Download What is the temporal and spatial variability in porewater chemistry?

Quantifying variability in
peatland porewater chemistry in
the context of a large-scale
climate change experiment
Natalie A. Griffiths1 and Stephen D. Sebestyen2
1Oak Ridge National Laboratory
2USDA Forest Service, Northern Research Station
Northern peatland ecosystems
- Saturated, nutrient-poor,
acidic ecosystems.
- Disproportionately large
carbon pools.
- Primarily located in northern
latitudes.
- Important to understand the
effects of climate change on
carbon dynamics and
associated processes in
peatlands.
(Yu et al. 2010)
Large-scale climate change experiment: SPRUCE
Elevated CO2 (+500 ppm)
Ambient CO2
+0°C
+4.5°C
+6.75°C
+6.75°C
+2.25°C
+4.5°C
+0°C
+9°C
+9°C
+2.25°C
mnspruce.ornl.gov
phenocam.sr.unh.edu/webcam
Water chemistry measurements in SPRUCE
Porewater chemistry (0-3m)
Lateral outflow (stream) chemistry
How does climate change affect
porewater chemistry?
H1: Elevated temperature will increase production and
decomposition rates and result in higher DOC and nutrient
concentrations in porewater.
H2: Increased CO2 concentrations will increase production and result
in greater leaching of DOC into porewater.
What is the temporal and spatial
variability in porewater chemistry?
Provides key information on bog ecosystem function.
Baseline data necessary to determine sampling regime.
Estimate the change needed to detect a significant effect.
How does porewater chemistry in
the study bog compare to that in
nearby peatlands?
Important to assess the representativeness of an experimental site for
upscaling and when incorporating processes into Earth System Models.
Porewater chemistry at multiple scales
Temporal variability
Spatial variability
Cross-peatland variability
Accounting for variability when
interpreting treatment effects
Focus on total organic carbon (TOC).
Temporal variability in porewater chemistry
0
1
2
3
0
20
40
60
(C) 2013
0
6/1/12
6/4/12
7/13/12
8/2/12
9/7/12
11/16/12
1
Depth (m)
8/16/11
9/2/11
9/19/11
9/27/11
10/15/11
10/22/11
10/29/11
11/11/11
11/19/11
11/26/11
12/3/11
12/10/11
2
3
80
6/28/13
7/26/13
8/27/13
10/3/13
11/1/13
1
2
3
0
Total organic carbon (mg/L)
Total organic carbon (mg/L)
Depth (m)
(B) 2012
Typical WL
fluctuations
Depth (m)
(A) 2011
0
20
40
60
Total organic carbon (mg/L)
80
0
20
40
60
80
Total organic carbon (mg/L)
- TOC concentrations decrease with
depth possibly due to decomposition.
80
70
60
- Greater variation in TOC in deeper peat.
Drivers of variation not clear. Vertical
flow paths slow, but present.
50
40
2011
2012
2013
30
20
50
100
150
200
250
Day of Year
300
350
400
- Surface water TOC seasonally variable
likely reflecting recently produced
photosynthate.
Spatial variability in porewater chemistry
- Chemistry can vary at small
spatial scales (10’s m). TOC
concentrations variable across
the bog.
- Lower TOC concentrations in
the southern part of the bog
may reflect processing of
labile OC along lateral flow
paths.
Depth (m)
0
1
2
South
North
3
20
30
40
50
60
70
80
Total organic carbon (mg/L)
South
90
Cross-peatland variability in porewater chemistry
Bog Lake Fen
S1 bog
S2 bog
Depth (m)
Bog Lake Fen
S2 bog
0
Depth (m)
0
S1 bog
1
2
Bog Lake Fen
S1 bog
S2 bog
1
2
0
1
2
3
4
Ammonium (mg N/L)
5
6
20
30
40
50
60
70
80
90
100
Total organic carbon (mg/L)
- Peatlands fall along a gradient from ombrotrophic bogs to minerotrophic
fens.
- S1 bog porewater chemistry resembles a weakly ombrotrophic bog.
Porewater at 0 m
Total organic carbon (mg/L)
Total organic carbon (mg/L)
At what scale is porewater chemistry most variable?
80
70
60
50
40
30
temporal
spatial
cross-site
- In near-surface porewater,
variability in TOC is largest over
time and across sites.
- In deeper porewater, variability in
TOC is similar across scales.
Porewater at 2 m
70
60
50
40
30
20
temporal
spatial
cross-site
Pools (kg)
Min
Mean Max
TOC (temporal)
1981 2716
3412
TOC (spatial)
2412 3105
3758
- Estimated TOC pools can vary by
almost 2-fold; highlighting the
importance of sampling over
space and time when upscaling.
Total organic carbon (mg/L)
Initial porewater responses to warming
160
140
120
100
80
+0°C (plot 6)
+0°C (plot 19)
+2.25°C (plot 11)
+2.25°C (plot 20)
+4.5°C (plot 4)
+4.5°C (plot 13)
+6.75°C (plot 8)
+6.75°C (plot 16)
+9°C (plot 10)
+9°C (plot 17)
pre-treatment
warming
60
40
20
0
01/13
07/13
01/14
07/14
01/15
07/15
01/16
- TOC concentrations increased in 0 m porewater with warming, possibly due
to increased leaching or mineralization.
- Small or no changes observed at deeper depths.
- Some changes occurred prior to warming; possibly due to lack of lateral
outflow from plots resulting in concentration of TOC.
Conclusions: variation
- Porewater chemistry is spatially and temporally variable. Variation
within a peatland can be as great as variation among peatlands.
- S1-bog porewater chemistry resembles a weakly ombrotrophic bog;
important for upscaling.
- Interpreting effects of ecosystem experiments requires spatially and
temporally resolved measurements in the pre-treatment period.
Conclusions: warming responses
- Up to a doubling of near-surface porewater TOC concentrations with
whole-ecosystem warming, possibly due to increased leaching or
mineralization.
- Continued sampling will examine longer-term responses, including
changes in solute fluxes and hydrology.
Acknowledgements
Keith Oleheiser, Robert Nettles, John Larson, Ben Munson,
Reid Peterson, Paul Hanson, Randy Kolka, the SPRUCE team.
Funding: U.S. Department of Energy’s Office of Science, Biological and
Environmental Research and the USDA Forest Service.