The effects of excessive moisture
on soil C and N mineralization and
site productivity
Toktam Sajedi
Faculty of Forestry
An ecological enigma
CH
HA
Poor conifer regeneration
and low nutrient supply in both oldgrowth and clearcut cedar-hemlock (CH)
forests but not in adjacent hemlockamabilis fir (HA) sites.
Role of moisture
Evidence indicated that excessive moisture may
contribute to low nutrient supply in CH sites:
• Frequent hydromors
• Lower fauna abundance
• Presence of copepods
• > 80% water in organic matter
• Frequent gleyed horizons
• Slightly lower slope position
Three approaches were taken to test the
“excessive moisture” theory:
1. Field evidence of excessive moisture (field
research)
2. Effects of excessive moisture on C and N
mineralization (laboratory research)
3. Evaluating drainage effects on forest
productivity and nutrient cycling (drainage
trial)
Field research - Objectives and hypotheses
Do CH and HA differ in soil and vegetation properties
consistent with excessive moisture?
Hypothesis: Compared with HA sites, CH sites will have:
•
•
•
•
Higher moisture content
Lower redox potential (especially in lower soil horizons)
Shallower aerated depth
Greater frequency of anoxic conditions in the forest floor
and mineral soil
Sampling
CH
HA
Port McNeill
F
H
A
B
Clearcut
Forest
Field research - Measurements
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Moisture (volumetric)
Redox potential (Eh-combination electrode)
pH
Aerated depth (iron rods)
Mottling
Bulk density (excavation method)
Rooting depth
Layer thickness
Mass loss (cellulose filter paper)
Field capacity (pressure plate method)
Soil particle size
Total C and N (total combustion method)
Composition of plant species
Plant species coverage (%)
Field research - Soil moisture and aeration
CH forests were wetter and less aerated than HA forests
340
a
a
500
a
b
400
b
330
b
b
CH
300
HA
200
a
a
100
Redox potential (mV)
Moisture % (dry-wt basis)
600
b
320
310
a
a
a
a
a
a
300
290
280
CH
270
HA
260
0
250
F
H1
H2
B
Anaerobic
condition
F
H1
H2
More than 40% of the sampling plots in CH forests
had an average redox value less than +300 mV
B
Field research - Aerated depth
72% of the rods put in the CH sites indicate presence of
anoxic conditions, compared to 46% in HA sites.
CH
FF
Soil surface
HA
FF
37 cm
CH
56 cm HA
Oxic
Anoxic
Field research-Results
Principal component analysis (PCA) of understory plant species
Field research - Soil variability in CH/HA sites
PCA-soil variables
Group 3: well-drained soils moist soils,
highest Eh, deep oxic layer with sandy Sand
texture
Forest type
CH
HA
CH-Group 3
OXD
EhB
OMTH
Group 1: anaerobic soils with high
moisture, lowest Eh and shallow
oxic layer caused by pan layers
EhHU
Axis 2
pHB
HA
pHHU
MCHU
CH-Group 1
Silt
Clay
CH-Group 2
Axis 1
Group 2: poorly drained soils with
high amounts of silt and clay and
signs of mottling
Field research - Results
Canonical correspondence analysis of soil variables in
relation to plant species
Cumulative percentage of
Environmental variables
canonical variance accounted
for by the first 3 axes
Moisture
17
Aeration (Eh and oxic depth)
25
Moisture and Aeration
27
10 soil variables
31
Field research-Harvesting effects
HA clearcuts were less aerated and likely to be anaerobic
compared with HA forests.
340
b
b
Redox potential (mV)
320
300
a
a
a
a
a
a
ab
a
aB
a
280
260
240
CH
HA
220
200
Also, compared with HA forests HA clearcuts had shallower
aerated depth, rooting depth and slower decomposition rate
Incubation study- Objectives and hypotheses
How does excessive moisture affect mineralization
of carbon and nitrogen?
Hypothesis 1: Microbial biomass and CO2 evolution will
decrease under saturated conditions in humus and soil from
both site types.
Hypothesis 2: NO3 concentrations and the proportion of N in
the form of SIN (i.e SIN:SON) will decrease under saturated
conditions in humus and soil from both site types.
Incubation study-Methods
Site 1
Site 2
Site 5
CH
HA
humus
mineral
……
H
B
200% (field capacity)
350%
500%
600% (saturation)
60% (field capacity)
90%
110%
130% (saturation)
Measurements:
 C mineralization (CO2 efflux)
 Microbial biomass C and N
 N mineralization -NH4 & NO3
 Soluble organic N
Incubation study-Methods
Incubated samples at 20 οC
and dark condition
CO2 measurement with
LI-COR auto-analyzer
Incubation study-Results
Redox potential declined sharply above 500%
moisture threshold
Redox potential (mV)
300
Moisture threshold
250
CH
HA
200
150
100
50
R² = 0.6539; p<0.0001
0
0
200
400
600
Moisture (% dry wt)
800
Incubation study-Results
mg C/ g soil C
Mineralization of carbon declined after reaching maxima
at intermediate moisture levels
HA
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
R² = 0.5661
R² = 0.0918
0
200
400
600
Moisture (%dry-wt based)
800
Incubation study-Results
3.0
R2=0.1968; p=0.006
2.5
2.0
1.5
1.0
CH
HA
0.5
0.0
0
200
400
600
Soil moisture (%dry wt)
H horizon
800
Microbial iomass C (mg C/ g soil)
Microbial iomass C (mg C/ g soil)
Microbial biomass C declined with increasing
moisture
0.8
R² = 0.3225; p=0.0002
0.7
0.6
0.5
0.4
CH
0.3
HA
0.2
0.1
0.0
0
50
100
150
Soil moisture (% dry wt)
B horizon
200
Incubation study-Differences in N forms in humus
HA-field
CH-field
SON
NH4
NO3
NH4
SON
NO3
HA-saturated
CH-saturated
SON
NH4
NH4
NO3
SON
NO3
Incubation study-Summary
Excessive moisture resulted in anaerobic
conditions, less microbial biomass and activity
and lower SIN:SON in cedar-hemlock forest soils
Drainage study- Study site
Drainage study- Study site
Average tree height (m)
Drainage significantly improved growth of
regenerating trees.
6
5
b
a
4
3
2
1
0
Control
Drained
Drainage study- Results
9
8
7
6
5
4
3
2
1
0
8
a
b
Control
Drained
Total CO2 emission (mg/gC)
Microbial biomass C (mg/g)
Lower microbial biomass and mineralization of C in
drained areas
a
7
6
5
a
4
3
2
1
0
Control
Drained
Drainage study-Results
Soils were better aerated but still anaerobic in drained areas
600
Moisture (% dry mass)
a
Eh (mv) - CaCl2
500
b
400
300
200
a
a
100
0
0.5
Control
1
1.5
Drained
2
2.5
Overall Conclusions
• Excessive moisture and resulting anaerobic
conditions in soil play a fundamental role in the
low nutrient supply in cedar-hemlock forests in
coastal B.C.
• The effect of moisture on nutrient supply
appears to be indirect, and arises through
interactions with vegetation and litter quality
High precipitation
Sand
Upper slope
Lower slope
Silt and clay
HA
CH
Well drained
Poorly drained
Well aerated
Vegetation
(Vaccinium, fireweed)
More easily
decomposable
material
Decomposition
Humification
Binding of N
Poorly aerated
Soil chemistry
Soil chemistry
More O2
Low O2
Oxidized form
of elements &
Reduced form
of elements &
SON, NO3
SON, NO3
Aerobic microbes
More microbial biomass
Greater enzyme activity
High nutrient supply
Facultative anaerobes
Less microbial biomass
Lower enzyme activity
Vegetation
(Cedar, salal, deer fern)
More phenolic
compounds
Decomposition
Humification
Binding of N
Low nutrient supply
Low productivity
High productivity
High litter N
Pan
High N mineralization
Low N mineralization
Low litter N
Management implications
1. Clearcut harvesting may shift HA ecosystems towards
more anaerobic conditions similar to CH forests.
2. Drainage could be a useful silvicultural practice for
improving the productivity of cedar-swamp ecosystems in
coastal B.C.
3. It may be possible to improve tree growth without
stimulating loss of soil C using drainage. This requires
maintaining redox levels within a range below +300 mV in
the bulk soil.