Constructing an Optimized Baseline Soil Carbon Map for the Hawaii Islands.

The ability to predict terrestrial ecosystem carbon (C) response to future changes in climate, land use, and management, depends in part on establishing accurate baseline assessments of current C resources. The Hawaiian Islands span a large diversity of soils with unique physical, chemical, and mineralogical properties which affect the potential for belowground C sequestration statewide, making it particularly challenging to provide an accurate resource assessment. This study examines the discrepancies that occur by spatially projecting soil C stock estimates derived by different means, and suggests a useful way of combining estimates for a more accurate projection of soil C stocks throughout the state. Three approaches to spatial projection of soil C were examined: two NRCS-Soil Survey products and one product derived from additional sources to interpolate soil C stocks through geostatistical methods. The first approach maps point field data from the Rapid Carbon Assessment (RaCA) initiative, the second approach projects soil C estimates from a pre-summarized attribute table within the Gridded Soil Survey Geographic (gSSURGO) database, and the third method combines values from the Kellogg Soil Survey Laboratory database with published literature values to interpolate soil C stocks through kriging. Each method produced similar, but not identical estimates where the largest discrepancies between products occurred primarily in areas with low sampling, steep changes in topography, or differences in land use. When total soil C stock was calculated for the Big Island of Hawaii, an estimate of 373.9 Tg of C was calculated from the kriging product, but the gSSURGO product calculated 100.0 Tg of C, resulting in a difference of 273.9 Tg of C. This methods comparison is useful for creating a final map product which combines the strengths of each approach to best estimate the soil C resource across this diverse landscape and predict its response to future conditions.