Environmental Engineering Reference
In-Depth Information
regulated partly by the fuel load, the existing vegetation,
and soil moisture regimen. The latter factor is especially
crucial in determining the areal and vertical extent of
fi re; soils subject to drought lead to severe fi res, result-
ing in a relatively signifi cant release of Hg, C and nutri-
ents (Kasischke and Stocks, 2000). Burning of SOM also
reduces the C:N ratio that is favorable for plant growth
and results in the loss of thermally labile C and excess
accumulation of ligninlike C in the soil (Harden et al.,
2004). In most cases, fi re promotes soil microbial N fi xa-
tion and brings about a change in N speciation toward
increasing soil NH
4
, NO
2
and NO
3
concentrations that
are more readily available for plant uptake (Chandler et
al., 1983; Acea and Carballas, 1996). Soil cations, how-
ever, are stable at high temperatures, and their mass
remains relatively constant after the fi re. The burning of
vegetation produces ash rich in cations, which increases
soil fertility (Harden et al., 2004), especially in nutrient-
poor soils such as those in the Amazon (Farella et al.,
2006). Increased soil cation concentrations also promote
microbial and fungal growth, which has direct implica-
tions for Hg methylation. Cations exchange with soil H
,
the extent of which depends on the soil cation exchange
capacity. This leads to the observation that especially
in the acidic forest soils, pH increases relatively signifi -
cantly after a fi re (Wright and Bailey, 1982; Chandler et
al., 1983). A combination of higher pH and nutrient avail-
ability in postfi re soils promotes soil microbial activity
that may in turn result in a greater level of Hg methyla-
tion. Cation input into soils also exchange with soil Hg
and mobilize it into receiving waters, as evidenced by the
postfi re increase in lake sediment Hg content (Garcia and
Carignan, 1999; Caldwell et al., 2000).
Biswas et al. (2007) contrasted soil Hg contents in three
recently burned sites and adjacent unburned sites. Chosen
sites represented a range of fi re severities from low to mod-
erate to high in coniferous, deciduous (aspen), and meadow
plots. The fi re resulted in the loss of Hg and SOM down to
a depth of 6-8 cm, which was attributed to the severity as
well as the duration of the fi re. Statistical analysis showed
that down to a depth of 4 cm, fi re severity and vegetation
type were the primary factors controlling Hg release. In the
coniferous plots, high-, moderate-, and low-severity fi res
released 75-87%, 62%, and 22% of the Hg soil, respectively.
In the deciduous plots, moderate and low severity fi res
released 63% and 17% of soil Hg, respectively. Given the
higher Hg content in the unburned coniferous as compared
with the unburned deciduous soils as mentioned above,
the coniferous soils released a signifi cantly higher mass of
Hg than the deciduous soils (7-28 vs. 4-13 g ha
1
; Biswas
et al., 2007).
Engle et al. (2006) estimated that forest soils
in the Sierran forests (California) released 2-5 g ha
1
of soil
Hg because of prescribed and wild fi res, whereas a desert
sagebrush fi re released 0.36 g ha
1
of soil Hg. In all cases,
the most important sources of the released Hg were the
litter and vegetation.
Role of Forest Fires and Other Disturbances
Forest Fires
Forest and agricultural fi res together contribute to nearly
30% of the global atmospheric Hg (Brunke et al., 2001;
Sigler et al., 2003; Wiedinmyer and Friedli, 2007). In the
United States alone, this amounts to an estimated release
of 19 to 64 metric tons of Hg yr
1
from burning of 2.7
10
6
ha of forest and shrubland, which represents 13 to 42% of
the U.S. anthropogenic Hg emissions (Biswas et al., 2007).
For the lower 48 states, Wiedinmyer and Friedli (2007)
estimated that forest fi res release 44 metric tons of Hg per
year. Fire results in the emission of a large part of soil and
biomass C and Hg to the atmosphere, the extent of which
depends in part on the severity of fi re (Friedli et al., 2003;
Biswas et al., 2007, 2008; Burke et al., 2010; Woodruff and
Cannon, 2010). Friedli et al. (2007), for example, showed
that heating organic soil at 300°C for 5 min results in a
nearly complete loss of Hg, whereas heating the same soil
at 100°C for 45 min results in the release of only 10% of
Hg. In a study of the effect of fi re on the soil Hg budget
at the Experimental Lakes Area in Northwestern Ontario,
Canada, Mailman and Bodaly (2005) reported that com-
plete burning resulted in the loss of 97% and 94% of total
Hg and MeHg in plants, and 79% and 82% of total Hg and
MeHg in the upland soil. The practice of suppressing forest
fi res in the United States has resulted in the accumulation
of fuel load, as well as Hg, and may increase the future Hg
release to the atmosphere (Biswas et al., 2007).
In addition to atmospheric Hg emissions, forest fi res have
signifi cant consequences for soil Hg storage and cycling,
and Hg release into the receiving water bodies. Following
a disturbance, forest soils can continue to release Hg, as
soil changes due to the disturbance can reduce their abil-
ity to retain stored Hg (Burke et al., 2010). Hg contained in
ash is also more readily mobilized via wind and water ero-
sion, resulting in high Hg concentrations in lake sediments
(Caldwell et al., 2000). It has been suggested that even
though soil erosion is generally enhanced because of fi re,
excessive erosion may occur only for several years immedi-
ately following the fi re because of the presence of roots that
stabilize the soil (Wright and Bailey, 1982). The extent of
erosion depends on factors such as slope and vegetation, as
well as soil properties such as texture and moisture content.
Considering that a substantially larger mass of Hg is
stored in forest soil than in vegetation (Grigal, 2003),
more Hg released by fi re could originate from the soil,
depending on fi re severity. The postfi re soil physical and
chemical environments that in turn control the fate of
Hg are affected by factors such as the extent of soil heat-
ing; the removal of vegetation, litter, and part of the O
horizon; and increased soil hydrophobicity because of
the formation of charcoal (Chandler et al., 1983). Burned
soils also tend to be thinner and more discontinuous
than unburned soils. Soil temperature during the fi re is
