Soil Organic Matter Dynamics in an Intensively Managed Douglas-fir Forest.

Forest harvest residues and forest floor materials are significant sources of mineral soil organic matter and nutrients for regenerating and establishing forests.  Harvest residues in particular are occasionally removed, piled, or burned following harvesting.  We have examined the effects of varying forest floor and harvest residual levels on the productivity, soil carbon content, and soil carbon composition in an intensively managed 2 year old Douglas-fir (W. Oregon) stands.  To quantify soil organic carbon fluxes we have measured DOC in lysimeter solutions, soil respiration, and soil carbon pools pre-, immediate post-, and 2-yrs post-treatment.  To determine the mechanisms of change we examined the soil with size/density fractionation and stable isotopic composition. Using these techniques we are able to test hypotheses regarding the role of residual root systems and higher mineralization rates in soil carbon and nitrogen pools. We will discuss how the stable isotopic composition can be used to examine the long-term sustainability of intensive biomass harvesting in Douglas-fir forests.