SOIL ORGANIC-CARBON IN A MOUNTAINOUS, FORESTED REGION - RELATION TO SITE CHARACTERISTICS

Soil organic C content (SOC, kilograms C per square meter) and its rel ation to site characteristics are important in evaluating current regi onal, continental, and global soil C stores and projecting future chan ges. Data were compiled for 499 pedons in the largely forested, mounta inous western Oregon region. The SOC of mineral soil ranged from 0.9 t o 24 kg C m(-2) (mean = 6.5) for 0- to 20-cm depth and 2.3 to 88 kg C m(-2) (mean = 15.8) for 0- to 100-cm depth. Variability in each of the three terms that determine SOC - C concentration, bulk density, and r ock volume - contributed substantially to SOC variation. Regression an alysis of 134 forest pedons indicated that combinations of site charac teristics explained up to 50% of the SOC variability. The SOC increase d with annual temperature, annual precipitation, actual evapotranspira tion, clay, and available water-holding capacity and decreased with sl ope. Relations for western Oregon differed qualitatively and quantitat ively from those for other regions and contrasted with the decrease in SOC associated with increased temperature in Great Plains grasslands. Of the variability not explained by regression analysis, one-half may be due to the combined uncertainty associated with measurements of C concentrations, built density, and rock volume; natural within-site va riability; and site-characteristic measurements. Other unexplained var iability is probably due to potentially important but poorly documente d site characteristics, such as recent vegetation composition, geomorp hic disturbance regime, and fire history.