Environmental Engineering Reference
In-Depth Information
(2007). Coniferous canopies are especially effi cient fi lters
for the removal of atmospheric particulates and gases,
exhibiting two to fi ve times the scavenging effi ciency as
compared with deciduous canopies (Grigal et al., 2000).
Demers et al. (2007) studied Hg dynamics in a conifer-
ous and a deciduous forested catchment in the Adirondacks
region of New York and observed a net increase in the lit-
terfall's total Hg content that could not be explained via
throughfall inputs. They attributed this increase to Hg
translocation from the forest organic horizon into the lit-
ter, thereby retarding Hg transport into the mineral hori-
zon. The authors proposed that this Hg accumulation in
the leaf litter may delay the recovery of surface waters by
increasing the Hg residence time in the forest.
Throughfall and litterfall have been proposed to deliver
different forms of Hg to the forest fl oor. Throughfall con-
tains Hg that is primarily associated with the leaf surface
during dry deposition in the form of particulate Hg or
the RGM (Mosbaek et al., 1998) that may originate from
local and regional sources (Demers et al. 2007). Litterfall
Hg, on the other hand, is originally derived from elemen-
tal Hg that is taken up by stomata (Rea et al., 2000, 2001,
2002). Demers et al. (2007) argued that Hg delivered to the
forest fl oor via litterfall is more likely incorporated into
the soil than Hg delivered via throughfall, which may be
largely volatilized. They observed larger Hg pools in both
the whole pedon and the O horizon in the deciduous (22.2
and 4.45 mg m
2
, respectively) than in the coniferous (9.64
and 2.05 mg m
2
) forest despite the larger Hg fl ux to the
forest fl oor in the deciduous forest. This difference was
partly attributed to the larger litter mass and a larger litter
Hg accumulation rate in the deciduous forest (Demers et
al., 2007). To account for this difference, the authors pro-
posed that since Hg input into the coniferous forest fl oor
is primarily via throughfall, Hg (re)emission from the for-
est fl oor is likely an important loss mechanism, whereas in
deciduous forests, Hg input to the forest fl oor is primarily
due to the litterfall, which has a lower likelihood of Hg
emission losses. The soil Hg concentrations in this study
peaked in the Oe layer (~1-4 cm depth) with 394.6 ng g
1
and 413.4 ng g
1
in the deciduous and coniferous stands,
respectively.
In the forest fl oor at Acadia National Park (Maine),
Sheehan et al. (2006) found that coniferous vegetation had
the highest Hg concentration (58.8
Vegetation type could also affect the lability and reactiv-
ity of SOM in the O horizon, in turn infl uencing Hg cycling.
Amirbahman et al. (2004) observed that SOM from the O
horizon of the coniferous soil was more soluble than that of
an adjacent deciduous soil, although this contrast could also
have been infl uenced by the deciduous forest being burned
a half century earlier. Laboratory experiments showed that
DOM from the coniferous O horizon had a higher Hg bind-
ing strength than that of the deciduous O horizon. Taken
together with the higher solubility of SOM from the conif-
erous versus the deciduous soil, Hg mobilization from the
coniferous watershed has a higher potential than from the
deciduous watershed. These differences could be related
to differences in the average age of the SOM, as well as to
contrasting vegetation composition in the two watersheds.
In both cases, however, Hg showed a higher affi nity for
DOM than the insoluble SOM. This difference in Hg affi n-
ity has also been observed in Fe-humus podzol O horizon
soils (Schlüter, 1997) and in peat soils from the Everglades
(Florida), where DOM from peat exhibited higher Hg con-
ditional stability constants than solid peat (Drexel et al.,
2002).
Soils in deciduous forests generally possess a higher pH
and litter quality than in the coniferous forests (Cronan
and Reiners, 1983). Deciduous forests also have faster
nutrient cycling and decomposition rates than coniferous
forests, as observed by higher rates of N mineralization
in the presence of deciduous species (Johnson, 1995; Hill
and Shackleton, 1989; Nadelhoffer et al., 1995; Campbell
et al., 2000). The higher MeHg concentrations observed by
Amirbahman et al. (2004) and by Schwesig and Matzner
(2000) in deciduous as compared with coniferous for-
est soils could be partly due to the faster rate of microbial
metabolism and relatively higher pH, as microbial activ-
ity, especially that of sulfate-reducing bacteria, tends to
increase with increased nutrient availability and decrease
with decreasing pH (Maier et al., 2000). MeHg also binds
more favorably to organic matter with increasing pH up
to the circumneutral pH range (Amirbahman et al. 2002),
which may partly explain a higher MeHg concentration in
a deciduous forest soil.
Differences in soil MeHg contents between watersheds
can also be attributed to temperature differences, as higher
temperatures increase microbial activity, thereby enhanc-
ing Hg methylation rate (Korthals and Winfrey, 1987; King
et al., 1999). Fernandez et al. (2007) observed consistently
higher mean soil temperatures in all soil horizons for decid-
uous as compared with coniferous stands. Temperature
differences by forest type increased with soil depth, and
greatest differences were observed during spring and sum-
mer. Coniferous soils exhibited a slower rate of temperature
change during spring and fall as compared with decidu-
ous soils in all horizons, suggesting less heat accumulation
in the former, which may be attributed to the lower light
penetration and moisture differences in coniferous stands
(Fernandez et al., 2007)
.
3.3 ng g
1
) followed
by mixed (41.7
2.8 ng g
1
), scrub (40.6
2.7 ng g
1
), and
2.6 ng g
1
) vegetation types. Previously,
Johnson (2002) had reported average litterfall Hg concen-
trations in the same coniferous and deciduous watersheds
at 52.9 and 39.7 ng g
1
, respectively. In contrast to the
study by Demers et al. (2007), however, the litter Hg fl ux
estimated by Sheehan et al. was similar among all vegeta-
tion classes, which in this case was attributed to the higher
softwood litter Hg concentration being balanced by the
larger hardwood litter mass (Grigal, 2002; Sheehan et al.,
2006).
deciduous (31.6
