Environmental Engineering Reference
In-Depth Information
emission. In the soil environment, a higher pH can bring
about a higher degree of microbial activity that in some
cases can enhance Hg(II) reduction.
of Johnson et al. (2007), who observed higher Hg and
MeHg fluxes in the runoff from the coniferous catchment
than in that from the deciduous catchment. This could be
partly attributed to the concentration and quality of DoM
released from each watershed.
The findings by Schwesig and Matzner (2000) regard-
ing the higher Hg accumulation in coniferous than in
deciduous soils were corroborated by Biswas et al. (2007),
who systematically studied the Hg content in adjacent
coniferous, deciduous (aspen), and meadow plots in the
rocky Mountains (Wyoming). For coniferous soils, they
observed Hg concentrations in the o horizon (0-4 cm)
ranging from 58.4 to 208 ng g
2
1
, and in the mineral hori-
zon (5-8 cm) ranging from 30.9 to 68.1 ng g
2
1
. In aspen
and meadow plots, the o-horizon Hg concentrations
ranged from 25.2 to 37.5 ng g
2
1
, and in the mineral hori-
zon it ranged from 19.5 to 39.2 ng g
2
1
. Biswas et al. (2007)
also observed significantly higher SoM concentrations
in coniferous plots than in aspen and meadow plots in
the o horizon. This may be attributed to the slower rate
of litter decomposition in lignin-rich coniferous forests
(Johnson, 1995).
Amirbahman et al. (2004) studied the distribution of Hg
and MeHg in the soils of a burned and unburned forested
watershed at Acadia National Park, Maine. Deciduous veg-
etation became the dominant forest type after an intense
fire in 1947, and coniferous forest dominated the unburned
watershed. As expected, total Hg concentrations were sig-
nificantly higher in the o horizon of the coniferous water-
shed as compared with the deciduous watershed (134
6
48
ng g
2
1
vs. 103
6
23 ng g
2
1
), but there were no significant
differences in total Hg concentrations in the mineral hori-
zons between the two watersheds (lowest in the Bh layer
at 60.2 ng g
2
1
to highest in the Bs layer at 79.6 ng g
2
1
). The
three comparative studies of Hg in coniferous and decidu-
ous soils cited above all report significantly higher soil Hg
concentrations under coniferous vegetation. The study
by Amirbahman et al. (2004) also showed a numerically
higher Bs Hg in the coniferous forest, although it was not
statistically significant. A lower Hg concentration in decid-
uous forests is expected because of the relatively high rates
of organic matter turnover as compared with coniferous
forests, which leads to a lower SoM and soil Hg content
(Grigal, 2003). The more lignin-rich character of the conif-
erous vegetation renders its degradation kinetics slower
(Johnson, 1995), and its lower nutrient concentrations con-
tributes to this effect. In addition, atmospheric inputs of
Hg under a coniferous forest are typically higher than in
deciduous forests because of the relatively higher specific
surface area and the generally longer persistence of needles
as compared with the deciduous leaves (rustad et al., 1994;
Bailey et al., 1996; Grigal, 2002), and possibly other foliar
surface characteristics such as high surface roughness and
waxier cuticles noted for pine (rea et al., 2002). Higher
throughfall Hg concentrations in coniferous forests than
deciduous forests have been reported by Johnson et al.
Role of Vegetation in Mercury Accumulation
in forest Soils
The role of vegetation on soil Hg content is reviewed here
within the context of differences in coniferous versus
deciduous forests. Several studies have indicated the impor-
tance of litterfall and throughfall as the two most impor-
tant vectors for delivering Hg to the forest floor (Iverfeldt,
1991; rea et al., 1996; Lee et al., 2000; Grigal et al., 2000;
St. Louis et al., 2001; Ericksen et al., 2003; Grigal, 2003). Hg
mass balance in forested catchments suggests that vegeta-
tion type, the resulting differences in Hg delivery (litterfall
vs. throughfall) as described above, and processes of incor-
poration into the soil determine the fate of Hg, especially
with respect to its residence time and transport into surface
waters (Demers et al., 2007).
Schwesig and Matzner (2000) studied the pools and fluxes
of Hg and MeHg in a coniferous and deciduous catchment
in southeastern Germany. Total Hg concentrations in wet
deposition, throughfall, and litterfall were relatively similar
between the two catchments. MeHg concentrations in wet
deposition and throughfall were also similar between the
two catchments, but litterfall contained twice as much MeHg
in the coniferous as in the deciduous catchment. Despite
nearly double the mass of litterfall in the deciduous as com-
pared with the coniferous catchment, litterfall MeHg fluxes
were similar in the two catchments. Litterfall contributed
55% of total deposition of total Hg and MeHg in the decidu-
ous catchment, while it contributed 29% of total deposition
of total Hg and 55% of MeHg in the coniferous catchment.
The coniferous soil retained four times higher total Hg and
seven times higher MeHg as the deciduous soil in the top 60
cm (total Hg
5
89.1 vs. 19.3 mg m
2
2
; MeHg
5
0.43 vs. 0.06
mg m
2
2
), with total Hg concentrations peaking in the oa
layer at ~500 and ~160 ng g
2
1
for the coniferous and decidu-
ous catchments, respectively (Schwesig and Matzner, 2000).
MeHg concentration, on the other hand, peaked in the oi
layer, but the concentrations in both catchments remained
1 ng g
2
1
. Even though the soil storage:annual input ratio
was approximately five times larger in the coniferous as
compared with the deciduous watershed, the authors attrib-
uted the higher Hg accumulation in the coniferous water-
shed primarily to the different history of Hg deposition in
the two watersheds.
The deciduous and the coniferous catchments in this
study retained 95% and 85% of the deposited Hg, respec-
tively (Schwesig and Matzner, 2000). In the deciduous
catchment, 2.8 and 0.01 g m
2
2
yr
2
1
of total Hg and MeHg,
respectively, were exported via runoff. In the conifer-
ous catchment, total Hg and MeHg export in the runoff
were 6.8 and 0.05 g m
2
2
yr
2
1
, respectively (Schwesig and
Matzner, 2000). These observations correspond to those
