Thursday, November 16, 2006 - 8:30 AM

Soil C Cycling Following Timber Harvest in Response to Logging Debris and Herbicide Application.

Robert Slesak, Dept of Forest Engineering, Oregon State Univ, 204 Peavy Hall, Corvallis, OR 97330-1663, Stephen Schoenholtz, College of Natural Resources, Virginia Tech, 210 Cheatham Hall, Blacksburg, VA 24061-0001, and Tim Harrington, USDA Forest Service, PNW Research Station, 3625 93rd Ave SW, Olympia, WA 98512-9193.

We are assessing soil C cycling in response to three levels of logging-debris retention (0, 40, and 80% coverage) with or without annual herbicide application at two sites in the Pacific Northwest that differ in soil properties and annual precipitation.  In situ measurement of bulk soil and microbial respiration, and soil water DOC at a depth of 60 cm, have been collected on a monthly basis since initiation of the experiment in September 2005.  Repeated measures analysis indicates a significant effect of herbicide application on bulk soil (site 1 F=13.22, p=.005; site 2 F=8.84, p=.016) and microbial respiration (site 1 F=8.53, p=.017; site 2 F=6.80, p=.028) dependent on time (interaction terms significant at α = 0.05).  In contrast, there was no main effect associated with debris retention, but significantly higher respiration rates were observed at high debris retention relative to no debris retention during summer months (July and September) at site 2. Debris retention significantly increased (p=.007) soil water DOC concentrations at site 2, with the 0% and 80% debris treatments having mean DOC concentrations of 5.1 and 8.0 mg L-1 , respectively.  In contrast, the trend in DOC concentration at site 1 was opposite that of site 2 (mean 0% debris = 15.2 mg L-1, 80% debris = 9.6 mg L-1), but the trend was not significant.  Theses results, as well as those from ongoing laboratory incubations, will be discussed in context of the effects of management practices on soil C cycling and how they vary with site characteristics.