Download Nitrogen acquisition from different spatial distributions by six Great

Western North American Naturalist
Volume 61 | Number 1
Article 12
1-29-2001
Nitrogen acquisition from different spatial
distributions by six Great Basin plant species
Sara E. Duke
Utah State University
Martyn M. Caldwell
Utah State University
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Recommended Citation
Duke, Sara E. and Caldwell, Martyn M. (2001) "Nitrogen acquisition from different spatial distributions by six Great Basin plant
species," Western North American Naturalist: Vol. 61: No. 1, Article 12.
Available at: http://scholarsarchive.byu.edu/wnan/vol61/iss1/12
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Western North American Naturalist 61(1), © 2001, pp. 93–102
NITROGEN ACQUISITION FROM DIFFERENT SPATIAL DISTRIBUTIONS
BY SIX GREAT BASIN PLANT SPECIES
Sara E. Duke1 and Martyn M. Caldwell1,2
ABSTRACT.—Plants of different growth form may utilize soil nutrients in various spatial distributions through different
scales of foraging. In this study we evaluated the ability of 6 species commonly found in the Great Basin to utilize nitrogen (N) distributed in different patterns. Three growth forms were represented by these 6 species.
We applied 15N-labeled nitrogen in concentrated patches and over broader uniform areas (at approximately 1% the
concentration of the patches) in large, outdoor sand-culture plots. Six weeks after N was applied, 2 plants adjacent to the
patch (Patch Treatment) and 2 plants within the uniform application (Uniform Treatment) were harvested. One plant
35–45 cm from both applications (Distant Treatment) was also harvested. The proportion of application-derived N in the
leaf N pool was calculated and the mass of N this represented was estimated.
Winter annual species Aegilops cylindrica and Bromus tectorum utilized the concentrated patches to a greater extent
than did perennial species. The mass of N acquired by Patch-Treatment annual plants was significantly greater than by
Uniform- and Distant-Treatment plants. Annual plants in the Distant Treatment had very little application-derived N in
their leaf tissue. The perennial tussock grasses Agropyron desertorum and Pseudoroegneria spicata differed in utilization
of the N applications. Agropyron acquired a greater quantity of N from patches than from uniform applications, and Distant-Treatment plants acquired very little from treatment applications. On the other hand, Pseudoroegneria utilized N in
the 3 treatments equally. The shrub species Artemisia tridentata ssp. vaseyana and Chrysothamnus nauseosus also differed in their pattern of N acquisition. There were no differences in quantity of N acquired by plants from different
treatments for Chrysothamnus; all treatment plants acquired appreciable amounts of N from the applications. In contrast, Artemisia tridentata was very effective at acquiring large quantities of N from patches relative to Uniform- and
Distant-Treatment plants, and yet there was still appreciable acquisition of applied N by Distant-Treatment Artemisia
plants.
We compared our results for these species in utilizing N patches with their ability to utilize N pulses (Bilbrough and
Caldwell 1997). The annual grasses, Artemisia, and Agropyron were capable of effectively acquiring N from both pulses
and patches, whereas Pseudoroegneria was effective in exploiting pulses but not patches. Chrysothamnus was generally
not responsive to either patches or pulses.
Our results suggest that the 2 shrubs and 2 perennial grasses differed in the scale at which they foraged for nutrients. Some species exhibited a coarse-scale utilization of nutrients while others were clearly capable of fine-scale utilization of spatially distributed nutrient sources. This suggests the potential for at least some spatial niche separation among
these species.
Key words: spatial heterogeneity, soil nitrogen distribution, sagebrush steppe, Great Basin, Aegilops cylindrica,
Agropyron desertorum, Artemisia tridentata, Bromus tectorum, Chrysothamnus nauseosus, Pseudoroegneria spicata.
Campbell et al. 1991). Utilization of a nutrientenriched soil patch is commonly associated with
the ability of a plant to elevate root physiological uptake capacity and proliferate roots in
these local zones (Drew and Saker 1975, Jackson and Caldwell 1989). Plant communities
composed of species that vary in growth form
and size have been postulated to exhibit different strategies for acquiring nutrients when
resources are not uniform in space or time
(Campbell and Grime 1989, Grime 1994).
Coexisting species that potentially compete for
the same resources are thought to segregate
Supplies of soil resources to plants are
characterized by spatial heterogeneity and
temporal variation in many natural environments (Grime 1994). Plant characteristics that
allow plants to acquire resources from spatially or temporally variable nutrient supplies
can differ among species and likely depend on
the evolutionary history of the species (Grime
1977, Chapin 1980, 1988, Aarssen 1983, Tilman
1986). Rapid acquisition of nutrients from
pulses requires the presence of an active root
system to take up nutrients when they first
become available (Campbell and Grime 1989,
1Department of Rangeland Resources and the Ecology Center, Utah State University, Logan, Utah 84322-5230.
2Corresponding author.
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WESTERN NORTH AMERICAN NATURALIST
resource use both spatially and temporally
(Aarssen 1983, Veresoglou and Fitter 1984,
Fitter 1986, McKane et al. 1990).
Sagebrush-steppe communities of the semiarid Intermountain West receive moisture primarily from late fall through spring. Summers
are warm and dry. Plant nutrient availability
corresponds with soil moisture in fall and spring
and is greatly reduced in summer months as
soils dry (Nye and Tinker 1977, Crawford and
Gosz 1982, Barber 1995).
Phenological differences among species can
result in different degrees of temporal coupling between nutrient uptake and its use for
growth and reproduction. For example, in the
sagebrush-steppe, perennial tussock grasses
and winter annuals initiate growth in early
spring from overwintered tillers that began
growth in the fall (Nowak and Caldwell 1984).
Tiller growth accelerates and seed production
occurs in late spring and early summer, followed by death of annuals or dormancy of
perennials in early to midsummer. The shrubs
Artemisia tridentata and Chrysothamnus nauseosus initiate belowground growth early in
the spring (Fernandez and Caldwell 1975), but
vegetative growth first begins in early summer
and flowering occurs in late summer (Bilbrough
and Caldwell 1997). Thus, within a sagebrushsteppe community the timing of nutrient acquisition in spring and its subsequent use by different species can vary considerably.
Recently, Bilbrough and Caldwell (1997)
investigated the ability of 6 species common
to the Great Basin to procure nitrogen (N)
from nitrogen-rich pulses at different times
during the spring growing season. Plant growth
was monitored throughout the growing season
for each species. They concluded that most
species exhibited a greater growth advantage
by the onset of summer when they received a
single 4-day pulse of N in the spring than when
they received the same quantity of nitrogen
applied over 10 weeks in the spring. The
species differed as to how effectively they
acquired N from the pulses, but there were
only small differences in the timing when
pulses were best utilized among the species.
Therefore, temporal niche separation of N
acquisition among the species during spring
was apparently limited (McKane et al. 1990,
Bilbrough and Caldwell 1997).
Superimposed upon temporal variability in
nutrient resource distribution is spatial hetero-
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geneity (Charley and West 1977, Crawford and
Gosz 1982, Jackson and Caldwell 1993, Halvorson et al. 1995, Schlesinger et al. 1996). Spatially variable nutrient distribution occurs due
to redistribution of resources by perennial
species (Charley and West 1977, Hook et al.
1991), animal defecation, urination, and death,
as well as microbial activity (Stark 1994). Root
proliferation and changes in nutrient uptake
kinetics are thought to contribute significantly
to plant acquisition of nutrients from enriched
soil patches (Jackson and Caldwell 1989, Jackson et al. 1990, Robinson 1994).
Our objective was to evaluate the ability of
6 Great Basin species representing 3 life forms
to acquire N from different spatial distributions.
The relative capability of species for coarsescale and fine-scale foraging of nutrients is
one aspect of potential spatial niche separation
among coexisting species (Campbell et al.
1991, Grime 1994). We evaluated this potential by comparing the ability of these species
to acquire N from (1) locally enriched patches,
(2) a uniform distribution surrounding the plant,
and (3) enriched areas at a distance of 35–45
cm. Due to phenological differences among
the selected species, it was necessary to create
spatial patterns of N that would persist over a
sufficiently long period of time in the spring
such that all species would have the opportunity to experience N applications when they
were active. We used an 15N-labeled ammonium-loaded clinoptilolite zeolite as a slowreleasing N source (Ming and Mumpton 1989).
The 6-week experimental period overlapped
active growth stages of all 6 species (Bilbrough
and Caldwell 1997). We focused on total quantity of N acquired from zeolite applications
and translocated to current year’s leaf and the
proportion of the leaf N pool that this newly
gained N represented.
METHODS
The research was conducted in spring 1995
at Utah State University Green Canyon
Research Center, located approximately 4 km
northeast of Logan, Utah (41°45′N, 111°48′W,
1460 m elevation). Annual precipitation in
Logan comes largely as snow in the winter,
but spring months receive 44 mm of rain on
average.
Our experiment was conducted in large, unlined, sand-filled plots (5 m × 18 m, 1 m deep)
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NITROGEN ACQUISITION BY GREAT BASIN PLANTS
where plants were exposed to field conditions.
There were 4 sand pits each planted with 6
species common to the Great Basin of the
Intermountain West and used in earlier studies (Bilbrough and Caldwell 1997). The 4
perennial species were planted in 1992 as
monoculture subplots within each sand plot.
Subplots with perennial species were ~15 m
× 1.5 m and were arranged parallel to each
other along the length of the sand plots. Subplots with annual species were located at the
end of each sand plot. Within each species
subplot were 4 separate experimental units
(approximately 1.5 m2 for perennials and 0.5
m2 for annuals; Fig. 1). Experimental treatments
had been applied to the perennial species’ experimental units in previous years. Each species
had received N (but no 15N) in the form of a
pulse the previous 2 years, and these N pulses
were applied to various experimental units at
different times during the spring (Bilbrough
and Caldwell 1997). For each species, we used
2 prior experimental units from each subplot
in each sand plot (i.e., n = 8 experimental
units). Prior treatments were included in the
ANOVA as an experiment factor. The ANOVA
factor PRIOR and all interaction terms with
this factor were not statistically significant for
any response variable for any species. Furthermore, N tissue concentrations between plants
from different prior treatment experimental
units were not statistically different. Therefore,
we believe our N acquisition results were not
strongly influenced or confounded by these
prior N applications.
Perennial shrubs were transplanted as small
seedlings and perennial tussock grasses as
clumps of tillers in 6 parallel rows within each
subplot. We staggered plants in adjacent rows
with consistent spacing between all plants, creating a uniform distribution (Fig. 1). Spacing
between plants necessarily varied because of
size differences of the growth forms. The
indigenous shrubs Artemisia tridentata ssp.
vaseyana (Rydb.) Beetle and Chrysothamnus
nauseosus (Pallas) Britt. were planted at an
interplant distance of 25 cm. The introduced
and indigenous perennial tussock grasses
Agropyron desertorum (Fisch. ex Link) Schult
and Pseudoroegneria spicata ([Pursh] A. Löve),
respectively, were planted at 20-cm spacing.
After 2 years, however, the interplant distance
was approximately 15 cm in perennial grass
subplots. In early March 1995 seeds of the
95
Fig. 1. Diagram of plant location and N-amendment
placement in experimental units (EU). Represented here
are (A) a perennial grass EU, and (B) an annual grass EU.
Spacing among shrub plants was 25 cm, 20 cm among
perennial grass plants, and 5 cm among annual grass
plants. Each dot represents an individual plant. Solid-line
circles represent patch applications and solid-line rectangles represent uniform applications; dashed circles represent potential target plants to be harvested. Shrub EUs
were similar in layout and treatment application to perennial grass EUs. The Uniform Treatment in the shrub EU
covered 2 plants in adjacent rows; otherwise treatment
coverage was the same as in perennial grass EUs.
introduced exotic annual grasses Bromus tectorum (L.) and Aegilops cylindrica Host. were
planted at 5-cm spacing in two 0.5-m2 experimental units within each sand plot (Fig. 1B).
Because plants were growing in sand, they
required light watering daily. To maintain
some background nutrient availability in these
sand cultures, we applied a 20% Hoaglands
solution (with 5% N) through the drip irrigation system which was positioned between
plant rows. After a 45-minute watering period,
the entire surface of the sand was wetted.
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WESTERN NORTH AMERICAN NATURALIST
Plants showed no signs of nutrient stress this
year or in previous years.
We utilized the mineral clinoptilolite zeolite to create local areas of increased nitrogen
availability. Clinoptilolite zeolite is composed
of a crystalline lattice with a very high surface
area that is negatively charged with an affinity
for NH4+ (Ming and Mumpton 1989). Pulverized zeolite can easily be “loaded” with NH4+
by soaking in concentrated NH4Cl solution,
allowing the internal sites to become occupied
by the cation NH4+. We soaked 200 g of
crushed zeolite for 7 days on a shaker table in
a 2% 15NH4Cl-labeled 1-M NH4Cl solution.
We then repeatedly rinsed the zeolite with
deionized distilled water to remove the chloride. Zeolite was oven-dried for 3 days and
stored in a desiccator. Following the “loading”
process, 1 g of dry NH4+-loaded zeolite contained 2.3 mg of N. Zeolite released the NH4+
+ 15NH4+ in exchange for other cations, such
as Ca2+, K+, and Mg2+ in our diluted Hoaglands solution; thus, it was effective in creating a long-term nutrient patch in this sand system, since zeolite would release N slowly over
time and remain localized.
Due to plant size differences (e.g., 3-yearold shrubs versus 3-week-old annual seedlings),
we used different quantities of NH4+-loaded
zeolite and created different size application
areas for each growth form. The quantity of
zeolite used for each growth form was based
on the spacing of plants such that uniform
applications were approximately equal to previous years’ background nitrogen fertilization
(0.043 g N m–2). Patches for shrubs were
applied to 9.5-cm-diameter circular areas with
1.65 g zeolite placed on the surface in the
space equidistant between 3 plants (Fig. 1A).
Uniform applications were 900 cm2 with the
same quantity of zeolite covering the soil surface; this included 2 shrubs in separate rows
(modification of uniform application in Fig. 1A).
These application rates correspond to 5.3 g N
m–2 and 0.042 g N m–2 for the patch and uniform treatments, respectively. Perennial grass
circular patches were 7 cm in diameter to which
1.1 g of zeolite was applied over the soil surface in the space equidistant between 3 plants.
Uniform applications were 600 cm2 with the
same amount of zeolite and encompassed 3
plants in 2 adjacent rows (Fig. 1A). Application
rates for perennial grasses corresponded to 6.6
g N m–2 and 0.042 g N m–2 for the patch and
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uniform treatments, respectively. Patches in
annual plots were 4.5 cm in diameter and uniform applications were 300 cm2, each with 0.55
g zeolite, corresponding to application rates of
8.0 g N m–2 and 0.042 g N m–2, respectively
(Fig. 1B).
Patch and uniform zeolite applications were
created within each experimental unit on 3
May 1995. Plants were harvested 14–20 June.
This experimental period was long enough for
zeolite to release much of the 15N-labeled
ammonium (J.L. Boettinger unpublished) and
overlapped the growth period of all 6 species
(Bilbrough and Caldwell 1997). In each perennial species experimental unit, 2 plants adjacent
to the patch (Patch Treatment), 2 plants within
the uniform area (Uniform Treatment), and 1
plant 35–45 cm away from both applications
(Distant Treatment) were harvested. Tissues
for the 2 individuals harvested from Patch and
Uniform Treatments were not pooled for N
analysis. Two or 3 plants were between the
Distant Treatment plant and zeolite applications in each experimental unit (Fig. 1). Immediately following harvest, aboveground tissue
for shrubs was partitioned into current year’s
growth and prior years’ growth. Perennial
grasses were separated into living tiller material and seed. For the annual grasses there was
only leaf tissue.
Soil cores were taken from the interspace
among plants in the Patch and Uniform Treatments and adjacent to the single distant plant
to assess local root morphological parameters.
Cores for shrubs and perennial grasses were
10 and 7.5 cm in diameter, respectively, slightly
larger than the diameter of patch treatments.
All soil cores were 20 cm deep. Roots were
separated from the soil cores and washed free
of sand and other debris. All plant material
was processed immediately, frozen in liquid
N2 in the field, and kept on dry ice until processing.
Plant material was freeze-dried for 48 hours
and then weighed. A subsample of roots was
rehydrated overnight and root length for each
sample was assessed using a Comair root length
scanner (Melbourne, Australia). The current
year’s leaf material was then finely ground.
Total nitrogen content and 15N enrichment of
leaf and root tissues were determined by a
continuous flow direct combustion mass-spectrometer (ANCA 2020 system, Europa Scientific Inc., Cincinnati, OH). Nitrogen acquired
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NITROGEN ACQUISITION BY GREAT BASIN PLANTS
by plants from the experimental treatments
was determined based on 15N enrichment in
the leaf tissue in excess of background 15N
(which we assumed to be constant at 0.36599
atomic %). Total N in the tissue was estimated
based on total tissue mass and tissue concentration. Since zeolite was 2% enriched with
15N, we could calculate the proportion of total
tissue N derived from our treatments. Mass of
N acquired from treatments and translocated
into current year’s growth was calculated as
the proportion of treatment-derived N multiplied by total tissue N for each plant.
Statistical Analysis
Statistical analyses were performed at the
species level to test for differences among N
levels in treatment plants. Comparisons among
species and growth forms were qualitative. We
performed all analyses as a randomized block
design with prior pulse treatments and distribution treatments (Patch, Uniform, and Distant) as fixed effects in the model. Tests for
normality were performed on each response
variable. Proportion data were transformed with
a modified log transformation, and amounts of
total N acquired were transformed with a
square root transformation to approximate
normality. ANOVA analyses were performed
on transformed variables using a mixed-model
analysis (Proc Mixed; SAS 1993) for the variables proportion of N acquired, mass of N
acquired, and specific root length. P-values
presented for explicit comparisons of means
among treatments were Bonferroni adjusted.
Means and standard errors for each variable
were back-transformed for graphical presentation of the data. Statistical power was low for
this experiment because of small sample sizes;
therefore, we did not adhere to a strict interpretation of statistical significance for P-values
≤ 0.05. P-values are presented in the results,
allowing readers to form their own conclusions.
There was high variability in the quantity
of N acquired between the 2 Artemisia plants
within each experimental unit of our Patch
Treatments, and this contributed to large standard errors for this variable. We therefore
defined a new variable, total N acquired by
the 2 plants harvested for each treatment, by
summing the mass of N acquired from the
treatment applications for the 2 plants in the
Patch and the 2 plants in the Uniform Treatments within each experimental unit. We ana-
97
lyzed this new variable in the same manner as
described above.
RESULTS
All 6 species acquired N from patch and
uniform applications. The proportion of N in
leaf tissue derived from zeolite applications
was greater for annual species (Figs. 2a, b)
than for perennials (Figs. 3a, b, 4a, b). The
proportion was significantly greater for both
species of annual plants adjacent to patches
(0.20 and 0.28 for Aegilops and Bromus, respectively, P = 0.0001) than for these annuals
within the uniform distribution (0.06 and 0.12,
respectively, P = 0.0001, Figs. 2a, b). For the
annual species, nitrogen acquired by plants
adjacent to patches and within uniform applications was significantly greater than N
acquired by Distant-Treatment plants (P ≤
0.03). A similar pattern was apparent for the
mass of N procured. Aegilops and Bromus
plants both acquired a greater quantity of N
from localized patches than from the uniform
application (P = 0.0001, Figs. 2c, d). DistantTreatment plants did acquire N from the
applications (P = 0.001), although significantly
less than Patch- and Uniform-Treatment plants
(P = 0.0001).
For the perennial grasses, Agropyron PatchTreatment plants exhibited a significantly
greater proportion of new N in leaf tissues and
also a greater mass of acquired N than Uniformand Distant-Treatment plants (P = 0.002 and
P = 0.001, Figs. 3a, c, respectively). However,
proportions of N in the leaf tissue and mass of
N acquired from applications were not significantly different among the 3 treatments for
Pseudoroegneria (P = 0.25 and P > 0.26, Figs.
3b, d, respectively). This suggests that zeoliteN applications were utilized with equal effectiveness by all Pseudoroegneria plants within
the experimental unit regardless of their proximity to the application area.
The 2 shrubs also differed in their acquisition of N from the treatments. There were no
differences in average mass of leaf N acquired
by plants from the 3 treatments for Chrysothamnus (P = 0.64, Fig. 4d). Artemisia exhibited
significant differences among treatments for
both the proportion of N and the quantity of N
derived from zeolite applications (P = 0.05
and P = 0.06, Figs. 4a, c, respectively). PatchTreatment plants acquired a greater mean
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WESTERN NORTH AMERICAN NATURALIST
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Fig. 2. Mean proportion of leaf N pool (a, b) and mean
mass of N (c, d) derived from zeolite-N application
appearing in leaf tissues for Aegilops and Bromus. Open
bars represent plants adjacent to the patch application,
solid bars represent plants in the uniform application, and
hatched bars represent plants 35–45 cm from the application. Vertical bars are standard errors based on n = 8
EUs. Differences among means (P < 0.05) are indicated
by different letters.
Fig. 3. Mean proportion of leaf N pool (a, b) and mean
mass of N (c, d) derived from zeolite-N applications appearing in leaf tissues for Agropyron and Pseudoroegneria.
Open bars represent plants adjacent to the patch application, solid bars represent plants in the uniform application, and hatched bars represent plants 35–45 cm from
the application (n = 8). Vertical bars are standard errors
based on n = 8 EUs. Differences among means (P < 0.05)
are indicated by different letters.
quantity of N than did Uniform- or DistantTreatment plants (P = 0.12, Fig. 4c). But Distant-Treatment plants for both shrub species
had a significant quantity of applicationderived N in their leaf tissues (Figs. 4c, d).
In the Artemisia data plant-to-plant variability within treatments was particularly large.
Of the 2 plants harvested in Patch Treatments,
one often acquired a much greater quantity of
N than its neighbor. Analysis performed on
total N acquired by the 2 plants revealed that
Artemisia procured more N from the Patch
than from the Uniform Treatment (P = 0.005).
Such high plant-to-plant variability within
treatment pairs was not apparent for any other
species; therefore, the same analysis was not
warranted.
among treatments for one species. Artemisia
was the only species that showed a significant
increase in SRL in the Patch Treatment compared with the Distant (P = 0.01) and Uniform Treatments (P = 0.08, Fig. 5).
Root Proliferation
There were no differences in root mass
within soil cores between the patch- or uniform-application areas and the unamended soil
for any of the species. However, specific root
length (SRL), root length per mass, did differ
DISCUSSION
At least one species of each growth form
obtained a greater quantity of N from patches
than from the uniform distribution: Artemisia,
Agropyron, and both annual grass species
(Figs. 2, 3, 4, 6). The shrub Artemisia and the
tussock grass Agropyron both utilized the concentrated patch better than the uniform application (ratio of N acquired from patch:uniform
> 2, Fig. 6), whereas the shrub Chrysothamnus and the tussock grass Pseudoroegneria utilized N from the patch and uniform applications equally (ratio of N acquired from patch:
uniform ~1, Fig. 6). Thus, for the perennial
species, effective acquisition of a localized
source of N did not simply correspond with
growth form (Campbell et al. 1991).
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NITROGEN ACQUISITION BY GREAT BASIN PLANTS
99
Fig. 5. Mean specific root length (SRL), root length per
mass, for the 4 perennial species for the Patch, Uniform,
and Distant Treatments. Means represent 8 soil cores.
Vertical bars are standard errors based on n = 8 EUs. Differences among means (P < 0.05) are indicated by different letters.
Fig. 4. Mean proportion of leaf N pool (a, b) and mean
mass of N (c, d) derived from zeolite-N applications appearing in leaf tissues for Artemisia and Chrysothamnus. Open
bars represent plants adjacent to the patch application,
solid bars represent plants in the uniform application, and
hatched bars represent plants 35–45 cm from the application (n = 8). Vertical bars are standard errors based on n
= 8 EUs. Differences among means (P < 0.05) are indicated by different letters.
Root characteristics that provide the capacity to utilize spatially concentrated N patches
may not necessarily be the same as those that
enable effective utilization of an N pulse. Our
study and that of Bilbrough and Caldwell
(1997) conducted in the same sand-plot system provide the opportunity to compare the
ability of these species to utilize patches and
pulses of N. Five of 6 species we tested were
common to both studies. Bilbrough and Caldwell (1997) used another aggressive exotic
annual grass, Teaniatherum caput-medusae, for
their N-pulse study rather than Aegilops cylindrica as used in our study. Of the species
tested for capacity to utilize N pulses here, all
except Chrysothamnus exhibited a growth
advantage when they received a 4-day concentrated pulse of N compared to a 10-week
continuous-control N application; however,
this varied in degree (Fig. 7). It appears from
our results and those of Bilbrough and Caldwell (1997) that Bromus has the ability to
effectively procure N from both concentrated
pulses and patches. The 2 annual grasses not
common to both studies, Aegilops and Teania-
therum, behaved similarly to Bromus in effectively acquiring N from patches and pulses,
respectively. These 3 exotic annual grasses
have similar growth habit and phenology and
are common aggressive weeds in agricultural
fields and rangelands (Donald and Ogg 1991,
Young 1992, Billings 1994). Their flexibility in
effectively using patches and early pulses of N
may contribute to their effectiveness in competing with Great Basin perennials.
Both perennial grasses appeared capable of
translating N acquisition from spring pulses
into a significant growth advantage (Fig. 7);
however, these 2 grass species differed in their
capacity to procure N from patches, as mentioned above (Fig. 6). Agropyron proved to be
facile in exploiting patches while Pseudoroegneria did not respond to patches, but appeared
to possess a more broadly distributed active
root system (Fig. 3). It was better able to take
advantage of N in distant patches than was
Agropyron. Shrub species behaved differently
in their ability to utilize both N pulses and
patches. Artemisia responded well to both
pulses and patches, while Chrysothamnus
acquired little N from different spatial distributions and did not respond to pulses. In a
similar vein, Chrysothamnus was not able to
utilize soil moisture in upper soil layers derived
from summer precipitation events (Ehleringer
et al. 1991, Lin et al. 1996). In general, it
appears this species is rather unresponsive to
resource opportunities in upper soil layers. In
contrast, Artemisia effectively utilized N pulses
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WESTERN NORTH AMERICAN NATURALIST
Fig. 6. Ratio of mean mass of N in leaf tissues acquired
by plants adjacent to patches to mean mass of N in leaf
tissues acquired by plants in the uniform applications.
in the spring (Bilbrough and Caldwell 1997),
and roots in upper soil layers remained responsive to pulses of moisture following periods of drought (Ehleringer et al. 1991, Lin et
al. 1996). Based on its ability to obtain N effectively from all 3 distributions, Artemisia apparently has a broad distribution of roots capable
of both coarse- and broad-scale nutrient foraging. Although the 2 shrub species occur in
similar habitats and have a generally similar
general growth form and phenological progression, they appear to be functionally different
in use of shallow soil resources (Ehleringer et
al. 1991, Donovan and Ehleringer 1994, Lin et
al. 1996, Bilbrough and Caldwell 1997).
The ability to allocate a large proportion of
actively absorbing roots in nutrient-rich soil
volumes should be of advantage in acquiring
N from such patches. Campbell et al. (1991)
found that dominant species that captured the
most mineral resources were those exhibiting
high belowground potential growth rates. These
fast-growing species were not, however, particularly precise in allocating roots to resourcerich patches. Grime (1994) maintained that
slower-growing species with high precision in
allocating roots to resource-rich patches would
be subdominant species that retain their position in the community by their precision in
root foraging. However, in our study relative
growth rates of the species were not inversely
correlated with precision in foraging. Fast-growing species in our study, such as the annual
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Fig. 7. Greatest growth exhibited from 1 of 3 spring N
pulses expressed as an increase in plant biomass relative
to control plants. Data were extracted from Bilbrough and
Caldwell (1997).
grasses and Artemisia, exhibited a high precision in foraging in that they clearly acquired
the greatest proportion of N from enriched
patches. Also, both perennial grasses in our
study have similar growth rates (Caldwell et
al. 1991, Bilbrough and Caldwell 1997) but
differ strikingly in their precision of foraging.
Results from our large, outdoor sand-culture experiment allow us to identify potential
patterns of spatial soil resource use by these
co-occurring Great Basin species. We do not
suggest that these are necessarily the tactics
employed by these species in nature. However, our results do identify differences among
species use of localized nutrient patches and
precision of foraging. Differences in spatial use
of mineral resources may contribute to spatial
niche separation in Great Basin communities.
However, temporal niche separation expressed
in the ability of these species to utilize pulses
of N through time is not necessarily correlated
with spatial foraging patterns.
ACKNOWLEDGMENTS
This research was supported by the National
Science Foundation (DEB 9208212, DEB
9807097) and the Utah Agricultural Experiment Station. We greatly appreciate the field
and laboratory assistance of Tim Hudelson,
Valerie Tinney, and Holly Wolsey. Further, we
gratefully acknowledge the statistical advice of
Susan Durham and Dr. Richard Cutler and
comments on earlier drafts of this manuscript
by Dr. Neil West.
2001]
NITROGEN ACQUISITION BY GREAT BASIN PLANTS
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Received 7 July 1999
Accepted 7 January 2000