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WATER-SOLUBLE CHEMISTRY
FOLLOWING SIMULATED BURNING
OF SOIL-LITTER OF BIG SAGEBRUSH,
SQUIRRELTAIL, CHEATGRASS, AND
MEDUSAHEAD
Robert R. Blank
FayL.Allen
James A. Young
the raw biochemical materials that when heated would
form organic acids. It was unclear, however, if the interspace soils reached elevated temperatures of long
enough duration to foster the synthesis of organic anions
as temperatures greater than 200 oc apparently are required (unpublished research, USDA-ARS, Reno, NV).
This research was thus initiated to determine:
ABSTRACT
Wildfires induce chemical changes in the soil surface.
The magnitude of these chemical changes is influenced by
the nature of the vegetative litter being burned. This paper
reports on the water-soluble chemistry following simulated
burning of soil-litter of big sagebrush (Artemisia tridentata
ssp. tridentata), squirreltail (Elymus hystrix), cheatgrass
(Bromus tectorum), and medusahead (Taeniatherum caput
medusae ssp. asperum). The soil was a fine, montmorillonitic, mesic Typic Chromoxerert collected approximately
20 km east of Susanville, CA Results indicate that length
of simulated burn and temperature of simulated burn interact to generate a particular water-soluble chemistry. Differences in postsimulation inorganic chemistry were noted
among the soil-litters, which suggest inherent differences
in the soil-litters. The lack of variation in levels of certain
organic acids among the soil-litters, however, suggests they
were synthesized via pyrolysis of the soil humic fraction,
rather than originating from the plant litter itself.
a. If simulated burning of soil-litters dominated by
annual grasses yields a different soil chemistry than that
of native perennial grasses or shrubs.
b. How the interactive effects of length of bum time
and temperature influence water-soluble chemistry
among the aforementioned soil-litters.
FIELDWORK
INTRODUCTION
Wildfires considerably influence the water-soluble chemistry of surface soils (DeBano and Conrad 1978; Wright and
Bailey 1982). In general, levels of water-soluble nitrate decrease and levels of water-soluble cations increase (Isichei
an~ Sanford 198~; Khanna and Raison 1986; Raison 1979;
Snuth 1970; White and others 1973). The magnitude of
these wildfire-induced changes is influenced by the chemical composition and biomass of the plant community
(Wright and Bailey 1982).
Recent work in the sagebrush-steppe of northeastern
California showed that high levels of water-soluble organic
anions, especially carboxylic acids, formed as a consequence
of wildfires in sagebrush subcanopy soils·, shrub
.
1nterspaces, largely occupied by cheatgrass, were unaffected (unpublished research, USDA-ARS, Reno, NV).
It was speculated that cheatgrass litter did not contain
The study area is approximately 20 km east of Susanville,
CA. The dominant soil is a fine, montmorillonitic, mesic
Typic Chromoxerert (table 1). Vegetation consists of
shrubs of big sagebrush (Artemisia tridentata ssp. tridentata) and Lahontan sagebrush (Artemisia arbuscula ssp.
longicaulis). Big sagebrush is largely confined to Vertisols;
Lahontan sagebrush seems limited to less churning,
clayey soils that have a thin, sandy, eolian veneer (E horizon). The most common perennial bunchgrass is squirreltail (Elymus hystri%). Alien annual grasses, cheatgrass
(Bromus tectorum) and medusahead (Taeniatherum
caput-medusae ssp. asperum), occur in small isolated
patches.
Samples were collected in December 1991. Collection
areas were limited to places where litter had accumulated
to completely cover the soil surface. Depth of collection
was essentially the depth of the highly aggregated surface
A horizons (table 1). Thus, the material collected is a mixture of soil and litter, which will be referred to as soillitter. Collections of soil-litters were replicated in two
nearby areas of similar soil.
LABORATORY ANALYSES
Collected soil-litter was immediately dried at 50 °C,
then passed through a 2-mm sieve. Coarse litter was
crushed to pass through the sieve. The material was
homogenized and stored in paper cartons prior to simulated burning.
Paper presented at the Symposium on Ecology Management and Restoration of Intermo~nta~ Annual Rangelands, B~ise, ID, May iS-22, 1992.
Robert R. Blank IS Soil Scientist; Fay L. Allen is Research Technician·
James A Young is Range Scientist, U.S. Department of Agriculture Agrlcultural Research Service, Conservation Biology of Rangelands Uni~ 920
Valley Road, Reno, NV 89512.
220
'able 1--Belected soil chemical and physical properties of the Typic
Chromoxerert soil
lorizon Depth
A
BA
Bss
Bw1
Bw2
Sand
Silt
Organic Total
Clay carbon
N
em
- - - - - - - - - Percent - - - - - - - 0-5
30.5
36.5 0.72
33.0
5-20
24.6
24.7
50.7
.51
20-46
23.1
23.8
53.1
.35
46-86
23.6
47.5
28.8
.35
86-122
23.6
47.9
.29
28.5
There were other commonalties among soil-litters following simulated burning. Levels of water-soluble sulfate
increased with increasing temperature reaching the highest levels at 450 oc for both simulated burning times. Nitrate levels generally decreased with increasing simulation temperatures and declined to zero between 250 and
350 oc at the 15-min simulation time.
Heat-induced chemistry for some water-soluable species differed among the soil-litters. Levels of calcium varied considerably among soil-litters. Amounts of watersoluble potassium in unheated samples of big sagebrush
were far greater than the other untreated soil-litters and
remained high at all simulated burning temperatures.
A most unusual response to simulated burning occurred
for water-soluble orthophosphate. At 5-min simulation
time, levels of orthophosphate generally decreased with
increasing temperature for soil-litters of cheatgrass, squirreltail, and big sagebrush. For medusahead soil-litter,
however, levels increased rapidly from 250 to 350 oc, becoming higher than the other soil-litters except for big
sagebrush.
CEC
mg/kg cmollkg
502
353
243
213
156
31.7
33.0
49.4
52.0
50.8
For simulated burning, 15 g of soil-litter was placed in a
5-mL crucible. The crucibles were placed in a preheated
1uftle furnace, uncovered, using the following time and
!mperature matrix: 5 and 15 min exposure time at temeratures of 150, 250, 350, and 450 °C. Previous research
as shown that these simulations can produce a water>luble chemistry closely matching that of a wildfire (unublished research, USDA-ARS, Reno, NV).
After simulated burnings, crucibles were cooled and
·eighed to record weight loss. Material was immediately
laced in a 50-mL polypropylene centrifuge tube to which
~-mL of deionized water was added. Tubes were shaken
1r 30 min on a reciprocating shaker, centrifuged, and fil!red through 0.22-J.UD nylon membrane filters.
Samples were chemically analyzed within 1 week of the
mulated burnings and were kept refrigerated prior to
ttd between analyses. Samples were chemically analyzed
~ing high-performance ion exchange chromatography.
nions were separated using an Omnipac PAX-500 colmn (Dionex Corp.). The eluant was a 0. 75-mM to 50-mM
aOH gradient. Detection was by suppressed conductivY and spectrophotometry at 210 nm connected in series.
bromatographic peaks were deemed positively identified
hen both of the following conditions were met: (1) the
1tention time of an unknown peak matched a standard
~ak (all standards reagent grade); and (2) for applicable
temical species, the ratio of the conductivity signal to
tat of the spectrophotometric signal was similar for the
rlknown peak and the standard. Cations were separated
1ing a HPIC-CS3 column (Dionex Corp.). The isocratic
uant was 27 .5-mM HCl, 2.25-mM diaminopropionic acid
onohydrochloride, and 2.25-mM histidine monohydrolloride. Detection was by suppressed conductivity.
DISCUSSION
There are both similarities and differences in the response of the various soil-litters to simulated burning.
From the data presented in this paper and other simulated burnings we have completed, patterns are evident.
Widely varying levels of water-soluble potassium,
magnesium, and calcium, among original untreated soillitters, suggest long-term differential plant cycling..Big
sagebrush soil-litter is especially enriched in potassium
relative to other soil-litters. Differential elemental cycling among plants and among plant communities is well
established (Rodin and Bazilevich 1965). These initial,
presimulation, differences in soil elemental content confound interpretation of postsimulation chemistry.
Another pattern is the response of the metal cations
sodium, potassium, magnesium, and calcium to the simulated burnings. Levels of these cations decreased substantially at temperatures greater than 350 oc (at 15-minsimulation time). Volatilization temperatures of the
metal cations are far above the simulation temperatures
used (Raison and others 1984; Wright and Bailey 1982).
Alternative explanations of this phenomenon include:
(a) at high temperatures additional cation sorption sites
are formed, thereby decreasing cation extraction with
water, and (b) low-temperature volitization of the cations
via metal-ligand complexation.
Levels of most organic anions show a similar trend with
temperature as do cations. Unexpectedly, for a given temperature and time, simulated burning produced similar
levels of organic acids among all the soil-litters. If these
organic anions were derived via pyrolysis of plant litter,
then big sagebrush soil-litter, which has a greater proportion of plant litter and is biochemically unique among the
soil-litters, would likely respond differently to simulated
.
burning. These data suggest that either (a) the orgamc
anions are derived from raw biochemical building blocks
common to all the soil-litters (cellulose for example), or (b)
they originate from the soil organic fraction. A clue to the
pathway for organic anion synthesis can be gleaned from
~SULTS
Graphic representation of water-soluble ion concentra>n versus temperature-simulated burnings for selected
,tions and anions is presented in figure 1. When comtred with replications (data not presented), several
ends can be noted in these data.
At long simulated burning times (15 min), levels of
ilter-soluble cations, extracted from all soil-litters, exbited a bell-shaped distribution from low to high temlratures. Maximum levels occurred at 350 oc for calllDl and sodium and magnesium (not shown) and at
iO or 350 oc for potassium. At the short simulation time
min), however, levels of water-soluble cations increased.
eadily to 450 °C. Similar trends occurred for the orgaruc
ids, formate, acetate, and maleate.
221
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TEMPERATURE °C
140
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Figure 1-Varlatlon of levels of several watersoluble anions and cations upon simulated
buming of soil-litter of big sagebrush (ARTR),
squirreltall (ELHY), medusahead (TAAS), and
cheatgrass (BRTE). Solid black squares at a
temperature of 50 oc refer to presimulatlon
levels.
4.50
TEMPERATURE °C
222
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TEMPERATURE
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Figure 1 (Con.)
TEMPERATURE °C
223
the information on maleic acid; all soil-litters had nearly
identical levels of maleic acid. Polymaleic acid has been
proposed as a model for fulvic acid (Anderson and Russell
1976). This suggests that pyrolysis of soil humic materials is responsible for the formation of maleate and likely
other organic anions.
Our simulations indicate that, given appropriate temperatures and time, all soil-litters tested produced organic
anions including the carboxylic acids formate, acetate,
and maleate. In contrast, the lack of water-soluble organic anions, postwildfire, in cheatgrass-occupied shrub
interspaces (unpublished research, USDA-ARS, Reno,
NV) is likely a consequence of insufficient temperatures
or exposure times to promote these reactions.
Christensen, N. L.; Muller, C. H. 1975. Effects of fire on
factors controlling plant growth in Adenostoma chaparral. Ecological Monographs. 45: 29-55.
DeBano, L. F.; Conrad, C. E. 1978. The effect of fire on nutrients in a chaparral ecosystem. Ecology. 59: 489-497.
Gibson, D. J.; Hartnett, D. C.; Merrill, G. L. S. 1990. Fire
temperature heterogeneity in contrasting fire prone
habitats: Kansas tallgrass prairie and Florida sandhill.
Bulletin of the Torrey Botanical Club. 117:349-356.
Isichei, A 0.; Sanford, W. W. 1980. Nitrogen loss by burning from Nigerian grassland ecosystems. In: Rosswall,
T., ed. Nitrogen cycling in West African ecosystems.
Stockholm: SCOPEIUNEP International Nitrogen Unit,
Royal Swedish Academy of Sciences: 325-331.
Keeley, J. E.; Morton, B. A; Pedrosa, A.; Trotter, P. 1985.
Role of allelopathy, heat and charred wood in the germination of chaparral herbs and sutfrutescents. Journal of
Ecology. 73: 445-458.
Khanna, P. K.; Raison, R. J. 1986. Effect of fire intensity
on solution chemistry of surface soil under a Eucalyptus
pauciflora forest. Australian Journal of Soil Research.
24:423-434.
McKell, C. M.; Wilson, A M.; Kay, B. L. 1962. Effective
burning of rangeland infested with medusahead.
Weeds.10: 125-131.
·
Melgoza, G.; Nowak, R. S. 1991. Competition between
cheatgrass and two native species after fire: implications from observations and measurements of root distribution. Journal of Range Management. 44:27-33.
Moreno, J. M.; Oechel, W. C. 1991. Fire intensity effects
on germination of shrubs and herbs in southern
California chaparral. Ecology. 72: 1993-2004.
Raison, R. J. 1979. Modification of the soil environment
by vegetation fires, with particular reference to nitrogen transformations: a review. Plant & Soil. 51:73-108.
Raison, R. J.; Khanna, P. K.; Woods, P. V. 1984. Mechanisms of element transfer to the atmosphere during
vegetation fires. Canadian Journal of Forest Research.
15: 132-140.
Rodin, L. E.; Bazilevich, W. I. 1965. Production and mineral cycling in terrestrial vegetation. Translated from
Russian by Scripta Technica Ltd. London: Oliver and
Boyd.
Smith, D. W. 1970. Concentrations of soil nutrients before
and after fire. Canadian Journal of Soil Science. 50:
17-29.
Went, F. W.; Juhren, G.; Juhren, M. C. 1952. Fire and biotic factors affecting germination. Ecology. 33:351-363.
White, E. M.; Thompson, W. W.; Gartner, F. R.1973. Heat
effects on nutrient release from soils under ponderosa
pine. Journal of Range Management. 26: 22-24.
Wright, H. A.; Bailey, A W. 1982. Fire ecology, United
States and southern Canada. New York: John Wiley &
Sons. 501 p.
Wright, H. A; Bunting, S.C.; Neuenschwander, L. F.
1976. Effect of fire on honey mesquite. Journal of Range
Management. 29:467-471.
Young, J. A; Evans, R. D.; Eckert, R. E., Jr.; Kay, B. L.
1987. Cheatgrass. Rangelands. 9:266-270.
IMPLICATIONS
Pathways of plant succession following wildfires are influenced by many factors including fire intensity, destruction of allelopathic compounds, reduction of competition,
release of seed dormancy, seed death, and the production
of germination cues-compounds created by wildfire that
stimulate certain seeds to germinate (Christensen and
Muller 1975; Keeley and others 1985; Moreno and Oechel
1991; Went and others 1952). These research findings
pose a question in regard to wildfires in alien annual
grass systems as compared with wildfires in native shrubgrass systems.
Is fire intensity significantly less in annual grass wildfires as compared to shrub-grass systems, and if so, does
this difference, in part, alter pathways of succession?
Available data indicate that annual grass fires will be of
lower temperature and shorter duration than sagebrush
shrub fires (Gibson and others 1990; McKell and others
1962; Wright and others 1976). Successional pathways on
alien annual grasslands in the Intermountain West, subjected to repeated wildfires, lead to an annual grass monoculture, an impoverished ecosystem (Billings 1991). The
success of these alien annual grasses is largely due to
their plasticity, germination characteristics, and competitiveness (Melgoza and Nowak 1991; Young and others
1987). We speculate, however, that loss of species diversity in these alien annual grasslands occurs, in part, because the cooler and less intense fire regime, as compared
with shrub microsites, does not engender qualities in the
soil necessary for the germination and establishment of
certain species. We will test this hypothesis in future
experiments.
REFERENCES
Anderson, H. A; Russell, J.D. 1976. Possible relationship
between soil fulvic acid and polymaleic acid. Nature.
260:597.
Billings, W. D. 1991. Bromus tectorum, a biotic cause
of ecosystem impoverishment in the Great Basin. In:
Woodwell, G. M., ed. The earth in transition: patterns
and processes of biotic impoverishment. New York:
Cambridge University Press: 301-322.
224