1. In Search of the Mechanisms Behind Soil Respiration of a Douglas-
Fir Forest in Complex Terrain Using Natural Abundance 13C
Day of Year
4/1/06 5/1/06 6/1/06 7/1/06 8/1/06 9/1/06 10/1/06 11/1/06
Soil
Respiration
(mol
m
-2
s
-1
)
0
1
2
3
4
5
6
7
8
9
10
11
Valley
Midslope
Ridge
2006 Data
Circle s= ridge
triangles = midslope
diamonds = valley
Solid line and symbols= sf
Dashed line and empty symbols= nf
4/1/06 5/1/06 6/1/06 7/1/06 8/1/06 9/1/06 10/1/06 11/1/06
C
R-s
(‰)
-29
-28
-27
-26
-25
-24
-23
-22
Valley
Midslope
Ridge
4/1/06 5/1/06 6/1/06 7/1/06 8/1/06 9/1/06 10/1/06 11/1/06
C
R-s
(‰)
-33
-32
-31
-30
-29
-28
-27
-26
SF Slope
NF Slope
OBJECTIVE
To use a naturally-occurring
gradient in microclimate across a
small, steep catchment to identify
and quantify biotic and abiotic
factors that influence soil
respiration and its isotopic
signature (13CR-s).
Soil respiration rates exhibited high temporal and spatial variability.
Ridge plots respired on average 0.8 mol m-2s-1 more than
midslope and valley plots.
METHODS
Site: Watershed 1 in HJ Andrews
Forest, OR.
Six highly instrumented plots across a
small, steep catchment were
categorized by topography (slope
position and slope aspect). On these
plots we measured :
Soil moisture and temperature, VPD,
and transpiration across plots.
Soil respiration using a LiCor 6200
with soil chamber.
Isotopic signature of soil respiration
with soil gas wells (Kayler et al. 2008)
Distinct period of enrichment in 13CR-s during the early growing
season could be indicative of an increase in heterotrophic breakdown
of soil carbon or autotrophic breakdown of plant reserves. Signal
gradually becomes more depleted during the seasonal drought
followed by another period of enrichment.
Uncorrected estimates of 13CR-s
13CR-s is not always at steady-state. A fractionation of 4.4‰
due to diffusive gas transport is associated with estimates of
13CR-s that use belowground samples. Thus, all field estimates
should line up on the 4.4 ‰ regression line which is clearly not
the case.
2005 samples taken during the day are enriched. This could
be due to a change in the background signal or atmospheric
incursion into the soil profile (e.g. Millard et al. 2008).
2006 samples taken during the evening are depleted with
reference to the steady-state model. This is likely caused by
advective gas transport.
Only estimates made during steady-state were used in the
subsequent analysis
Figure details: Comparison of uncorrected mixing model
estimate with steady-state model (Amundson et al. 1998).
Two regression lines are shown: one is a 1:1 line with the steady-
state model estimate, the second uses the same model estimates
but they are enriched by 4.4‰.
Uncorrected mixing-model estimates (y-axis) are plotted against
the steady-state estimate (x-axis).
Zachary Kayler1,4, Elizabeth Sulzman2, Holly Barnard1, Barbara Bond1, Alan Mix3
4zachary.kayler@oregonstate.edu
Affiliations: 1 Department of Forest
Science, 2Crop and Soil Science
Department, 3College of
Oceanography and Atmospheric
Science.
13CR-s seasonal patterns remain approximately the same even
when non steady-state estimates are removed. The SF slope
13CR-s values are enriched by 0.42‰ relative to the NF slope
(p<0.01). Enriched signal could be due to SF slope soil organic
matter at 15 and 30 cm (table inset).
Soil Depth NF slope s.e. SF slope s.e.
5 cm -26.7 0.1 -26.4 0.2
15 cm -26.4 0.2 -25.7 0.2
30 cm -25.7 0.2 -25.2 0.1
Soil 13C
_
_
0 3 6 9 12
-1.0
-0.7
-0.4
-0.1
0.1
0.3
0.5
0.7
0.9
_
_
0 3 6 9 12
-1.0
-0.7
-0.4
-0.1
0.1
0.3
0.5
0.7
0.9
_
_
0 3 6 9 12
-1.0
-0.7
-0.4
-0.1
0.1
0.3
0.5
0.7
0.9
_
_
0 3 6 9 12
-1.0
-0.7
-0.4
-0.1
0.1
0.3
0.5
0.7
0.9
***
**
***
***
***
***
***
***
**
***
***
*
**
**
_
_
0 3 6 9 12
-1.0
-0.7
-0.4
-0.1
0.1
0.3
0.5
0.7
0.9
_
_
0 3 6 9 12
-1.0
-0.7
-0.4
-0.1
0.1
0.3
0.5
0.7
0.9
_
_
0 3 6 9 12
-1.0
-0.7
-0.4
-0.1
0.1
0.3
0.5
0.7
0.9
_
_
0 3 6 9 12
-1.0
-0.7
-0.4
-0.1
0.1
0.3
0.5
0.7
0.9
***
**
***
***
***
***
***
***
**
***
***
*
**
**
Flux
response
13CR-s
response
*
p<0.01
p<0.05
p<0.10
**
***
ABSTRACT:
Soil respiration and its isotopic
signature (13CR-s) varied spatially
and temporally over the growing
season (1)
13CR-s was not always at steady-state
(2)
Transpiration was the principle driver
of soil respiration and 13CR-s (3)
1. Soil Respiration and 13CR-s Seasonal Patterns 2. Observations of Non Steady-State 13CR-s Dynamics 3. Transpiration as a Driver of 13CR-S
Transpiration was positively correlated with soil flux and negatively
correlated with 13CR-s indicating the importance of recent
photosynthates to this system.
First peak in correlation (0-6 lag days) could correspond to root
respiration and microbial breakdown of root exudates.
2006 soil respiration rates
Dashed Line: North Facing (NF) slope
Solid Line: South Facing (SF) slope
Dashed Line: North Facing (NF) slope
Solid Line: South Facing (SF) slope
13CR-s at steady-state corrected by -4.4‰
Lag Days
Pearson’s
Correlation
Coefficient
r
-32
-30
-28
-26
-24
-22
-20
-36 -34 -32 -30 -28 -26 -24 -22 -20
Steady-State Estimate of 13
CR-s (‰)
Uncorrected
Mixing-Model
Estimate
of
13
C
R-s
(‰)
2005
2006
4.4‰
steady-state model
estimate offset by 4.4‰
1:1 line of steady-
state model estimate
Measured during the day
Measured during the night
References: Amundson, R., L. Stern, T. Baisden and Y. Wang.1998. The isotopic composition of soil and soil-
respired CO2. Geoderma 82(1-3): 83-114.
Kayler, Z. E., E. W. Sulzman, J. D. Marshall, A. Mix, W. D. Rugh and B. J. Bond.2008. A laboratory comparison of two methods used to
estimate the isotopic composition of soil CO2 efflux at steady state. Rapid Communications in Mass Spectrometry 22(16): 2533-2538.
Millard, P., A. J. Midwood, J. E. Hunt, D. Whitehead and T. W. Boutton.2008. Partitioning soil surface CO2 efflux into autotrophic and
heterotrophic components, using natural gradients in soil delta C-13 in an undisturbed savannah soil. Soil Biology & Biochemistry 40(7):
1575-1582.
