1. Soil C Cycling Following Timber Harvest in Response to Logging Debris Retention and Herbicide Application Robert Slesak – Oregon State University Stephen Schoenholtz – Virginia Tech Timothy Harrington – PNW Research Station, USFS

2. Background Uncertainty in response of soil microbial respiration (SMR) to harvesting (disturbance) Net effect of management practices on soil C pools Site Productivity - Reduced soil C (soil OM) may reduce longterm soil quality

3. Study Objectives Treatment effects on SMR Importance of modified soil enviro. or C input from treatments on response Determine if DOC concentrations vary with treatment Relationships between DOC and soil respiration (see you in New Orleans)

4. Experimental Approach Experimental unit = individual tree (4m 2 area centered on tree) Logging debris at either 0, 40, 80% ocular coverage With or without sustained herbicide application 2 sites with contrasting soil characteristics and precipitation Treatments applied in March 2005

5. Measures Monthly in situ SMR Soil temperature (0-10 cm) Soil water DOC with tension lysimeters (60 cm) Lab incubations (3 annually) - constant temp and moisture - SMR and DOC

6. Matlock – microbial respiration Effect F valuep value Herbicide8.34.018 Month118.5<.001 Herb*month3.69<.001 Debris*month2.74<.001 Treatment MeanPercent increase NWC2.6926.9 WC2.12 0% debris2.6327.8 80% debris2.06

7. Molalla – microbial respiration Effect F valuep value Herbicide6.41.032 Month81.3<.001 Herb*month6.19<.001 Debris*month2.13.001 Treatment MeanPercent increase NWC3.0538.0 WC2.21 0% debris3.0036.4 80% debris2.20

8. Matlock Molalla Lab-field comparison for April No significant differences in lab or field No significant differences in lab Sig. main effect of herbicide (p<.05) In field Suggests effect in field at Molalla probably due to modified environment

9. Lab-Field comparison for July Significant effect of debris (p=.05) In field No significant differences in lab No significant differences in field (herb p=.11) Significant effect of debris (p=.04) and herb (p=.03) in lab Matlock Molalla Matlock – field effects probably due to modified environment Molalla – field environment may be inhibiting treatment effect - variance may mask field effect (may be C effect from herbicide)

10. Soil Water DOC Matlock Molalla Debris F=2.19, p=.12 Debris F=5.94, p<.01

11. Conclusions Control of competing vegetation and logging debris reduces microbial respiration for part of the growing season - length of time varies by site Modified SMR due primarily modified soil environment - temperature alone does not explain response - soil moisture most likely plays important role Some evidence for C input effect or modified microbial community DOC “loss” greatest in the absence of logging debris (non-sig. Matlock) Combined (SMR+DOC) C flux greatest for 0% debris retention - losses may be offset by C inputs (NWC) - losses may not be greater than reference (WC) Total soil C at end of experiment will determine net change in pool

12. Questions?