Soil C Cycling Following Timber Harvest in Response to Logging Debris Retention and
Herbicide Application
Robert Slesak – Oregon State UniversityStephen Schoenholtz – Virginia Tech
Timothy Harrington – PNW Research Station, USFS
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
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)
Experimental Approach• Experimental unit = individual tree (4m2 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
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
Matlock – microbial respirationEffect F value p value
Herbicide 8.34 .018
Month 118.5 <.001
Herb*month 3.69 <.001
Debris*month 2.74 <.001
Treatment Mean Percent increase
NWC 2.69 26.9
WC 2.12
0% debris 2.63 27.8
80% debris 2.06
Molalla – microbial respirationEffect F value p value
Herbicide 6.41 .032
Month 81.3 <.001
Herb*month 6.19 <.001
Debris*month 2.13 .001
Treatment Mean Percent increase
NWC 3.05 38.0
WC 2.21
0% debris 3.00 36.4
80% debris 2.20
Matlock
Molalla
Lab-field comparison for April
No significant differences in labor 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
Lab-Field comparison for JulySignificant 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) andherb (p=.03) in lab
Matlock
Molalla
Matlock – field effects probably due to modified environmentMolalla – field environment may be inhibiting treatment effect - variance may mask field effect (may be C effect from herbicide)
Soil Water DOC
0
5
10
15
20
25
Dec-0
5
Jan-
06
Feb-0
6
Mar-0
6
Apr-0
6
May-0
6
Jun-
06
Jul-0
6
DO
C (m
g L-1
)
NWC-0 NWC-40 NWC-80 WC-0 WC-40 WC-80
0
10
20
30
40
50
60D
OC
(mg
L-1) Matlock
Molalla
Debris F=2.19, p=.12
Debris F=5.94, p<.01
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
Questions?
