Landscape-scale patterns in N biogeochemistry of high-elevation watersheds
Chuck Rhoades1, Eugene Kelly2, Mary Stromberger2 & Robert L. Sanford Jr3
1U.S.
Forest Service, Rocky Mountain Research Station, Ft. Collins,
Collins, Colorado, crhoades@fs.fed.us
of Crop and Soil Sciences, Colorado State University; 3Department of Biology, University of Denver
2Department
Alpine Biogeochemistry
Influences
Watershed N Export
Redistribution
150
Prevailing
Wind
b
200
Snowpack
Depth (cm)
30
0
Leeward
East
Nitrate
Windward
Ammonium
0.25
Initial Pools (mg N /L)
0 .5 0
0.20
e
t
ne
pin eli res
Al Tre Fo
0.05
Nov
Jan
Mar
May
July
Nov
Jan
Mar
May
July
25
Treeline
Net N Turnover (mg / L / 28 d)
Net Nitrification
0.5
S
ne
pi
Al
lo
pe
T
n
eli
re
eA
e
in
lp
T
n
eli
re
eF
es
or
40
50
60
70
The strong relationship between
streamwater inorganic N & alpine cover
in Fraser watersheds (d) demonstrates
the importance of alpine
biogeochemistry on watershed N export.
15
It is unclear to what extent landscapelandscapescale variability in snow accumulation &
soil development contribute to this
pattern.
Windward /
Erosional
10
5.0
5
4.5
4.0
5
ge
st
ne orest
pe
lpi
ore
Rid
Slo
eF
eA
eF
ine
lin eelin
ine
pin
e
l
p
Alp
l
e
a
r
A
b
T
Tr
Su
Subalpine
When incubated at
constant moisture &
temperature alpine
topography had little
effect on nitrification.
Substrate supports
comparable nitrifier
activity on both alpine
aspects.
Similar vegetation
occurs on both
aspects, so it appears
that differences
originate from factors
controlling nitrate
production or
leaching.
e
15
Alpine
As expected, resinresinavailable nitrate is
higher in the alpine.
Environmental factors
may be more critical
than organic matter
quality in regulating
alpine microbial
processes.
Leeward /
Depositional
t
b
Su
i
alp
ne
re
Fo
st
Treeline differences in N dynamics relate to the :
- Short alpine growing season & low plant N demand
- WeaklyWeakly-developed alpine soils & limited residence
time of snowmelt within groundwater flow paths
- Tundra v.s.
v.s. forest differences in litter & soil organic
matter quality
B Ochric
Alpine
C
Alpine
Alpine
Tundra
Alpine
Tundra
Subalpine
Forest
This project samples transects on 3
ridges in central Colorado where
treeline occurs from 3340 - 3630 m.
Subalpine
Physical processes, such as
cryoturbation, consume H+
& may buffer against
increased acidification.
In contrast, on leeward &
subalpine slopes, greater
organic acids from litter
combined with increased
snowpack & soil moisture
to favor chemical
weathering & lower pH.
Organic
A
E Albic
Leeward /
Depositional
Subalpine
Forest
Alpine
Soil acidity & nitrate display
similar patterns, possibly
indicating a shift in relative
importance of physical vs.
chemical weathering
across the alpine ridge.
Soil Horizons
C%
N%
Windward
Windward
Leeward
Leeward
6.0
5.5
10
Windward /
Erosional
1.0
e
30
20
Subalpine
Subalpine
Soil nitrate & nitrification decline abruptly below treeline
Net Mineralization
dg
Ri
C:N
20
Treeline
2.0
20
P e r c e n t A lp in e
Leeward
Within the alpine landscape,
windblown areas differ from
depositional areas
Treeline
0.10
10
30
0.15
ne
pi
Al
Subalpine
y = 0.06 + 0.006x + 0.0002x2
0
-10
0
0.0
Adj. = 0.77
p < 0.001
0 .0 0
Windward
Windward
Leeward
Leeward
0
More interestingly,
windward & leeward
alpine soils differ by
about 100%.
r2
0 .2 5
-10
st
e ine
re
lin
Fo
ee Alp
Tr
Treeline Patterns
1.5
S t r e a m w a te r
N itr a te (m g /L )
0 .7 5
0
Total Mineral Soil C and N
East
Wind Direction
400
0
North
Windward
West
1 .5 0
1 .0 0
10
South
d
1 .7 5
1 .2 5
Alpine Tundra
Windswept
Tundra
Subalpine
Forest
Alpine
Tundra
Windswept
Tundra
Depositional
Slope
Depositional
Subalpine Slope
Forest
10
West
c
Soil Temperature
Soil Temperature (oC)
20
50
600
Alpine
20
100
6.5
0
30
Soil pH
Net Nitrification
25
Windward
Leeward
200
Our preliminary work on alpine ecosystems at the Fraser
Experimental Forest in central Colorado indicates that nitrogen
nitrogen
dynamics & the soil chemical environment differ between
windswept & leeward alpine slopes & across the alpinealpine-subalpine
ecotone.
a
7.0
30
Resin Nitrate
800
μg / bag
In contrast, increased snow accumulation on leeward slopes (b)
dampens subnivean temperature fluctuations (c) and maintains
adequate conditions to sustain wintertime soil microbial activity.
activity.
Soil Processes
1000
(μg /g/28d)
Alpine Landscape Patterns
Removal of snow from windward slopes exposes these landscapes to
colder wintertime soil temperatures & higher frequency of freeze
freeze-thaw events (a).
Hypothetical model (e) of highhigh-elevation soil
development on windward & leeward aspects.
Cryoturbation (dashed arrows) occurs more
frequently on windswept slopes resulting in
poorlypoorly-defined horizons.
On leeward sites with persistent snowpack,
snowpack,
combined chemical & physical weathering result
in more distinct soil horizons.
Pedogenic
Links
This onon-going study will
characterize the distinct
biogeochemical &
pedological conditions
within alpine landscapes &
across the treeline
ecotone in order to
explain patterns of
watershed N export &
improve predictions
regarding the impact of
climate change on
watershed processes.