Unprocessed Atmospheric Nitrate in Waters of the Northern Forest Region in the U.S. and Canada.

Little is known about the regional extent and variability of nitrate from atmospheric deposition that is transported to streams without biological processing in forests. We measured water chemistry and isotopic tracers (δ18O and δ15N) of nitrate sources across the Northern Forest Region of the U.S. and Canada and reanalyzed data from other studies to determine when, where, and how unprocessed atmospheric nitrate was transported in catchments. These inputs were more widespread and numerous than commonly recognized, but with high spatial and temporal variability. Only 6 of 32 streams had high fractions (>20%) of unprocessed atmospheric nitrate during baseflow. Seventeen had high fractions during stormflow or snowmelt, which corresponded to large fractions in near-surface soil waters or groundwaters, but not deep groundwater. The remaining 10 streams occasionally had some (<20%) unprocessed atmospheric nitrate during stormflow or baseflow. Large, sporadic events may continue to be cryptic due to atmospheric deposition variation among storms and a near complete lack of monitoring for these events. A general lack of observance may bias perceptions of occurrence; sustained monitoring of chronic nitrogen pollution effects on forests with nitrate source apportionments may offer insights needed to advance the science as well as assess regulatory and management schemes.

Modeled ecohydrological responses to climate change at seven small watersheds in the northeastern United States.

A cross-site analysis was conducted on seven diverse, forested watersheds in the northeastern United States to evaluate hydrological responses (evapotranspiration, soil moisture, seasonal and annual streamflow, and water stress) to projections of future climate. We used output from four atmosphere-ocean general circulation models (AOGCMs; CCSM4, HadGEM2-CC, MIROC5, and MRI-CGCM3) included in Phase 5 of the Coupled Model Intercomparison Project, coupled with two Representative Concentration Pathways (RCP 8.5 and 4.5). The coarse resolution AOGCMs outputs were statistically downscaled using an asynchronous regional regression model to provide finer resolution future climate projections as inputs to the deterministic dynamic ecosystem model PnET-BGC. Simulation results indicated that projected warmer temperatures and longer growing seasons in the northeastern United States are anticipated to increase evapotranspiration across all sites, although invoking CO2 effects on vegetation (growth enhancement and increases in water use efficiency (WUE)) diminish this response. The model showed enhanced evapotranspiration resulted in drier growing season conditions across all sites and all scenarios in the future. Spruce-fir conifer forests have a lower optimum temperature for photosynthesis, making them more susceptible to temperature stress than more tolerant hardwood species, potentially giving hardwoods a competitive advantage in the future. However, some hardwood forests are projected to experience seasonal water stress, despite anticipated increases in precipitation, due to the higher temperatures, earlier loss of snow packs, longer growing seasons, and associated water deficits. Considering future CO2 effects on WUE in the model alleviated water stress across all sites. Modeled streamflow responses were highly variable, with some sites showing significant increases in annual water yield, while others showed decreases. This variability in streamflow responses poses a challenge to water resource management in the northeastern United States. Our analyses suggest that dominant vegetation type and soil type are important attributes in determining future hydrological responses to climate change.

Arctic Vortex changes alter the sources and isotopic values of precipitation in northeastern US.

Altered atmospheric circulation, reductions in Arctic sea ice, ocean warming, and changes in evaporation and transpiration are driving changes in the global hydrologic cycle. Precipitation isotopic (δ(18)O and δ(2)H) measurements can help provide a mechanistic understanding of hydrologic change at global and regional scales. To study the changing water cycle in the northeastern US, we examined the longest (1968-2010) record of precipitation isotope values, collected at the Hubbard Brook Experimental Forest in New Hampshire, US (43(°)56'N, 71(°)45'W). We found a significant reduction in δ(18)O and δ(2)H values over the 43-year record, coupled with a significant increase in d-excess values. This gradual reduction in δ(18)O and δ(2)H values unexpectedly occurred during a period of regional warming. We provide evidence that these changes are governed by the interactions among the Atlantic Multidecadal Oscillation, loss of Arctic sea ice, the fluctuating jet stream, and regular incursions of polar air into the northeastern US.

Calcium and aluminum cycling in a temperate broadleaved deciduous forest of the eastern USA: relative impacts of tree species, canopy state, and flux type.

Ca/Al molar ratios are commonly used to assess the extent of aluminum stress in forests. This is among the first studies to quantify Ca/Al molar ratios for stemflow. Ca/Al molar ratios in bulk precipitation, throughfall, stemflow, litter leachate, near-trunk soil solution, and soil water were quantified for a deciduous forest in northeastern MD, USA. Data were collected over a 3-year period. The Ca/Al molar ratios in this study were above the threshold for aluminum stress (<1). Fagus grandifolia Ehrh. (American beech) had a median annual stemflow Ca/Al molar ratio of 15.7, with the leafed and leafless values of 12.4 and 19.2, respectively. The corresponding Ca/Al molar ratios for Liriodendron tulipifera L. (yellow poplar) were 11.9 at the annual time scale and 11.9 and 13.6 for leafed and leafless periods, respectively. Bayesian statistical analysis showed no significant effect of canopy state (leafed, leafless) on Ca/Al molar ratios. DOC was consistently an important predictor of calcium, aluminum, and Ca/Al ratios. pH was occasionally an important predictor of calcium and aluminum concentrations, but was not a good predictor of Ca/Al ratio in any of the best-fit models (of >500 examined). This study supplies new data on Ca/Al molar ratios for stemflow from two common deciduous tree species. Future work should examine Ca/Al molar ratios in stemflow of other species and examine both inorganic and organic aluminum species to better gauge the potential for, and understand the dynamics of, aluminum toxicity in the proximal area around tree boles.

Consequence of altered nitrogen cycles in the coupled human and ecological system under changing climate: The need for long-term and site-based research.

Anthropogenically derived nitrogen (N) has a central role in global environmental changes, including climate change, biodiversity loss, air pollution, greenhouse gas emission, water pollution, as well as food production and human health. Current understanding of the biogeochemical processes that govern the N cycle in coupled human-ecological systems around the globe is drawn largely from the long-term ecological monitoring and experimental studies. Here, we review spatial and temporal patterns and trends in reactive N emissions, and the interactions between N and other important elements that dictate their delivery from terrestrial to aquatic ecosystems, and the impacts of N on biodiversity and human society. Integrated international and long-term collaborative studies covering research gaps will reduce uncertainties and promote further understanding of the nitrogen cycle in various ecosystems.

Isotopic evidence for determining the sources of dissolved organic sulfur in a forested catchment.

Understanding sulfur (S) biogeochemistry, especially in those watersheds subject to elevated levels of atmospheric S inputs, is needed for determining the factors that contribute to acidification, nutrient losses and the mobilization of toxic solutes (e.g., monomeric aluminum and methylmercury). S is found in a variety of both organic and inorganic forms undergoing a range of biotic and abiotic transformations. In watersheds with decreasing atmospheric S inputs, internal cycling is becoming dominant in affecting whether there is net loss or retention of S. Little attention has been given to the role of dissolved organic S (DOS) in affecting S biogeochemistry. DOS originates from assimilatory and bacterial dissimilatory S reduction (BDSR), the latter of which produces (34)S-depleted S. Within groundwater of the Archer Creek Catchment in the Adirondack Mountains (New York) there was reoxidation of reduced S, which was an important source of SO4(2-). DOS in surface waters had a higher variation of δ(34)S-DOS values (-6.0 to +8.4‰) than inorganic S with δ(34)S-SO4(2-) values ranging from +1.0 to +5.8‰. Inverse correlations between δ(34)S values of SO4(2-) and DOS suggested that BDSR played an important role in producing DOS.

Winter climate change affects growing-season soil microbial biomass and activity in northern hardwood forests.

Understanding the responses of terrestrial ecosystems to global change remains a major challenge of ecological research. We exploited a natural elevation gradient in a northern hardwood forest to determine how reductions in snow accumulation, expected with climate change, directly affect dynamics of soil winter frost, and indirectly soil microbial biomass and activity during the growing season. Soils from lower elevation plots, which accumulated less snow and experienced more soil temperature variability during the winter (and likely more freeze/thaw events), had less extractable inorganic nitrogen (N), lower rates of microbial N production via potential net N mineralization and nitrification, and higher potential microbial respiration during the growing season. Potential nitrate production rates during the growing season were particularly sensitive to changes in winter snow pack accumulation and winter soil temperature variability, especially in spring. Effects of elevation and winter conditions on N transformation rates differed from those on potential microbial respiration, suggesting that N-related processes might respond differently to winter climate change in northern hardwood forests than C-related processes.

The effects of phenoseason and storm characteristics on throughfall solute washoff and leaching dynamics from a temperate deciduous forest canopy.

Seasonal variations in the washoff and leaching dynamics of throughfall ionic fluxes represent a significant process affecting the biogeochemical cycling of forested ecosystems-particularly for temperate deciduous forests with distinct phenological seasons (or "phenoseasons"). Most studies on temperate deciduous forests aggregate seasonal throughfall fluxes to the leafed (growing) and leafless (dormant) periods, yet the phenological conditions controlling seasonality demand finer-scale demarcations that include the transitional phenoseasons (leaf senescence and emergence). To fill these gaps our study examines the washoff and leaching dynamics of Na(+), Mg(2+), K(+), Ca(2+), Cl(-), SO(4)(2-), and NO(3)(-) throughfall derived from bulk and sequentially sampled rain events across leafed, leafless and both transitional phenoseasons over a 3-year period (2008-2010). As throughfall washoff and leached solute fluxes are also closely-coupled to rainfall conditions, we further examine the effects of storm characteristics on phenoseasonal washoff-dominated (Na(+) and Cl(-)) and leaching-dominated (K(+), Ca(2+), Mg(2+)) fluxes through intrastorm event comparison plots and factorial MANOVA. Highly significant differences in leached and washoff solute fluxes were found across meteorological conditions (p<0.001) nested within phenoseasonal divisions (p<0.00001). Phenoseasonal washoff Na(+) and Cl(-) fluxes seemed to be more closely related to leaf area; whereas, leaching flux and canopy exchange of all solutes to correspond more with major phenological changes (when the canopies tend to be most metabolically active). The greatest differences in leached Mg(2+), K(+), Ca(2+), and SO(4)(2-) fluxes were not between the full leafed and leafless phenoseasons (33-80% difference), but between the transitional periods (80 to 200 fold greater during leaf senescence than leaf emergence). Intrastorm average canopy NO(3)(-) leaching, however, ranged from low losses (1 µmol(c)m(-2)h(-1)) to canopy uptake (-2 µmol(c)m(-2)h(-1)) during both transitional phenoseasons. K(+), Ca(2+), Mg(2+) were all markedly more exchangeable during senescence, with Ca(2+) and Mg(2+) being more tightly held by the canopy. Leaching rates and fluxes for all measured solutes were negligible to negative during emergence, except for K(+) and SO(4)(2-). Our results indicate that much of the variance in timing and magnitude of throughfall solute fluxes to forest soils within temperate deciduous ecosystems may be ascribed to phenologically-delineated seasons and storm conditions.

Comparison of sample preparation methods for stable isotope analysis of dissolved sulphate in forested watersheds.

Pretreatment methods for measuring stable sulphur (δ(34)S) and oxygen (δ(18)O) isotope ratios of dissolved sulphate from watersheds have evolved throughout the last few decades. The current study evaluated if there are differences in the measured stable S and O isotope values of dissolved sulphate from forested watersheds when pretreated using three different methods: Method 1 (M1): adsorb sulphate on anion exchange resins and send directly to isotope facility; Method 2 (M2): adsorb sulphate on anion exchange resins, extract sulphate from anion exchange resins, and send the produced BaSO(4) to the isotope facility; and Method 3 (M3): directly precipitate BaSO(4) without anion exchange resins with the precipitates being sent to the isotope facility. We found an excellent agreement of the δ(34)S(sulphate) values among all the three methods. However, some differences were observed in the δ(18)O(sulphate) values (M1 versus M2:-1.5 ‰; M1 versus M3:-1.2 ‰) associated with possible O contamination before isotope measurement. Several approaches are recommended to improve the pretreatment procedures for δ(18)O(sulphate) analysis.

Soil organic carbon of degraded wetlands treated with freshwater in the Yellow River Delta, China.

Supplying freshwater is one of the important methods to help restore degraded wetlands. Changes in soil properties and plant community biomass were evaluated by comparing sites with freshwater treatment versus reference sites following freshwater addition to wetlands of the Yellow River Delta for 7 years. The results indicated that soil organic carbon (SOC) was significantly increased in all wetland sites that were treated with freshwater compared to the reference sites. The treatment wetlands had greater total nitrogen (TN), lower pH and electrical conductivity and higher water content in the soil compared to the reference wetlands. In general, the upper soil layer (0-20 cm) had greater SOC than the lower soil layer (20-40 cm). The increase of SOC in the freshwater reintroduction wetlands was higher in the Suaeda salsa plant community (mean ± standard error) (6.89 ± 0.63 g/kg) and Phragmites communis plant community (4.11 ± 0.12 g/kg) than in the Tamarix chinensis plant community (1.40 ± 0.31 g/kg) in the upper soil layer. The differences were especially marked between the treated and reference wetlands for SOC and TN in the P. communis plant communities. The C:N ratio of the soil was significantly greater in the treated compared to the reference wetlands for the S. salsa plant community. Although the C: N ratios increased after treatment, they were all <25 suggesting that N availability was not limiting soil organic matter decomposition. Our results indicate that freshwater addition and the concomitant increase in soil moisture content enhances the accumulation of SOC in the Yellow River Delta.

Watershed sulfur biogeochemistry: shift from atmospheric deposition dominance to climatic regulation.

North American atmospheric S emissions peaked in the early 1970s followed by a dramatic decrease that resulted in marked declines in sulfate (SO4²â»)) concentrations in precipitation and many surface waters. These changes in S biogeochemistry have important implications with respect to the mobilization of toxic (Al(n⁺), H⁺) and nutrient (Ca²âº, Mg²âº, K⁺) cations and the acidification of watersheds. We used the continuous long-term record for watersheds 1, 3, 5, and 6 (37-44 years from 1965 through 2008) of SO4²â» 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 (SO4²â» 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 SO4²â» export in streamwater of forested watersheds has shifted from atmospheric S deposition to climatic factors by affecting soil moisture.

Potential effects of climate change and variability on watershed biogeochemical processes and water quality in Northeast Asia.

An overview is provided of the potential effects of climate change on the watershed biogeochemical processes and surface water quality in mountainous watersheds of Northeast (NE) Asia that provide drinking water supplies for large populations. We address major 'local' issues with the case studies conducted at three watersheds along a latitudinal gradient going from northern Japan through the central Korean Peninsula and ending in southern China. Winter snow regimes and ground snowpack dynamics play a crucial role in many ecological and biogeochemical processes in the mountainous watersheds across northern Japan. A warmer winter with less snowfall, as has been projected for northern Japan, will alter the accumulation and melting of snowpacks and affect hydro-biogeochemical processes linking soil processes to surface water quality. Soils on steep hillslopes and rich in base cations have been shown to have distinct patterns in buffering acidic inputs during snowmelt. Alteration of soil microbial processes in response to more frequent freeze-thaw cycles under thinner snowpacks may increase nutrient leaching to stream waters. The amount and intensity of summer monsoon rainfalls have been increasing in Korea over recent decades. More frequent extreme rainfall events have resulted in large watershed export of sediments and nutrients from agricultural lands on steep hillslopes converted from forests. Surface water siltation caused by terrestrial export of sediments from these steep hillslopes is emerging as a new challenge for water quality management due to detrimental effects on water quality. Climatic predictions in upcoming decades for southern China include lower precipitation with large year-to-year variations. The results from a four-year intensive study at a forested watershed in Chongquing province showed that acidity and the concentrations of sulfate and nitrate in soil and surface waters were generally lower in the years with lower precipitation, suggesting year-to-year variations in precipitation as a key factor in modulating the effects of acid deposition on soil and surface water quality of this region. Results from these case studies suggest that spatially variable patterns of snow or summer precipitation associated with regional climate change across NE Asia will have significant impacts on watershed biogeochemical processes and surface water quality, in interactions with local topography, land use change, or acid deposition.

Influences of a calcium gradient on soil inorganic nitrogen in the Adirondack Mountains, New York.

Studies of the long-term impacts of acidic deposition in Europe and North America have prompted growing interest in understanding the dynamics linking the nitrogen (N) and calcium (Ca) cycles in forested watersheds. While it has been shown that increasing concentrations of nitrate (NO3-) through atmospheric deposition or through nitrification can increase Ca loss, the reciprocal effects of Ca on N transformation processes have received less attention. We studied the influence of soil Ca availability on extractable inorganic N (NO3- + NH4+) across a Ca gradient in the Adirondack Mountains, New York, USA. Our results did not show the direct Ca-N interaction that we had expected, but instead showed that exchangeable Ca coupled with soil moisture, soil organic matter, and ambient temperature accounted for 61% of the variability in extractable inorganic N across 11 sites over two growing seasons. Soil Ca concentrations were, however, positively related to sugar maple (Acer saccharum) and American basswood (Tilia americana) basal areas and negatively related to American beech (Fagus grandifolia) basal area. Based on litter chemistry differences among these tree species and reported potential N mineralization values, we suggest that the influence of Ca on soil inorganic N is through a multistep pathway: reciprocal interactions between soil Ca concentrations and species composition, which in turn affect the quality of litter available for N mineralization. If chronic soil Ca depletion continues, as reported in some forested ecosystems, potential shifts in biotic communities could result in considerable alterations of N cycling processes.

Seasonal and event variations in delta34S values of stream sulfate in a Vermont forested catchment: implications for sulfur sources and cycling.

Stable sulfur (S) isotope ratios can be used to identify the sources of sulfate contributing to streamwater. We collected weekly and high-flow stream samples for S isotopic analysis of sulfate through the entire water year 2003 plus the snowmelt period of 2004. The study area was the 41-ha forested W-9 catchment at Sleepers River Research Watershed, Vermont, a site known to produce sulfate from weathering of sulfide minerals in the bedrock. The delta(34)S values of streamwater sulfate followed an annual sinusoidal pattern ranging from about 6.5 per thousand in early spring to about 10 per thousand in early fall. During high-flow events, delta(34)S values typically decreased by 1 to 3 per thousand from the prevailing seasonal value. The isotopic evidence suggests that stream sulfate concentrations are controlled by: (1) an overall dominance of bedrock-derived sulfate (delta(34)S approximately 6-14 per thousand); (2) contributions of pedogenic sulfate (delta(34)S approximately 5-6 per thousand) during snowmelt and storms with progressively diminishing contributions during base flow recession; and (3) minor effects of dissimilatory bacterial sulfate reduction and subsequent reoxidation of sulfides. Bedrock should not be overlooked as a source of S in catchment sulfate budgets.

Sulfate exports from multiple catchments in a glaciated forested watershed in western New York, USA.

Sulfate (SO4(2-)) concentrations and fluxes were studied for multiple storm events in the Point Peter Brook watershed, a glaciated, forested watershed located in Western New York, USA. Investigations were performed across one large (696 ha) and three small (1.6-3.4 ha) catchments with varying extent of riparian and wetland areas. Concentrations of SO4(2-) in groundwater sources (mean values: 238-910 micromol(c) L(-1)) were considerably greater than concentrations recorded for rainfall (60 micromol(c) L(-1)) and throughfall (72-129 micromol(c) L(-1)). Seasonality in SO4(2-) concentrations was most pronounced for valley-bottom riparian waters with maximum concentrations in late winter-spring (February-March) and a minimum in late summer (August). Concentrations of SO4(2-) in wetland water were considerably less than riparian water indicating the likelihood of SO4(2-) reduction in anoxic wetland conditions. Storm events displayed a dilution pattern in SO4(2-) concentrations with a minimum coinciding with the maximum in throughfall contributions. End member mixing analysis (EMMA) was able to predict the storm event concentrations of SO4(2-) for four of the six comparisons. Concentrations of SO4(2-) at the outlet of the large (696 ha) catchment were much greater than values recorded for the smaller catchments. Exports of SO4(2-) in streamflow exceeded the inputs from atmospheric deposition suggesting that watersheds like Point Peter Brook may not show any immediate response to decreases in atmospheric SO4(2-) deposition.

Processes affecting oxygen isotope ratios of atmospheric and ecosystem sulfate in two contrasting forest catchments in Central Europe.

Sulfate aerosols are harmful as respirable particles. They also play a role as cloud condensation nuclei and have radiative effects on global climate. A combination of delta18O-SO4 data with catchment sulfur mass balances was used to constrain processes affecting S cycling in the atmosphere and spruce forests of the Czech Republic. Extremely high S fluxes via spruce throughfall and runoff were measured at Jezeri (49 and 80 kg S ha(-1) yr(-1), respectively). The second catchment, Na Lizu, was 10 times less polluted. In both catchments, delta18O-SO4 decreased in the following order: open-area precipitation > throughfall > runoff. The delta18O-SO4 values of throughfall exhibited a seasonal pattern at both sites, with maxima in summer and minima in winter. This seasonal pattern paralleled delta18O-H2O values, which were offset by -18 per thousand. Sulfate in throughfall was predominantly formed by heterogeneous (aqueous) oxidation of SO2. Wet-deposited sulfate in an open area did not show systematic delta18O-SO4 trends, suggesting formation by homogeneous (gaseous) oxidation and/or transport from large distances. The percentage of incoming S that is organically cycled in soil was similar under the high and the low pollution. High-temperature 18O-rich sulfate was not detected, which contrasts with North American industrial sites.

Winter-time climatic control on dissolved organic carbon export and surface water chemistry in an Adirondack forested watershed.

Although most of forested watersheds in temperate and boreal regions are snow-covered for a substantial portion of the year, responses of biogeochemical processes under the snow pack to climatic fluctuations are poorly understood. We investigated responses of dissolved organic carbon (DOC) and surface water chemistry in stream and lake discharge waters draining the Arbutus Lake Watershed in the Adirondacks of New York State to climatic fluctuations during the snow-covered months from December through April. Interannual variability in stream discharge corresponded to changes in air temperature and snow pack depth across the winter months. Concentrations of DOC in stream water draining a subcatchment showed immediate positive responses to rising temperatures and subsequent increases in runoff during most snowmelt events. Increases in DOC concentrations usually coincided with decreases in pH and increases in total aluminum (Al) concentrations, while the correlations between concentrations of DOC and SO4(2-) or base cations were negative. Although changes in air temperature, snow pack depth, and runoff were all significantly correlated with stream water concentrations of major solutes, stepwise linear regression found that runoff was the best predictor of solute concentrations. Results of stepwise linear regression with long-term monthly monitoring data collected at the lake outlet showed weaker but still consistent climatic effects on interannual variations in concentrations of DOC and other solutes. Over the 17 winter periods from December 1983 through April 2000, changes in seasonal average concentrations of DOC, H+, and Al in lake discharge generally corresponded to interannual variations in temperature, precipitation, and runoff, while SO4(2-) and base cations displayed an opposite trend. The results suggest that snowmelt-mediated DOC responses to temperature fluctuations during the winter months might offset increases in the surface water pH caused by decreasing acidic deposition and pose a potential hazard of Al toxicity in surface waters.

Factors affecting acid neutralizing capacity in the Adirondack region of New York: a solute mass balance approach.

High rates of acidic deposition in the Adirondack region of New York have accelerated acidification of soils and surface waters. Annual input-output budgets for major solutes and acid-neutralizing capacity (ANC) were estimated for 43 drainage lake-watersheds in the Adirondacks from 1998 to 2000. Sulfate was the predominant anion on an equivalent basis in both precipitation and drainage export. Calcium ion had the largest cation drainage export, followed by Mg2+. While these watersheds showed net nitrogen (N) retention, the drainage losses of SO4(2-), Cl-, base cations, and ANC exceeded their respective inputs from precipitation. Land cover (forest type and wetlands) affected the export of SO4(2-), N solutes, and dissolved organic carbon (DOC). The relationships of solute export with elevation (negative for base cations and Cl-, positive for NO3- and H+) suggest the importance of the concomitant changes of biotic and abiotic watershed characteristics associated with elevational gradients. The surface water ANC increased with the sum of base cations and was greatest in the lakes with watersheds characterized by thick deposits of glacial till. The surface water ANC was also higher in the lake-watersheds with lower DOC export. Some variation in lake ANC was associated with variability in acidic deposition. Using a classification system previously developed for Adirondack lakes on the basis primarily of surficial geology, lake-watersheds were grouped into five classes. The calculated ANC fluxes based on the major sinks and sources of ANC were comparable with measured ANC for the thick-till (I) and the medium-till lake-watersheds with low DOC (II). The calculated ANC was overestimated for the medium-till with high DOC (III) and the thin-till with high DOC (V) lake-watersheds, suggesting the importance of naturally occurring organic acids as an ANC sink, which was not included in the calculations. The lower calculated estimates than the measured ANC for the thin-till lake-watersheds with low DOC (IV) were probably due to the mobilization of Al as an ANC source in these watersheds that were highly sensitive to strong acid inputs. Our analysis of various drainage lakes across the Adirondacks on the basis of solute mass balances, coupled with the use of a lake classification system and GIS data, demonstrates that the lake-watersheds characterized by shallow deposits of glacial till are highly sensitive to acidic deposition not only in the southwestern Adirondack region where previous field-based studies were intensively conducted but also across the entire Adirondack region. Moreover, the supply of organic acids and Al mobilization substantially modify the acid-base status of surface waters.

Chemical response of lakes in the Adirondack Region of New York to declines in acidic deposition.

Long-term changes in the chemistry of wet deposition and lake water were investigated in the Adirondack Region of New York. Marked decreases in concentrations of SO4(2-) and H+ in wet deposition have occurred at two sites since the late 1970s. These decreases are consistent with long-term declines in emissions of sulfur dioxide (SO2) in the eastern United States. Changes in wet NO3- deposition and nitrogen oxides (NOx) emissions have been minor over the same interval. Virtually all Adirondack Lakes have shown marked decreases in concentrations of SO4(2-), which coincide with decreases in atmospheric S deposition. Concentrations of NO3- have also decreased in several Adirondack lakes. As atmospheric N deposition has not changed over this period, the mechanism contributing to this apparent increase in lake/watershed N retention is not evident. Decreases in concentrations of SO4(2-) + NO3- have resulted in increases in acid-neutralizing capacity (ANC) and pH and resulted in a shift in the speciation of monomeric Al from toxic inorganic species toward less toxic organic forms in some lakes. Nevertheless, many lakes continue to exhibit pH values and concentrations of inorganic monomeric Al that are critical to aquatic biota. Extrapolation of rates of ANC increase suggests that the time frame of chemical recovery of Adirondack Lakes will be several decades if current decreases in acidic deposition are maintained.

Role of soil freezing events in interannual patterns of stream chemistry at the Hubbard Brook Experimental Forest, New Hampshire.

Soil freezing is a disturbance of the below ground environment, potentially resulting in increased losses of NO3- and surface water acidification. Here, we report the effects of soil freezing on interannual variation in stream chemistry at the Hubbard Brook Experimental Forest, New Hampshire. Data from 1970 to 1997 of soil frost depth, snow cover, precipitation, air temperature, and stream discharge and chemistry were used in a stepwise linear regression model to select the variables that best predicted deviations of annual stream concentrations from 4-year running averages. Variables quantifying soil freezing severity were selected as significant predictors of short-term fluctuations in stream K+, NO3-, Ca2+, and Mg2+ concentrations from 1970 to 1989, explaining 59 and 47% of the short-term variability in K+ and NO3-, respectively. Fine-root mortality and disturbance of root-soil-microbe interactions, with subsequent effects on decomposition and nutrient uptake, likely contributed to the mobilization of K+ and NO3- to streamwater following severe soil freezing events. The relationship between soil freezing and stream chemistry, however, weakened during the period 1990-1997. Because soil freezing has had inconsistent effects on stream chemistry during the period 1970-1997, it is unclear whether future changes in the frequency, duration, and depth of soil freezing events as the result of changes in the snow cover regime under a warmer climate will have significant impacts on the losses of NO3- and nutrient-base cations from temperate northern ecosystems.