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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 5267
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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 5268
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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 5269
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’ 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. ’ REFERENCES (1) Reuss, J. O.; Johnson, D. W. Acid Deposition and Acidification of Soils and Waters; Springer-Verlag: New York, 1986. (2) CanadaUnited States Air Quality Agreement: 2008 Progress Report; ISBN 9781100105161; 2009; http://www.ec.gc.ca/ Publications/default.asp?lang=En&xml=D8E5913D-0A85-4BE4-AD67B627D0C0FE87. (3) Sickles, J. E.; Shadwick, D. S. Changes in air quality and atmospheric deposition in the eastern United States: 19902004. J. Geophys. Res. 2007, 112, D17301; DOI 10.1029/2006JD007843. (4) Jeffries, D. S.; Clair, T. A.; Couture, S.; Dillon, P. J.; Dupont, J.; Keller, W.; McNicol, D. K.; Turner, M. A.; Vet, R.; Weeber, R. Assessing the recovery of lakes in southeastern Canada from the effects of acidic deposition. Ambio 2003, 32, 176–182. (5) Stoddard, J. L.; Jeffries, D. S.; Lukewille, A.; Clair, T. A.; Dillon, P. J.; Driscoll, C. T.; Forsius, M.; Johannessen, M.; Kahl, J. S.; Kellogg, J. H.; Kemp, A.; Mannio, J.; Monteith, D. T.; Murdoch, P. S.; Patrick, S.; Rebsdorf, A.; Skjelkva, B. L.; Stainton, M. P.; Traaen, T.; van Dam, H.; Webster, K. E.; Wieting, J.; Wilander, A. Regional trends in aquatic recovery from acidification in North America and Europe. Nature 1999, 401, 575–577. (6) Likens, G. E.; Driscoll, C. T.; Buso, D. C.; Mitchell, M. J.; Lovett, G. M.; Bailey, S. W.; Siccama, T. G.; Reiners, W. A.; Alewell, C. The biogeochemistry of sulfur at Hubbard Brook. Biogeochemistry 2002, 60, 235–316. (7) Likens, G. E.; Driscoll, C. T.; Buso, D. C. Long-term effects of acid rain: response and recovery of a forest ecosystem. Science 1996, 272, 244–246. (8) Driscoll, C. T.; Postek, K. M. The chemistry of aluminum in surface waters. In The Environmental Chemistry of Aluminum; Sposito, G., Ed.; CRC: Boca Raton, FL, 1995; pp 363418. (9) Bailey, S. W.; Horsley, S. B.; Long, R. P. Thirty years of change in forest soils of the Allegheny Plateau, Pennsylvania. Soil Sci. Soc. Am. J. 2005, 69, 681–690. (10) Siver, P. A.; Ricard, R.; Goodwin, R.; Giblin, A. E. Estimating historical in-lake alkalinity generation from sulfate reduction and its relationship to lake chemistry as inferred from algal microfossils. J. Paleolimnol. 2003, 29, 179–197. (11) Gauci, V.; Dise, N. B.; Howell, G.; Jenkins, M. E. Suppression of rice methane emission by sulfate deposition in simulated acid rain. J. Geophys. Res. 2008, 113, G00A07; DOI 10.1029/2007JG000501. (12) Gilmour, C. C.; Henry, E. A.; Mitchell, R. Sulfate stimulation of mercury methylation in freshwater sediments. Environ. Sci. Technol. 1992, 26, 22812287; DOI 10.1021/es00035a029. (13) IPCC Climate Change 2001: A Scientific Basis, Intergovernmental Panel on Climate Change; Houghton, J. T., Ding, Y., Griggs, D. J., Noguer,
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