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?