temperature regime, depth to restrictive layer, and drainage class using an expert system.  Then for each of the17 MO regions, a hierarchical clustering analysis was done to group soils into 8 – 20 groups. Those groups were then mapped using the SSURGO published map units and intersected with the US Geological Survey national land cover dataset to tabulate the approximate extent of each group-land cover/use combination.  An algorithm was used to assign the number of sites to sample across groups and land use/cover for approximately 390 per MO.  Each site will have 5 pedons samples within at 30m radius. The samples from those pedons will have bulk density measured on soil organic and inorganic carbon predictions made.  The result of this project will be to have an aggregate estimate of soil carbon stocks at a regional and national scale and a comparison of soil carbon stocks in land cover/uses on similar soils that can be extrapolated using soil maps.