Environmental Engineering Reference
In-Depth Information
importance of limiting nutrients versus Hg release from
burned catchments is due to fire characteristics, such as fire
severity, proportion of catchment burned, and timing and
intensity of runoff.
mailman and Bodaly (2005) proposed that burning land
before flooding it would limit the extent of methylation
and meHg release into water due to the drastic reductions
in som and Hg. They suggested that loss of som would
limit the availability of the organic substrate to methylat-
ing organisms, while loss of Hg would limit its availability
for methylation. The remaining ash also possesses a higher
adsorption capacity and a lower solubility that promotes
the adsorption and hinders the release of Hg and meHg
(amirbahman et al., 2004). in a later study using meso-
cosms with burned and unburned soil and plant samples,
mailman and Bodaly (2006) observed that even though
burning reduced meHg concentrations in the flooded
water, it did not lower meHg accumulation in the lower
food web. They attributed this to the lower dom concen-
trations in mesocosms containing burned soil and vegeta-
tion, and the fact that a lower dom concentration brought
about a higher biota:water meHg ratio.
lorey and driscoll (1999) showed that net retention of Hg
in adirondack forested watersheds has declined from 95%
in 1850 to 78% in more recent years, as the anthropogenic
Hg emission has increased. This has resulted in an increase
in the sedimentary Hg flux in the studied lakes significantly
during this period. The reason behind this trend is not
understood, even though it has resulted in an increase in
the level of Hg contamination in surface waters. This trend
is perhaps rooted in the terrestrial Hg interactions involv-
ing soils and litterfall (demers et al., 2007), and its elucida-
tion is vital to understanding the response of watersheds to
changes in Hg deposition as well as the climate.
The effect of climate change on Hg storage and cycling
in soils deserves attention. it is difficult to predict with any
certainty how the modern era of accelerated climate change
will influence ecosystem function, let alone Hg dynamics spe-
cifically. There is consensus that we will experience climatic
warming on a local and global scale (intergovernmental Panel
on climate change [iPcc], 2007). regional predictions such as
for the northeastern united states can suggest both increased
annual average precipitation as well as increased frequency
of soil moisture stress at certain times in the growing season
(Hayhoe et al., 2006), along with an overall intensification of
the hydrologic cycle (Huntington, 2006), including earlier ice
out in lakes (Hodgkins et al., 2002). These trends do not define
changes in Hg cycling but give us insight into places to look for
the first signs of effects on Hg. Warmer growing-season tem-
peratures are likely to decrease overall soil moisture, particu-
larly during the late growing season, leading to lower water
tables, perhaps increased rates of decomposition, Hg mobiliza-
tion, and methylation. a warmer soil also leads to greater soil
Hg emission, but on the other hand, dom export from water-
sheds increases because of warming, enhancing Hg mobili-
zation. changing forest growth and composition over short
and long timeframes can alter landscapes from coniferous to
deciduous vegetation, leading to changing Hg deposition and
cycling as discussed above. sustained droughts brought about
by warming trends together with the current practice of forest
fire suppression lead to the accumulation of surface fuel, and
result in an increase in the magnitude and frequency of for-
est fires. This would in turn result in the release of higher Hg
levels into the atmosphere.
Conclusions
The large capacity of soils for binding Hg causes them to
act as a long-term Hg reservoir and source for freshwater
ecosystems. Grigal (2003) estimated that the mass of Hg
in mineral soils was significantly larger than that in the
forest floor and living vegetation. Given that atmospheric
Hg deposition is the main input into the forest floor
and that mineral horizons in forest soils are the domi-
nant ecosystem storage compartment for Hg, it follows
that soil Hg is indeed mobile between horizons because
of the downward migration of organic matter following
litter decomposition and pedogenic processes. Harris
et al. (2007) suggested that whereas reductions in anthro-
pogenic mercury emissions would result in a relatively
rapid decrease in fish mercury content on the order of
years, a full recovery in fish Hg content could take centu-
ries because of the gradual release of Hg from upland and
wetland soils.
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