Soil organic carbon in a mountainous, forested region: relation to site characteristics

Soil organic C content (SOC, kilograms C per square meter) andits relation to site characteristics are important in evaluating currentregional, continental, and global soil C stores and projecting futurechanges. Data were compiled for 499 pedons in the largely forested,mountainous western Oregon region. The SOC of mineral soil rangedfrom 0.9 to 24 kg C m-2 (mean = 6.5) for 0- to 20-cm depth and2.3 to 88 kg C m-2 (mean = 15.8) for 0- to 100-cm depth. Variabilityin each of the three terms that determine SOC — C concentration,bulk density, and rock volume — contributed substantially to SOCvariation. Regression analysis of 134 forest pedons indicated thatcombinations of site characteristics explained up to 50% of the SOCvariability. The SOC increased with annual temperature, annual pre-cipitation, actual evapotranspiration, clay, and available water-holdingcapacity and decreased with slope. Relations for western Oregondiffered qualitatively and quantitatively from those for other regionsand contrasted with the decrease in SOC associated with increasedtemperature in Great Plains grasslands. Of the variability not explainedby regression analysis, one-half may be due to the combined uncertaintyassociated with measurements of C concentrations, bulk density, androck volume; natural within-site variability; and site-characteristicmeasurements. Other unexplained variability is probably due to poten-tially important but poorly documented site characteristics, such asrecent vegetation composition, geomorphic disturbance regime, andfire history.