Disruption of sulfur cycling and acid neutralization in lakes at low pH

Experimental acidification of a softwater lake to below pH 5 fundamentally changed the sulfur cycle and lowered internal alkalinity generation (IAG). Prior to reaching pH 4.5, the balance of sulfur reduction and oxidation reactions within the lake was in favour of reduction, and the lake was a net sink for sulfate. In the four years at pH 4.5 the balance of reduction and oxidation reactions was in favour of oxidation, and there was a net production of sulfate (SO₄ ^2–) within the lake. Evidence indicating a decrease in net SO₄ ^2– reduction at pH 4.5 was also obtained in an anthropogenically acidified lake that had been acidified for many decades. In both lakes, the decrease in net SO₄ ^2– reduction appeared to be linked not to a simple inhibition of SO₄ ^2– reduction but rather to changes in benthic ecosystem structure, especially the development of metaphytic filamentous green algae, which altered the balance between SO₄ ^2– reduction and sulfur oxidation.At pH's above 4.5, net SO₄ ^2– reduction was the major contributor to IAG in the experimental lake, as it is in many previously studied lakes at pH 5 and above. At pH 4.5, the change in net annual SO₄ ^2– reduction (a decrease of 110%) resulted in a 38% decrease in total IAG. Because of the important role of net SO₄ ^2– reduction in acid neutralization in softwater lakes, models for predicting acidification and recovery of lakes may need to be modified for lakes acidified to pH <5.     

Dimethylsulfide and methane thiol in sediment porewater of a Danish estuary

Seasonal variation of dimethylsulfide (DMS) and methane thiol (MSH) concentrations in sediment porewater was determined in a Danish estuary. Dimethylsulfide (DMDS) was never found. Detectable DMS levels of up to 0.1 M were found only in the summer and only within the upper 5 cm of the sediment. The DMS accumulation was probably associated with decomposing fragments of macro-algae in the surface layer. Significant MSH accumulation of up to 1 M was found only in the deep, CH₄-rich sediment below the SO₄ ^2- zone. With depth, a detectable MSH level could thus be observed below the 1 mM SO₄ ^2--isopleth which also marked the SO₄ ^2--CH₄ transition. The transition zone was located deeper in the sediment in winter (20–25 cm depth) than in summer (5–10 cm depth). The absence of MSH in the SO₄ ^2- zone could be due to rapid utilization of the compound by SO₄ ^2--reducing bacteria. A possible involvement of MSH in anaerobic CH₄ oxidation at the transition zone is discussed; CH₄ and sulfide (HS^- form, pH 7) are proposed to form MSH and H₂ which in turn may be metabolized by, e.g. SO₄ ^2--reducing bacteria.     

Sulfate reduction in fine-grained sediments in the Eastern Scheldt,southwest Netherlands

Sulfate reduction and sulfide accumulation were examined in fine-grained sediments from rapidly accreting abandoned channels and mussel culture areas in the Eastern Scheldt, which covered 4 and 5% of the total surface area, respectively.Reduction rates were measured in batch experiments in which the SO₄ ^2– depletion was measured during anoxic incubation. The reduction rates in summer varied between 14–68 mmol SO₄ ^2– m^–2 day^–1 and were related to the sedimentation rate. In the most rapidly accreting channels, SO₄ ^2– was exhausted below 15–50 cm and methanogenesis became the terminal process of organic carbon oxidationOne-dimensional modelling of sulfate profiles in mussel banks indicated that the subsurface influx of SO₄ ^2– was almost of the same order as the diffusive flux at the sediment-seawater interface, during the initial stages of the mussel bank accretion. The energy dissipation of waves and tidal currents on the mussel bank surface increased the apparent sediment diffusivity up to 3-fold, especially in the winterThe results indicate that acid volatile sulfide (AVS) was the major, in-situ reduced, sulfur compound in the sediment. The sulfidation of easily extractable iron was nearly complete. Pyrite concentrations (40–80 M S cm^–3) were as high as the AVS concentrations, but there was apparently no in-situ transformation of AVS into pyrite. The detrital pyrite originated from eroding marine sediments elsewhere     

Cycling of inorganic and organic sulfur in peat from Big Run Bog,West Virginia

Total S concentration in the top 35 cm of Big Run Bog peat averaged 9.7 mol·g — wet mass^–1 (123 mol·g dry mass^–1). Of that total, an average of 80.8% was carbon bonded S, 10.4% was ester sulfate S, 4.5% was FeS₂­S, 2.7% was FeS­S, 1.2% was elemental S, and 0.4% was SO₄ ^2–­S. In peat collected in March 1986, injected with³⁵S­SO₄ ^2– and incubated at 4 °C, mean rates of dissimilatory sulfate reduction (formation of H₂S + S⁰ + FeS + FeS₂), carbon bonded S formation, and ester sulfate S formation averaged 3.22, 0.53, and 0.36 nmol·g wet mass^–1·h^–1, respectively. Measured rates of sulfide oxidation were comparable to rates of sulfate reduction. Although dissolved SO₄ ^2– concentrations in Big Run Bog interstitial water (< 200 µM) are low enough to theoretically limit sulfate reducing bacteria, rates of sulfate reduction integrated throughout the top 30–35 cm of peat of 9 and 34 mmol·m^–2·d^–1 (at 4 °C are greater than or comparable to rates in coastal marine sediments. We suggest that sulfate reduction was supported by a rapid turnover of the dissolved SO₄ ^2– pool (average turnover time of 1.1 days). Although over 90% of the total S in Big Run Bog peat was organic S, cycling of S was dominated by fluxes through the inorganic S pools.     

Reduction of Sulfur Compounds in the Sediments of a Eutrophic Lake Basin

Concentrations of various sulfur compounds (SO₄²⁻, H₂S, S⁰, acid-volatile sulfide, and total sulfur) were determined in the profundal sediments and overlying water column of a shallow eutrophic lake. Low concentrations of sulfate relative to those of acid-volatile sulfide and total sulfur and a decrease in total sulfur with sediment depth implied that the contribution of dissimilatory sulfur reduction to H₂S production was relatively minor. Addition of 1.0 mM Na₂³⁵SO₄ to upper sediments in laboratory experiments resulted in the production of H₂³⁵S with no apparent lag. Kinetic experiments with ³⁵S demonstrated an apparent K_m of 0.068 mmol of SO₄²⁻ reduced per liter of sediment per day, whereas tracer experiments with ³⁵S indicated an average turnover time of the sediment sulfate pool of 1.5 h. Total sulfate reduction in a sediment depth profile to 15 cm was 15.3 mmol of sulfate reduced per m² per day, which corresponds to a mineralization of 30% of the particulate organic matter entering the sediment. Reduction of ³⁵S⁰ occurred at a slower rate. These results demonstrated that high rates of sulfate reduction occur in these sediments despite low concentrations of oxidized inorganic compounds and that this reduction can be important in the anaerobic mineralization of organic carbon.     

The influence of lime and biological activity on sediment pH,redox and phosphorous dynamics

The addition of powdered limestone to intact sediment cores from oligotrophic, acid Lake Hovvatn caused pH to increase, redox potential (E₇) to drop, and permitted net precipitation of phosphorous (P) from the water column. Significant pH increase was found to a sediment depth of 6 cm and a maximum increase in pH from 4.9 to 6.5 was found at a depth of 0.5 cm when dosed with 36 g m^–2 of lime. Such pH increase creates important changes in sediment equilibrium chemistry and enhances habitat suitability. In the case of Hovvatn, however, sediments would consume only 5 kg of the 91 tons of applied limestone. Superficial sediments remained oxidized, but below 0.5 cm, E₇ in limed sediment declined significantly more than in unlimed sediments, with a maximum difference of 102 mV versus –66 mV at a depth of 6 cm in unlimed and limed cores, respectively. Abiotic reactions account for 82 ± 54% of this reduction and the remainder is due to the oxidation of organic matter by bacteria. Precipitation of CaSO₄, reduction of the sediments by organic compounds at elevated pH and inhibition of the downward diffusion of O₂ by the limestone powder are potential abiotic mechanisms which could drive E₇ down. Enhanced P release was not found at lowered E₇, and supernatent TP concentrations dropped from 11.7 to 4.4 µg P l^–1. More P was swept from solution in cores which recieved larger lime doses. The presence of chironomids caused sediment pH to increase by as much as 1.2 pH units, presumably due to NH₄ release, reduced sediment E₇ by as much as 171 mV and facilitated TP release during the first 17 d of core incubation. Field measurements of vertical distributions of sediment pH and E₇ before and after the liming of Hovvtn corroborated laboratory findings.     

Sulfate reduction and S-oxidation in a moorland pool sediment

In an oligotrophic moorland pool in The Netherlands, S cycling near the sediment/water boundary was investigated by measuring (1) SO₄ ^2– reduction rates in the sediment, (2) depletion of SO₄ ^2– in the overlying water column and (3) release of³⁵S from the sediment into the water column. Two locations differing in sediment type (highly organic and sandy) were compared, with respect to reduction rates and depletion of SO₄ ^2– in the overlying water.Sulfate reduction rates in sediments of an oligotrophic moorland pool were estimated by diagenetic modelling and whole core³⁵SO₄ ^2– injection. Rates of SO₄ ^2– consumption in the overlying water were estimated by changes in SO₄ ^2– concentration over time in in situ enclosures. Reduction rates ranged from 0.27–11.2 mmol m^–2 d^–1. Rates of SO₄ ^2– uptake from the enclosed water column varied from –0.5, –0.3 mmol m^–2 d^–1 (November) to 0.43–1.81 mmol m^–2 d^–1 (July, August and April). Maximum rates of oxidation to SO₄ ^2– in July 1990 estimated by combination of SO₄ ^2– reduction rates and rates of in situ SO₄ ^2– uptake in the enclosed water column were 10.3 and 10.5 mmol m^–2 d^–1 at an organic rich and at a sandy site respectively.Experiments with³⁵S^2– and³⁵SO₄ ^2– tracer suggested (1) a rapid formation of organically bound S from dissimilatory reduced SO₄ ^2– and (2) the presence of mainly non SO₄ ^2–-S derived from reduced S transported from the sediment into the overlying water. A³⁵S^2– tracer experiment showed that about 7% of³⁵S^2– injected at 1 cm depth in a sediment core was recovered in the overlying water column.Sulfate reduction rates in sediments with higher volumetric mass fraction of organic matter did not significantly differ from those in sediments with a lower mass fraction of organic matter.Corresponding author     

Sulfur cycling in the water column of Little Rock Lake, Wisconsin

The S cycle in the water column of a small, soft-water lake was studied for 9 years as part of an experimental study of the effects of acid rain on lakes. The two basins of the lake were artificially separated, and one basin was experimentally acidified with sulfuric acid while the other served as a reference or control. Spatial and seasonal patterns of sulfate uptake by plankton (53–70 mmol m^–2 yr^–1), deposition of sulfur to sediments in settling seston (53 mmol m^–2 yr^–1), and sulfate diffusion (0–39 mmol m^–2 yr^–1) into sediments were examined. Measurements of inputs (12–108 mmol m^–2 yr^–1) and outputs (5.5–25 mmol m^–2 yr^–1) allowed construction of a mass balance that was then compared with rates of S accumulation in sediments cores (10–28 mmol m^–2 yr^–1) and measured fluxes of S into the sediments. Because of the low SO₄ ^2– concentrations (µmole L^–1) in the lake, annual uptake by plankton (53–70 mmol m^–2 yr^–1) represented a large fraction (>50%) of the SO₄ ^2– inventory in the lake. Despite this large flux through the plankton, only small seasonal fluctuations in SO₄ ^2– concentrations (µmole L^–1) were observed; rapid mineralization of organic matter (half-life <3 months) prevented sulfate depletion in the water column. The turnover time for sulfate in the water column is only 1.4 yr; much less than the 11-yr turnover time of a conservative ion in this seepage lake. Sulfate diffusion into and reduction in the sediments (0–160 µmole m^–2 d^–1) caused SO₄ ^2– depletion in the hypolimnion. Modeling of seasonal changes in lake-water SO₄ ^2– concentrations indicated that only 30–50% of the diffusive flux of sulfate to the sediments was permanently incorporated in solid phases, and about 15% of sulfur in settling seston was buried in the sediments. The utility of sulfur mass balances for seepage lakes would be enhanced if uncertainty about the deposition velocity for both sulfate aerosols and SO₂, uncertainty in calculation of a lake-wide rate of S accumulation in sediments, and uncertainty in the measured diffusive fluxes could be further constrained.     

Carbon and Electron Flow in Mud and Sandflat Intertidal Sediments at Delaware Inlet, Nelson, New Zealand

An investigation of carbon and electron flow in mud and sandflat intertidal sediments showed that the terminal electron acceptor was principally sulfate and that the carbon flow was mainly to CO₂. Studies with thin layers of sediment exposed to H₂ showed that methane production accounted for virtually none of the H₂ utilized, whereas sulfate reduction accounted for a major proportion of the gas uptake. At all sampling sites except one (site B7), rates of methanogenesis were low but sulfate concentrations in the interstitial water were high (>18 mM). At site B7, the sulfate concentrations declined with depth from 32 mM at 2 cm to <1 mM at 10 cm or below, and active methanogenesis occurred in the low-sulfate zone. Sulfate-reducing activity at this site initially decreased and then increased with depth so that elevated rates occurred in both the active and nonactive methanogenic zones. The respiratory index (RI) [RI = ¹⁴CO₂/(¹⁴CO₂ + ¹⁴CH₄)] for [2-¹⁴C]acetate catabolism at site B7 ranged from 0.98 to 0.2 in the depth range of 2 to 14 cm. Addition of sulfate to sediment from the low-sulfate zone resulted in an increase in RI and a decrease in methanogenesis. At all other sites examined, RI ranged from 0.97 to 0.99 and was constant with depth. The results suggested that although methanogenesis was inhibited by sulfate (presumably through the activity of sulfate-reducing bacteria), it was not always limited by sulfate reduction.     

Inorganic sulfur turnover in oligohaline estuarine sediments

Inorganic sulfur turnover was examined in oligohaline (salinity < 2 g kg^-1) Chesapeake Bay sediments during the summer. Cores incubated for < 3 hr exhibited higher sulfate reduction (SR) rates (13–58 mmol m^-2 d^-1) than those incubated for 3–8 hr (3–8 mmol m^-2 d^-1). SR rates (determined with³⁵SO ₄ ^2- ) increased with depth over the top few cm to a maximum at 5 cm, just beneath the boundary between brown and black sediment. SR rates decreased below 5 cm, probably due to sulfate limitation (sulfate < 25 μM). Kinetic experiments yielded an apparent half-saturating sulfate concentration (K_s) of 34 μM, ≈ 20-fold lower than that determined for sediments from the mesohaline region of the estuary. Sulfate loss from water overlying intact cores, predicted on the basis of measured SR rates, was not observed over a 28-hr incubation period. Reduction of³⁵SO ₄ ^2- during diffusion experiments with intact core segments from 0–4 and 5–9 cm horizons was less than predicted by non-steady state diagenetic models based on³⁵SO ₄ ^2- reduction in whole core injection experiments. The results indicate that net sulfate flux into sediments was an order of magnitude lower than the gross sulfur turnover rate. Solid phase reduced inorganic sulfur concentrations were only 2–3 times less than those in sediments from the mesohaline region of the Bay, despite the fact that oligohaline bottom water sulfate concentrations were 10-fold lower. Our results demonstrate the potential for rapid SR in low salinity estuarine sediments, which are inhabited by sulfate-reducing bacteria with a high affinity for sulfate, and in which sulfide oxidation processes replenish the pore water sulfate pool on a time scale of hours.     

Anoxic degradation of biogenic debris in sediments of eutrophic subalpine Lake Bled (Slovenia)

Anoxic degradation of sedimentary biogenic debris using closed sediment incubation experiments was studied in eutrophic subalpine Lake Bled (NW Slovenia) which, for most of the year, has an anoxic hypolimnion. Production rates of dissolved inorganic carbon (DIC), NH₄ ⁺, PO₄ ^3- and dissolved Si, and reduction rates of SO₄ ^2- were measured and anoxic mineralization rates were modelled using G-model. The depth profiles indicated major mineralization of biogenic debris and SO₄ ^2- reduction near the sediment surface. A comparison between depth integrated anoxic mineralization rates and diffusive benthic fluxes of DIC, NH₄ ⁺ and PO₄ ^3- showed that the anoxic incubation experiments provide a good estimate of N degradation of biogenic debris. The contributions of SO₄ ^2- reduction and acetate fermentation in NH₄ ⁺ production are about 30 and 70%, respectively. The DIC production accounted for only 15% of DIC benthic flux, indicating that methanogenesis and oxidation of methane provides 80% of this flux. Only about 30% of PO₄ ^3- was released because phosphate precipitated in the closed incubation experiments. The depth integrated production of Si accounts for 70–80% of Si benthic fluxes indicating intense dissolution of biogenic Si in the surficial lake sediment.     

Localization of sulfate reduction in planted and unplanted rice field soil

Rates of in situ sulfate reduction (SRR) in planted and unplanted rice fieldsoil were measured by the ³⁵SO^2– ₄-radiotracermethod using soil microcosms. The concentration of ³⁵SO^2– ₄ decreased exponentially with time.However, time course experiments indicated that incubation times of10–30 min were appropriate for measurements of SRRusing a single time point in routine assays. Unplanted microcosmsshowed high SRR of 177 nmol cm^-3 d^-1 inthe uppermost centimeter where average sulfate concentrations were<33 µM. Fine scaled measurements (1 mmresolution) localized highest SRR (<100 nmol cm^-3d^-1) at the oxic/anoxic interface at 2–5 mmdepth. In planted rice field soil, SRR of <310 nmolcm^-3 d^-1 were observed at 0–2cm depth. Sulfate reduction rates were determined at a millimeter-scalewith distance to a two dimensional root compartment. The SRR was highestat 0–1.5 mm distance to the root layer with rates up to500 nmol cm^-3 d^-1, indicating a highstimulation potential of the rice roots. SRR seemed to be mainlydependent on the in situ sulfate porewater concentrations. At thesoil surface of unplanted microcosms sulfate concentration decreasedfrom <150 µM to <10 µM within the first 8 mm of depth. In planted microcosmssulfate concentration varied from 87–99 µMsulfate at the 0–3 mm distance to the root layer to48–62 µM sulfate at a root distance>4 mm from the roots.The depth distribution of inorganic sulfur compounds was determinedfor planted and unplanted rice field soil. Sulfate, acid volatilesulfide (AVS) and chromium reducible sulfide (CRS) were up to 20 foldhigher in planted than in unplanted microcosms. CRS was the majorinsoluble sulfur fraction with concentrations >1.7µmol cm^-3. Organic sulfur accounted for25–46% of the total sulfurpresent (269 µg/g dw) in an unplanted microcosm.The biogeochemical role of sulfate reduction forshort-term accumulation of inorganic sulfur compounds(FeS, FeS_{_2} and S°) in rice soil wasdetermined in a time course experiment with incubationperiods of 5, 10, 20, 30 and 60 min. The relativedistribution of CRS and AVS formation showedlittle depth dependence, whereas the formation of³⁵S° seemed to be the highest in themore oxidized upper soil layers and near the root surface.AV³⁵S was the first major product of sulfatereduction after 20–30 min, whereas CR³⁵Swas formed, as AV³⁵S and ³⁵S°decreased, at longer incubation periods of >30 min.     

Sulfate Reduction in Freshwater Sediments Receiving Acid Mine Drainage

One arm of Lake Anna, Va., receives acid mine drainage (AMD) from Contrary Creek (SO₄²⁻ concentration = 2 to 20 mM, pH = 2.5 to 3.5). Acid-volatile sulfide concentrations, SO₄²⁻ reduction rates, and interstitial SO₄²⁻ concentrations were measured at various depths in the sediment at four stations in four seasons to assess the effects of the AMD-added SO₄²⁻ on bacterial SO₄²⁻ reduction. Acid-volatile sulfide concentrations were always an order of magnitude higher at the stations receiving AMD than at a control station in another arm of the lake that received no AMD. Summer SO₄²⁻ reduction rates were also an order of magnitude higher at stations that received AMD than at the control station (226 versus 13.5 mmol m⁻² day⁻¹), but winter values were inconclusive, probably due to low sediment temperature (6°C). Profiles of interstitial SO₄²⁻ concentrations at the AMD stations showed a rapid decrease with depth (from 1,270 to 6 μM in the top 6 cm) due to rapid SO₄²⁻ reduction. Bottom-water SO₄²⁻ concentrations in the AMD-receiving arm were highest in winter and lowest in summer. These data support the conclusion that there is a significant enhancement of SO₄²⁻ reduction in sediments receiving high SO₄²⁻ inputs from AMD.     

Kinetic Studies of Bacterial Sulfate Reduction in Freshwater Sediments by High-Pressure Liquid Chromatography and Microdistillation

Indirect photometric chromatography and microdistillation enabled a simultaneous measurement of sulfate depletion and sulfide production in the top 3 cm of freshwater sediments to be made. The simultaneous measurement of sulfate depletion and sulfide production rates provided added insight into microbial sulfur metabolism. The lower sulfate reduction rates, as derived from the production of acid-volatile ³⁵S²⁻ only, were explained by a conversion of this pool to an undistillable fraction under acidic conditions during incubation. A mathematical model was applied to calculate sulfate reduction from sulfate gradients at the sediment-water interface. To avoid disturbance of these gradients, the sample volume was reduced to 0.2 g (wet weight) of sediment. Sulfate diffusion coefficients in the model were determined (D_s = 0.3 × 10⁻⁵ cm² s⁻¹ at 6°C). The results of the model were compared with those of radioactive sulfate turnover experiments by assessing the actual turnover rate constants (2 to 5 day⁻¹) and pool sizes of sulfate at different sediment depths.     

Methane Metabolism in a Temperate Swamp

Comparisons between in situ CH₄ concentration and potential factors controlling its net production were made in a temperate swamp. Seasonal measurements of water table level and depth profiles of pH, dissolved CH₄, CO₂, O₂, SO₄^2-, NO₃^-, formate, acetate, propionate, and butyrate were made at two adjacent sites 1.5 to 2 m apart. Dissolved CH₄ was inversely correlated to O₂ and, in general, to NO₃^- and SO₄^2-, potential inhibitors of methanogenesis. At low water table levels (August 1992), maximal CH₄ (2 to 4 μM) occurred below 30 cm, whereas at high water table levels (October 1992) or under flooded conditions (May 1993), CH₄ maxima (4 to 55 μM) occurred in the top 10 to 20 cm. Higher CH₄ concentrations were likely supported by inputs of fresh organic matter from decaying leaf litter, as suggested by high acetate and propionate concentrations (25 to 100 μM) in one of the sites in fall and spring. Measurements of potential CH₄ production (and consumption) showed that the highest rates generally occurred in the top 10 cm of soil. Soil slurry incubations confirmed the importance of organic matter to CH₄ production but also showed that competition for substrates by nonmethanogenic microorganisms could greatly attenuate its effect.     

Sulfate reduction processes in sediments at different sites in Lake Kinneret,Israel

Lake Kinneret, Israel, is a warm (13–30°C) monomictic lake that stratifies in April and turns over in December. Between January and June each year, a heavy bloom (up to 250 g wet weight n^–2 2) of the dinoflagellate Peridinium gatunense dominates the phytoplankton biomass. In early summer, the bloom collapses, and the sinking Peridinium biomass serves as a trigger for intense sulfate-reduction activity throughout the hypolimnion and within the sediments. The availability of organic matter and sulfate was high shortly after the bloom crash and the beginning of stratification and was lowest in December before overturn. Sulfate-reduction rates at three different sites in the lake were studied. In the sediments, the rates varied seasonally and among stations from 5 to 1600 nmol SO₄ ^–2 reduced cm^–3 day^–1, with respect to the distance from the Jordan River, depth, organic content, and stratification period. During years of low lake water levels, intense sulfate reduction occurred in the hypolimnion, resulting in anoxia and high concentrations of H₂S (>400 m). In years with high water levels, early bloom, and delayed stratification, higher rates of sulfate reduction were recorded in the sediments, probably as a result of a greater fraction of the primary production (organic matter) reaching the bottom. Correspondence to: O. Hadas.     

Sulfur dynamics in mineral horizons of two northern hardwood soils. A column study with 35S

Sulfur dynamics of two Spodosols were ascertained using soil columns constructed from homogenized mineral soil from nothern hardwood ecosystems at the Huntington Forest (HF) in the Adirondack Mountains of New York and Bear Brook Watershed in Maine (BBWM). Columns were leached for 20 weeks with a simulated throughfall solution with³⁵SO₄ ^2-. Sulfur constituents were similar to those of other Spodosols, with the organic S fractions (C-bonded S and ester sulfate) constituting over 90% of total S. HF soil columns had higher total S (14.9 mol S g^-1) than that for the BBWM soil columns (7.4 mol g^-1) primarily due to higher C-bonded S in the former.Initially, adsorbed SO₄ ^- accounted for 5 and 4% of total S for the BBWM and HF soil columns, respectively. After 20 weeks, adsorbed SO₄ ^2- decreased (81%) in BBWM and increased (33%) in HF soil columns. For both HF and BBWM soil columns, C-bonded S increased and ester sulfate decreased, but only for HF columns was there a net mineralization of organic S (5.6% of total S). The greatest decrease in ester sulfate occurred at the top of the columns.Leaching of³⁵S was less than 0.5% of the³⁵S added due to its retention in various S constituents. There was an exponential decrease in³⁵S with column depth and most of the radioisotope was found in C-bonded S (70–88 and 70–91% for BBWM and HF, respectively). The rapid turnover of adsorbed SO^2- ₄ was reflected in its high specific activity (834 and 26 kBq mol^-1 S for BBWM and HF, respectively). The lower specific activity of adsorbed SO₄ ^2- in HF was attributable to greater isotopic dilution by non-radioactive SO^2- ₄ derived from greater organic S mineralization in the HF versus the BBWM columns.Both soil columns initially had high levels of NO^- ₃ which resulted in the generation of H⁺ and net retention of SO₄ ^2- in the early phase of the experiment due to pH dependent sulfate adsorption; later NO₃ ^- decreased and SO₄ ^2- was desorbed. Leaching of NIO₃ ^- and SO₄ ^2- was correlated with losses of Mg²⁺ and Ca²⁺ of which the latter was the dominant cation.Analyses using both S mass balances and radioisotopes corroborate that for BBWM soil columns, SO^2- ₄ adsorption-desorption dominated the S biogeochemistry while in HF soil columns, organic S mineralization-immobilization processes were more important. It is suggested that similar techniques can be applied to soils in the field to ascertain the relative importances of SO₄ ^2- adsorption processes and organic S dynamics.     

Input-output budgets at Langtjern,a small acidified lake in southern Norway

Precipitation and streamwater volume and chemical composition have been measured since 1974 at Langtjern, a small, acid (pH 4.6–4.8) lake on granitic-gneissic bedrock in coniferous forest located ca. 100 km north of Oslo, Norway. The area receives acid precipitation (weighted average pH 4.28). The 7-year input-output budgets for major ions at two terrestrial subcatchments indicate that for Na, K, SO₄ and Cl outputs approximately equal inputs, for H⁺, NH₄ and NO₃ outputs are much less than inputs, and for Ca, Mg and Al outputs greatly exceed inputs. The sulfate budgets (which include estimated dry deposit) indicate that the terrestrial catchment retains about 20% of the incoming sulfate, perhaps due to absorption in the soil, plant uptake, reduction and storage in peaty areas or reduction and release of H₂S to the atmosphere. The budgets for Langtjern lake itself indicate that for most components output equals inputs to within 10%, i.e. these compounds simply pass through the lake. For H⁺, and possibly NH₄ and NO₃, inputs exceed outputs. Because gaseous phases are not measured the N budgets are uncertain. A mechanism that leads to ‘retention’ of both H⁺ and SO₄ is sulfate reduction and incorporation of sulfides in the lake sediments. Such has been documented in the experimentally-acidified Lake 223, Experimental Lakes Area, Ontario, Canada. Although there is no evidence suggesting the development of anoxic bottom waters at Langtjern, such reduction might occur at the water-sediment interface and in the sediments. The budgets for the pollutant components H⁺ and SO₄ at Langtjern differ substantially from those at the relatively unaffected Lake 239, in the Experimental Lakes Area.     

Acid-base chemistry and aluminum transport in an acidic watershed and pond in New Hampshire

Cone Pond is one of the few acidic, clear-water ponds in the White Mountains of New Hampshire, a region dominated by high inputs of strong acids from atmospheric deposition and low base content of bedrock. Monitoring was conducted for 13 months to compare and contrast the acid-base chemistry of the terrestrial and aquatic portions of the watershed. Variations in Al concentration and speciation in drainage waters were correlated with changes in the supply of naturally occurring organic ligands. During the study period, the pond retained 28% of Al inputs, including nearly half of the inputs of organically complexed Al. Chemical equilibrium calculations indicated that the entire water-column was oversaturated with respect to the solubility of synthetic gibbsite during summer, as was the hypolimnion during winter. Retention of Al resulted from an increase in pH in the hypolimnion concomitant with SO₄ ^2– reduction, and from loss of organic anions in epilimnetic waters. Acid neutralizing capacity (ANC) generated in the pond primarily through SO₄ ^2– reduction and base cation (C _B ) release was balanced by ANC consumed as a result of Al retention.     

Dynamics of Bacterial Sulfate Reduction in a Eutrophic Lake

Bacterial sulfate reduction in the surface sediment and the water column of Lake Mendota, Madison, Wis., was studied by using radioactive sulfate (³⁵SO₄²⁻). High rates of sulfate reduction were observed at the sediment surface, where the sulfate pool (0.2 mM SO₄²⁻) had a turnover time of 10 to 24 h. Daily sulfate reduction rates in Lake Mendota sediment varied from 50 to 600 nmol of SO₄²⁻ cm⁻³, depending on temperature and sampling date. Rates of sulfate reduction in the water column were 10³ times lower than that for the surface sediment and, on an areal basis, accounted for less than 18% of the total sulfate reduction in the hypolimnion during summer stratification. Rates of bacterial sulfate reduction in the sediment were not sulfate limited at sulfate concentrations greater than 0.1 mM in short-term experiments. Although sulfate reduction seemed to be sulfate limited below 0.1 mM, Michaelis-Menten kinetics were not observed. The optimum temperature (36 to 37°C) for sulfate reduction in the sediment was considerably higher than in situ temperatures (1 to 13°C). The response of sulfate reduction to the addition of various electron donors metabolized by sulfate-reducing bacteria in pure culture was investigated. The degree of stimulation was in this order: H₂ > n-butanol > n-propanol > ethanol > glucose. Acetate and lactate caused no stimulation.