Changes in the concentrations and speciation of aluminum in response to an experimental addition of ammonium sulfate to the bear Brook Watershed,Maine, USA

An understanding of the biogeochemistry of aluminum (Al) in acid-sensitive terrestrial and aquatic ecosystems is critical to assessments of the effects of acidic deposition. Bear Brook Watershed, Maine, USA includes paired watersheds, East Bear and West Bear. Starting in November 1989, experimental additions of ammonium sulfate ((NH₄)₂SO₄; 900 mol/ha-yr) have been made to West Bear Brook Watershed. Chemical analysis of soil and stream waters were conducted to evaluate the speciation of Al prior to (1987–89) and following (1989–92) the experimental treatments. Before the treatments, soilwater Al occurred largely as inorganic monomeric Al (Ali). Concentrations of organic monomeric Al (Alo), Ali and dissolved organic C (DOC) were high in soil solutions draining the E horizon, and decreased in the lower mineral soilwater (Bs horizon) and streamwater. Streamwater concentrations of monomeric Al (Alm) were largely in the form of Alo. After the (NH₄)₂SO₄ treatments were initiated in the West Bear Brook Watershed, concentrations of Alm increased in soilwater and streamwater, largely as Ali. These increases in Al accompanied decreases in pH and increases in concentrations of SO₄ ^2? and NO₃ ^? in drainage waters. Increases in stream concentrations of Al were particularly evident during high flow events. This pattern, coupled with the increases in concentrations of Ali in upper soilwaters in response to the (NH₄)₂SO₄ addition, suggests that episodic increases in Ali were due to inputs of water entering the stream from shallow hydrologic flowpaths.     

Temporal and Spatial Variations in Soil Water Chemistry at Three Acid Forest Sites

As acid deposition declines, recovery from acidification is delayed by the fact that the soil processes that earlier buffered against acidification are now being reversed. Monitoring of within catchment processes is thus desirable. However, soil sampling is destructive and not suitable for long-term monitoring at a single site, whereas sampling of soil water with suction lysimeters may be more suitable. In this paper we evaluate 8–11 years of soil water chemistry from E- and B-horizons in three acid forest soil plots within monitored catchments. Five years of sampling also included the C-horizon. To our knowledge, this is the first long-term lysimeter study including the E-horizon showing recovery from acidification, and one of few studies including the B-horizon. Soil water concentrations of SO₄ decreased significantly between –9.5 and –1.4 μeq L^-1 yr^-1, with much higher rates of change at two southern sites compared to a northern site, where levels and changes of deposition were lower. The average annual bulk deposition of S ranged between 3 kg S ha^-1 at the northernmost site to 11 kg S ha^-1 at the southernmost site. The SO₄ decline in E-horizons was smaller than the decline in deposition, which indicated leaching of SO₄ from the O-horizon. At the two southern sites, a weaker decline in SO₄ in the B-horizon compared to the E-horizon indicated desorption of SO₄. The negative trends in SO₄ were to a large extent balanced by decreases in base cations but there were also tendencies of recovery from acidification in soil solution at the southern sites by increasing pH and ANC. However, these were contradicted by increasing Al concentrations. A high influence of marine salts in the early 1990s may have delayed the recovery. Decreasing trends of the Ca/(H⁺)² ratio in the soil solution, most pronounced at one of the southern sites, suggested that the soils were becoming more acidic, although the soil solution tended to recover.     

Estimation of Dry Deposition by Using a Filter Pack Method at Chunchon, Korea

Atmospheric gases and particulates were collected using four-stage filter-pack in Chunchon from January through December in 1999. Particulate SO₄ ^2? and NO₃ ^?, and gaseous HNO₃, SO₂ and NH₃ were analyzed. Annual average concentration of SO₄ ^2?(S), NO₃ ^?(S), HNO₃ (g), SO₂(g) and NH₃(g) were 5.75µg/m³, 4.98µg/m³, 0.33ppb, 1.52ppb and 7.25ppb, respectively. Annual dry deposition fluxes were estimated using the measured concentration and dry deposition velocity published by other research group. Annual dry deposition of S was 287kg · (km)^?2·y^?1, which accounted for about 30% of total S deposition. For N deposition, dry deposition is predominant; about 70% of total N deposition was through dry process mostly as forms of NH₃ and HNO₃.     

Recovery of acidified catchments in the extremely polluted Krusne hory Mountains,Czech Republic

Runoff and atmospheric chemistry in the Krusne hory Mts. have changed significantly from 1978 to 1994. Forest die-back related deforestation resulted in decreased dry deposition of SO₂ and changes in streamwater chemical composition. Atmospheric sulphur (S) deposition decreased from extremely high values of 66.6 kg S ha^?1 year^?1, in the early 1980s to 35.5 kg S ha^?1 year^?1 in 1994. Decreasing S input is reflected in decrease of streamwater sulphate (SO₄ ^2?) concentrations, which decreased from 1560 μeq l^?1 to 1164 μeq l^?1. Runoff export of S was 53 kg S ha^?1 year^?1 in 1993, S is not retained in the catchments. Nitrogen (N) budget indicates accumulation in the catchment, which is attributed to forest regrowth.     

Long-Term Annual and Seasonal Patterns of Acidic Deposition and Stream Water Quality in a Great Smoky Mountains High-Elevation Watershed

The recovery potential of stream acidification from years of acidic deposition is dependent on biogeochemical processes and varies among different acid-sensitive regions. Studies that investigate long-term trends and seasonal variability of stream chemistry in the context of atmospheric deposition and watershed setting provide crucial assessments on governing biogeochemical processes. In this study, water chemistries were investigated in Noland Divide watershed (NDW), a high-elevation watershed in the Great Smoky Mountains National Park (GRSM) of the southern Appalachian region. Monitoring data from 1991 to 2007 for deposition and stream water chemistries were statistically analyzed for long-term trends and seasonal patterns by using Seasonal Kendall Tau tests. Precipitation declined over this study period, where throughfall (TF) declined significantly by 5.76?cm?year^?1. Precipitation patterns play a key role in the fate and transport of acid pollutants. On a monthly volume-weighted basis, pH of TF and wet deposition, and stream water did not significantly change over time remaining around 4.3, 4.7, and 5.8, respectively. Per NDW area, TF SO₄ ^2- flux declined 356.16?eq?year^?1 and SO₄ ^2- concentrations did not change significantly over time. Stream SO₄ ^2- remained about 30???eq L^?1 exhibiting no long-term trends or seasonal patterns. SO₄ ^2- retention was generally greater during drier months. TF monthly volume-weighted NH₄ ⁺ and NO₃ ^- concentrations significantly increased by 0.80???eq L^?1?year^?1 and 1.24???eq L^?1?year^?1, respectively. TF NH₄ ⁺ fluxes increased by 95.76?eq?year^?1. Most of NH₄ ⁺ was retained in the watershed, and NO₃ ^- retention was much lower than NH₄ ⁺. Stream monthly volume-weighted NO₃ ^- concentrations and fluxes significantly declined by 0.56???eq L^?1?year^?1 and 139.56?eq?year^?1, respectively. Overall, in NDW, inorganic nitrogen was exported before 1999 and retained since then, presumably from forest regrowth after Frazer fir die-off in the 1970s from balsam wooly adelgid infestation. Stream export of NO₃ ^- was greater during winter than summer months. During the period from 1999 to 2007, stream base cations did not exhibit significant changes, apparently regulated by soil supply. Statistical models predicting stream pH, ANC, SO₄ ^2-, and NO₃ ^- concentrations were largely correlated with stream discharge and number of dry days between precipitation events and SO₄ ^2- deposition. Dependent on precipitation, governing biogeochemical processes in NDW appear to be SO₄ ^2- adsorption, nitrification, and NO₃ ^- forest uptake. This study provided essential information to aid the GRSM management for developing predictive models of the future water quality and potential impacts from climate change.     

Effects of acid sulphate on DOC release in mineral soils: the influence of SO42− retention and Al release

Dissolved organic carbon (DOC) in acid‐sensitive upland waters is dominated by allochthonous inputs from organic‐rich soils, yet inter‐site variability in soil DOC release to changes in acidity has received scant attention in spite of the reported differences between locations in surface water DOC trends over the last few decades. In a previous paper, we demonstrated that pH‐related retention of DOC in O horizon soils was influenced by acid‐base status, particularly the exchangeable Al content. In the present paper, we investigate the effect of sulphate additions (0–437 µeq l^?1) on DOC release in the mineral B horizon soils from the same locations. Dissolved organic carbon release decreased with declining pH in all soils, although the shape of the pH‐DOC relationships differed between locations, reflecting the multiple factors controlling DOC mobility. The release of DOC decreased by 32–91% in the treatment with the largest acid input (437 µeq l^?1), with the greatest decreases occurring in soils with very small % base saturation (BS, < 3%) and/or large capacity for sulphate (SO₄^2?) retention (up to 35% of added SO₄^2?). The greatest DOC release occurred in the soil with the largest initial base status (12% BS). These results support our earlier conclusions that differences in acid‐base status between soils alter the sensitivity of DOC release to similar sulphur deposition declines. However, superimposed on this is the capacity of mineral soils to sorb DOC and SO₄^2?, and more work is needed to determine the fate of sorbed DOC under conditions of increasing pH and decreasing SO₄^2?.     

Seasonal,annual and long-term variability in the water chemistry of a remote high mountain lake: Acid rain versus natural changes

Seasonal fluctuations as well as long-term trends in water chemistry were studied in Schwarzsee ob Sölden (Tyrol, Austria), an oligotrophic softwater lake situated at 2796 m a.s.l. The catchement is composed of granite, plagioclase and micaschists containing considerable amounts of sulphur, with little soil cover. The lake is ice covered for about nine months, during this time the deepest layers (>16m) become anoxic. During summer overturn, alkalinity (ALK) is lowest (?8 μeq l^?1) in the whole water column, whereas pH reaches its minimum (4.88) at the surface during snowmelt. A decrease of pH from 5.8 to 5.4 during winter is caused by CO₂ oversaturation, but deep water ALK increases to up to 130 μeq l^?1 due to in-lake ALK generation by reductive processes and base cation (BC) release. The seasonal pattern of ALK in SOS is driven by in-lake processes in winter, the snowmelting in spring and watershed processes and precipitation during summer. Since 1989 summer sulfate concentrations in SOS, originating mainly from the catchment, show a tendency to increase presumably caused by enhanced weathering. In contrast, SO₄ ^2? concentrations in other high mountain lakes which are dominated by atmospheric depositions show a decreasing trend. SOS is a good example for the complexity of interactions between catchment and in-lake processes which act at different time scales and depend on climate changes and atmospheric inputs.     

Sulphate Mobilization and Pore Water Chemistry in Relation to Groundwater Hydrology and Summer Drought in two Conifer Swamps on the Canadian Shield

Variations in sulphate (SO₄ ^2-) concentration of porewater and net SO₄ ^2- mobilization were related to differences in water level fluctuations during wet and dry summers in two conifer swamps located in catchments which differed in till depth and seasonality of groundwater flow. Sulphate depletion at the surface and in 20 cm porewater coincided with anoxia and occurred mainly during the summer when water levels were near the peat surface and water flow rates were low in both catchments. There was an inverse relationship between net SO₄ ^2- mobilization and water level elevation relative to the peat surface, explaining variation in SO₄ ^2- dynamics between the swamps during summer drought periods. Aeration of peat to 40 cm and a large net SO₄ ^2- mobilization (10–70 mg SO₄ ^2- m^-2 d^-1) occurred during a dry summer in which the water level dropped to 60 cm below the surface in the swamp receiving ephemeral groundwater inputs from shallow tills within the catchment. This resulted in high SO₄ ^2- concentrations in the surface water and porewater (30–50 mg L^-1), and elevated SO₄ ^2- concentrations remained through the fall and winter. In contrast, within the swamp located in the catchment with greater till depth (> 1 m), continuous groundwater inputs maintained surface saturation during the dry summer, and SO₄ ^2- mobilization and concentrations of SO₄ ^2- in the pore water during the following fall did not increase. Susceptibility to large water table drawdown and mobilization of accumulated SO₄ ^2- is influenced by the occurrence of ephemeral vs. continuous groundwater inputs to valley swamps during dry summer periods in the Canadian Shield landscape. This study reveals that extrapolation of results of SO₄ ^2- cycling from one wetland to another requires knowledge of the hydrogeology of the catchment in which the wetlands are located.     

Finnish lake survey: the role of organic and anthropogenic acidity

A lake survey consisting of 987 randomly selected lakes was conducted in Finland in autumn 1987. The survey covered the whole country, and the water quality of the lakes can be considered as representative of the approximately 56 000 lakes larger than 0.01 km² in Finland. The median TOC concentration is 12 mg L^-1 and the median pH 6.3. The proportion of lakes with TOC concentrations > 5 mg L^-1 in the whole country is 91 %. Organic anion is the main anion in the full data set (median 89 μeq L^-1). The high organic matter concentrations in Finnish lakes are associated with catchment areas containing large proportions of peatlands and acid organic soils under coniferous forest. The survey demonstrated that organic matter strongly affects the acidity of lakes in Finland. The decreasing effect of organic matter on the pH values was demonstrated by both regression analysis and ion balances. At current deposition levels of ^*SO₄ the pH of humic lakes in Finland is determined to a greater extent by high TOC concentrations than by ^*SO₄ in most areas. In lakes with pH values lower than 5.5 the average organic anion contribution is 56 % and non-marine sulfate contribution 39 %. However, in the southern parts of the country, where the acidic deposition is highest, the minerogenic acidity commonly exceeds the catchment derived organic acidity.     

Acid-base chemistry of high-elevation streams in the great smoky mountains

Longitudinal and temporal variations in water chemistry were measured in several low-order, high-elevation streams in the Great Smoky Mountains to evaluate the processes responsible for the acid-base chemistry. The streams ranged in average base flow ANC from ?30 to 28 μeq L^?1 and in pH from 4.54 to 6.40. Low-ANC streams had lower base cation concentrations and higher acid anion concentrations than did the high-ANC streams. NO₃ ^? and SO₄ ^2? were the dominant acid anions. NO₃ ^? was derived from a combination of high leaching of nitrogen from old-growth forests and from high rates of atmospheric deposition. Streamwater SO₄ ^2? was attributed to atmospheric deposition and an internal bedrock source of sulfur (pyrite). Although dissolved Al concentrations increased with decreasing pH in the study streams, the concentrations of inorganic monomeric Al did not follow the pattern expected from equilibrium with aluminum trihydroxide or aluminum silicate phases. During storm events, pH and ANC declined by as much as 0.5 units and 15 μeq L^?1, respectively, at the downstream sites. The causes of the episodic acidification were increases in SO₄ ^2? and DOC.     

Atmospheric Deposition of Acidifying Components to a Japanese Cedar Forest

One-year field measurements were conducted in a Japanese cedar (Cryptomeria japonica) forest, located in Gunma Prefecture, Japan. On the basis of the meteorological and atmospheric concentration data, the dry deposition of SO₂, HNO₃, NO₂ and HCl was estimated using the inferential method. The annual dry deposition of H⁺ was estimated at 721 eq ha^?1yr^?1, which was 40% larger than the measured annual wet deposition of H⁺ (514 eq ha^?1yr^?1). Therefore, dry deposition is an important pathway for the atmospheric input of H⁺ to the forest in the study site. The contribution of each gas to the dry deposition of H⁺ was as follows: SO₂, 25%; HNO₃, 32%; NO₂, 10%; and HCl, 33%. The extremely high contribution of HCl appeared to be caused by the high emission intensity of HCl due to waste incineration in the site region. The differences between estimated deposition and throughfall and stemflow measurements indicated that about 80% of the total deposition of H⁺ was taken up by the canopy.     

Nitrogen and Sulfate Export from High Elevation Catchments of the Sierra Nevada,California

Based on studies of high-elevation, Sierra Nevada catchments during the period from 1983 through 1996, we describe temporal variations in the concentrations of NO₃ ^- and SO₄ ^2- in surface waters. During snowmelt, some catchments had a pattern of NO₃ ^- increase to a plateau between the start of snowmelt and some weeks before runoff peaked, and a decline as runoff increased to its maximum. In other catchments, NO₃ ^- concentrations peaked during the autumn and winter. Long-term trends in surface water chemistry were evident in only two catchments: an increase in SO₄ ^2- concentrations in surface waters of the Ruby Lake basin, and a lowering of annual maxima and minima of NO₃ ^- concentrations at Emerald Lake. From October 1987 through April 1994, SO₄ ^2- concentrations increased from about 6 µeq L^-1 to about 12 µeq L^-1 in Ruby Lake, and in Emerald Lake, NO₃ ^- maxima declined by 25-50 %.     

Experimental acidification of alpine catchments at Sogndal,Norway: Results after 8 years

Manipulations with whole catchments were initiated in Norway in 1983 (RAIN project Reversing Acidification In Norway) to obtain direct experimental evidence relating to the reversibility of soil and water acidification, rate of change, and the relative roles of sulfur and nitrogen. We present here results for soil and runoff chemistry during 8 years of acid addition at Sogndal, a pristine acid-sensitive site in central Norway characterized by gneissic bedrock, thin and patchy soils, and alpine vegetation. Catchment SOG2 receives 100 meq m^?2 yr^?1 H₂SO₄, catchment SOG4 receives a 1∶1 mixture of H₂SO₄ and HNO₃, while catchments SOG1 and SOG3 serve as untreated controls. Acid is applied to the snowpack in April and in 5 portions of 11 mm of pH 3.2 acidified lakewater during the snowfree period. The 8-years of acid addition have caused major changes in runoff chemistry. Concentrations of sulfate and base cations have increased while acid neutralizing capacity (ANC) has decreased. Henriksen's F-factor (change in concentration of non-marine Ca+Mg divided by change in concentration of non-marine SO₄) is about 0.35, but is expected to decrease as soil acidification proceeds. Runoff is acidic, aluminum-rich, and toxic to fish and other aquatic organisms. Repeated soil sampling indicates no dramatic trends related to treatment. Year-to-year variations are large, and mask changes expected. The input-output budgets indicate that over the 8-yr period Ca has been depleted by about 5% of the total soil pool of exchangeable Ca. The observed trends are consistent with response predicted by MAGIC, a process-oriented model of soil and water acidification. The gradual increase in nitrate flux from catchment SOG4 may be the first indication of ‘nitrogen saturation’ induced simply by increasing nitrogen deposition.     

Dry Deposition of Sulfur Containing Species to the Water Surface Sampler at Two Sites

Sulfate dry deposition increases the deteriorating effects on environment. Sulfate can be deposited from atmosphere to water via both particulate (SO₄ ² :sulfate)and a gas(SO ₂:sulfurdioxide)form.In this research, the fluxes of gaseous(SO ₂)and particulate(SO ₄ ²)species were measured employing a water surface sampler(WSS)and glass fiber filters(GFFs)ontheknife?edge surrogate surface(KSSs)in the campus of Uludag University and the city of Bursa, Turkey.Sampling program was conducte dinter mittently between September2004and March2005.Average to talsulfate fluxes measured with the WS Satthe Uludag University campus and in the city of Bursa were58 ± 41and235 ± 43?mgm ^?2 d ^?1, respectively.The to talsulfate fluxe smeasure dat Bursa were highe rand this was probably due to greater sulfur containing species in it satmosphere.The dry deposition of gas eous SO ² flux was calculated by sub tracting the particulate flux collected with the KSS s from the total flux(particulate sulfate plus SO ₂ flux)obtained by the WSS.Anautomatic SO ₂ analyzer was used concurrently to measure the ambient concentration of gas eous SO ₂. The average SO₂ gas fluxes and ambient SO ₂ concentrations were18 ± 28and54 ± 48?mgm ^?2 day ^?1 and11 ± 7and49 ± 14?μgm ^?3 for the campus and the city, respectively.The measured gaseous SO ₂ fluxes and ambient concentrations were used to calculate the mass transfer coefficient.The calculated MTC values for the campus and the city were0.8 ± 1.0and1.2 ± 1.1?cms ^?1, respectively.The sevalues wereinag reement with previously reported dry deposition velocities for SO ₂.     

Specifics and Temporal Changes in Air Pollution in Areas Affected by Emissions from Oil Shale Industry, Estonia

Atmospheric air pollution levels and long-term effects on the environment caused by simultaneous presence of SO₂ and oil shale alkaline fly ash during the last five decades (since 1950) were investigated. The annual critical value of SO₂ for forest (20 µg m^?3) was surpassed in 1% (～35 km²) of the study area where the load was 30–40 µg m^?3. No effect of long-term SO₂ concentrations of up to 10–11 µg m^?3 (0.5-h max up to 270 µg m^?3) and simultaneous fly ash loads of up to 95 µg m^?3 (1000 µg m^?3) on the growth and needle longevity of Pinus sylvestris was established. The yearly deposition (average load up to 20–100 kg S ha^?1) was alkaline rather than acidic due to an elevated base cation deposition in 1960–1989. Since 1990, the proportion of SO₂ in the balance of components increased: about 70–85% of the total area was affected while the ratio of annual average concentrations of SO₂ to fly ash was over 1. The limit values of fly ash for Sphagnum mosses and conifers in the presence of SO₂ are recommended.     

Response to a smelter closure in Cascade mountain lakes

A statistically significant decrease in sulfate was observed in high elevation Cascade lakes during 1983 through 1988. The total decrease averaged 2.2 μeq L^?1 in two slow-flush lakes and 4.2 μeq L^?1 in three fast-flush lakes for 1983–1985 vs 1986–1988, respectively. Coincident with these changes in sulfate concentrations were a sharp decrease of SO₂ emissions from the ASARCO smelter (100 km SE of the lakes), from 87 to 70 kt yr^?1 during 1983–1984 to 12 in 1985, the year of its closure, and a gradual change in SO₂ emissions from Mt. St. Helens, from 39 to 27 during 1983–1984 to 5 in 1988. The sharpest decreases occurred in non-marine sulfate in fast-flush lakes from 1984 to 1985 (about 2 μeq L^?1) and in slow-flush lakes from 1985 to 1986 (1 μeq L^?1, which point to the ASARCO closure as the sole cause. However, some of the more gradual decline in non-marine sulfate observed during 1983 through the 1988 sampling periods may have been due to a slow washout of sulfate enriched ash from the 1980 Mt. St. Helens' eruption. Sulfate concentrations in precipitation also declined significantly by about 2 μeq L^?1, but changes in volume-weighted sulfate content were not significant. Lake alkalinity did not show a consistent increase in response to decreased sulfate. This was probably due to either watershed neutralization of acidic deposition or the greater variability in alkalinity measurements caused by small changes in acidic deposition making it difficult to detect changes.     

Influence of 2-chloro-6 (trichloromethyl) pyridine and dicyandiamide on nitrous oxide emission under different soil conditions

Abstract

Two experiments were conducted to evaluate the inhibitory effects of 2-chloro-6 (trichloromethyl) pyridine (nitrapyrin) and dicyandiamide on nitrous oxide (N₂O), a greenhouse gas, emission from soils amended with ammonium sulfate. In the two experiments, samples of an Andosol and a Gray Lowland soil were kept in glass vessels sealed with a butyl rubber cap and incubated at 25°C. In the first experiment, nitrapyrin (1 µg g^?1 dry soil) and dicyandiamide (10 µg g^?1 dry soil) were applied to samples of a water-saturated Andosol and a Gray Lowland soil to which ammonium sulfate had been applied at a rate of 0.1 mg N g^?1 dry soil. Nitrapyrin decreased N₂O emissions from the Andosol and the Gray Lowland soil by 71% and 24%, respectively. Dicyandiamide decreased N₂O emissions from the Andosol and Gray Lowland soil by 31% and 18%, respectively. In the second experiment, nitrapyrin (1 µg g^?1 dry soil) was applied to samples of an Andosol at 51% water-filled pore space to which ammonium sulfate had been applied at rates of 0.01, 0.1 and 0.5 mg N g^?1 dry soil. Nitrapyrin decreased N₂O emissions by 62%, 83% and 74%, respectively. Changes in the NH⁺ ₄ and NO^? ₂ + NO^? ₃ concentrations in soil showed that nitrapyrin and dicyandiamide slowed down the nitrification process, but did not completely stop the process at any time. The results reveal the potential of nitrification inhibitors to decrease N₂O emission from fertilized soil in a wide range of moisture conditions and nitrogen levels.     

Ion leaching from a sugar maple forest in response to acidic deposition and nitrification

Year-to-year variation in acidic deposition within a mature sugar maple-dominated forest and in leaching of ions from the associated podzolic soil were examined at the Turkey Lakes Watershed between 1981 and 1986. Below-canopy inputs to the soil of SO₄ ^2? and NO₃ ^? in throughfall averaged 640 and 295 eq. ha^?1 yr^?1; the corresponding ranges were 493–917 and 261–443 eq. ha^?1 yr^?1. The contribution of atmospheric deposition to SO₄ ^2? NO₃ ^? and Ca²⁺ leaching decreased over the six years. During the study period, the mean annual volume-weighted NO₃ ^? concentration decreased in throughfall and forest-floor percolate and increased in the mineral-soil solution collected below the effective rooting zone. A substantial shift in the balance between SO₄ ^2? and NO₃ ^?leaching from the mineral soil was observed; leaching of SO₄ ^2?decreased and NO₃ ^? leaching increased with time. Leaching of Ca²⁺ and Mg²⁺ from the soil was increased as a result of excess NO₃ ^? production in the soil. The calculated output of NO₃ ^? from the soil, which averaged 1505 eq. ha^?1 yr^?1, considerably exceeded the atmospheric deposition of NO₃ ^?, whereas SO₄ ^2? outputs were only moderately greater than inputs.     

Recovery of Surface Waters in the Northeastern U.S. from Decreases in Atmospheric Deposition of Sulfur

A simple mass flux model was developed to simulate the response of SO₄ ^2- concentrations in surface waters to past and anticipated future changes in atmospheric deposition of SO₄ ^2-. Values of bulk (or wet) SO₄ ^2- deposition and dry deposition of S determined from measured air concentrations and a deposition velocity were insufficient to balance watershed SO₄ ^2- export at the Hubbard Brook Experimental Forest, NH and for a regional survey of watersheds in the northeastern U.S. We propose two explanations for the unmeasured S source: 1) a significant underestimation of dry S deposition, and/or 2) internal watershed S sources, such as weathering and/or mineralization of soil organic S. Model simulations based on these two mechanisms agreed closely with measured stream SO₄ ^2- concentrations at Hubbard Brook. Close agreement between measured and model predicted results precluded identification of which of the two mechanisms controlled long-term trends in stream SO₄ ^2-. Model simulations indicated that soil adsorption reactions significantly delayed the response of stream water to declines in SO₄ ^2- inputs since 1970, but could not explain the discrepancy in watershed S budgets. Extrapolation of model predictions into the future demonstrates that uncertainty in the source of the S imbalance in watersheds has important implications for assessments of the recovery of surface water acid neutralizing capacity in response to anticipated future reductions in SO2 emissions.     

Trends in Sulfate,Base Cations and H+ Concentrations in Bulk Precipitation and Throughfall at Integrated Monitoring Sites in Finland 1989-1995

Temporal trends in sulfate, base cation (Ca2+ + Mg2+ + K+), and H+ ion concentrations in bulk precipitation and throughfall samples collected over a seven year period (1989-95) in four forested catchments in Finland are presented. The catchments are in remote locations and span the boreal zone (61-69 °N). The stands represent old, undisturbed forests, and are composed of varying proportions of Scots pine, Norway spruce and deciduous species (mainly Betula spp.). Monthly SO₄ ^2- and H+ ion concentrations in bulk precipitation averaged over the study period and catchments were: 18.7 µmol L^-1 and 32.3 µmol L^-1. The corresponding values for throughfall were: 37.4 µmol L^-1 and 32.4 µmol L^-1. Sulfate and H+ ion concentrations in bulk precipitation and throughfall both showed negative linear trends, which were significant (p < 0.05) for the three southernmost catchments. Concentrations and trend slope decreased northwards (e.g., bulk precipitation SO₄ ^2- slope estimates: ^-1.6 to ^-1.0 µmol L^-1 yr^-1). The decline was greater for throughfall than for bulk precipitation, indicating a proportionally greater reduction in dry deposition than wet. The sum of base cation concentrations averaged 12.1 µmol(+) L^-1 in bulk precipitation and 83.1 µmol(+) L^-1 in throughfall. There were no significant trends in the sum of base cations (p > 0.05). It is concluded that the reported reduction in S emissions over the study period has resulted in a significant reduction in the acidity and SO₄ ^2- concentration of bulk precipitation, and this reduction has has been reflected in throughfall concentrations. The greatest reduction has taken place in the southern part of the country.