CHEMICAL CHANGES IN THE SOIL
INDUCED BY FIRE IN A COMMUNITY
DOMINATED BY SHRUB-GRASS
Robert R. Blank
James A. Young
and the potential catalytic activity of soil minerals on
precursor compounds created by plant pyrolysis, creates
secondary chemical compounds. These compounds may
inhibit or enhance seed germination or affect seedling
vigor and establishment potential. This paper is only a
preliminary report of a long-term study on the fire ecology
in a sagebrush-grass plant community. We feel theresearch is timely, given the history of fire in sagebrushgrass plant communities as a consequence of cheatgrass
invasion.
ABSTRACT
The role of fire as an ecological factor is well established,
but an understanding of the effects of postfire conditions
on seed germination and seedling vigor in sagebrush-grass
plant communities is lacking. One hypothesis is that fireinduced chemical changes in the soil influence seed germinability and seedling vigor. This study along the eastern Sierra front north of Reno, NV, included recently
burned and unburned soils with shrubs as the dominant
life form. Chemical differences between burned and
unburned soils were largely confined to the upper 5 em
in dune (shrub-influenced) positions. Burned soils were
significantly (P ~ 0.05) different in several quantitative
measures than unburned soils. Preliminary laboratory
evidence suggests that burned soil inhibited root
elongation.
FIELDWORK
At present, we have confined our research to coarsetextured granitic soils along the eastern Sierra Nevada
front, from Reno, NV, to Honey Lake, CA. Our principal
study area is near Doyle, CA, which is approximately
40 miles north of Reno on U.S. Highway 395. Most
of the soils in this study area are weakly developed
(Haploxerolls, Torriothents, Torripsamments) and occur
on alluvial fans that, in places, have been partially reworked by lake water during the last pluvial period of
Lake Lahontan. The prefire vegetation was dominantly
sagebrush (Artemisia tridentata), bitterbrush (Purshia
tridentata), and cheatgrass (Bromus tectorum) set in a
dune-interdune microcomplex. Other native species included desert peach (Prunus andersonii), needle-andthread (Stipa comata), and Indian ricegrass (Oryzopsis
hymenoides ).
In August 1986, an intense fire swept through the
Doyle area, generally stopped on its eastern march by
U.S. 395. The general uniformity of soils in the area
enabled us to compare similar burned and unburned
soils on either side of U.S. 395. One week after the fire
and before any precipitation event, we set out 16 plots
in the burned soils and four plots in the unburned soils.
Each plot consisted of an adjacent dune and interdune
microsite. At each microsite of each plot, we collected
soil at three depths: 0-5 em, 5-10 em, and 10-20 em. The
<2-mm fraction was reserved for later analyses. We
have undertaken satellite sampling of more recent burns.
INTRODUCTION
The role of fire as an ecological agent of change is well
established (Odum 1971). As an agent of change, fire
affects the successional patterns of plants, soil organisms
(Ahlgren 1974), soil chemical and physical properties
(Wells and others 1978), seed viability, and seed germination potential (Went and others 1952; Komarova 1985).
In studies of fire-prone plant communities, notably the
chaparral of California, an interesting factor of fire ecology has emerged; fire-induced chemical changes in the
soil influence seed ecology. Muller (1966) determined that
allelopathic agents from herbaceous plants accumulated
in soils to such levels that seed germination and growth
of some plants were inhibited. The toxins decreased following fires, allowing other plant species to germinate
and proliferate. Keeley and others (1984) and Keeley and
others (1985) found that the charred wood of some chaparral shrubs had a significant stimulating effect on the germination of certain dormant seeds. Keeley and Pizzorno
(1986) determined that the stimulative compound was
water soluble and appeared to be an alteration product
of:xylan and glucuronic acid. They speculated that the
compound was an oligosaccharide.
Our working hypothesis expands on previous work.
We propose that heat, acting on complex plant materials,
LABORATORY ANALYSES
To obtain water-soluble soil extracts, 10 g of soil was
placed in a 50-mL polypropylene centrifuge tube and
35 mL of an extracting solution was added. We experimented with several types of extracting solutions including 0.15 percent CaC12, 0.15 percent KCl, and 0.015 percent KCl. Each solution had a particular advantage for
subsequent analytical measurements. The tubes were
Paper presented at the Symposium on Cheatgrass Invasion, Shrub DieOff, and Other Aspects of Shrub Biology and Management, Las Vegas, NV,
April 5-7, 1989.
Robert R. Blank and James A. Young are Soil Scientist and Range
Scientist, respectively, U.S. Department of Agriculture, Agricultural
Research Service, Landscape Ecology of Rangelands Unit, 920 Valley
Road, Reno, NV 89512.
256
This file was created by scanning the printed publication.
Errors identified by the software have been corrected;
however, some errors may remain.
8
shaken for 30 min on a reciprocating shaker, then centrifuged. The nearly clear supernatant was decanted, then
filtered with 0.22-micron nylon filters.
Compounds in the extracts were qualified and quantified by ion chromatography using established procedures
(Weiss 1986; Dionex Corporation 1987). All standards
were prepared with reagent-grade chemicals. To positively identify certain organic acids, we used two separate
chromatographic methods: ion exchange chromatography
and ion exclusion chromatography. If the retention time
of an unknown organic acid matched a standard under
both chromatographic conditions, we were confident it
was a positive match.
10
6
A
RESULTS AND DISCUSSION
A variety of water-soluble compounds were created by
fire (fig. 1). The presence of organic acids, especially acetic, glycolic, and formic, was a principal characteristic of
the burned soils. This finding should not be considered
unusual, as acetic acid is a product of pyrolytic distillation
of wood products, and dune positions, especially, contain
many shrub fragments (Streitwieser and Heathcock 1976,
p. 446). Another characteristic of the burned soils was
a general increase in levels of organic acids with time
(fig. 1). Two mechanisms could explain this finding.
Microbes, proliferating in the postfire soil environment,
may have synthesized the organic acids. We discount this
possibility because: (1) the soil was already dry during
this part of the study; and (2) short-chained aliphatic carboxylic acids would be readily utilized by soil bacteria and
thus decrease. Wind may have deflated materials from
the burned soils, concentrating the organic acids. Field
evidence that supports this mechanism was observed, included the winnowing of mineral soil particles and partial
removal of charcoal debris from dune positions.
We quantified differences in quality and quantity
of water-soluble constituents at the Doyle study site
(table 1). The data indicate that differences in watersoluble compounds between burned and unburned treatments largely occurred at depths of0-5 em, in dune positions, directly beneath shrubs. Burned soils contained
significantly more (P ~ 0.05) water-soluble nitrate and
orthophosphate. Undoubtedly, the burned soils contained
more of other organic acids than the unburned controls,
but at this time we have not performed quantitative comparisons of these compounds. In the chaparral soils studied by DeBano and others (1979), burning decreased
nitrate-Nand increased ammonia-N beneath the litter
layer. This only occurred in a dry soil. We determined
that levels of ammonia-N were very low in burned soils
immediately after the fire and in unburned soils sampled
at the same time. In measurements at satellite burns,
however, levels of ammonia-N were much higher 1 month,
2 months, and even 4 months after the burn.
The Doyle sites were sampled 1 year after the burn,
and levels of organic acids were below detection limits.
Apparently, winter precipitation, with subsequent microbial proliferation and utilization of the organic acids,
leaching of the anionic organic acids through the soil profile, or both, reduced the levels of organic acids. In several satellite locations sampled in the summer and fall,
4
B
10
c
0
30
MINUTES
Figure 1-Typical chromatograms of water-soluble
extract (0.0015 percent KCI solution) from burned
and unburned soils collected at a sampling site
near Hallelujah Junction, CA. All soil samples
were collected in dune positions at depths of 0-5
em. Conditions were as follows: Column, AS5A5u; Eluent, 0.75 to 200 mM NaOH gradient; Detection, suppressed conductivity. The soils sampled
are as follows: (A) unburned soil, (B) soil collected
the day following the fire, (C) soil collected approximately 100 days following the fire. The numbered
peaks refer to compounds qualified thus far: (1)
acetate, (2) glycolate, (3) butyric acid, (4) formate,
(5) chloride, (6) nitrate, (7) succinate, (8) sulfate,
(9) oxalate, (1 0) orthophosphate, (11) citrate. Shift
in retention time among chromatograms is the
result of variations in room temperature.
257
Table 1--Concentration of several water-soluble inorganic and organic ions extracted from burned and unburned soils at the Doyle, CA, study area1
Nitrate
Dune lnnerdune
Ulf U B
Depth
Orthophosphate
Dune lnnerdune
1fB
U B
Sulfate
Dune lnnerdune
Ulf UB
Potassium
Magnesium
Dune lnnerdune Dune lnnerdune
UB UB
Ulf UB
Acetate
Dune lnnerdune
1fB U B
Formate
Dune lnnerdune
1fB U B
- - - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -mglkg soil- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 44 210
0-5cm
7 8
25 210
tr tr
10 239
5-10 em
10
4
2 28
7
5
tr tr
4
10-20 em
3
3
3 3
3
3
tr tr
4 2
2
139 tr tr
3 9
202 142
54 45
60 48
42 34
tr
2 4
140
85
32 41
49 45
34 36
tr
tr
dl dl
tr
tr
dl dl
90
37
44 44
46 38
37 38
dl
dl
dl dl
dl
dl
dl dl
5
3
tr tr
1
The soils were extracted with a 0.15 percent KCI solution. Nitrate, orthophosphate, and sulfate were quantified using an AS4A separator column and 1.80 mM
1.70 mM NaHC03 eluent. Acetate and formate were quantified using an AS4A separator column and a 2.0 mM N~8407 eluent. Below detection limit are
designated by dl; trace level amounts are designated by tr.
2
Denotes significant differences (F-test, P < 0.05) between burned and unburned treatment means by microsite and depth.
N~C03 -
however, levels of organic acids remained the same or
increased 6 months after the burn even though several
precipitation events had occurred.
To determine if burned soil affected seed germinability
or seedling vigor, we germinated several grasses in recently burned soil and unburned control soil (table 2).
There were no significant differences in the percentage
of germination between any of the burned and unburned
soil treatments. However, the root length after 2 weeks
of incubation time was significantly less (P ~ 0.05) in the
burned soils than in the unburned controls. To explain
this finding we draw on the work of Lynch (1978) and
Harper and Lynch (1982). They determined that under
anaerobic conditions, phytotoxic water-soluble organic
acids were synthesized and one of these organic acids
(acetic acid) suppressed root elongation.
We hypothesize that acetic, formic, and glycolic acids
at the levels found after the fire suppress root growth.
The long-term effects of this root stunting on plant
survivability and competitiveness will be tested in future
experiments.
The presence of certain organic acids in burned soils
presents a speculative, but interesting, possibility
that they may predispose a plant to certain biochemical
changes. Gal (1938) determined that wheat seeds treated
with particular aliphatic organic acids exhibited higher
levels of asorbic acid production in seedlings than untreated controls. Spring wheat (Triticum aestivum)
seeds treated with solutions of succinic and fumaric
acid had a significant effect on catalase activity, unsaponfied chlorophyll content, and water content ofleaves in
mature plants as compared with untreated controls
(Blagoveshchenskii and Petrochenko 1959). Succinic acid
was positively identified in the burned soils of the study,
but the levels measured were much lower than those used
by Blagoveshchenskii and Petrochenko.
Table 2-Mean percent germination and root length
of four grass species incubated in burned
and unburned soiP
Species
Soil
treatment
Germination 2
Root
length
STCO
STCO
Unburned
Burned
62
61
TAAS
TAAS
Unburned
Burned
95
98
BRTE
BRTE
Unbutned
Burned
92
85
ORHY
ORHY
Unburned
Burned
67
80
CONCLUSIONS
em
Percent
3
3
Fire in a sagebrush-grass plant community created high
levels of organic acids in dune soil positions. Root elongation of several grasses was inhibited in the burned soil.
3.4
2.7
3.5
2.6
REFERENCES
3.3
3
2.5
3
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2.9
2.1
1
The soil used in these experiments was field
collected from 0-5 em in dune positions. For the
duration of the experiment the soil was maintained
at a soil water potential of approximately 0.1 MPa.
2
Germination and root length measurements performed after 2 weeks incubation time at 15
3
Denotes significant differences { T-test, P s 0.05)
between burned and unburned means.
oc.
258
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259