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Quantification of Nitrate Uptake by Riparian Forests
1
and Wetlands in an Undisturbed Headwaters Watershed
2
3
4
Jonathan Rhodes, C.M. Skau , Daniel Greenlee, David L. Brown
5
Three years of research on a headwaters watershed has
shown this area to be capable of removing over 99% of the
incoming nitrate nitrogen. Riparian vegetation nitrate uptake and output, and microbial denitrification will be incorporated into hydrologically-based nutrient transport models.
INTRODUCTION
Currently, investigations are attempting to
describe nitrate delivery mechanisms at the individual watershed level. Working at an experimental watershed immediately adjacent to the
Lake Tahoe basin, the goal is to develop a
hydrologically-based nutrient transport model
which can be used to predict the extent to which
watersheds will release nutrients. Riparian
wetland areas are emerging as one of the most
important parameters in controlling nutrient
transport.
Lake Tahoe in the northern Sierra Nevada
has been a focus of national attention due to
its steadily declining water quality. This
oligotrophic lake is nitrogen deficient with
respect to plant growth; however, its famed
clarity is disappearing at a rate of 0.6 m/year
in response to algal growth fueled by accelera ted ni tra te inpu ts from man-disturbed wa tersheds. Research was initiated by the University
of Nevada, Reno in the late 1960's to determine
the nutrient sources and delivery mechanisms
which are impacting Lake Tahoe.
Nitrate can enter an undisturbed watershed
from two major sources: precipitation and
nitrogen-fixation by alder and other plants
(Swank, 1984). Upon entering the soil, nitrate
can undergo a variety of transformations which
result in assimilation into tissues in microbes
and plants (uptake), microbial reduction into
nitrous oxide and nitrogen gas (denitrification), and adsorption onto soil particles
(mineralization). With the exception of sediment-adsorbed nitrates, these transformations
remove dissolved nitrate from soil and groundwater before they enter the stream, thereby
minimizing the watersheds impact on Lake Tahoe.
Of the forest nutrients, nitrate and
phosphate are of greatest concern since they are
the eutrophication rate-limiting compounds
(Kramer, 1972). In the Tahoe region, these two
nutrients are found in approximately equal
amounts in undisturbed watersheds (Brown, et
al., 1973). Since the required nitrate to
phosphate ratio for organisms is about 16:1,
nitrate is the limiting nutrient, and is hence
the major focus of Tahoe pollution research. It
is important to differentiate between the
nitrate concentration (mass per volume), and the
load (the total mass) delivered by the wa tershed, since the latter is what will actually
impact Lake Tahoe for example.
These processes are all closely linked with
the hydrologic system operating on local hillslope watersheds. Major hydrologic variables
include: inputs from rain and snowmelt, changes
in soil water storage, losses and gains from
fracture flow in the granite bedrock, and output
in the creek. Riparian and wetland areas are
key components of the watershed, contributing to
streamflow generation and response to rain/snowmelt events (Hewlett and Hibbert, 1967).
1
Paper presented at the North American
Riparian Conference. [University of Arizona,
Tucs02' April 16-18, 1985]
Jonathan Rhodes is Graduate Research
Assistant in Forest Hydrology, University of
Washi~gton, Seattle, WA.
C.M. Skau is Professor of Forestry and
Watershed Management, University of Nevada,
Reno'4 NV •
Daniel Greenlee is Graduate Research
Assistant in Hydrology/Hydrogeology, University
of Negada, Reno, NV.
David L. Brown is Graduate Research Fellow
in Hydrology/Hydrogeology, University of Nevada,
Reno, NV.
DESCRIPTION OF STUDY SITE
The study site is a 79.6 hectares (200
acre) headwaters watershed located on the east
slope of the Sierra Nevada near Spooner Summit
(above Carson City, Nevada). The watershed has
a base elevation of 2012 m (6800 ft) rising to
slightly over 2500 m (8200 ft). Northern
175
25
V/:/:\I Wet meadows
IDillillm Dense riparian zone
(dominated by
large alder and
\<Jill ow)
~
t;.!i..!.J Riparian zone with
ferns, small alders
and wi 11 ows
Figure 1.
2072
Clear Creek study watershed riparian and wetland areas.
aspec ts predomina te along this eas t-wes t
trending watershed with the majority of slopes
ranging from 20 to 50 percent.
These species are found along the perennial and
ephemeral channels, surrounding seep zones and
springs, and in the wet and dry meadows.
The watershed receives approximately 76
centimeters (30 inches) of precipitation annually. Over 80 percent of this amount occurs as
snow, with the remainder usually in the form of
summer and autumn rain events. The precipitation input of nitrates averages a fairly constant concentration of 34 mcg/l in fresh snow
(Rhodes, 1985). This value showed little variance, ranging between 15-50 mcg/l for both
record and below normal yearly precipitation
totals. For 1982/83, a record high snow year,
the total incoming load from rain and snow was
449 g/ha (Rhodes, 1985).
INSTRUMENTATION
The Clear Creek watershed was instrumented
beginning in 1982 to monitor the saturated subsurface system (46 piezometers), precipitation
(a recording precipitation gage and accessory
snowboards), streamflow (a recording gage on a
1.5 foot H-flume), and soil water content (36
neutron probe access tubes). The presence and
levels of nitrate in the saturated/unsaturated
zone were monitored using 40 pressure-vacuum
soil water lysimeters. Water analysis was performed on a Dionex ion chromatograph in an EPA
certified laboratory at the University of Nevada
Desert Research Institute.
The study area is drained by a perennial
first order tributary of Clear Creek. Also
present are several ephemeral and intermittent
channels and springs. Flows in the creek range
from 0.2 cfs in the late Summer and early Fall
to a maximum of 1.0 cfs during snowmelt. Sampling the creek (1982/83) at the outlet flume
showed an average nitrate concentration of 1
mcg/l (detection limits) which produces an
outlet load of 2.5 g/ha.
Riparian and wetland areas are the most
intensively instrumented and monitored sites in
the entire watershed. Since they respond to
hydrologic inputs in a very rapid manner, it was
deemed necessary to develop a monitoring system
which could measure small changes in groundwater
and soil moisture levels on a continuous basis.
The granodioritic soils support a mixed
pine-fir forest. Soils in the riparian and
spring zones are gleyed and mottled with
fractions of organic matter and clay larger than
the majority of the watershed. This is indicative of periodic, transient saturation.
A telemetered, remote data collection and
transmission system was developed with a local
firm, Scientific Engineering Instruments, Inc.
(SEI). The telemetry portion of the system has
been incorporated into the Soil Conservation
Service's SNOTEL network, and has been very
reliable under the demanding winter conditions
encountered at high altitudes. This system
delivers data directly to the University Watershed Lab, and thus allows real-time monitoring
of changes in the watershed.
The lower watershed is dominated by a 0.5
ha wet meadow on the north side of the creek and
a 1 ha wet meadow/seep zone on the south side
(fig. 1). Riparian species are predominately
willow (Salix spp.), red alder (Alnus tenuifolia), horse tail (Equise turn arvense), ferns
~rium felix-femina), sedges, and grasses.
In November, 1984, a transect in the lower,
northside meadow was instrumented to monitor
176
groundwater levels and soil moisture content
(fig. 1). SEI adapted an acoustical sensing
device to measure the depth to groundwater
inside the standard piezometers deployed at the
study site. Readings with 3 cm resolution are
taken at 10 minute intervals.
Direct measurements of denitrification were
carried out in the north-side wet meadow at the
bottom of the watershed using the acetylene
blockage technique (Ryden, et al., 1979a;
1979b). For the 111 samples taken thus far,
denitrification rates ranged between 0.138-2.41
grams nitrogen lost per hour per hectare
(0.341-7.265 g N/hr/acre).
Three sites comprise the transect and
follow the gradient of the hill roughly parallel
to the creek. The middle site has sensors on a
nested pair of piezometers in order to measure
the hydraulic head gradient. The upper and
lower sites both contain one sensor each. Subsurface flow pulses moving into and through the
meadow and riparian zones can thus be tracked as
the watershed response to snowmelt or rain.
Assuming the most conservative denitrifying
conditions (8 hours per day, June through
November), the 1984 to tal deni trif ica tion ra te
would have been 1770 g N/ha. While an order of
magnitude less than for low elevation
agricul tural fields (Ryden e t a1., 1979a), these
values are significant given the comparably low
concentration and load of nitrate contained in
the precipitation.
Changes in soil moisture, particularly the
downward advance of wetting fconts due to rain
infiltration, can be measured using a stack of
Coleman moisture/temperature probes with an SEI
interface module. The probes are emplaced at 25
cm, 40 cm, and 60 cm depths. Another anticipated use of these probes and the entire system
is to refine soil water sampling efforts by more
accurately detecting the presence of hydrologic
processes targeted for study, especially rain
events.
Another important finding of this study
concerns the conditions under which denitrification can occur in high elevation watersheds.
While soil water contents decrease during the
Summer and into the Fall, sufficient amounts of
saturated microsites remain so as to allow continuous denitrification in the meadows.
Microbial populations encountered in this
study also appear to have adapted to lower soil
temperatures than are reported for agricultural
fields. Low but significant (approximately
0.484 g N/hr/ha) rates of denitrification were
measured in November under a 15 cm snowpack at
soil temperatures of 1.6 (10cm), 2.9 (25cm), and
4.7 (50cm) degrees Centigrade.
NITRATE REMOVAL MECHANISMS
The difference between the precipitation
input and the stream's output represents a 99
percent removal of nitrate by an undisturbed
watershed. The magnitude of this removal is
even greater since the contribution of nitrogen-fixation has yet to be fully quantified.
This situation suggests a system which is either
biologically nutrient-starved or which has an
extremely high nitrate storage capacity. Even
though the ni tra te concen tra tions wi thin the
watershed and delivered in the stream are low,
it is the overall efficiency of removal that is
locally significant. The average nitrate concentration in the clearest center of Lake Tahoe
is correspondingly low, at about 14.6 mcg/l
(Goldman, 1982).
Apparent adaptations to high altitude
conditions indicate that denitrification could
be occurring year-round. Low level winter
denitrification rates might even increase during
mid-winter melts which periodically occur around
the meadow edges. The riparian meadows and
other wetlands which support populations of
denitrifying bacteria thus seem td be critical
natural controls in the transport of nitrate.
They also offset the nitrogen-fixation inputs of
nitrate from riparian and other species which
can amount to several kilograms of nitrogen per
hectare per year (Swank, 1984). An estimation
of this input will be made for Clear Creek.
This preliminary nutrient budget suggests
that outputs other than streamflow must be
investigated. Riparian and wetland nitrate removal appear to be the most important means of
nitrate depletion.
Coniferous Forest Nitrate Uptake
Nitrate assimilation quring the addition of
new forest growth may also be a significant
removal process. Using forest mensuration
techniques to measure annual biomass production,
a conservative estimation of the conifer nitrate
uptake for the Clear Creek watershed will be
attempted (Budy, 1985).
Biological Nitrate Removal
Microbial Denitrification
Of all the myriad biochemical transformations involving nitrogen in the nitrate (NO -)
form, denitrification - the reduction of ni~rate
to nitrous oxide (N 20) and N2 gas - has been
demonstrated as a hIghly significant removal
mechanism (Greenlee, 1985), accounting for a
nitrogen loss an order of magnitude higher than
the inputs from precipitation. Denitrifying
bacteria are active in the anaerobic saturated
zone or in saturated microsites in the unsaturated soil zone.
Ni tra te Storage in Soils
Working with the US Agricultural Research
Service in Reno, Nevada, efforts are under way
to measure the watershed's potential for nitrate
mineralization. Sediment nutrient adsorption is
also being studied in determining if nitrate is
being transported in the stream without showing
up in water quality analysis. This process is
177
especially relevant to the role of riparian
saturated zones since they are the major source
of sediment in local watersheds.
of the watershed. This effectively prevents any
significant nitrate removal, and increases the
pollution impacts downstream. Ongoing studies
with the US Geological Survey and the US Forest
Service Central Sierra Snow Lab are trying to
determine the extent of fractionation in Sierra
snowpacks. Results to date have been
inconclusive.
HYDROLOGIC ROLE OF RIPARIAN AND WETLAND
AREAS IN NUTRIENT TRANSPORT
Variable Source Areas
In addition to serving as nitrate removal
mechanisms, riparian and wetland areas may also
actually act as conduits which rapidly transmit
nitrates to the stream in response to rain
even ts, to ra in-accelerated snowmel t, or to
subsurface flow inputs from the upper watershed.
Described by Hewlett and Hibbert (1967),
riparian areas are usually at or near saturated
and thus respond hydraulically much more rapidly
than other parts of the watershed. Also, since
these saturated areas expand and contract in
response to the influx of water, they are both
temporally and spatially variable in their
influence on the stream.
CONCLUSION
The importance of riparian and wetland
areas in a watershed's nutrient transport system
is that of a net process, balancing vegetative
inputs, potential sediment outputs, microbial
and plant removal mechanisms, and hydrologic
transmissive properties. These areas appear to
be able to "clean up" nitrate-containing waters
with a very high degree of efficiency, and are
thus of major value in providing natural pollution controls for sensitive waters such as Lake
Tahoe. The major challenge is to further quantify parameters such as denitrification rates,
snow fractionation, plant nitrate assimilation,
and sediment adsorption potentials.
Coats, et al. (1976) and Melgin (1985)
concluded that nitrate removal mechanisms were
"short-circuited" when saturated areas prevented
water from coming into contact with the
soil-biological complex. In effect, these
variable source areas severely reduce water's
residence time in the soil, and thereby limit or
elimina te the possibil1. ty of biological ni tra te
uptake and removal. Paradoxically, it seems
that both extensive riparian/wetland areas and
extended water residence times are important
parameters in watershed nutrient transport.
Additional hillslope hydrologic research is
needed regarding the watershed flow characteristics - especially in the unsaturated zones.
Snow-related mechanisms such as melt, water
movement through snow, and the development of
macropores will be the focus of continuing
research to refine their role in nutrient transport. Despite their complexity, the hydrologic,
biologic, chemical, geologic, and atmospheric
processes which interact in a watershed must be
integrated in order to discern those key parameters which yield the greatest control over
nutrient transport. Once identified, these
controls can be developed into a predicative
tool which can identify those watersheds with
the greatest potential for the delivery of
pollutants into oligotrophic waters such as Lake
Tahoe.
Water Delivery Mechanisms
Overland flow is generally absent in the
Sierra Nevada since infiltration capacities
usually exceed rainfall and snowmelt rates.
Riparian and wetland soils show the most rapid
saturation in response to rain given their
higher antecedent moisture contents and shallow
water tables. These soils also show a fairly
high percentage of saturation (50 to 80 percent
has been measured) during winter months due to
groundmelt of the snowpack. This relatively low
level, slow melt occurs as a result of the
earth's heat loss, and causes wetlands to
saturate rapidly in response to the Spring
snowmelt.
LITERATURE CITED
Brown, J.C., C.M. Skau, and W. Howe. 1973.
Nutrient and sediment production from
forested watersheds. Proceedings: ASAE
Annual Meeting. (University of Kentucky,
Lexington, KY, June 17-20, 1973]
Budy, J. 1985. Personal conversation. Range,
Wildlife and Forestry Department, University of Nevada, Reno, NV.
Coats, R.N., R.L. Leonard, and C.R. Goldman.
1976. Nitrogen uptake and release in a
forested watershed, Lake Tahoe Basin,
California. Ecology 51:995-1004.
Goldman, C.R., R.P. Axler, and F.E. Reuter.
1982. Interagency Tahoe Monitoring
Program, water year 1981 preliminary
report. Prepared for Brown and Caldwell
Consulting Engineers. Sacramento, CA.
Greenlee, Daniel. 1985. Denitrification rates
of a mountain meadow near Lake Tahoe. M.S.
thesis (in progress), University of Nevada,
Reno, NV.
Snow Processes
Snowpacks can exhibit many properties similar to soil profiles. In some ways, snow is
much more dynamic than soil since snow morphology can alter drastically or even disappear completely in a very short time. One particular
snow property which may affect nutrient transport is the fractionation process.
Studies have shown that this process can
concentrate nitrates at the bottom of the pack
over the Winter (Johannessen and Henriksen
1978). Spring melt and subsequent runoff causes
a "pulse" of ni tra tes to be sen t through and ou t
178
Rhodes, Jonathan J. 1985. A reconnaissance of
hydrologic nitrate transport in an undisturbed watershed near Lake Tahoe. M.S.
Thesis, University of Nevada, Reno, NV.
Ryden, J.C., L.J. Lund, and D.D. Focht. 1979a.
Direct measurement of denitrification loss
from soils: I. Laboratory evaluation of
acetylene inhibitation of nitrous oxide
reduction. Soil Science American Journal
43:104-110.
Ryden, J.C., L.J. Lund, J. Le tey , and D.D.
Focht. 1979b. Direct measurement of
denitrification loss from soils: II.
Development and application of field
methods. Soil Science American Journal
43:110-118.
Swank, Wayne T. 1984. Atmospheric contributions to forest nutrient cycling. Water
Resources Bulletin 20(3):313-321.
Hewlett, J.D., and A.R. Hibbert. 1967. Factors
affecting the response of small watersheds
to precipitation in humid areas. In International Symposium on Forest Hydrology. 813
p. Pergammon Press, Oxford.
Johannessen, J., and A. Henriksen. 1978.
Chemistry of snow meltwater: Changes in
concentration during melting. Water
Resources Research 14:614-619.
Kramer, James R. 1972. Phosphorus: analysis
of water, biomass and sediment. p. 51-100.
In Nutrients in natural waters. 457 D.
John Wiley and Sons, Inc., New York,
Melgin, Wendy. 1985. The influence of hillslope hydrology on nitrate transport in a
forested watershed, near Lake Tahoe. M.S.
Thesis (in progress), University of Nevada,
Reno, NV.
NY.
179