Initial Response of Soil Carbon and Nitrogen to Harvest Intensity and Competing Vegetation Control in Douglas-Fir (Pseudotsuga menziesii) Plantations of the Pacific Northwest
We assessed the effect of harvest type (bole-only or whole-tree) and vegetation control treatments (initial or annual application of herbicide) on soil C and N at two contrasting sites in the Pacific Northwest. Pretreatment (2003) and posttreatment (2005) soil samples were collected by depth to 60 cm, and a stratified sampling approach based on four surface conditions was used for posttreatment sampling in surface soils. Surface condition had a significant effect on soil C and N concentrations, generally decreasing with decreasing amounts of logging debris and increasing soil disturbance. There was no difference between harvest treatments in the change in soil C and N content despite differences in surface condition coverage between harvest types, indicating estimates of C and N change determined from the stratification approach were imprecise. Soil C and N content tended to increase regardless of treatment, but increases were significant only in the bole-only harvest at one site and in the whole-tree harvest at the other site. Initial vegetation control caused significantly greater positive change in soil C and N than the annual vegetation control treatment, with effects limited to surface soil at one site and all sample depths at the other site. Much of these increases occurred in deeper (>20 cm) parts of the soil profile, indicating that deep soil sampling is necessary for assessment of harvest-related change in soil C and N.