Deposition of air pollutants in a wind-exposed forest edge

The atmospheric deposition of air pollutants at a forest edge was studied by means of monitoring canopy throughfall at the edge and at five different parallel lines in the forest behind the edge. The investigation was carried out at a pine forest on the Swedish west coast. Throughfall and bulk deposition samples were analyzed for volume, SO ₄ ^2? , NO ₃ ^? , Cl^?, NH ₄ ⁺ , Na⁺, K⁺, Mg²⁺, Ca²⁺, and for pH. The results show that the throughfall flow at the edge was increased substantially for most ions. The ratios in throughfall flows between the edge and the line 50 m into the forest were for SO ₄ ^2? , 1.5, NO ₃ ^? 2.9, NH ₄ ⁺ 2.7, and Na⁺ 3.1. Since this effect is not only valid for forest edges but also for hillsides, hilltops, and edges between stands of different age, etc., there might be substantial areas which get much larger total deposition than the normally considered closed forest.     

Forest canopy transformation of atmospheric deposition

Solute fluxes to the ground in open plots and under the forest canopy of different species were investigated in a number of long-term ecosystem studies in West Germany. From the canopy flux balance, rates of interception deposition and canopy/deposition interactions were assessed. Chemically, both open precipitation and throughfall are dilute solutions of H₂SO₄ and HNO₃ and their salts. For the sites investigated, mean pH in bulk precipitation ranged from 4.1 to 4.6, and in throughfall from 3.4 to 4.7. The increase in acidity after canopy passage at most sites indicates considerable interception deposition of strong acids to the forest stands, exceeding the rate of H+ buffering in the canopy. Evidence for buffering processes can be directly deduced from the fact that on sites with high soil alkalinity and high foliage base status, throughfall pH is usually higher than precipitation pH. Furthermore, the same idea can be concluded from changes in solution composition after canopy passage: the H⁺/SO _inf4 ^sup2? ratio is decreasing at most sites, while alkali earth cations from exchange processes occur in throughfall (Ca²⁺/SO _inf4 ^sup2? ratio increases). Solution composition and element flux data are presented for each of the sites, and the regional, orographical and site specific (species composition, ecosystem state) differentiations are discussed. A method for the assessment of total deposition and of canopy interactions such as H⁺-buffering and cation leaching is described, and results of calculations are shown. From these calculations it is concluded that forest ecosystems in Germany receive mean H⁺ loads of ca. 1 to 4 keq H⁺ · ha^?1 · a^?1 from atmospheric deposition. Acidity deposition rates seem to be related to a few key factors such as regional characteristics and ecosystem characteristics.     

Canopy leaching of cations in Central European forest ecosystems – a regional assessment

The leaching of Ca, Mg, and K from canopies is a major pathway of these cations into forest soils. Our aim was to quantify rates of canopy leaching and to identify driving factors at the regional scale using annual fluxes of bulk precipitation and throughfall from 37 coniferous and deciduous forests of North and Central Europe. Total deposition of Ca, Mg, K, and H⁺ was estimated with Na as an index cation. The median canopy leaching increased in the order: Mg (0.11 kmol_c ha^–1 a^–1) < Ca (0.31 kmol_c ha^–1 a^–1) < K (0.39 kmol_c ha^–1 a^–1). Canopy leaching of Ca and K was positively correlated with the calculated total H⁺ deposition and H⁺ buffered in the canopy, whereas canopy leaching of Mg was not. With contrasting effects, fluxes of SO₄‐S and NH₄‐N in throughfall explained to 64 % (P<0.001) of the Ca canopy leaching. Fluxes of NH₄‐N and Ca were negatively correlated, suggesting that buffering of H⁺ by NH₃ deposition reduced canopy leaching of Ca. Amount of bulk precipitation and SO₄‐S in throughfall were identified as much weaker driving factors for canopy leaching of K (r²=0.28, P<0.01). Our results show that Ca is the dominant cation in buffering the H⁺ input in the canopy. At the regional and annual scale, canopy leaching of Mg appears to be unaffected by H⁺ deposition and H⁺ buffering in the canopy.     

Long-term trends in the chemistry of precipitation and lake water in the Adirondack Region of New York,USA

Long-term changes in the chemistry of precipitation (1978–94) and 16 lakes (1982–94) were investigated in the Adirondack region of New York, USA. Time-series analysis showed that concentrations of SO₄ ^2–, NO₃ ^–, NH₄ ⁺ and basic cations have decreased in precipitation, resulting in increases in pH. A relatively uniform rate of decline in SO₄ ^2– concentrations in lakes across the region (1.81±0.35 eq L^–1 yr^–1) suggests that this change was due to decreases in atmospheric deposition. The decrease in lake SO₄ ^2– was considerably less than the rate of decline anticipated from atmospheric deposition. This discrepancy may be due to release of previously deposited SO₄ ^2– from soil, thereby delaying the recovery of lake water acidity. Despite the marked declines in concentrations of SO₄ ^2– in Adirondack lakes, there has been no systematic increase in pH and ANC. The decline in SO₄ ^2– has corresponded with a near stoichiometric decrease in concentrations of basic cations in low ANC lakes. A pattern of increasing NO₃ ^– concentrations that was evident in lakes across the region during the 1980's has been followed by a period of lower concentrations. Currently there are no significant trends in NO₃ ^– concentrations in Adirondack lakes.     

Ion mass budgets for small forested catchments in Finland

Ion mass and H⁺ budgets were calculated for three pristine forested catchments using bulk deposition, throughfall and runoff data. The catchments have different soil and forest type characteristics. A forest canopy filtering factor for each catchment was estimated for base cations, H⁺, Cl^? and SO ₄ ^2? by taking into account the specific filtering abilities of different stands based on the throughfall quality and the distribution of forest types. Output fluxes from the catchments were calculated from the quality and quantity of the runoff water. Deposition, weathering, ion exchange, retention and biological accumulation processes were taken into account to calculate catchment H⁺ budgets, and the ratio between external (anthropogenic) and internal H⁺ sources. In general, output exceeded input for Na⁺, K⁺, Ca²⁺, Mg²⁺, HCO ₃ ^? (if present) and A^? (organic anions), whereas retention was observed in the case of H⁺, NH ₄ ⁺ , NO ₃ ^? and SO ₄ ^2? . The range in the annual input of H⁺ was 22.8–26.3 meq m^?2 yr^?1, and in the annual output, 0.3–3.9 meq m^?2 yr^?1. Compared with some forested sites located in high acid deposition areas in southern Scandinavia, Scotland and Canada, the catchments receive rather moderate loads of acidic deposition. The consumption of H⁺ was dominated by base cation exchange plus weathering reactions (41–79 %), and by the retention of SO ₄ ^2? (17–49 %). The maximum net retention of SO ₄ ^2? was 87% in the HietajÄrvi 2 catchment, having the highest proportion of peatlands. Nitrogen transformations played a rather minor role in the H⁺ budgets. The ratio between external and internal H⁺ sources (excluding net base cation uptake by forests) varied between 0.74 and 2.62, depending on catchment characteristics and acidic deposition loads. The impact of the acidic deposition was most evident for the southern Valkeakotinen catchment, where the anthropogenic acidification has been documented also by palaeolimnological methods.     

Chemical Composition of Precipitation in Beijing Area, Northern China

Variations of anions (SO₄ ^2-, NO₃ ^-,NO₂ ^-, Cl^- and F^-),cations (K⁺,Na⁺, Ca²⁺, Mg²⁺ and ⁺) and pH values in precipitation, througfall and stemflow samples collected overa four-year period (1995–1998) in Beijing (two sitesZhongguancun and Mangshan) are presented. The annualvolume-weighted range of pH values were 6.57–7.11 inprecipitation, 5.46–6.86 in thoughfall and 5.32–6.41 instemflow. The fominant anion was , while Ca⁺and NH₄ ⁺ were the main cations in precipitation,throughfall and stemflow. Most of ion concentrations with precipitation, throughfall and stemflow volume showed negative correlation, except for some ones. Significant correlationvalues were also found between ions (SO₄ ^2-,NO₃ ^-, Cl^-, F^-, Ca²⁺,Mg²⁺ andNa⁺) in precipitation, throughfall and stemflow indicatedthe common sources of these ions such as coal combustion,automobile emission and fertilizers application. Compared toprecipitation, there was an increased ion concentration inthroughfall or in stemflow. Changes of ion concentrations werein Quercus liatungensis Koiz. and Pinus tabulaefornisCarr. throughfall (or stemflow) because of different crown andbark qualities of tree species.     

Geochemical fluxes in forested acidified catchments

The atmospheric deposition of some major components (e.g., NO₃-N 6.6 and 14.5; SO₄-S 17.8 and 42.4 kg ha^–1 a^–1) and trace elements (e.g., Cd 0.4 and 1.1, Cu < 1.9=" and=" 15,=" pb=" 43=" and=" 48=" g=">^–1 a^–1) in bulk and throughfall deposition respectively, shows a pronounced decline in recent measurements of total deposition in the eastern Erzgebirge (Germany). This is true for both bulk and throughfall deposition in 1992–1994 as compared to similar data from 1985–1989. The decline is a result of successful emission control strategies in central Europe and the shut down of plants and factories in the former GDR.The dry deposition at the highest (influenced by long-range transport) and the lowest station (local influences) shows distinct differences (e.g., Cd 0.6 and 0.3; Cu 17.4 and 7.3; Pb 13 and 31 ng m^–3). A comparison between total and dry deposition exhibits the different behavior of elements in respect to atmospheric concentrations and solubility in (rain)water. Anthropogenically released elements are mainly immited via wet deposition.     

Elevational trends in the fluxes of sulphur and nitrogen in throughfall in the Southern Appalachian Mountains: Some surprising results

From 1986–1989, a team of scientists measured atmospheric concentrations and fluxes in precipitation and throughfall, and modeled dry and cloudwater deposition in a spruce-fir forest of the Great Smoky Mountains National Park which is located in the Southern Appalachian Region of the United States. The work was part of the Integrated Forest Study (IFS) conducted at 12 forests in N. America and Europe. The spruce-fir forest at 1740 m consistently received the highest total deposition rates (～2200, 1200, and 700 eq ha^?1 yr^?1 for SO₄ ^2?, NO₃ ^?, and NH₄ ⁺). During the summers of 1989 and 1990 we used multiple samplers to measure hydrologie, SO₄ ^2?, and NO₃ ^? fluxes in rain and throughfall events beneath spruce forests above (1940 m) and below (1720 m) cloud base. Throughfall was used to estimate total deposition using relationships determined during the IFS. Although the SO₄ ^2? fluxes increased with elevation by a factor of ～2 due to higher cloudwater interception at 1940 m, the NO₃ ^? fluxes decreased with elevation by ～30%. To investigate further, we began year round measurements of fluxes of all major ions in throughfall below spruce-fir forests at 1740 m and at 1920 m in 1993–1994. The fluxes of most ions showed a 10–50% increase with elevation due to the ～70 cm yr^?1 cloudwater input at 1920 m. However, total inorganic nitrogen exhibited a 40% lower flux in throughfall at 1920 m than at 1740 m suggesting either higher dry deposition to trees at 1740 m or much higher canopy uptake of nitrogen by trees at 1920 m. Differential canopy absorption of N by trees at different elevations would have significant consequences for the use of throughfall N fluxes to estimate deposition. We used artificial trees to understand the foliar interactions of N.     

Throughfall quality and quantity in polluted and damaged ecosystems in Northern Bohemia

The interaction between high concentrations of polluting gases (SO₂ and NO_x) and damaged forest ecosystems was observed by studying throughfall precipitation in the Erzegebirge Mountains, Northern Bohemia. Qualitative and quantitative data on throughfall for the period November 1989–October 1990 are presented. Weighted averages of SO ₄ ^2? and NO ₃ ^? concentrations in the throughfall were 23.05 mg L^?1 and 13.61 mg L^?1 in a beech and 34.41 mg L^?1 and 11.03 mg L^?1 in a spruce forest respectively. Three variables (the molar ratios of K/Na, N_tot/S and N-NO₃/N-NE₄) were used to compare the spruce throughfall quality to that observed in areas with similar however, less damaged spruce stands. Both K/Na and N-NO₃/N-NH₄ ratios clearly decreased with increasing tree damage, the N_tot/S ratio increased. The results suggest that the throughfall in damaged ecosystems of the Erzegebirge region becomes more like a wet precipitation as the tree canopies get sparser and the trees reduce canopy leaching.     

Covered catchment experiment at Gårdsjön: changes in runoff chemistry after four years of experimentally reduced acid deposition

Atmospheric deposition was almost entirely excluded from the forested headwater catchment G1 ROOF, by means of a 7000 m² plastic roof that prevents rain and throughfall from reaching the ground. Under the roof an irrigation system was installed to simulate a natural precipitation regime. The intercepted throughfall was substituted by the same amount of clean pre-industrial throughfall. The experiment started in April 1991. During the four years of the treatment, 2960 mm (18 600 m³) of sprinkled solution was applied under the roof, which is about 15 times the mean water storage of the catchment. After four years of treatment major changes in runoff chemistry were observed. The exclusion of all non-marine sulphate (SO₄ ^2–) input to the catchment (i.e. ca 75 % of total SO₄ ^2– input excluded) resulted in significant decline of sulphate in runoff through all four years of treatment. During the fourth year, annual volume weighted SO₄ ^2– concentration was 46 % lower than the two years prior to the treatment. Concentrations of inorganic aluminium Al³⁺ declined 52 % and Mg²⁺ declined 54 %. No change of H⁺ concentration was detected. As the treatment proceeds there seems to be a trend towards less negative acid neutralising capacity in runoff.     

Relation between sulphur concentrations in the Scots pine needles and the air in northernmost Europe

The SO₂ emissions from the Kola Peninsula in Arctic Russia (totalling around 600 Gg(SO₂) yr^–1 at the beginning of the 1990s) produce an atmospheric SO₂ concentration gradient to the northernmost Europe. This gradient covers the range from >50 g m^–3 in the vicinity of the sources to 1 g m^–3 in Finnish Lapland. In the present study, the measured sulphur concentrations in Scots pine needles were compared with the estimated distribution of atmospheric SO₂. The total sulphur concentrations in the needles ranged from 741 to 2017 mg kg^–1. Strongly elevated concentrations (> 1200 mg kg^–1) were found within 40 km from the smelters corresponding to an area where the annual mean atmospheric SO₂ concentration exceeded 10 g m^–3. The foliar sulphur concentrations (total, organic and inorganic) show a high correlation with the estimated mean SO₂ concentration distribution in the air. Consequently the foliar sulphur concentrations reflected the atmospheric sulphur load well. The data presented here show that uptake via stomata is an important deposition pathway also in the arctic conditions with a short growing season.     

Interactions between anthropogenic sulphate and marine salts in the Bs horizons of acidic soils in Scotland

In laboratory adsorption experiments, the comparison of podzol Bs horizons from coastal and inland moderately-impacted catchments with those from a severely-acidified inland region has demonstrated the effect of marine inputs on SO₄ ^2– -retention. Moderate sea-salt inputs and low acid deposition leads to the retention of most of the SO₄ ^2– and the release of soluble Mg²⁺; increasing the marine salt loading causes the development of a selectivity towards retention of acidic SO₄ ^2– and the retention of Mg²⁺. In the highly-impacted soil, the marine input caused a decrease in SO₄ ^2– retention in open moorland soils. The opposite occurred under forest, due to the ion-exchange of marine Mg²⁺ for soil Al³⁺, increasing soil acidity towards the pH₀ (Gillman and Uehara, 1980), which is depressed below that of its moorland equivalent.     

Estimates of atmospheric deposition and canopy exchange for three common tree species in the Netherlands

During one year, dry and wet deposition onto thirty forest stands is studied by sampling throughfall and bulk precipitation. Nine measurement sites are situated in Douglas fir (Pseudotsuga menziesii Mirb. Franco) stands, ten in Scotch pine (Pinus sylvestris L.) and eleven in Oak (Quercus robur L.) stands. Because the stands are situated in each other's proximity (i.e. within a radius of approximately 1.4 km) it is assumed that they experience an approximately equal air pollution load. For the acidifying compounds SO₄ ^2?, NO₃ ^? and NH₄ ⁺ spatial variability in wet deposition was small within the area studied. Dry deposition, as estimated by net throughfall, displayed a much higher spatial variability. Significant differences existed between tree species and growing seasons. Douglas fir mostly displayed the highest, Oak the lowest and Scotch pine intermediate values for net throughfall fluxes of acidifying compounds. The annual net throughfall fluxes for nitrogen compounds were significantly higher for the coniferous tree species than the broadleaved tree species. For SO₄ ^2?, however, Oak showed a relatively high throughfall flux during the summer. By comparing the temporal pattern of net throughfall fluxes between the three tree species it was concluded that considerable canopy leaching occurred for SO₄ ^2?, Mg⁺, PO₄ ^3?, HCO₃ ^? and K⁺ in Oak stands during the sprouting of leaves in spring. From surface wash experiments in the laboratory it is concluded that canopy leaching of these ions may also be enhanced when Oak leaves are infected by Oak mildew, a fungal disease caused by the fungus Microshaera aliphilitoides.     

Measured and modelled retention of inorganic sulfur in soils and subsoils (Harz Mountains,Germany)

Atmospheric deposition has resulted in an accumulation of inorganic sulfur (S) in many forest soils. At Sösemulde (Harz Mountains) samples from 5–240 cm depth were analysed. Most sulfate (SO₄) is accumulated at about 30–60 cm depth: 8.5–9.5 mmol_c kg^–1. Large amounts can also be retained in > 100 cm. To assess changes in SO₄ dynamics in time,adsorption isotherms have been included in several process-oriented models, e.g., in MAGIC. The Lange Bramke (LB) Model is the first model used on the catchment scale containing solubility products for the hydroxosulfate minerals jurbanite and alunite. By reconstructing the long-term acidification history (140 years) both models were successfully calibrated to a 14-year deposition, soil and streamwater data set at Lange Bramke catchment (Harz Mountains). According to MAGIC the present accumulation of SO₄ in 0 –80 cm is 8.7 mmol_c kg^–1, while according to the LB-Model 10.2 mmol_c kg^–1 are stored as jurbanite. Both models predicted 4.5 mmol_c kg^–1 SO₄ in the subsoil layer, retained as alunite in the LB Model. These values correspond to the amounts measured in soil and subsoil samples at Sösemulde, respectively. However, for future scenarios with decreasing S inputs the models show different developments in SO₄ concentrations. Changes in MAGIC are gradual whereas the LB model predicts stepwise decreasing SO₄ values as soon as previously stored hydroxosulfates are fully dissolved. Such concentration jumps have not been observed.     

Atmospheric deposition in forest edges measured by monitoring canopy throughfall

Atmospheric dry deposition in two forest edges was studied by means of monitoring canopy throughfall in Douglas Fir stands. Throughfall fluxes in the first 50 to 100 m of forest edges were found to be substantially higher than fluxes in the interior of forest stands. Sodium and chloride showed the steepest throughfall flux gradients. Ions important for soil acidification and eutrophication showed relatively less steep but still significant gradients. The mean increase of the throughfall flux at 10 m, with respect to the flux at 200 m from the forest edge amounted to 150% for Na⁺, 119% for Cl^?, 54% for S0₄ ^2?, 38% for NO₃ ^? , and 39% for NH₄ ⁺ The enhancement of dry deposition in forest edges strongly depends on wind velocity and wind direction during dry deposition. Particularly trees in forest edges exposed to prevailing wind directions receive relatively large amounts of dry deposition.     

Rainfall,Stemflow, and Throughfall Chemistry at Urban- and Mountain-facing Sites at Mt. Gokurakuji,Hiroshima, Western Japan

Rainfall, stemflow, and throughfall were collected from 1996 to 1999 at two types of forest sites: (1) forests near the traffic roads and urban areas and (2) forests away from the urban areas at Mt. Gokurakuji, Hiroshima, western Japan in order to estimatethe effects of anthropogenic activities on atmospheric deposition. Rainfall deposition for major ions showed small differences between the sites. The NO₃ ^- and SO₄ ^2-concentrations in stemflow were higher at the urban-facing slope than at the mountain-facing slope. Throughfall deposition of NO₃ ^- and SO₄ ^2- was also higher at urban-facing slopes. Net throughfall (NTF) deposition (throughfall minus rainfall) of NO₃ ^- and SO₄ ^2- accounted for 77 and50% of the total throughfall deposition on urban-facing slopes, respectively, while it accounted for 44 and 23% on themountain-facing slopes, respectively. These results indicated a higher contribution from dry deposition on urban-facing slopes compared to mountain-facing slopes. Atmospheric N (NO₃ ^- +NH₄ ⁺) deposition from throughfall was estimated to be around 17–26 kg N ha^-1 yr^-1 on urban-facing slopes, which was greater than the threshold of N deposition that could cause nitrogen leaching in Europe and the United States. The highload of atmospheric N deposition may be one of the factors bringing about the decline of pine forests on urban-facing slopesof Mt. Gokurakuji.     

The dry deposition of gaseous and participate nitrogen compounds to a spruce stand

The dry deposition of particle-bound NH₄ ⁺ and NO₃ ^– and of gaseous NH₃ and HNO₃ to a 45 year old Norway Spruce forest was determined by the inferential method. Deposition velocities were calculated from meteorological data and plant morphology by a multi-layer model which combines the transfer of the trace substances within the canopy and their absorption probabilities due to a variety of deposition mechanisms. For gaseous deposition the same mathematical concept was adopted as for particles. That means that this approach is an alternative to the common concept of resistance analogy. The mean deposition velocities found were 0.47 cm s^–1 for NH₄ ⁺ and 1.85 cm s^–1 for NO₃ ^–. While these values are in the range found in other studies, the mean deposition velocities for the gases were quite large. For NH₃ 13 cm s^–1 were found and for HNO₃ 11 cm s^–1. It is suggested that the absorption of these gases on the plant surfaces is not as effective as assumed in the model.     

Sulphur isotope characteristics of two north Bohemian forest catchments

Sulphate concentrations and ³⁴S ratios were monitored in bulk precipitation, spruce throughfall, and soil water (depth of 30 and 90 cm) at ervená jáma (CER) and Naetín (NAC), two severely polluted sites in the Czech Republic, between December 1992 and September 1994. Throughfall [SO₄ ^2–], up to 80 mg L^–1 in winter and as low as 7 mg L^–1 in summer, was higher than [SO₄ ^2–] in bulk precipitation (annual average 6 mg L^–1). There was a distinct seasonaity in S isotope abundances, with lower ³⁴S_BULK in summer (+4 per mil CER, + 6 per mil NAC) and lower ³⁴S_TF in winter (+3 per mil CER, +4 per mil NAC). Wintertime ³⁴S_BULK was around +8 per mil at CER and +10 per mil at NAC, summertime ³⁴S_TF was close to +7 per mil at both sites. For only a 1- month period in spring, bulk precipitation S became isotopically lighter than throughfall S. Bulk precipitation data from CER were in good agreement with those from the nearby monitoring station Lesná (LES), typically differing by less than 10 mg L^–1 and 2 per mil in [SO₄ ^2–] and ³⁴S, respectively. Suction lysimeters (soil depth of 30 and 90 cm) yielded higher sulphate concentrations and lower ³⁴S ratios compared to both bulk and throughfall precipitation. Little seasonality was observed in [SO₄ ^2–] at 30 cm (around 40 mg L^–1); at 90 cm [SO₄ ^2–] was higher in winter (70 mg L^–1) than in summer (45 mg L^–1). ³⁴S at 90 cm was <+5 per=" mil=" in=" 1993=" and=" up=" to=" +7.5=" in=" 1994,=" lower=" in=" the=" first=" year=" and=" higher=" in=" the=" second=" year=" compared=" to=" the=" depth=" of=" 30=" cm.=" sulphur=" fluxes=" at=" cer=" and=" nac=" are=" characterized=" by=" distinct=" isotope=" compositions=" and=" can=" therefore=" be=" used=" to=" trace=" s=" pathways=" and=" transformations=" in=" the=" forest=">     

Acidic deposition,ecosystem processes,and nitrogen saturation in a high elevation Southern Appalachian watershed

High-elevation red spruce-Fraser fir forests in the Southern Appalachian mountains: 1) receive among the highest rates of atmospheric deposition measured in North America, 2) contain old-growth forests, 3) have shown declines in forest health, 4) have sustained high insect-caused fir mortality, and 5) contain poorly buffered soils and stream systems. High rates of nitrogen and sulphur deposition (1900 and 2200 Eq·ha^–1·yr^–1, respectively) are dominated by dry and cloud deposition processes. Large leaching fluxes of nitrate-nitrogen (100–1400 Eq·ha^–1·yr^–1) occur within the soil profile. We have expanded the study to the watershed scale with monitoring of: precipitation, throughfall, stream hydrology, and stream chemistry. Two streamlets drain the 17.4 ha Noland Divide Watershed (1676–1920m) located in the Great Smoky Mountains National Park. A network of 50 20x20 m plots is being used to assess stand structure, biomass, and soil nutrient pools. Nitrate is the predominant anion in the streamlets (weighted concentrations: 47 and 54 eq·L^–1 NO₃ ^–; 31 and 43 eq·L^–1 SO₄ ^2–). Watershed nitrate export is extremely high (1000 Eq·ha^–1 yr^–1), facilitating significant base cation exports. Stream acid neutralizing capacity values are extremely low (–10 to 20 eq·L^–1) and episodic acidifications (pH declines of a full unit in days or weeks time) occur. Annual streamwater sulfate export is on the order of 770 Eq·ha^–1yr^–1 or about one-third of total annual inputs, indicating there is net watershed sulfate retention. The system is highly nitrogen saturated (Stage 2, Stoddard, 1994) and this condition promotes both chronic and episodic stream acidification.     

Biogeochemistry of two Appalachian deciduous forest sites in relation to episodic stream acidification

Bulk precipitation, throughfall, and soil water chemistry were studied from November 1983 to November 1984 at two ridge-top Appalachian deciduous forest sites to isolate causes of differing episodic stream acidification. The Fork Mountain site in West Virginia, which exhibited little episodic stream acidification, had lower deposition of H⁺ and SO _inf4 ^sup2? and greater reductions of H⁺ in the water circulating through the forest canopy, forest floor, and mineral soil than the Peavine Hill site in Pennsylvania. Greater neutralization at Fork Mountain was linked to higher Ca and Mg carbonate contents in the sandstone and shale soil parent materials. Fork Mountain had greater amounts of exchangeable bases in the organic and mineral soil horizons. Neither site appeared to be accumulating SO _inf4 ^sup2? in the soil, with Peavine Hill losing 56% more than was received in bulk deposition. Anions in soil leachate at Fork Mountain were largely balanced by Ca²⁺ and Mg²⁺, while at the Peavine Hill site A1" was the dominant cation. Results suggest that the typically-low carbonate content of sandstone and shale soil parent materials commonly found in Appalachian forests may be a key parameter controlling soil and stream acidification. Data for the one-year period also suggest bulk deposition of H⁺ was 63% greater at Peavine Hill than Fork Mountain.