Heterogeneity in Soil C and N in
Late- and Mid-Seral SagebrushGrass Rangeland
J. D. Reeder
D. W. Grant
G. G. Jezile
R. D. Child
Abstract: Development of criteria and indicators that can be used
to evaluate nutrient cycling among plants, animals, and the soil is
essential for evaluating rangeland health and sustainability, as
well as for developing proper rangeland management practices. The
distribution of nutrients in space and time has been identified as a
useful criterion for evaluating the integrity of rangeland nutrient
cycles and energy flow in rangelands. Spatial distributions of soil
organic carbon (C) and organic and mineral nitrogen (N) were
measured in both mid- and late-seral stages of a sagebrush-grass
association. Vegetation cover differed between the two seral stages,
with grass cover significantly higher (15 versus 3 percent), and bare
ground area significantly lower (56 versus 69 percent) for the lateseral compared to mid-seral plant communities. The spatial distributions of soil organic C and N differed between the two seral stages
and corresponded to differences in vegetation cover. The proportion
of total soil N in the nitrate form was significantly higher in midseral soil than in late-seral soil, suggesting less efficient capture of
available N by the fragmented mid-seral plant community and an
uncoupling of the N cycle. Soil nitrate-N shows promise as a sensitive
indicator of seral stage in sagebrush-grass plant communities.
Introduction ___________________
In developing a recommended approach for evaluating the
ecological health of rangeland ecosystems, the National
Research Council (1994) recognized the fundamental importance of nutrient cycling to ecosystem health, and recommended that any comprehensive evaluation of rangeland
health and sustainability include indicators of the integrity
of nutrient cycles. They suggested that the best indicator of
rangeland nutrient cycling integrity is an assessment of the
distributions of nutrients in space and time. Soil organic
matter (SOM) is the primary source of plant-available nitrogen (N), the nutrient that most frequently limits primary
In: Hild, Ann L.; Shaw, Nancy L.; Meyer, Susan E.; Booth, D. Terrance;
McArthur, E. Durant, comps. 2004. Seed and soil dynamics in shrubland
ecosystems: proceedings; 2002 August 12–16; Laramie, WY. Proceedings
RMRS-P-31. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Rocky Mountain Research Station.
J. D. Reeder is a Soil Scientist and D. W. Grant a Biological Science
Technician, USDA-ARS, 1701 Centre Ave., Fort Collins, CO 80526, U.S.A.
FAX: (970) 492-7310, e-mail: jeanreeder@ars.usda.gov. G. G. Jezile was a
Graduate Student and D. W. Child is a Professor of the Department of
Rangeland Ecosystem Science, Colorado State University, Fort Collins, CO
80521, U.S.A.
USDA Forest Service Proceedings RMRS-P-31. 2004
and secondary productivity in rangeland ecosystems
(Woodmansee and others 1978). The distribution and turnover of SOM is therefore a major determinant of ecosystem
health and stability (Follett 2001).
Historically, rangeland studies have focused on the
aboveground components of the ecosystem, and relatively
few studies have evaluated belowground components and
processes. Basic knowledge is therefore limited concerning
the processes governing SOM distribution and turnover,
and the response of these processes to management or
disturbance. Additionally, detection of change in the mass or
distribution of SOM as an indication of change in rangeland
health is difficult because rangeland ecosystems are generally characterized by a high degree of spatial heterogeneity
in SOM (Burke and others 1999; Tongway and Ludwig
1994). The magnitude of change expected as the result of
disturbance or change in management is small relative to
the large SOM pool size and its spatial heterogeneity across
the landscape (Reeder 2002).
To address these limitations in our understanding of
rangeland nutrient cycling and change in cycle integrity in
response to management or disturbance, we compared soil
and vegetation characteristics of two seral stages of a
sagebrush-grass range to test the hypothesis that the
distributions and forms of SOM carbon (C) and N differ
between late- and mid-seral stages of a sagebrush-grass
plant community. The objectives were to (1) characterize
and quantify differences in soil C and N in late- and midseral stages of a Dry Mountain Loam sagebrush-grass
range; and (2) to identify forms of soil C and/or N that are
sensitive indicators of seral stage.
Methods and Materials __________
Study Site Description
The study was conducted on a Dry Mountain Loam Range
Site south of Walden, CO, at an elevation of 2,500 m on the
Cabin sandy loam soil series (fine loamy over sandy or
sandy-skeletal, mixed Argic Cryoboroll). Average annual
temperature is 2.8 ∞C, and average annual precipitation is
240 mm, with about two-thirds of annual precipitation
falling as rain during the growing season (NRCS 1981). The
Dry Mountain Loam Range Site consists of cool season
grasses and forbs that form a sparse stand beneath an open
69
Reeder, Grant, Jezile, and Child
stand of big sagebrush (Artemisia tridentata Nutt.). Estimated annual production ranges from 450 to 700 kg ha–1
(NRCS 1981).
Soil and vegetation sampling was conducted on two seral
stages (mid-seral and late-seral) with different management
histories. The areas selected for this study were based on the
range condition classification outlined by the Natural Resource
Conservation Service (NRCS 1981), and seral stages assigned
in 1997 by the Owl Mountain Partnership, Walden, CO.
Historic grazing management of the late-seral stage consisted of decades of heavy-to-moderate season-long grazing
prior to 1988, when the Bureau of Land Management implemented an allotment improvement program to improve
livestock distribution and provide rest periods from grazing.
Since 1988, grazing management has consisted of light-tomoderate grazing using a rotational grazing system that
limits annual grazing to 3 to 6 weeks per year, with the season
of grazing cycling annually among early season (beginning in
mid June), mid season (beginning in early to mid July), and
late season (beginning in mid August or early September).
Soil and vegetation sampling was conducted 12 years after
implementation of the new grazing management, at which
time the plant community had improved from “fair” to “good”
condition (NRCS 1981). At the time of sampling, late-seral
vegetation was composed of streambank wheatgrass (Agropyron dasystachyum), sheep fescue (Festuca ovina), junegrass
(Koeleria macrantha), sandberg bluegrass (Poa secunda),
muttongrass (Poa fendleriana), needlegrass (Stipa spp.),
squirretail (Sitanion hystrix Nutt.), bluebunch wheatgrass
(Pseudoroegneria spicata), big sagebrush, snake weed
(Gutierrezia sarothrae Pursh), pussytoes (Antennaria rosea),
stonecrop (Sedum lanceolata), hoods phlox (Phlox hoodii
Richards), moss phlox (Phlox bryoides), five fingers (Potentilla diversiflora), sedges (Carex spp.), lupine (Lupinus spp.),
false buckwheat (Eriogonum jamesii), clover (Trifolium spp.),
prickly gilia (Leptodactylon pungens), narrow leaved
mertensia (Mertensia lanceolata), and other forbs.
Mid-seral plant species were similar to those of the late
seral, but differed in abundance. Mid-seral vegetation consisted of increased big sagebrush, the warm season grass
blue grama (Bouteloua gracilis H.B.K., Lag. Ex Steud.), and
less palatable forbs, while cool season grasses were infrequent (NRCS 1981). Current and historic grazing management of the mid-seral stage consists of moderate-to-heavy
nonrotational grazing.
Field Plot Establishment and Sampling
Three 100-m transects were established in both seral
stages. Transect sites were selected on the basis of standard
soil classification techniques to assure that all transects
were positioned on the same soil series (Cabin sandy loam).
Species composition at peak production (late June) was
determined from visual estimates of ground cover from
twenty 0.1-m2 rectangular quadrats spaced 5 m apart along
each transect (Jezile and others, in press). The amounts of
bare ground, gravel, stones, ground litter, and standing
litter were recorded for each quadrat. Species cover was
expressed as a percentage of total cover, and estimates of
percentage composition of each species were used to calculate an NRCS range condition rating. Herbage yield was
estimated by randomly selecting and harvesting five of the
70
Heterogeneity in Soil C and N in Late- and Mid-Seral Sagebrush-Grass Rangeland
20 quadrats along each transect. For both seral stages,
clipped plant material was separated by dominant species,
dried for 48 hours at 60 ∞C and then reweighed to determine
the percentage dry weight of the dominant species (Jezile
and others, in press).
Soil samples were collected from two of the three transects
of both seral stages. Cores were obtained at the five randomly selected 0.1-m2 quadrat locations where herbage
yields had been sampled. At each coring location along each
transect, separate cores were obtained from three landscape
microsites: bare ground, within a grass patch, and under a
sagebrush canopy. Soil samples were taken with a 76.2-mmdiameter coring tube. Each soil core was partitioned into
three depth increments: 0 to 5, 5 to 10, and 10 to 20 cm.
Estimates of soil bulk density were obtained by the doublecylinder coring method described by Blake and Hartge
(1986) from three equally spaced locations along each
transect. Soil samples were placed in sealed plastic bags,
and transported in coolers to the laboratory for processing.
Laboratory Analyses
Soil samples were passed through a 2-mm screen to
remove plant crowns and visible roots. A subsample of fieldmoist soil was taken for analyses of gravimetric water
content and field-moist water soluble organic C (WSOC); the
remaining soil was air dried and used for all other analyses.
Total C and N were determined by dry combustion with a
Carlo Erba NA 1500 automatic carbon-nitrogen analyzer as
described by Schepers and others (1989). Inorganic C was
analyzed by a modified pressure-calcimeter method (Sherrod
and others 2002), and soil organic C was calculated by
subtracting inorganic C from total C. WSOC was determined
by the procedure of Davison and others (1987) with the
following modifications: the extraction ratio used was 5 mL
deionized water to 1 g soil, and filtrates were analyzed with
a Shimadzu TOC-5050A Soluble C analyzer. Exchangeable
nitrate (NO3–N) and ammonium (NH4–N) were extracted
with 2 M KCl and determined with an auto analyzer (Keeney
and Nelson 1982).
Statistical Methods
Differences in soil C and N among landscape microsites
(bare ground, beneath grass, and beneath shrub canopy)
were analyzed by depth and by cumulative depth with
analysis of variance and mean separation by least significant differences (LSD, P < 0.05) (Steele and Torrie 1980).
Differences in soil C and N between the two seral stages
were determined by depth and by cumulative depth with
paired t-tests (Steele and Torrie 1980).
Results _______________________
Vegetation
Based on estimates of plant species composition, calculated NRCS range condition ratings were 35 percent (“fair”
condition) for the mid-seral, and 57 percent (“good” condition)
for the late-seral stages (NRCS 1981). Jezile and others (in
press) reported that late-seral vegetation covered 44 percent
USDA Forest Service Proceedings RMRS-P-31. 2004
Heterogeneity in Soil C and N in Late- and Mid-Seral Sagebrush-Grass Rangeland
Soil organic C distribution
(0 to 20 cm)
100
80
Percent
of the soil surface (21 percent shrubs and canopy, 15 percent
grasses, and 7 percent forbs), while the remaining 56 percent
of the soil surface was comprised of exposed bare soil, gravel,
or litter-covered soil. In comparison, mid-seral vegetation
cover was 31 percent (23 percent shrubs, 3 percent grasses,
5 percent forbs), and 69 percent exposed soil, gravel, or littercovered soil. Mid-seral grass cover and total forage (grasses +
forbs) cover were significantly lower (P < 0.10), and bare
ground significantly higher, than in the late-seral stage
(Jezile and others, in press).
Reeder, Grant, Jezile, and Child
60
Bare
Grass
Shrub
40
20
Soil Properties
Masses of SOM-C and SOM-N in the late-seral stage were
significantly lower in bare areas than in grass or shrub
areas, whereas C and N in the mid-seral stage were not
significantly different among the three landscape microsites
(table 1). Spatial distribution of total SOM-C differed between the two seral stages, with significantly more C associated with bare-ground areas, and less associated with
grass areas, in the mid-seral than in the late-seral landscape
(fig. 1). Similar distribution patterns were observed for total
SOM-N (data not shown).
Labile fractions of soil C and N differed between the two
seral stages. WSOC was significantly lower, and nitrate-N
significantly higher, in the mid-seral compared to the lateseral landscape (table 1). Similar trends were observed in
the proportion of total SOM-C in water soluble forms (fig. 2),
and the proportion of total N in nitrate-N form (fig. 3). In
both the late- and mid-seral landscapes, mass of nitrate-N
was not significantly different among the three landscape
0
Late-seral
Mid-seral
Figure 1—Spatial distribution of soil organic C
in late- and mid-seral landscapes. (The two
seral stages differ (P < 0.01) in mass of soil C
in bare and grass areas.)
Ratio of soluble organic C to total soil organic C
(0 to 20 cm)
a
a
1.0
0.8
b
a
b
0.6
b
Late-seral
Mid-seral
0.4
Bare
Grass
Shrub
LSD (0.05)
Total soil organic C, Mg ha–1
Late-seral
Mid-seral
29.0
37.3
37.3
40.6
40.4
41.8
3.4
ns
pa
<0.01
.20
.30
Bare
Grass
Shrub
LSD (0.05)
Total soil organic N, Mg ha–1
Late-seral
Mid-seral
2.62
3.43
3.22
3.51
3.32
3.39
0.33
ns
pa
<0.01
.17
.37
–1
Bare
Grass
Shrub
LSD (0.05)
Bare
Grass
Shrub
LSD (0.05)
a
Water soluble organic C, kg ha
Late-seral
Mid-seral
2.16
1.84
3.36
2.28
3.94
3.02
.51
.33
Nitrate N, kg ha–1
Late-seral
Mid-seral
.98
3.10
.85
2.87
.97
2.72
ns
ns
pa
<0.01
<0.01
<0.01
pa
<0.01
<0.01
<0.01
0.2
0.0
Bare
Grass
Shrub
Figure 2—The ratio of mass of water soluble
organic C to mass of total organic C (x 104) in the
0- to 20-cm soil profile.
Ratio of Nitrate-N to total soil N
(0 to 20 cm)
1.0
NO3-N/total N (x 1000)
Table 1—Masses of carbon and nitrogen in the soil profile (0 to 20 cm)
of late- and mid-seral stages of a sagebrush-grass plant
community.
a
0.8
a
a
0.6
0.4
Late-seral
Mid-seral
b
b
b
Grass
Shrub
0.2
0.0
Bare
Figure 3—The ratio of mass of nitrate-N to mass
of total soil N (x 103) in the 0- to 20-cm soil profile.
Comparison of late- and mid-seral values (paired t-test).
USDA Forest Service Proceedings RMRS-P-31. 2004
71
Reeder, Grant, Jezile, and Child
microsites, but mass of WSOC was lowest in bare ground
areas, intermediate in grass areas, and highest beneath
shrub canopies (table 1).
Discussion ____________________
Jezile and others (in press) reported that SOM-C and
SOM-N concentrations were not significantly different between late- and mid-seral stages of this sagebrush-grass
plant community. When converted to a mass basis, we found
that levels of SOM-C and SOM-N were comparable between
late- and mid-seral landscapes in grass and shrub areas, but
levels of C and N were lower in bare-ground areas of the lateseral landscape than in the mid-seral landscape (table 1).
These data, plus visual field observations, suggest that the
late-seral landscape had experienced more historic soil loss
by erosion than had the mid-seral landscape.
Similar to the trends in C and N concentrations observed
by Jezile and others (in press), we found that total SOM-C
and SOM-N masses were not good indicators of seral stage.
Soil properties such as SOM are the result of millennia of
soil-forming processes (Jenny 1980). Thus, even though
significant changes in the plant community may be observed
in response to a particular management history spanning
decades, corresponding changes in most soil properties usually will not be observed unless erosional soil losses are
substantial (Lal and others 1999).
Although SOM consists primarily of a large stable pool of
C and nutrients that is resistant to decomposition, it also
includes small subpools of more readily decomposable or
labile organic compounds (Collins and others 1997) that may
be easily changed in response to perturbation such as grazing, or to grazing-induced changes in plant community
composition (Burke and others 1997; Reeder and others
2001). Assessing the integrity of nutrient cycles as an indicator of change in rangeland health (NRC 1994) can therefore be more readily accomplished by measuring the quality
rather than the quantity of SOM C and N in the soil. We
selected WSOC as an indicator of the labile pool of SOM-C
because it is a measure of root exudates that provide a
readily available C source for soil microorganisms (Cheng
and others 1993, 1996), and is strongly correlated with
microbial biomass and microbial activity (Burford and
Bremner 1975; Davidson and others 1987). We selected the
mineral forms of N in the soil, ammonium (NH4–N), and
nitrate (NO3–N), as indicators of the labile pool of soil N,
because these forms are easily measured, are the end products of the microbial mineralization processes that decompose SOM-N (Bartholomew 1965), and are the forms of soil
N that are available for direct uptake by plants (Harmsen
and Kolenbrander 1965).
We found that while comparisons of total masses of
SOM-C and N were not good indicators of seral stage and
thus of rangeland health, the proportions of total SOM-C
and N that were in labile forms clearly separated late-seral
from mid-seral landscapes in this sagebrush-grass plant
community. The proportion of total SOM-C that was soluble
was consistently higher (fig. 2), and the proportion of soil N
that was in the nitrate form consistently lower (fig. 3) in the
late-seral compared to the mid-seral landscape. These data
suggest that the N cycle is more tightly coupled in the late-seral
72
Heterogeneity in Soil C and N in Late- and Mid-Seral Sagebrush-Grass Rangeland
landscape. The higher level of WSOC observed in the lateseral soil suggests a higher level of root exudation and
associated increase in microbial activity to utilize the readily
available C (Cheng and others 1996). Stimulation of microbial activity by C-rich root exudates results in a N-limited
environment in which microorganisms immobilize soil N
(Kuzyakov 2002); thus levels of nitrate-N in the late-seral
soil are low. In the mid-seral landscape, higher levels of
nitrate-N and lower levels of WSOC indicate a loss of
integrity, or uncoupling, of the N cycle. The mid-seral landscape is characterized by a higher percentage of bare-ground
area and a more fragmented grass and forb plant community. In bare-ground and sparse-plant areas, warmer soil
surface temperatures in combination with pulsed rainfall
events enhance microbial decomposition of SOM (Bowman
and others 1990). In the absence of growing plants or in
areas of sparsely growing plants, root exudation of WSOC
compounds is low, so less stimulation of microbial activity by
C-rich root exudates occurs, resulting in net mineralization
rather than net immobilization of soil N, and nitrate-N
accumulation in the absence of sufficient plants to take up
excess mineral N.
Rangeland ecosystems continually respond to temporary
changes in both physical and biotic conditions. An assessment of change in rangeland health must be able to distinguish between changes that result in the crossing of an
ecological threshold from those that are temporary and the
result of normal fluctuations in physical and biotic conditions (NRC 1994). Our results demonstrate that assessments of labile forms of SOM, and of the landscape-level
spatial distribution of SOM-C and SOM-N, distinguished a
mid-seral from a late-seral stage of this sagebrush-grass
plant community. Measurement of soil nitrate shows promise as an indicator of the integrity of the N cycle and change
in successsional stage. Future studies are needed to evaluate the seasonality of differences in nitrate-N content between different seral stages, and to evaluate whether a
change in nitrate-N can serve as an early indicator of change
in rangeland health.
Acknowledgments _____________
We thank Ernest Taylor and Lucretia Sherrod for their
laboratory assistance with this research.
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