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

Soil organic C content (SOC, kilograms C per square meter) and its relation to site characteristics are important in evaluating current regional, continental, and global soil C stores and projecting future changes. Data were compiled for 499 pedons in the largely forested, mountainous western Oregon region. The SOC of mineral soil ranged from 0.9 to 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 rock volume - contributed substantially to SOC variation. Regression analysis of 134 forest pedons indicated that combinations of site characteristics explained up to 50% of the SOC variability. The SOC increased with annual temperature, annual precipitation, actual evapotranspiration, clay, and available water-holding capacity and decreased with slope. Relations for western Oregon differed qualitatively and quantitatively 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 variability; and site-characteristic measurements. Other unexplained variability is probably due to potentially important but poorly documented site characteristics, such as recent vegetation composition, geomorphic disturbance regime, and fire history.