Mesoscale spatial variability of sulfate in air and rainwater at St. Louis

Sulfate concentrations in rainwater and in air measured on four summer days at St. Louis were highly variable, both spatially or temporally. Maximum/minimum ratios of aerosol SO _inf4 ^sup? varied by up to a factor of 9, and those in rainwater by a factor of 3 on the average. Generally, SO _inf4 ^sup? concentration patterns in air and rainwater were similar, and consistent with wind direction and the location of sources. Direct relationships between SO _inf4 ^sup? in air and in water were evident on two of the individual days, but not for all days together. The non-uniformity of the SO _inf4 ^sup? pattern plus consideration of possible sources of SO _inf4 ^sup? suggests that nucleation of SO _inf4 ^sup? particles must be a major cause of S scavenging, with some possible influence from sub-cloud impaction.     

A method for evaluation of acidic sulfate and nitrate in precipitation

A simple method is presented and used to estimate the portions of SO _inf4 ^sup2? and NO _inf? ^sup3 that contribute to the strong acidity in weekly precipitation samples collected at three NADP sites in the eastern United States. The method assumes that, in general, the difference between SO _inf4 ^sup2? and NH _inf+ ^sup4 represents acidic sulfate and the difference between NO _inf? ^sup3 and soil-derived materials (the sum of Ca²⁺, Mg²⁺, and K⁺) represents acidic nitrate. Acidic sulfate and nitrate are considered to be the predominant source of H⁺ (determined from laboratory pH) in the weekly precipitation samples. Most of the acidity for all three sites was attributed to acidic sulfate. The highest fraction of acidic SO _inf4 ^sup2? to H⁺ wet deposition values was for the east-central Tennessee site (0.95) and the northeastern Illinois site (0.90), and the lowest fraction occurred at the central Pennsylvania site (0.75). The Tennessee site had the greatest acidic fraction of sulfate (0.84) and the Pennsylvania site had the greatest acidic fraction of nitrate (0.59).     

The geographical distribution and temporal variations of acidic deposition in Eastern North America

Data from two national precipitation chemistry monitoring networks, and several regional air and precipitation chemistry networks are used to describe some broad-scale features of acidic deposition in eastern North America. In northeastern North America, the coefficient of variation is shown to increase from 10–16% for annual averages to nearly 100% for daily values. There is a strong annual cycle in H⁺, SO _inf4 ^sup= and NH _inf4 ^sup+ deposition and some of the other ions although these cycles are not all in phase. The wet NO _inf3 ^sup? deposition contributes relatively more than SO _inf4 ^sup= to the acidity of snow as compared to rain. Wet deposition is highly “episodic” with about 50% to 70% of the total annual deposition of SO _inf4 ^sup= and NO _inf3 ^sup? accumulating in the highest 20% of the days. Estimates made in various ways indicate that, over eastern North America as a whole, dry deposition is approximately equal to wet for both SO _inf4 ^sup= and NO _inf3 ^sup? . Dry may exceed wet in the high emissions zone but drops to about 20% of the total deposition in more remote areas. Deposition via fog or low cloud impaction is an important input to high elevation forests, but more data are required to quantify the magnitude and regional extent of this.     

Interaction between nitrogen fertilization,rainfall and groundwater pollution in sandy soil

A 2 yr field study on the influence of N fertilization and rainfall on groundwater pollution was carried out in the sandy area of Belgium. The NO _inf3 ^sup? -N and Cl^? content of the groundwater at 0.5, 1.0, 1.5, and 2.0 m depths was monitored every two weeks on a field, grown with barley in 1980 and with maize in 1981. Turnips for cattle feed were grown in between the two crops. The total annual rainfall during the period under study was about 800 mm. The NO _inf3 ^sup? -N content at all depths was at all times above 11.3 mg NO _inf3 ^sup? -N dm^?3, the WHO safe limit. Fluctuation of the NO _inf3 ^sup? -N content occurred mainly at 0.5 and 1.0 m. The concentration at 1.5 and 2.0 m depths was higher most of the time than at 0.5 and 1.0 m. Leaching of NO _inf3 ^sup? -N into deeper layers occurred when there was heavy rainfall. There was no important loss of NO _inf3 ^sup? -N through denitrification at 1.5 and 2.0 m depths.     

Analysis of airborne particles sampled in the southern Appalachian mountains

Over 3 yr of particulate measurements were made at two high elevation sites in the southern Appalachian Mountains of Tennessee and Virginia. Both dichotomous samplers and filter packs were used to obtain day and night, week-long samples for subsequent elemental and ionic analysis. Total No _inf3 ^sup? (HNO₃ + No _in3 ^sup? ) and SO _inf4 ^sup2? averaged, respectively, 1.1 and 5.0 µg m^?3 at Look Rock, Tennessee and 2.0 and 6.4 µg m^?3 at Whitetop Mountain, Virginia. At Whitetop Mountain, the spring and summer seasons had the highest average SO _inf4 ^sup2? concentrations. Seasonally, total N03 varied little. The diurnal variation of elements and SO _inf4 ^sup2? was small. Only total NO _inf3 ^sup? varied substantially with highest values during the day. The fine fraction (particle diameter < 2.5 µm) accounted for about 67% of the total mass. Fine mass and elemental concentrations were generally higher at Look Rock. The elements comprising the principal mass fraction of the coarse samples (2.5 gm < particle diameter < 10 to 15 µm) were of crustal origin (e.g., Al, Si, Ca, Fe) while the element comprising the principal mass fraction of the fine samples (i.e., S) was of manmade origin. Cluster analysis identified two groups of elements at Whitetop Mountain. These groups, in both the coarse and fine fraction, were associate with a soil and an automobile emission component. At Look Rock, only a soil component was obvious.     

An observational study on precipitation chemistry data as a function of surface wind direction

Precipitation chemistry data were collected at 8 sites in the western part of the Netherlands over a period of 151/2 yr. Using specially-designed wind direction-dependent rain collectors, it is demonstrated that levels of ion constituents in rain water vary with wind direction, which can be assigned to different sources of contamination. The strongest variation was measured for Na⁺. As expected, trajectories over sea contributed most. Continental sources slightly reduced the variation for Cl^? and Mg²⁺. For both NH _inf4 ^sup+ and excess SO _inf4 ^sup2? , maxima were found in southeasterly rainfall. It is assumed that polluted air from the large industrial Ruhr Area crosses a region with strong NH₃ emissions in the Southeast of the Netherlands. The presence of ammonium sulfate was suggested. Minor variations were measured for NO _inf3 ^sup? (due to diffusely-spread emissions and possible sampling artefact) and for H⁺ and Ca²⁺ (dry-deposition effects). Over the period 1973–1987 excess-SO _inf4 ^sup2? levels decreased significantly (?3.3% yr^?1, p<0.01) which is consistent with reduced S emission in Western Europe. Over the last decade an upward trend in NO _inf3 ^sup? levels has been observed (3.2% yr^?1, p<0.05) which is possibly related to the increased usage of cars.     

Temporal variation in aluminum speciation and concentration during snowmelt

Precipitation samples in Alberta were collected and analyzed monthly from six Alberta Environment stations. Samples were collected with Sangamo samplers and analyzed for the major ions, pH and acidity. The data were tabulated and analyzed for spatial distribution, seasonal variation, temporal trends, ionic character and wet sulphate deposition. The major ionic species in Alberta precipitation are Ca²⁺, SO _inf4 ^sup2? , NH _inf4 ^sup+ and N0 _inf3 ^sup? . The spatial distribution shows a slight decrease in pH from southern Alberta (pH 6.0) to northern Alberta (pH 5.4). The seasonal variation shows higher hydrogen ion content in the summer months (pH 5.4 in summer and pH 5.8 in winter). Temporal trends are not apparent over the five year period investigated. The five year average wet sulphate deposition rate in Alberta is 9.1 kg ha^?1 yr^?1.     

Mobilization of heavy metals from river sediments of Northern Greece by complexing agents

The influence of NTA, EDTA, STPP, Triton X100, PO _inf4 ^sup3 and NO _inf3 ^sup? on the mobilization of Pb, Cd, Cu, Cr and Mn from sediments of two rivers located in Northern Greece was studied. The release caused by all examined complexing agents was higher in deionized water than either Axios or Aliakmon river water due to the lack of competition of Ca and Mg cations with the heavy metals for the studied complexing agents, and the decrease of ionic strength. From all examined agents NTA and EDTA showed the greater mobilization ability. Copper showed the greater tendency for remobilization by all examined agents, (according to the order: EDTA?NTA, Triton X1004 PO _inf4 ^sup3? > NO _inf3 ^sup3? ?STPP) while Cr and Mn the smallest following the orders: NTA, PO _inf4 ^sup3? >> NO _inf3 ^sup? , Triton X1004 EDTA, STPP and STPP > EDTA > NTA > Triton X 100 ? PO _inf4 ^sup3? NO₃, respectively. An increase in mobilization was noticed with an increase of agent concentration and time of shaking.     

Resistance of the caribou lichen Cladina stellaris (Opiz.) brodo to growth reduction by simulated acidic rain

Natural mats of C. stellaris growing in the subarctic lichen woodlands of northern Québec were treated in a randomized complete block design with solutions of simulated rain at pH 2.5, 3.0, 3.5, 4.0, 4.5, 5.0 and 5.6. These solutions were acidified by addition of mixtures of sulfuric and nitric acids to give both 2 : 1 and 6 : 1 μequivalent ratios of SO _inf4 ^sup= : NO _inf3 ^sup? . After two years of acidification there was no significant effect of either pH or SO _inf4 ^sup= : NO _inf3 ^sup? ratio on the growth of C. stellaris, but thallus discoloration was evident below pH 3.5. After three years of acidification marginally significant (p = 0.05) but erratic depression of growth occurred under the 6 : 1 but not the 2 : 1 acidification regime, especially at pH 4.5 or less. Acid precipitation therefore only very gradually impairs the growth of C. stellaris, and the deleterious effects of acidification may be partially offset by nitrogen enrichment when precipitation is relatively rich in nitrate compared to sulfate ions.     

Precipitation chemistry in Nova Scotia: 1978–1987

More than 1400 precipitation samples were collected weekly from 5 sites in Nova Scotia between 1978 and 1987. High concentrations of H⁺, non-marine SO _inf4 ^sup= (^*SO₄) and NO _inf3 ^sup- were observed in 1978 and 1986. In 1983, concentrations of all three parameters were the lowest in the data record. Fluctuations in emissions for SO₂ are insufficient to account for the variability observed in concentration and deposition values. Mean annual concentrations in 1983 were 13, 16, and 6 ueq L^-1 for H⁺, ^*SO₄, and NO _inf3 ^sup- , respectively. In 1986 the values were 35, 28, and 13 ueq L^-1. Concentrations in 1978 were 31, 38, and 16 ueq L^-1. Average pH of precipitation was 4.61 during the 10 yr study. The two most acidic years were 1979 (4.47) and 1986 (4.46). In 1983, the average pH was 4.89. The ratio (equivalents) of NO _inf3 ^sup- to ^*SO4 was 0.41, so most acidity in the precipitation results from H₂SO₄ However, multiple regression analysis revealed that H⁺ is more sensitive to changes in NO₃-concentrations than ^*SO₄. Ratios of summer (JJA) vs winter (JFM) average concentrations were examined. During summer months, ^*SO₄ and H⁺ were 1.8 times winter values. Summer to winter ratios for NO _inf3 ^sup- and NH _inf4 ^sup+ were 1.4 and 2.5, respectively.     

An estimation of atmospheric deposition input of sulphur and nitrogen oxides to the Kejimkujik watershed : 1979–1987

Daily measurements of the concentrations of major ions in ambient air and in precipitation at Kejimkujik National Park, Nova Scotia, Canada over the period May 1979 to December 1987 are used to estimate the wet, dry and total deposition to the watershed. Variations on three time-scales are apparent. The strongest variation, of up to two orders of magnitude occurs on a day to day basis resulting in a coefficient of variation in the range of 110 to 140%. Deposition is highly episodic with the highest 20% of the daily events accounting for 55 to 60% of the long-term deposition. The most systematic variation is the annual cycle observed for many of the species. The air concentration of SO₂ has the most pronounced cycle with a winter maximum and a summer minimum. The SO _inf4 ^sup= air concentrations show a smaller amplitude and are out-of-phase with SO₂, showing a summer maximum. Air concentrations of HNO₃ and particulate N0 _inf3 ^sup- also have an out-of-phase annual cycle, with a summer maximum and summer minimum respectively. Wet deposition of SO _inf4 ^sup= shows a broad maximum through the summer months, but for NO _inf3 ^sup- no systematic cycle is evident. On an ion equivalent basis, NO _inf3 ^sup- contributes as much as SO _inf4 ^sup= to the acidity of winter precipitation, but only one-third as much in the summer months. Although 8.7 yr is too short a time-scale to establish long term variations with any certainty, there does appear to be an overall downward trend in S concentrations and deposition, but not for N. This is not inconsistent with the trends in the emissions of SO₂ and NO_X in the regions upwind of Nova Scotia. The fraction of the S input to the watershed as dry deposition is estimated to average 22% of the total.     

Chemical composition of precipitation in a forest area of Chongqing,Southwest China

Experiments were carried.out in Chongging-a city seriously damaged by acid precipitation in southwest China — to explore chemical compositions of open bulk precipitation, throughfall and stemflow in a Masson pine (Pinus massoniana) forest. The results showed that annual mean pH values of and annual ion depositions in the three types of rain water were 4.47 and 50.6 g m^?2, 3.82 and 69.7 g m^?2, and 2.92 and 0.215 g m^?2 respectively. pH values demonstrated an obvious seasonal variation; they were lower in winter than in the rest of the year. Ca²⁺ and NH _inf4 ^sup+ together made up more than 80% of the total cation, while SO _in4 ^sup2? alone contributed over 90% to the total anion. This high level of SO _in4 ^sup2? in rain water in Chongqing, which outran those found in other cities in China, was closely related to the combustion of locally produced coal that contains 3 to 5% sulphur. Thus, acid precipitation in Chongqing is of a typical sulphuric-acid type.     

Episodic acidification of a coastal plain stream in Virginia

This study investigates the episodic acidification of Reedy Creek, a wetland-influenced coastal plain stream near Richmond, Virginia. Primary objectives of the study were to quantify the episodic variability of acid-base chemistry in Reedy Creek, to examine the seasonal variability in episodic response and to explain the hydrological and geochemical factors that contribute to episodic acidification. Chemical response was similar in each of the seven storms examined, however, the ranges in concentrations observed were commonly greater in summer/fall storms than in winter/spring storms. An increase in SO _inf4 ^sup2? concentration with discharge was observed during all storms and peak concentration occurred at or near peak flow. Small increases in Mg²⁺, Ca²⁺, K⁺ concentrations and dissolved organic carbon (DOC) were observed during most storms. At the same time, ANC, Na⁺ and Cl^? concentrations usually decreased with increasing discharge. In summer/fall storms, the absolute increase in SO _inf4 ^sup2? concentration was one-third to 15 times the increase observed in winter/spring storms; the decrease in ANC during summer/fall storms was usually within the range of the decrease observed in winter/spring storms. In contrast, the decrease in Na⁺ and Cl^? concentrations during winter/spring storms was much greater than that observed during summer/fall storms. Data show that while base flow anion deficit was higher in summer/fall than in winter/spring, anion deficit decreased during most summer/fall storms. In contrast, base flow anion deficit was lower in spring and winter, but increased during winter/spring storms. Increased SO _inf4 ^sup2? concentration was the main cause of episodic acidification during storms at Reedy Creek, but increased anion deficit indicates organic acids may contribute to episodic acidification during winter/spring storms. Changes in SO _inf4 ^sup2? concentration coincident with the hydrograph rise indicate quick routing of water through the watershed. Saturation overland flow appears to be the likely mechanism by which solutes are transported to the stream during storm flow.     

Comparison of RADM dry deposition algorithms with a site-specific method for inferring dry deposition

Data collected during 1986 at seven widely separated sites in the eastern United States were used to estimate weekly averages of deposition velocities for SO₂, O₃, HNO₃, and SO _inf4 ^sup2? with both a modified version of the RADM dry deposition module and a site-specific inferential technique developed by the Atmospheric Turbulence and Diffusion Division of the National Oceanic and Atmospheric Administration's Air Resources Laboratory. The results show some systematic differences between the two techniques, even when the module uses distributions of landuse types that match as closely as possible the observed vegetation coverages used in the inferential technique. When one ignores the systematic differences that easily could be removed by minor changes in the algorithms for computing resistances to deposition, weekly averages of the deposition velocities calculated with the two methods are within approximately ±30% of each other for SO₂ and O₃. Overall, the relative differences in the deposition velocities for HNO₃ and SO _inf4 ^sup2? are about ±30 and ±50%, respectively. Use of the module with landuse types extrapolated to areas as large as RADM grid cells (approximately 80 km square) around the measurement stations produces weekly averages within ±20% of the site-specific estimates for SO₂, O₃, and SO _inf4 ^sup2? and approximately ±30% for HNO₃ if one avoids landuse types such as urban and water areas that are both nonrepresentative and have very different characteristics from the measurement sites. These estimates are not complete measures of the true uncertainty associated with the two techniques because they do not account for such effects as differences in the siting of instrumentation for collecting input data and the inability of the computational algorithms to include the many surface nonuniformities that typically exist in the areas surrounding measurement sites.     

Nitrate reduction coupled with microbial oxidation of sulfide in river sediment

Purpose

Nitrate (NO ₃ ^? ) is often considered to be removed mainly through microbial respiratory denitrification coupled with carbon oxidation. Alternatively, NO ₃ ^? may be reduced by chemolithoautotrophic bacteria using sulfide as an electron donor. The aim of this study was to quantify the NO ₃ ^? reduction process with sulfide oxidation under different NO ₃ ^? input concentrations in river sediment.

Materials and methods

Under NO ₃ ^? input concentrations of 0.2 to 30?mM, flow-through reactors filled with river sediment from the Pearl River, China, were used to measure the processes of potential NO ₃ ^? reduction and sulfate (SO ₄ ^2? ) production. Molecular biology analyses were conducted to study the microbial mechanisms involved.

Results and discussion

Simultaneous NO₃ ^? removal and SO₄ ^2? production were observed with the different NO ₃ ^? concentrations in the sediment samples collected at different depths. Potentially, NO ₃ ^? removal reached 72 to 91?% and SO ₄ ^2? production rates ranged from 0.196 to 0.903?mM?h^?1. The potential NO ₃ ^? removal rates were linearly correlated to the NO ₃ ^? input concentrations. While the SO ₄ ^2? production process became stable, the NO ₃ ^? reduction process was still a first-order reaction within the range of NO ₃ ^? input concentrations. With low NO ₃ ^? input concentrations, the NO ₃ ^? removal was mainly through the pathway of dissimilatory NO ₃ ^? reduction to NH ₄ ⁺ , while with higher NO ₃ ^? concentrations the NO ₃ ^? removal was through the denitrification pathway.

Conclusions

While most of NO ₃ ^? in the sediment was reduced by denitrifying heterotrophs, sulfide-driven NO ₃ ^? reduction accounted for up to 26?% of the total NO ₃ ^? removal under lower NO ₃ ^? concentrations. The vertical distributions of NO ₃ ^? reduction and SO ₄ ^2? production processes were different because of the variable bacterial communities with depth.     

Long-term sulfate dynamics at lange bramke (Harz) used for testing two acidification models

At Lange Bramke (Harz) soil solution and runoff concentrations of major elements were observed over 16 yr. During this period acid deposition was high but showed a marked decrease of H⁺ and SO ₄ ^2? both in concentrations and fluxes over the last five years. Among others, this record reveals the following patterns: seasonality in the signals for SO ₄ ^2? and NO ₃ ^? in runoff which are synchronous; an accumulation of SO ₄ ^2? in the soil, initially up to 50% of the deposition fluxes; apparently no correlation between runoff and SO ₄ ^2? concentration, and no long-term trend in runoff concentration of SO ₄ ^2? . In this paper we use these patterns in the data set from Lange Bramke to test two established acidification models. The test criterion is that the algorithms employed by the SO ₄ ^2? modules of these models must be able to reproduce these features. To that end, both models need not to be run as it can be shown that even with completely unrestricted parameter values the two algorithms are unable to match the observed SO ₄ ^2? dynamics. The MAGIC model (Cosbyet al., 1985) is unable to reproduce, given the existence of net SO ₄ ^2? accumulation, the constant SO ₄ ^2? concentration in runoff during the last 16 years. The second model, BEM (Prenzel, 1986), is succesful in reconstructing the constant SC>4~ levels in runoff. However, on a monthly time scale BEM predicts a shift between the periodic maximum concentrations of SO ₄ ^2? and NO ₃ ^? which is not observed in the data.     

Sulfur and oxygen isotope ratios in sulfate during an acidification reversal study at Lake Gårdsjön,Western Sweden

The reversibility of acidification is being investigated in a full scale catchment manipulation experiment at Lake Gårdsjön on the Swedish west coast using isotopes as environmental tracers. A 6300 m² roof over the catchment enables researchers to control depositional variables. Stable S isotope values were determined in bulk deposition, throughfall, runoff, groundwater and soil-extracted water during one year prior to and two years of experimental control. Data collected prior to experimental control suggest that the inorganic SO ₄ ^2? pool within the catchment has a homogeneousδ ³⁴S value of about+5.5‰. Sprinkling of water spiked with small amounts of sea-water derived SO ₄ ^2? started in April 1991. Theδ ³⁴S value of this SO ₄ ^2? is around+19.5‰. Since April 1991, the SO ₄ ^2? concentration in runoff has decreased by some 30%, however, theδ ³⁴S value have increased by only 0.5‰. This suggests mixing of sprinkling water S with a large reservoir of S in the catchment. Oxygen isotopes in SO ₄ ^2? suggest that less than one third of the SO ₄ ^2? in runoff is secondary SO ₄ ^2? formed within the soil profile. This is, however, no evidence for net mineralization of S. The SO ₄ ^2? in runoff in the roofed catchment is a mixture of SO ₄ ^2? previously adsorbed in the soil, mineralized organic S and SO ₄ ^2? from the sprinkler water. Calculations based on isotope data indicate that the turnover time of S within the catchment is on the order of decades. Since SO ₄ ^2? facilitates base cation flow, the acidification reversal will take a much longer time than concentration decreases of SO ₄ ^2? would suggest.     

Trends in surface water acidification at ecological monitoring sites in southeastern Canada (1981–1993)

Atmospheric deposition and surface water chemistry have been monitored intensively at 5 geologically “sensitive” sites in southeastern Canada. The sites receive differing acid inputs that span the entire range found in Canada. Surface water data collected at 9 stations from 1981 to 1993 for SO ₄ ^2? , NO ₃ ^? , Alkalinity, DOC, pH, Ca²⁺ and Mg²⁺ have been analyzed to detect monotonic trends. Similarities between the temporal patterns and trends for SO ₄ ^2? in deposition and surface water suggest that they are strongly linked at our sites. Our 13-year datasets showed significant negative SO ₄ ^2? trends at the 3 Ontario sites and a positive trend in Nova Scotia. A climatically-induced SO ₄ ^2? increase in northwestern Ontario has been reversed. Mobilization and export of adsorbed SO ₄ ^2? and/or reoxidized S from the basins of central Ontario sites is delaying their recovery. Two of our 9 stations (in Quebec and central Ontario) are continuing to acidify. The 2 Nova Scotia stations have the highest DOC levels and both exhibit a decreasing trend. Ionic compensation for declining SO ₄ ^2? varies from station to station, sometimes involving an Alk increase, sometimes a base cation decrease, and sometimes more complex combinations. Additional factors (e.g. climatic variation) also influence variable trends, and data records longer than those presently available will be needed to unequivocally verify acidification recovery.     

Mid-term trends in acid precipitation,streamwater chemistry and element budgets in the strengbach catchment (Vosges Mountains,France)

In the Vosges Mountains (NE of France), integrated plot-catchment studies have been carried out since 1985 in the Strengbach basin to study the influence of acid atmospheric inputs on surface water quality and element budgets. In this paper, available mid-term time series (1985–1991) have been considered to detect obvious trends, if any, in surface water chemistry and element budgets. Air quality data showed a slight decline for SO₂, whereas NO₂ slightly increased over the period, but these trends are not very significant. This is in agreement with increased N concentration (mainly as NH ₄ ⁺ ) and with the stability of SO ₄ ^2? in open field precipitation. Because of a significant decrease in rainfall amount over the period, only inputs of NH ₄ ⁺ increased significantly whereas H⁺ and SO ₄ ²⁺ inputs declined. In spring and streamwaters, pH and dissolved Si concentration increased mainly as a result of a reduced flow. Na⁺, K⁺, Cl^? and HCO-3~^? concentrations remained stable whereas Ca²⁺, Mg²⁺ and SO ₄ ²⁺ concentrations declined significantly. Only NO ₃ ^? concentration increased significantly in springwaters. The catchment budgets revealed significant losses of base cations, Si and SO ₄ ^2? . These losses decreased over the period. Nitrogen was retained in the ecosystem. However, a longer record is needed to determine whether or not changes in surface water chemistry have resulted from short-term flow reductions or long-term changes in input-output ion budgets. This is specially true with N because the decline in SO ₄ ^2? output was accompanied by N accumulation.     

Effects of nitrogen dioxide on selected soil processes

Nitrogen dioxide gas was rapidly absorbed by soil. After a 15 min incubation at 25°C, soil at a moisture content of 16% absorbed 99% of the NO₂ introduced into the gas-phase volume of a closed system. The presence of microorganisms hatl no influence on the rate of absorption of the gas by soil. The absorption of NO₂ by sandy clay loam soil was not an oxygen- or temperature-dependent process nor did it depend upon the moisture content of the soil. These physical factors acquired significance only in determining the initial rate of absorption of the gas and the rate at which NO₂ diffused through the soil. Exposure of soil to NO₂ resulted in substantial increases in the levels of NO _inf2 ^sup? N in the soil. Chemical oxidation of the NO _inf2 ^sup? N resulted in an increase in NO _inf3 ^sup? N levels. During a 14-day incubation, NO _inf2 ^sup? N concentrations in sterile soil exposed to an atmosphere containing 100 μg ml^?1 of NO₂ decreased from 190 μg g^?1 of soil to 105 μg g^?1 with an accompanying increase in NO _inf3 ^sup? N from 2 μg g^? ₁ to 63 μg g^?1 of soil. Nitrogen dioxide severely inhibited the growth of both aerobic and anaerobic asymbiotic N₂-fixing bacteria in soil. After a 48 h incubation at 25°C, soil aggregates exposed to an atmosphere containing 100 μg ml^?1 of NO₂ contained 88% and 98% fewer aerobic and anaerobic N₂-fixing bacteria, respectively. C₂H₂-reduction measurements showed that nitrogenase synthesis and activity in artificial soil aggregates amended with 2% glucose were inhibited by 20% and 48%, respectively, when exposed to atmospheric concentrations of 35 and 3.5 μg ml^?1 of NO₂, respectively.