Environmental Engineering Reference
In-Depth Information
In the forested watersheds studied by Amirbahman et al.
(2004), soil pH was signifi cantly higher in all horizons of
the burned soils than in the unburned soils because of
the incorporation of ash into the soil profi le. This differ-
ence was especially signifi cant in the O horizon, where on
average, burned soils had a pH of approximately 0.40 unit
higher than the unburned soils (3.41
0.22 vs. 2.99
0.13).
Total Hg concentrations were higher in the O horizon of
the unburned soils than in the burned soils, most likely
as a result of the fi re volatilizing the SOM-bound Hg.
Methylmercury concentrations, with respect to both soil
mass and C content, were higher in the burned versus the
unburned soils, also most likely as a result of the fi re, even
though no statistically signifi cant differences between
MeHg concentrations in the O and mineral horizons within
each soil were observed. The MeHg concentration in the O
horizon for the burned and unburned soils were 0.2
0.13
and 0.07
0.07 ng g
1
, respectively. Higher MeHg con-
centrations are generally a result of the enhanced micro-
bial, especially sulfate-reducing, activity. An enhance-
ment in microbial activity in burned soils was attributed
to the increased nutrient availability and decreased soil
acidity (Maier et al., 2000). Even though soils studied by
Amirbahman et al. (2004) were generally well drained,
and as such, well oxygenated, soil anaerobic microsites are
known to serve as Hg methylation hot spots (Compeau and
Bartha, 1985). However, microbial Hg methylation is slower
in unsaturated systems, such as well-drained forest soils,
than in saturated systems, such as wetlands (Rudd, 1995).
The higher MeHg concentrations in the burned soils could
also be attributed to the more favorable binding of MeHg to
SOM at a less acidic pH (Amirbahman et al., 2002). MeHg
concentrations with respect to the C content were higher
in the mineral horizons than in the O horizon, which is
indicative of the smaller MeHg than C loss during SOM
mineralization. This can also be attributed to the presence
of more humifi ed organic matter in the mineral horizons.
(2004) studied forest and deforested (pasture) soils with a
similar composition and distance from any Hg source. They
observed signifi cantly higher Hg concentrations in forest
soils (average, 61.9 ng g
1
) than in pasture soils (average,
33.8 ng g
1
), with a cumulative Hg burden in the former
soils nearly double that of the latter soils in the top 10 cm,
pointing to the importance of deforestation in soil Hg mobi-
lization. However, they observed no signifi cant differences
in the soil Hg down to a depth of 10 cm in both undisturbed
(forest) and disturbed (pasture) Amazon forest soils.
Soil Hg in the Amazon is either associated with the sur-
face organic horizon or horizons dominated by Fe and Al
minerals (Roulet et al., 1998; Fostier et al., 2000; Oliveira et
al., 2001), and as such, can be mobilized by the oxidation
of SOM or degradation of the mineral duricrust (Lacerda et
al., 2004). Several studies have shown that deforestation in
the Amazon enhances both processes, and hence, results in
soil Hg mobilization (Roulet et al., 1998, 1999; Fostier et al.,
2000; Oliveira et al., 2001).
Higher Hg concentrations in forested as compared with
pasture soils can be attributed not only to signifi cant soil
erosion in the latter soils (Roulet et al., 1998, 1999; Fostier
et al., 2000), but also to the interception of dry deposition
via throughfall and litterfall by the canopy in forested
watersheds. Soil exposure also results in higher Hg emis-
sion rates (Zhang and Lindberg, 1999), with pasture soils
exhibiting an order of magnitude larger emission rates than
forest soils (Lacerda et al., 2004). Increased solar exposure
in deforested soils leads to elevated soil temperatures that
can also increase the Hg emission rate (Zhang et al., 2001).
The effect of soil disturbance on receiving water qual-
ity can be complicated by characteristics of the watershed
as well as the water body. As such, extrapolating water-
body response to wildfi re or deforestation, especially with
respect to nutrient and DOM loading, may not be war-
ranted for other water bodies with different watershed
and physicochemical conditions (Carignan and Steedman,
2000). Enhanced transport of Hg from disturbed soils
increases its loading to a water body. Hg loading is one
of the key factors determining the extent of Hg methyla-
tion in waters (Winfrey and Rudd, 1990; St. Louis et al.,
1994). Studies of the effect of forest harvesting on lakes in
Québec, Canada, have shown elevated Hg concentrations
in zooplankton and northern pike, which was attributed
to the increase in the DOM export to the receiving lakes
(Garcia and Carignan, 1999, 2000). In a study of a par-
tially burned catchment, Kelly et al. (2006) observed up
to a fi vefold increase in the fi sh Hg concentration as com-
pared with a reference catchment. They proposed that fi re
causes a signifi cant but short-term release of Hg and MeHg
into streams and lakes that enhances fi sh Hg accumula-
tion. However, they also proposed that the major enhance-
ment in fi sh Hg accumulation could be due to food-web
restructuring brought about by increased nutrient concen-
trations in lakes and streams that supports primary produc-
tion. Kelly et al. (2006) further suggested that the relative
Other Disturbances
Forest harvesting, especially by clear-cutting, also has impor-
tant implications for soil Hg budgets and cycling (Porvari
et al., 1993). The impacts, however, would be different from
those of a wildfi re (Carignan and Steedman, 2000). Even
though both harvesting and wildfi res result in the release of
nutrients from the watershed, harvesting can result in a larger
release of DOM, whereas fi re can result in a larger release of
NO
3
from the soil, and released nitrogen being predomi-
nantly organic in the case of harvesting and inorganic in the
case of fi re (Carignan and Steedman, 2000).
In the Amazon, land-use change, especially the conver-
sion of forest into agricultural and pasture land, has been
suggested as one of the most important causes of soil Hg
mobilization (Lacerda et al., 2004). Deforestation results in
the degradation of the organic horizon, which in turn can
cause signifi cant erosion of the mineral layer. Lacerda et al.
