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pubs.acs.org/est
Watershed Sulfur Biogeochemistry: Shift from Atmospheric
Deposition Dominance to Climatic Regulation
Myron J. Mitchell*,† and Gene E. Likens‡
†
‡
SUNY ESF, 1 Forestry Drive, Syracuse, New York 13210, United States
Cary Institute of Ecosystem Studies, Box AB, Millbrook, New York 12545-0129, United States
ABSTRACT: North American atmospheric S emissions peaked in the early 1970s
followed by a dramatic decrease that resulted in marked declines in sulfate (SO42-)
concentrations in precipitation and many surface waters. These changes in S
biogeochemistry have important implications with respect to the mobilization of
toxic (Alnþ, Hþ) and nutrient (Ca2þ, Mg2þ, Kþ) cations and the acidification of
watersheds. We used the continuous long-term record for watersheds 1, 3, 5, and 6
(3744 years from 1965 through 2008) of SO42- concentrations and fluxes at
Hubbard Brook Experimental Forest in New Hampshire (U.S.) for evaluating S
budgets. Analysis revealed that the annual discrepancies in the watershed S
budgets (SO42- flux in drainage waters minus total atmospheric S deposition)
have become significantly (p < 0.001) more negative, indicating the increasing
importance of the release of S from internal sources with time. Watershed wetness,
as a function of log10 annual water flux, was highly significant (p < 0.001) and
explained 57% (n = 157) of the annual variation for the combined results from watersheds 1, 3, 5, and 6. The biogeochemical control
of annual SO42- export in streamwater of forested watersheds has shifted from atmospheric S deposition to climatic factors by
affecting soil moisture.
’ INTRODUCTION
Atmospheric deposition of sulfur (S), resulting from S emissions, has been the dominant factor in causing widespread,
deleterious impacts of acidic deposition on aquatic and terrestrial
ecosystems.1 North American atmospheric S emissions peaked in
the early 1970s followed by a dramatic decrease (Figure 1a) that
resulted in marked declines in sulfate (SO42-) concentrations in
precipitation and many surface waters including those at the
Hubbard Brook Experimental Forest (HBEF) in the White
Mountains of NH (U.S.) (Figure 1b). Much of the decline of
anthropogenic emissions of sulfur dioxide (SO2) in North
America was driven by the enactment of the U.S. Clean Air
Act (CAA) in 1970 and subsequent Title IV Amendments of
the CAA in 1990, as well as other regulatory controls on SO2
emissions. Similarly, implementation of the Eastern Canada
Acid Rain Program reduced Canadian emissions such that total
CanadaU.S. emissions of SO2 were 14 million tonnes in 2006: a
50% reduction relative to 1980 levels.2 These historical emission
trends are matched by changes in precipitation concentration
and S deposition.3 The decreases in atmospheric deposition have
also resulted in decreases in SO42- concentrations in surface
waters with notable decreases across southeastern Canada4 and
northeastern U.S.5,6
Elevated atmospheric S deposition has been closely linked
with the acidification of surface waters and soils.7 This acidification has resulted in the mobilization of toxic cations (e.g., Alnþ,
Hþ)8 and the depletion of soil nutrient cations (e.g., Ca2þ,
Mg2þ, Kþ)9 that have resulted in deleterious impacts to both
r 2011 American Chemical Society
aquatic and terrestrial ecosystems.6 Temporal changes in SO42concentrations of surface waters have also been related to
changes in microbial dissimilatory SO42- reduction,10 an increase
in methane (CH4) production,11 and the methylation of mercury
(Hg).12 Increased concentrations of atmospheric CH4 are a
concern due to the high heat-trapping capacity of this “greenhouse gas”.13 Methyl Hg is bioaccumulated along food chains,
and this chemical form of Hg is highly toxic to biota, including
humans.14
Mitchell et al.15 evaluated the S budgets of 15 well-studied
watersheds in southeastern Canada and northeastern U.S. and
found that, especially for those watersheds subjected to elevated
levels of atmospheric S deposition, S outputs in drainage waters
significantly exceeded atmospheric inputs from 1985 to 2002 by
∼16 kg S ha1 yr1. Large amounts of S have likely accumulated in the soil from past, large inputs of atmospheric
deposition.16 Now, with declining inputs of atmospheric S, the
higher outputs of SO42- in drainage waters relative to precipitation inputs are driven by the S stored in the soil. Some studies
have suggested the potential importance of the role of drying and
wetting in affecting SO42- mobilization of this stored S.17,18
The objectives of the current study were to use the long-term
record at the Hubbard Brook Experimental Forest in New
Received: March 14, 2011
Accepted: May 9, 2011
Revised:
May 4, 2011
Published: May 19, 2011
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Figure 1. (a) SO2 emissions for the entire United States and regions
15 (eastern U.S.) from 1900 to 2008 with the study period from 1965
to 2008 highlighted. The estimates of annual SO2 emissions are derived
from the U.S. EPA estimates from Acid Rain Program sources for total
emissions and those emissions located in eastern U.S. (EPA Regions 1
through 5), the latter of which would most likely influence S deposition
in the northeastern U.S., http://camddataandmaps.epa.gov/gdm/index.
cfm?fuseaction=emissions.wizard. (b) Monthly concentrations of SO42in watershed 6 discharge at the Hubbard Brook Experimental Forest
(HBEF) in the White Mountains of New Hampshire (U.S.) from 1966
through 2008. Details on watershed 6 and the other watersheds at the
HBEF can be found at http://www.hubbardbrook.org/.
Hampshire of climate, hydrology, and atmospheric deposition of
S and discharge losses of SO42-. These data were used to evaluate
the role of climate change as a function of discharge in affecting
discrepancies in the S mass balances of watersheds. Our analyses
focused on the role of changing water availability in affecting the
amount of S mobilized from internal watershed sources and
resultant changes in the S budget discrepancies over time.
’ METHODS
Our current study provides a comprehensive analysis of the
relationships between atmospheric deposition and hydrology in
affecting SO42- losses in drainage waters at the HBEF in the
White Mountains of New Hampshire (U.S.) which has the
longest and most complete historical record of S watershed
biogeochemistry.6,19 We used the long-term historical record
from the HBEF for which information on precipitation amount,
bulk precipitation chemistry, streamflow, and streamwater chemistry was available. We focused our analyses on the biogeochemical reference watershed (watershed 6), which had the longest
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Figure 2. Annual calendar-year sulfur budgets for watershed 6 at the
Hubbard Brook Experimental Forest in the White Mountains of New
Hampshire from 1966 through 2008. (a) Total deposition is the sum of
bulk precipitation measurements as well as dry deposition estimates
using CASTNET measurements as developed elsewhere.15 (b) Annual
discharge of inorganic SO42-. (c) The sulfur budget discrepancy is the
difference between total deposition and discharge of inorganic SO42-.
Regression lines and associated statistics are provided (n = 44).
record from 1965 through 2008 (44 years), but also utilized
results from adjacent watersheds 1 and 3 (1971 through 2008;
38 years) and watershed 5 (19722008; 37 years) to support our
findings. Our more detailed and extensive analysis of the
influences due to atmospheric deposition and hydrology differed
from a more limited study where data from only 18 years
(19852002) were available and only watershed 6 at the HBEF
was evaluated.15
Annual deposition of S was estimated from the measured total
in bulk deposition (weekly measurements)19,20 combined with
annual dry deposition estimates using established formulations.15
Annual streamwater SO42- fluxes were based upon the summation of weekly sampling of chemistry and continuous monitoring
of stage height converted into daily flow values (Figure 2a).
Chemical measurement techniques were identical for the precipitation and streamwater determinations.19,20
’ RESULTS AND DISCUSSION
There was a significant decrease in the annual precipitation
and streamwater SO42- fluxes during the study as shown for
watershed 6 (Figure 2a,b). Also, for almost all years there was a
net negative discrepancy in the difference between atmospheric
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Figure 3. Relationship between annual sulfur budget discrepancies
normalized to concentration15 and log10 annual hydrological discharge
for watersheds 1, 3, 5, and 6 at the HBEF. Dotted lines are regression
lines for individual watersheds (all significant p < 0.001). The solid black
line and the resultant equation are the combined results using data from
all four watersheds.
deposition and stream export (Figure 2c) as has been found
previously at HBEF6 and many other watersheds in eastern
North America,15 as well as Europe.21 For a few watersheds such
as Sleepers River in Vermont22 and Arbutus Watershed in New
York,23 the weathering of S minerals is also an important source
of SO42- in drainage waters. Our current study shows for
watershed 6 at the HBEF that these discrepancies have become
significantly more negative at an average rate of 0.094 kg S ha1
yr1 (Figure 2c), indicating the increasing importance of the
release of S from watershed internal sources with time.
Internal S sources in watersheds may include the weathering of
S minerals, mineralization of organic S, and desorption of
adsorbed SO4.26,15 We evaluated this discrepancy further by
converting kg S ha1 yr1 to annual SO42- concentration to
normalize these data with respect to annual water discharge15
(Figure 3). An evaluation of the relative proportion of this
discrepancy versus time revealed that the increase in this
discrepancy was highly significant (p < 0.001) showing that the
relative importance of internal S sources is increasing over time.
There was considerable annual variation in this discrepancy, and
hence, we evaluated a range of factors that could be affecting this
internal source. We found that watershed wetness, as a function
of log10 annual water flux, was highly significant (p < 0.001) and
explained 58% (n = 45) and 57% (n = 157) of the annual variation
for watershed 6 and combined results for watersheds 1, 3, 5, and
6, respectively (Figure 3). A multiple linear regression using two
independent variables, the log10 of annual water discharge flux
and normalized annual concentration of SO42- (assuming that all
S deposition was converted to SO42- in precipitation), was highly
significant (p < 0.001) and explained 72% (n = 44) and 67%
(n = 157) of the annual variation in the annual budget SO42concentration discrepancy for watershed 6 alone and the combined results from watersheds 1, 3, 5, and 6, respectively.
Within the northeastern U.S. as well as other regions of the
world there is considerable evidence that the climate is changing
with respect to temperature and precipitation with resultant
effects on the amount of surface water discharge.24,25 Results at
the HBEF showed that discharge has significantly (p = 0.08)
increased (þ 3.5 mm yr1) from water years 1964 to 2008 for
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watershed 626 suggesting increasing soil wetness for a total
increase of ∼154 mm of discharge for this period. These analyses
demonstrate that this hydrologic, climatic component is becoming the predominant controlling factor for S outputs at HBEF.
These patterns observed at the HBEF, however, may vary in
other locations that differ in the temporal precipitation patterns
and hydrological responses. Hence, further analyses across a
broader range of sites are warranted. Regardless, the changes in
the sulfur budget at the HBEF are markedly different compared
to earlier years during which atmospheric deposition of S was the
major determinant of the export of SO42- in drainage waters.6,15
The importance of hydrologic regulation of SO42- loss from
internal sources of S can be attributed to the close linkage
between soilwater increases and the mobilization of SO42- by
SO42- desorption, weathering of S bearing minerals, and organic
S mineralization, all of which can contribute to SO42- mobilization to drainage waters as shown in the conceptual model
provided in the abstract. Greater amounts of soil water, associated with high drainage water losses, will result in SO42desorption,27 but other studies have suggested that within the
northeastern United States the desorption of SO42- cannot
account for discrepancies in sulfur budgets.28 The importance
of SO42- adsorption and desorption in affecting S budgets at
HBEF was clearly shown in forest harvesting experiments that
resulted in enhanced nitrification, acidification, and SO42- adsorption, followed by reduction of nitrification, acidification, and
SO42- desorption.29 Although the presence of S bearing minerals
has been established at the HBEF,30 these S minerals are believed
to be a minor internal source of S.6 However, it would still be
expected that greater amounts of soil water would enhance the
weathering of these S minerals.31 Soil organic S is the largest
(generally >90% of total ecosystem S content) S pool at the
HBEF and forested watersheds throughout the world.16 The
major importance of mineralization of organic S in the S budget
has been suggested from mass balance and stable isotopic
analyses.6 The mineralization of this organic S is regulated by
soil microbial activity, and the importance of soil water in
affecting microbial activity has been clearly documented32 with
specific evidence on how increasing available soil moisture
enhances microbial activity at the HBEF.33 There is some
possibility at very high soil moisture conditions coupled with
reduced oxygen availability that there could be decreased microbial activity,34 but there is no evidence that these conditions are
important at the HBEF with its well drained soils.19
’ IMPLICATIONS
We predict that the role of climate in affecting the long-term as
well as short-term temporal patterns of SO42- drainage water
losses across watersheds in eastern North America and Europe
will continue to increase, especially for those watersheds, which
are recovering from previous inputs of high levels of atmospheric
S inputs and resultant increase in soil S pools. These biogeochemical responses at the HBEF and likely other watersheds with
stored S from past anthropogenic sources will be amplified with
further climate change including greater annual stream discharge
and watershed wetness.24,25,35 This climatic change will potentially increase SO42- mobilization and hence may slow the
resultant recovery from acidification of both aquatic and terrestrial ecosystems. This mobilization of internal S sources not only
has important implications in understanding long-term changes
in watershed mass balances, but also has ramifications on the
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recovery of surface waters from acidification and efficacy of policy
considerations associated with regulation of atmospheric emissions of sulfur.
’ AUTHOR INFORMATION
Corresponding Author
*Phone: 315-470-6765; fax: 315-470-6996; e-mail: mitchell@syr.
edu.
’ ACKNOWLEDGMENT
This work was supported by The Andrew W. Mellon Foundation, U.S. National Science Foundation including the LTER and
LTREB programs, New York State Energy Research Development Authority, Northeast Ecosystem Research Cooperative
(NERC), U.S. Forest Service. The help of Don Buso in helping
with analysis and data compilation was invaluable.
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