Distribution of soil organic and inorganic carbon pools by biome and soil taxa in Arizona

Arid systems represent an important component of the global soil C budget in that they cover 12% of the global land area and contain nearly 20% of global soil C stocks, both organic (SOC) and inorganic (SIC). The objectives of this study were to quantify SOC and SIC stocks in Arizona biomes, using Arizona as a model system for arid lands. Biome distribution was extracted from the Arizona Gap Analysis Project spatial vegetation dataset (GAP), while soil C date were extracted from the Arizona State Soil Geographic Dataset (STATSGO) at a scale of 1:250 000, and the western Yavapai County Soil Survey Geographic Dataset (SSURGO) at a scale of 1:24 000. Soil data were converted from a polygonal vector format to a raster format, and a raster-based method used to estimate SOC and SIC stocks by biome. Statewide, STATSGO soil C stocks indicate Arizona contains 0.5 and 1.5 Pg of SOC and SIC, respectively, with 27% of the SOC in pinyon-juniper biomes (PJ), and 34% of SIC in creosotebush-bursage biomes (CB). A comparison of soil C estimates between datasets indicates significantly greater estimates of biome SOC and SIC using SSURGO data relative to the STATSGO data. SSURGO soil C estimates varied considerably between the raster-based and soi taxa based method of data aggregation. Soil taxa data exhibited large ultra-unit variation in each biome. In addition, soil C differed substantially between biomes by soil taxa (e.g., Haplargid SOC of 40 and 13.5 kg m ^-2 in the paloverde-cacti (PC) and montane pine (MP) forest biomes, respectively). Raster based soil C estimations incorporate the spatial distribution and areal land cover of each soil type within a biome, providing a more accurate representation of soil C stocks.