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Fire Frequency and Soil Nutrient Status on the
Southern Gunnery Ranges at Fort Huachuca
Military Reservation, Arizona
Thomas B. Wilson, Principal Investigator, Department of Soil, Water,
and Environmental Science, University of Arizona; Robert H. Webb,
Co-Investigator, United States Geological Survey; and Thomas l.
Thompson, Co-Investigator, Dept of Soil, Water, and
Environmental Science, University of Arizona
G
rassland fires have important but often poorly understood effects on soil
nutrient levels, which may either aid or hinder plant reestablishment following fires. The objective of this study was to determine the relationship between
fire frequency and time elapsed since the most recent fire upon plant-available
phosphorus, total organic carbon, total nitrogen, ammonium, nitrate, and
aboveground plant biomass in a semidesert grassland in southeastern Arizona.
Eight sites with reasonably uniform physical characteristics but with fire frequencies ranging from less than 1 fire/decade to 5 fires/decade were analyzed at
the Fort Huachuca Army Base in the Huachuca Mountains. Soils are noncalcareous
coarse to fine sandy loams, classified as Aridisols with a granitic parent material,
and a high degree of physical uniformity. The sites contained two geomorphic
surfaces: the younger Garden Canyon Series, and the older Langue Series. Soil
samples were collected at 25 random locations within each site and analyzed for
total organic C, total N, ammonium, and nitrate. Aboveground biomass samples
were collected from 1 m 2 areas at 15 randomly selected locations within each site;
within each area, litter, forbs, shrubs, native grasses, and Eragrostis lehmanniana
(Lehmann love grass) were collected separately.
The trends between total carbon and total nitrogen closely paralleled each
other on each geomorphic surface. Total organic C values obtained for the younger
geomorphic surface ranged from 0.84% to 1.08%, while total N values ranged
between 0.07% and 0.08%; there were positive relationships between percent total
Nand total C and fire frequency (0, 2, and 4 burns/decade). At the older geomorphic surface, total organic C ranged from 0.67% to 1.10%, while total N ranged
between 0.06% to 0.13%. For both total organic C and N the 1 burn/decade site
had the highest values; the 3 burns/decade had the lowest values, while the 0, 2,
4, and 5 burn/decade sites had intermediate values.
Ammonium and nitrate concentrations also showed a similar relationship with
fire frequency on the younger geomorphic surface. Ammonium concentrations
ranged from 0.2 ppm (2 burns/decade) to 1.1 ppm (4 burns/decade), while
nitrate concentrations ranged from 1.2 ppm (2 burns/decade) to 3.3 ppm (4
burns/decade). On the older geomorphic surface ammonium concentrations decreased with increasing fire frequency, from 1.7 ppm to 0.2 ppm. The highest
nitrate concentration (4.7 ppm) was also at the lowest fire frequency, but on this
surface the 3 burns/decade site had the lowest concentration (1 ppm). Overall,
these are low values for plant-available nitrogen.
For the three fire frequencies evaluated on the younger geomorphic surface,
native grasses constituted about half of the total biomass collected. Lehmann
lovegrass biomass displayed a positive relationship with fire frequency, ranging
from 0% at the 0 burns/decade site to 25% at the 4 burns/decade site. Shrubs
USDA Forest Service Proceedings RMRS-P-10. 1999.
81
Wilson, Webb, and Thompson
Fire Frequency and Soil Nutrient Status, Fort Huachuca Military Reservation, Arizona
For the three fire frequencies evaluated on the younger geomorphic surface,
native grasses constituted about half of the total biomass collected. Lehmann
lovegrass biomass displayed a positive relationship with fire frequency, ranging
from 0% at the 0 burns/decade site to 25% at the 4 burns/decade site. Shrubs
were virtually nonexistent at the 3 sites. Forbs ranged from a high value of 12% at
the 0 burns/decade site to 5% at the 2 burns/decade site. Litter ranged from 41%
at the 2 burns/decade site to 25% at the 4 burns/decade site.
On the older geomorphic surface, Lehmann lovegrass also displayed a positive
relationship with fire frequency, with greater percentage values than for native
grasses at the 4 and 5 burns/decade sites. At the 4 burns/decade site, Lehmann
love grass constituted over 60% of the total, while native grasses at this site cop_stituted only 7% of the total. This site also possessed the lowest values for shrubs
(3%) and forbs (4%). In contrast, at the 1 fire/decade site, native grasses comprised 42% of the total, compared to 3% for Lehmann love grass. Shrubs displayed
a generally negative relationship with increasing fire frequency; at the 0 burns/
decade site they constituted 50% of the total biomass, followed by native grasses
(20%), litter (13%), Lehmann lovegrass (10%), and forbs (7%). At the 3 burns/
decade site, shrubs constituted just 3% of the total and the 4 burns/decade site
contained 2.5%.
Soil pH and plant-available phosphorus remain to be analyzed in the soil
samples. However, the existing results suggest that fire frequency plays an important role in determining the species composition of the aboveground biomass.
The sharp increase in the percentage of Lehmann lovegrass at the 4 and 5 burns/
decade sites on the older geomorphic surface, and the less dramatic increase at the
2 and 4 burns/decade sites on the younger geomorphic surface, suggest that
Lehmann lovegrass has a competitive advantage in rapidly establishing after disturbance events.
On the older geomorphic surface, the 3 burns/decade site contained the lowest concentrations of total organic C, total N, and nitrate; this may coincide with
the low percentage of shrubs (including nitrogen-fixing plants) and a relatively
uniform representation by other plants and litter, ranging from 30% native grasses
to 19% for forbs. In contrast, the highest concentrations of ammonium and nitrate
occurred on the 0 burns/decade site, possibly coinciding with the relatively high
composition of shrubs. To define this relationship more rigorously, more detailed
work needs to be conducted to evaluate the role these shrubs play in contributing
to nitrogen cycling within this plant community.
82
USDA Forest Service Proceedings RMRS-P-10. 1999.