Regional soil organic carbon storage estimates for western Oregon by multiple approaches

Soil is an important factor in regional and global C budgets becauseit serves as a reservoir of large amounts of organic C. In our study,we compared six approaches of estimating soil organic C (kg C m-2,not including the surface organic horizon, hereafter called soil C)and its spatial pattern in the mountainous, largely forested westernOregon region. The approaches were (i) USDA NRCS pedons, (ii)other pedons, (iii) the State Soil Geographic Data Base (STATSGO),(iv) the United Nations Soil Map of the World, (v) the National SoilGeographic Data Base (NATSGO), and (vi) an ecosystem-complexmap. Agreement between approaches varied with scale. For the entireregion (105 km2), estimates of average soil C varied from 4.3 to 6.8kg C m-2 for the 0- to 20-cm depth and from 12.1 to 16.9 kg C M-2for the 0- to 100-an depth. At the subregional scale 104 km2), allapproaches indicated higher soil C in the coastal area than in theinland southern area, but relative amounts in other subregions variedamong the approaches. At the subsubregional scale (AO' km2), soilC was consistent between individual STATSGO map units and NRCSpedons within those map units, but there was less agreement withother pedons. Rigorous testing of soil-C maps requires data frompedons that are located by objective criteria, in contrast to the subjec-tively located pedons now available. The uncertainty associated withregional soil-C amounts and spatial patterns should be consideredwhen soil-C maps are integrated into regional or global assessmentsof physical and biotic processes because simulation-model outputsmay be sensitive to soil C.