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POTENTIAL ROLE OF SOIL
MICROORGANISMS IN
MEDUSAHEAD INVASION
James D. Trent
James A. Young
Robert R. Blank
METHODS
Medusahead (Taeniatherum caput-medusae ssp.
asperum) is an annual grass that is invading western
rangelands. Once it invades native communities or replaces cheatgrass (Bromus tectorum) in degraded communities, utilizable forage for domestic livestock and seeds
for granivores are sharply reduced, wildfires are perpetuated, and secondary plant succession is disrupted. Medusahead and cheatgrass appear to respond favorably to nitrogen (N) addition to soils (Kay and Evans 1965) and may
occupy and dominate disturbed sites where N mineralization is high. Tilman (1988) proposes a model of succession
that describes changing plant dominance based on zero
net growth isolines of different plant species with different resource requirements.
We propose that vegetation removal and soil disturbance results in elevated levels of mineral N, which favors
medusahead over the perennial seedlings. A decrease in
soil mineral N over time may eventually favor perennial
seedlings that are more competitive at lower levels of mineral N. In addition, disturbance alters detrital food webs,
shifting food webs from those dominated by fungi to those
dominated by bacteria (Hendrix and others 1986). Bacterially dominated systems are characterized by rapid
decomposition and nutrient mineralization, while localization of organic matter in surface soils of less-disturbed
soils promotes fungal-dominated systems that are characterized by lower decomposition rates and net nutrient immobilization (Hendrix and others 1986). According to this
hypothesis, undisturbed sagebrush (Artemisia spp.) soils
should be characterized by fungal domination and net N
immobilization, while disturbed soils containing medusahead would be dominated by bacteria, resulting in net N
mineralization.
The purpose of this study was to: (1) determine whether
soil N status differs among heavily disturbed, moderately
disturbed, and undisturbed sites and to relate such differences to microbial dominance by either fungi or bacteria;
and (2) manipulate soil mineral N by adding either fertilizer, sugar, or a nitrification inhibitor to determine the effect of mineral N on medusahead seed germination.
An observational study and an N fertilization experiment were conducted. The observational study was conducted in Lahontan sagebrush (Artemisia arbuscula ssp.
longicaulis)/Sandberg bluegrass (Poa sandbergii) plant
communities near Susanville in northeastern California.
Three sites with different disturbance histories were
chosen for comparison: (1) an undisturbed site, which
lacked cheatgrass and medusahead (UNSAG soil
takenbeneath sagebrush; UNINT =soil taken in the
shrub interspaces); (2) a site moderately disturbed, with
cheatgrass and medusahead invading the sagebrush community (MODSAG =soil taken beneath sagebrush;
MODINT soil taken in the shrub interspaces); (3) a
heavily disturbed site, which historically had been subjected to heavy sheep grazing. This site had burned, so it
lacked sagebrush and was dominated by medusahead
with some recent squirreltail (Elymus hystri:c) colonists
(DISEL soil adjacent to Elymus; DISMED soil in
medusahead).
Soil was taken between 1 and 20 em below the litter
layer. Four replications of dune and interdune soils were
sampled on September 17, 1991. Soil smears ofbacteria
and actinomycetes were stained with Europium tritluoroacetonate and and fluorescent brightner (Anderson and
Slinger 1975) and counted at 1000x with a UV microscope.
Actinomycete& were less than 1 JUl1 in diameter and fungi
were generally greater than 1.5 JUl1 in diameter. Fungal
hyphae were stained and quantified (400x) using an
lrgalan black membrane filter technique. Organic carbon,
pH, anaerobic N mineralization, aerobic N mineralization,
and total N were determined using standard methods
(Page and others 1982). Microbial biomass carbon was
measured using a chloroform fumigation/potassium sulfate extraction procedure (Sparling and West 1988).
In a second study, five N manipulations were imposed
in the heavily disturbed medusahead soils in November
1991. In the first three treatments, soils were fertilized
with 30 kg/ha N ammonium sulfate, urea, or calcium nitrate. In the fourth treatment, sucrose was applied at
230 kg/ha in an effort to induce net immobilization of mineral N. The fifth treatment was an application of the nitrification inhibitor, N-serve. The treatments were replicated four times as a randomized complete block design.
On April 7, 1992, ocular estimates of percent cover, squirreltail leaf lengths, and seedling densities were quantified
using a 0.01-m2 quadrat.
=
=
=
Poster paper presented at the Symposium on Ecology, Management,
and Restoration of Intermountain Annual Rangelands, Boise, ID,
May 18-22, 1992.
James D. Trent is Soil Microbiologist, James A. Young is Range Scientist, and Robert R. Blank is Soil Scientist, Agricultural Research Service
U.S. Department of Agriculture, 920 Valley Road, Reno, NV 89512.
'
140
=
Table 1-Qrganic carbon and soil nitrogen parameters for undisturbed soils (UNSAG =soil beneath the sagebrush canopy; UNINT =soil
between sagebrush plants), moderately disturbed soils (MODSAG = soil beneath the sagebrush canopy; MODINT = soil between
sagebrush plants), and heavily disturbed soils (DISEL c soil adjacent to Elymus hystrix; DISMED = soil beneath medusahead litter)
Location
ORGCAR
Percent
ab1
UNSAG
UN INT
MODSAG
MODINT
DISEL
DISMED
0.96
.46c
1.24a
.64bc
.60bc
.50 be
P-VALUE2
0.016
ANAEROB
NKt·MIN
AEROBE
N·MIN
TOTN
HzO
pH
CaCiz
pH
15
18
14
17
6.85b
6.92b
6.78b
6.88b
7.50a
7.46a
6.31 be
6.17c
6.56b
6.65b
7.26a
7.27a
0.15
0.0001
0.0001
C:N
- - - - - - - - - - - - - - - - -(j.Jglg) - - - - - - - - - - - - - - 17.64 a
2.74b
19.93 a
1.14 b
7.52 ab
4.91 b
1.11
1.38
2.44
1.29
.91
.80
524ab
214b
869a
353b
509ab
298b
0.022
0.34
0.055
20
22
'Values In columns followed by the same letter are not significantly different at P < 0.05 according to mean separations by least Significant Differences.
Probability values obtained using ANOVA.
2
In the N manipulation study, medusahead germination
was significantly greater in all fertilization treatments
(table 3). Medusahead density was six, four, and four
times greater than controls in the calcium nitrate, ammonium sulfate, and urea treatments, respectively. Medusahead density in the sugar, N-serve, and control treatments was not significantly different. Squirreltailleaf
length was also stimulated by N fertilization and slightly
inhibited by N-serve application.
RESULTS
Differences in microbial populations and soil properties
among sites were found (tables 1 and 2). Soil pH was significantly greater in the heavily disturbed site. Soils beneath sagebrush had greater levels of organic carbon than
soils from other sites. Total N and anaerobic N mineralization was greatest in soils collected beneath sagebrush.
Total N and anaerobic N mineralization was least in soil
collected from sagebrush interdunes and heavily disturbed medusahead soils. Heavily disturbed soils containing squirreltail were intermediate. Aerobic N mineralization and C:N ratios were not significantly different
among sites. Bacterial numbers were not significantly
different among sites. Heavily disturbed, moderately disturbed, and undisturbed soils in general had similar fungal hyphallengths; however, undisturbed interdune soils
had distinctly lower hyphallengths. Actinomycete numbers and hyphallengths were significantly greater in
heavily and moderately disturbed soils, when compared to
undisturbed soils. Microbial biomass carbon was greatest
in soils adjacent to squirreltail and sagebrush, and least
in medusahead and interdune soils.
DISCUSSION
In contrast to what has been observed in agroecosystems after soil disturbance (Hendrix and others
1986), we did not find a significant shift from fungaldominated to bacterial-dominated systems with increased
disturbance. The low levels of fungal hyphallength would
suggest that nutrient cycling in general is largely dominated by bacteria at all sites, regardless of disturbance
history. The higher numbers of actinomycete& in disturbed soils may be a consequence of the change in soil
structure observed after disturbance. After fire and
heavy grazing, the loss of sagebrush and the thin sandy
Table 2-Microbial carbon (MIC CARB), bacterial numbers (BACT#), actinomycete numbers (ACTIN#),
actinomycete hyphallength (ACTIN), and fungal hyphallength (FUNGI) (location descriptions are
In table 1)
Location
MJCCARB
(X 101)
BACT#
(X 101)
ACTIN#
671 ab1
370c
852a
504bc
617 abc
480bc
P-VALUE2
0.026
FUNGI
--------rnVg--------
pglg
UNSAG
UN INT
MODSAG
MODINT
OISEL
DISMEO
ACTIN
1.78d
1.61 d
14.01 a
3.73cd
10.13 ab
6.67bc
66.0
43.8
114.0
101.0
113.3
87.2
0.0001
0.34
56.4c
26.6c
376.0a
95.8c
305.4ab
155.7 be
0.0031
3.5a
.7b
2.8ab
3.0ab
4.6a
3.6a
0.091
1Values tn columns followed by the same letter are not significantly different at P < 0.05 according to mean separations by
least Significant Differences.
2Probabllity values obtained using ANOVA.
141
Innovative methods in managing these microbial processes may allow us to reduce the competitive ability of
medusahead after soil disturbance or vegetation removal
by fire. The effect of nitrification inhibitors or organic
amendments on soil mineral N and medusahead germination should be investigated in greater detail. Manipulations that reduce soil nitrate should reduce medusahead
germination and favor perennial grasses such as squirreltail. In addition, further research into biocontrol of
medusahead and other weeds should consider the soil
microbial community structure of each particular ecosystem. In the heavily disturbed medusahead ecosystem,
bacteria and actinomycete& dominate over fungi, hence a
bacterial or actinomycete biocontrol agent may be more
effective than a fungal biocontrol agent.
Table 3-Medusahead density, cover, and Elymus hystrix leaf
length in relation to soil nitrogen manipulations.
Treatments imposed in November 1991, and
measurements taken April 7, 1992
Treatment
Medusahead
seedllngs/0.01 mz
Medusahead
cover
Percent
Control
Sugar
N-serve
NH4SO•
Urea
CaNO:s
1Valuas
at
1
5c
4c
6c
20b
20b
32a
6c
4c
5c
23b
25b
52 a
Elymus
leaf length
em
12.4b
11.6 be
9.7c
17.8a
18.1 a
19.1 a
In columns followed by tho same Iotter are not significantly different
P < 0.05 according to mean separations by Least Significant Differences.
REFERENCES
Anderson, J. R.; Slinger, J. M. 1975. Europium chelate
and fluorescent brightner staining of soil propagules
and their photomicrographic counting. I. Methods. Soil
Biology and Biochemistry. 7:205-209.
Hendrix, P. F.; Parmelee, R. W.; Crossley, D. A., Jr.;
Coleman, D. C.; Odum, E. P.; Grofl'man, P. M.1986.
Detritus food webs in conventional and no-tillage
agroecosystems. Bioscience. 36:374-380.
Kay, B. L.; Evans, R. A 1965. Effects of fertilization on
a mixed stand of cheatgrass and intermediate wheatgrass. Journal ofRange Management.18: 7-11.
Page, A L.; Miller, R. H.; Keeney, D. R.1982. Methods of
soil analysis. Part 2. Chemical and microbiological properties. 2d ed. ASA-SSSA Agronomy Monograph 9.
Sparling, G. P.; West, A W. 1988. A direct extraction
method to estimate soil microbial C: calibration in situ
using microbial respiration and 14C labelled cells. Soil
Biology and Biochemistry. 20: 337-343.
Stotzky, G. 1986. Influence of soil mineral colloids on
metabolic processes, growth, adhesion, and ecology of
microbes and viruses. In: Huang, P. M.; Schnitzer, M.,
eds. Interactions of soil minerals with natural organics
and microbes. SSSA Spec. Publ. 17. Madison, WI:
305-412.
Tilman, D. 1988. Plant strategies and the dynamics and
structure of plant communities. Princeton, NJ:
Princeton University Press.
veneer at these sites results in the exposure of smectitic
clays. Clays typically are more heavily colonized by bacteria and actinomycete& than fungi (Stotzky 1986).
Laboratory incubations indicated that soils beneath
sagebrush plants had a greater potential for nitrogen
mineralization than sagebrush interspace and medusahead soils. We have also shown that N fertilization can
stimulate medusahead germination. Hence, removal of
sagebrush by fire should result in patches of soil with
high levels of mineral N that should stimulate medusahead germination. Sugar additions to the medusahead
seedbed did not reduce medusahead seed germination.
Lack of a sugar effect could mean that (1) controls are already low in mineral N, resulting in minimum germination; (2) sugar is not an adequate carbon substrate for reducing mineral N below control levels; or (3) germinating
seeds already received nitrate priming from the previous
year and sugar would have a greater effect in the second
year of the study. N-serve inhibits the transformation of
ammonium to nitrate, which should reduce the pools of nitrate in the soil and lower germination. However, there
was no effect ofN-serve on germination. As with the
sugar treatment, germinating seeds may have already received enough nitrate from the previous year; therefore,
an N-serve effect may be more evident in the second year
of this study.
142