Project rain: Changing acid deposition to whole catchments. The first year of treatment

Project Rain (Reversing Acidification In Norway) is a 5-yr international research project aimed at investigating the effect on water and soil chemistry of changing acid deposition to whole catchments. The project comprises 2 parallel large-scale experimental manipulations -- artificial acidification at Sogndal and exclusion of acid rain at Risdalsheia. Treatment at Sogndal commenced April 1984 with the acidification of the snowpack by addition of H₂SO₄ (SOG2) and a 1:1 mixture of H₂SO₄ and HNO₃ (SOG4). Preliminary results indicate rapid and significant response in runoff chemistry to the acid treatment; pH decreased (to as low as 4.1 during snowmelt in 1984); SO₄, NO₃, and labile Al increased. Response during snowmelt 1985 was modest relative to 1984. At Risdalsheia treatment began in June 1984 with the mounting of the transparent panels on the roofs at KIM catchment (treatment by deacidified rain) and EGIL catchment (control with ambient acid rain). Preliminary data for the first year indicate that most runoff samples from KIM contain much lower NO₃ concentrations, about 20 to 30% lower SO₄ levels and pH 0.1 to 0.3 units higher than runoff from EGIL catchment. The treatments continue in 1985–87. Project RAIN provides experimental evidence bearing on target loading, reversibility of acidification, and the processes linking acid deposition, soil acidification and freshwater acidification.     

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.     

Extreme acidification in small catchments in southwestern Norway associated with a sea salt episode

During heavy storm events in January 1993 in the coastal areas of south-western Norway, a sea salt episode created extreme acidification in the afforested Svela catchment. Stream-water chloride increased sharply to eight times the normal concentration and the non-marine Na concentration was calculated to ?208 μeq L^?1. Negative values indicate that Na was retained in the soil profile. By ion-exchange processes this was largely compensated by an increase in stream-concentrations of Al and H⁺. Concentrations of inorganic monomeric Al increased from about 20 to 200 μeq L^?1 and pH decreased from 4.90 to 4.45. Due to the low pH and the dramatic increase in inorganic monomeric Al the water toxicity for aquatic organisms increased. Acidification associated with the storm was also observed in a forested and a non-forested catchment, but never reached the levels of the afforested catchment. The extra vulnerability of afforested catchments may be due to their ability to intercept larger amounts of sea salts than areas less dominated by dense tree stands. Although both pH and Al went back to normal levels for the area after 3–4 months the Na/Cl-relationship in cumulated transport values indicated a longlasting effect (> 2 years) on the soil profile. Reloading the soil profile with Al and H⁺ back to prestorm values will affect the catchments ability to mobilize these ions during future sea salt episodes. More frequent episodes will probably result in less acid and Al-rich stream-water during the episodes than documented here due to incomplete reacidification of the soils.     

Simulating acidification and recovery processes in experimental catchments with the ILWAS model

The Integrated Lake Watershed Acidification Study (ILWAS) model was used to simulate soil discharge chemistry at two neighboring experimental catchments. One catchment underwent deacidification because of the artificial application of deacidified precipitation whereas the other catchment received unaltered acidic precipitation. Simulated results reproduce the observed seasonal dynamics in the concentrations of base cations, NO ₃ ^? , Al, and H₄SiO ₄ ⁰ in soil discharges for both catchments. Simulated results also indicate that the export flux of base cations was decreased by 30% at the deacidification catchment in response to the decrease in acid deposition. However, simulated SO ₄ ^2? concentrations show decreases that are about 40% more rapid than were observed. Simulated organic acid concentrations were also substantially lower than those observed at the deacidification catchment, indicating that organic matter decomposition processes were not correctly simulated. Acid-base budgets for both 5 and 50-yr simulations indicate that acid displacement by base cations through ion exchange is the principal process delaying recovery of runoff alkalinity, whereas SO ₄ ^2? desorption has a minor role. Silicate weathering is the dominant acid-consuming process at both catchments. Criteria proposed here for assessing forecast reliability include reproducing seasonal dynamics in discharge chemistry, providing numerically accurate chemical concentrations when compared to monitoring data, and correctly predicting deacidification rate and extent. The ILWAS model generally meets these criteria, indicating that the model can produce a reliable forecast of the effects of acid deposition on the acid-base chemistry of surface waters given sufficient temporal data for confident optimization of the calibrated variables in the model.     

Water chemistry in forested catchments after topsoil treatment with liming agents in South Sweden

Critical loads of sulphur and nitrogen are exceeded in South Sweden, and nutritional imbalances are expected to appear with time in forests. During 1984 paired catchments were established in a northwestern-southeastern gradient in South Sweden. The aim was to study long-term liming effects on throughfall, soil water, groundwater and runoff. Dolomitic limestone and wood ash were tested at one locality, Hagfors (59° N). Three adjacent catchments were used; one reference area, one treated with dolomitic lime (0.5 kg/m²) in 1985, and one with wood fly ash (0.22 kg/m²) in 1988. The lime and the fly ash was granulated and applied by a helicopter in the end of May. Measurements concerning chemistry of the precipitation, throughfall, soil water and runoff has been conducted since spring 1984. The results showed that top-soil spreading of liming agents, besides the desired effects on soil chemistry, after some years also affected the quality of the recipient water. In the dolomitic lime treated catchment the positive effects were most obvious, with raised pH-, Ca-, and Mg-values and lowered Al-, Fe- and Mn-values. A positive trend regarding lower nitrogen (NO₃ ^?) leaching could also be calculated. Wood ash in the used amount affected only slowly, but after six years the runoff water indicated increased pH-values as well as increased Ca- and K-values and Ca/Al-ratios. Dolomitic lime in the amounts of 0.5 kg/m² was concluded to be sufficient to achieve positive effects in catchments of the present type. Wood ash in the amount of 0.22 kg/m² although enough for recycling purposes, was not sufficient enough in increasing pH in runoff to prevent acid leaching from the forest soils.     

Results from the Covered Catchment Experiment at Gårdsjön, Sweden, after Ten Years of Clean Precipitation Treatment

The Covered Catchment Experiment at Gårdsjön is a large scale forest ecosystem manipulation, where acid precipitation was intercepted by a 7000 m² plastic roof and replaced by `clean precipitation' sprinkled below the roof for ten years between 1991 and 2001. The treatment resulted in a strong positive response of runoff quality. The runoff sulphate, inorganic aluminium and base cations decreased, while there was a strong increase in runoff ANC and a moderate increase in pH. The runoff continued to improve over the whole duration of the experiment. The achieved quality was, however, after ten years still considerably worse than estimated pre-industrial runoff at the site. Stable isotopes of sulphur were analysed to study the soil sulphur cycling. At the initial years of the experiment, the desorption of SO₄ from the mineral soil appeared to control the runoff SO₄ concentration. However, as the experiment proceeded, there was growing evidence that net mineralisation of soil organic sulphur in the humus layer was an additional source of SO₄ in runoff. This might provide a challenge to current acidification models. The experiment convincingly demonstrated on a catchment scale, that reduction in acid deposition causes an immediate improvement of surface water quality even at heavily acidified sites. The improvement of the runoff appeared to be largely a result of cation exchange processes in the soil due to decreasing concentrations of the soil solution, while any potential change in soil base saturation seemed to be less important for the runoff chemistry over the short time period of one decade. These findings should be considered when interpreting and extrapolating regional trends in surface water chemistry to the terrestrial parts of ecosystems.     

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.     

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.     

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.     

RAIN project: Role of organic acids in moderating ph change following reduction in acid deposition

The RAIN project (Reversing Acidification In Norway) entails catchment-scale experimental manipulations to investigate the effect on water and soil chemistry of drastic changes in precipitation chemistry. At Risdalsheia in southernmost Norway wet deposition of acid is excluded from a 860-m² headwater catchment by means of a roof and “clean” precipitation is added beneath. Four years of acid exclusion (through June 1988) have resulted in lower concentrations of the strong acid anions NO₃ (from 35 to 7 ueq L^-1) and SO₄ (from 110 to 53 ueq L^-1) in runoff. The decline in strong acid anion concentrations has been compensated partially by a decrease in concentrations of base cations (55%) and partially by an increase in alkalinity (45%). pH has increased only slightly from 4.0 to 4.1. Organic acids have become increasingly important for the pH of runoff. Runoff from the shallow organic soils contains 10 to 20 mg C L^-1 total organic carbon (TOC). The concentration of organic anions (estimated from the ionic balance) has increased from about 22 ueq L^-1 in 1984 to 49 ueq L^-1 in 1987. This increase is due to increased dissociation of organic acids and not to change in TOC concentrations. The organic C in these acid samples apparently has a maximum charge density of about 4.5 ueq mg C^-1 and pK of about 4.     

Effects of Land Use Conversion from Native Forests to Exotic Plantations on Nitrogen and Phosphorus Retention in Catchments of Southern Chile

In six small catchments located at the Cordillera de la Costa in southern Chile (40° S), concentrations and fluxes of NO₃-N, NH₄-N, organic-N, total-N and total-P in bulk precipitation and runoff water were measured. The main objective of this study was to compare nitrogen and phosphorus retention of catchments with varying land cover of native forest and exotic plantations, in order to evaluate possible effects of land use change. Nitrate-N was the dominant fraction (>50%) of nitrogen loss, especially in the catchments dominated by exotic plantations. In the catchment with native forests, NO₃ ^? only contributed with 34% of the nitrogen loss and DON was the main output with 55%. Annual NO₃ ^? export was lower in the catchment with native forest compared to the catchments with exotic plantations where the streamflow output exceed the precipitation input. Average inputs of total-N were 2.6 kg ha^?1 year^?1 (DIN?=?1.4 kg ha^?1 year^?1, DON?=?1.2 kg ha^?1 year^?1) and outputs were 1.7 kg ha^?1 year^?1 (DIN?=?1.2 kg ha^?1 year^?1, DON?=?0.5 kg ha^?1 year^?1). Annual retention of total nitrogen fluctuated between 61% in a catchment dominated by native forests to 15% in catchments dominated by exotic plantations of Eucalyptus sp. Nitrogen retention was positively related with native forest coverage. The N retention capacity of the catchments could be both attributed to consequences of clear cutting practices and differences in vegetation cover.     

The significance of in-lake production of alkalinity

Alkalinity production in terrestrial and aquatic ecosystems of Canada, the U.S.A., Norway and Sweden is calculated from either strong acid titrations or budgets for base cations and strong acid anions, using mass-balance budgets. Where alkalinity budgets for lakes and their catchments are calculated in acid-vulnerable geological settings, in-lake processes often contribute more to lake alkalinity than yield from terrestrial catchments. Nitrate and sulfate removal, and Ca exchange with sediments are the predominant alkalinity generating mechanisms in lakes. Nitrate and sulfte removal rates increase as the concentrations of NO^? ₃ and SO₄ ^2? in lake water increase, so that in-lake acid neutralizing capacity increases as acid deposition increases. Both processes occur in sediments overlain by oxic waters, at rates which seem to be controlled primarily by diffusion.     

Response of spodic B horizons to acid precipitation in northernmost Fennoscandia

In northernmost Fennoscandia there is concern about the possible environmental effects of the sulphur emissions from Russian nickel smelters on the Kola Peninsula. The purpose of this study was to investigate to what extent the soils of this region may delay the response to pH changes through sulphate adsorption, and whether there are evidence for strong soil acidification effects. To this end 26 spodic B horizons were collected along a transect from northernmost Sweden to north-easternmost Norway, only 10 km from the Pechenganikel smelter. As the pH(H₂O) was > 4.8 in all soils, and as the exchangeable Ca/Al ratio was high, there were no evidence for strong soil acidification effects. Water-extractable SO₄ was clearly affected by the S deposition and thus SO₄ was at least partly mobile in the soils; it is therefore possible that soil solutions close to the smelter may have been acidified. In spite of this, sulphate adsorption was found to be more important than cation exchange reactions as a delaying process against soil acidification, at least in the top 10 cm of the B horizon. For the top 20 cm of the B horizon it was estimated that S04 adsorption can neutralize, on average, 700 mmol_c, m^?2 of acid before the pH is decreased to 4.4. Thus if the S deposition remains unchanged, decades are required to severely acidify most soils in the affected parts of Norway and Finland.     

Solute fluxes and sulfur cycling in forested catchments in SW Germany as influenced by experimental (NH4)2SO4 treatments

Results are presented from the research project Arinus which investigates biogeochemical cycling in Norway spruce (Picea abies KARST.) ecosystems in the Black Forest (SW Germany) and effects of experimental (NH_{4 2}SO₄ additions. The interaction of the terrestrial and aquatic system is assessed using an integrated approach which combines flux measurements in representative plots on the stand level with input-output budgets of small catchments. The approach, field installations and experimental manipulations are described. Results from element flux measurements in the untreated systems are presented and processes controlling N and S transformations are discussed for two catchments representing contrasting site conditions. Even though the S budget is negative for both systems there is a distinct difference in the relation between organicvs. inorganic S fractions in the soil. Sulfate mineralization and desorption, respectively are discussed as controlling processes. Sulfate retention is not only a function of soil properties, but also of water fluxes and pathways. The uptake of added SO ₄ ^2? was highly controlled by the counter-cation. Microbial N retention in the soil was highly influenced by the site management history. The extent of streamwater acidification was highly dependent on the transformations and mobility of N and S in the soils which in turn controlled cation leaching and alkalinity.     

Influence of Catchment Characteristics and Flood Type on Relationship Between Streamwater Chemistry and Streamflow: Case Study from Carpathian Foothills in Poland

The study aimed to determine the influence of catchment characteristics and flood type on the relationship between streamflow and a number of chemical characteristics of streamwater. These were specific electrical conductivity (SC), pH, the concentrations of main ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO ₃ ^? , SO ₄ ^2? , and Cl^?), and nutrients (NH ₄ ⁺ , NO ₂ ^? , NO ₃ ^? , and PO ₄ ^3? ). These relationships were studied in three small catchments with different geological structure and land use. Several flood types were distinguished based on the factors that initiate flooding and specific conditions during events. Geological factors led to a lower SC and main ion concentrations at a given specific runoff in catchments built of resistant sandstone versus those built of less resistant sediments. A lower concentration of nutrients was detected in the semi-natural woodland catchment versus agricultural and mixed-use catchments, which are strongly impacted by human activity. The strongest correlation between streamflow and the chemical characteristics of water was found in the woodland catchment. Different types of floods were characterized by different ion concentrations. In the woodland catchment, higher SC and higher concentrations of most main ions were noted during storm-induced floods than during floods induced by prolonged rainfall. The opposite was true for the agricultural and mixed-use catchments. During snowmelt floods, SC, NO ₃ ^? , and most main ion concentrations were higher when the soil was unfrozen in the agricultural and mixed-use catchments versus when the soil was frozen. In the case of the remaining nutrients, lower concentrations of NH ₄ ⁺ were detected during rain-induced floods than during snowmelt floods. The opposite was true of PO ₄ ^3? .     

Factors influencing fluoride concentrations in Norwegian lakes

The sources and mechanisms regulating fluoride (F) in Norwegian lake waters are studied using data from regional surveys of precipitation chemistry, lake water chemistry and bedrock geology. Fluoride concentrations in Norwegian lakes range from < 5 to 560 μg L^?1. Fluoride content in the bedrock is the most important factor controlling F levels in lake waters, as shown by significant differences in median value of F concentrations between lakes situated in different geological provinces. There are also weak but significant correlations between F in the lakes and components typical for weathering such as non-marine Ca, Mg, Na and K. The regional picture of F concentration in lake water shows elevated F concentrations in the acidified areas in southern and southeastern Norway compared to other regions of the country with comparable geology. There is a weak but significant correlation between F and SO₄, a typical indicator of acidification in surface water. Mass balance calculations in three catchments show that F is retained in soils in pristine areas, while F output exceeds precipitation input in acidified areas. This both demonstrates the strong retention capacity for F in soils and indicates that anthropogenic F added through polluted rain is a minor source of F in surface waters. Fluoride is mobilized in acidifies areas, probably due to complexation with Al.     

Impacts of acid precipitation on coniferous forest ecosystems

This paper summarizes the results from current studies in Norway. One main approach is the application of artificial acid ‘rain’ and of lime to field plots and lysimeters. Application during two growth seasons of 50 mm mo^?1 of ‘rain water’ of pH 3 to a podzol soil increased the acidity of the humus and decreased the base saturation. The reduction in base saturation was mainly due to leaching of Ca and Mg. Laboratory experiments revealed that decomposition of pine needles was not affected by any acid ‘rain’ treatment of the field plots. Liming slightly retarded the decomposition. No nitrification occurred in unlimed soils (pH 4.4-4.1). Liming increased nitrification. The soil enchytraeid (Ohgochaeta) fauna was not much affected by the acidification. Germination of spruce seeds in acidified mineral soil was negatively affected when soil pH was 4.0 or lower. Seedling establishment was even more sensitive to increasing soil acidity. Analysis of throughfall and stemflow water in southernmost Norway reveals that the total deposition of H₂SO₄ beneath spruce and pine is approximately two times the deposition in open terrain. A large part of this increase is probably due to dry deposition. Increased acidity of the rain seems to increase the leaching of cations from the tree crowns. Tree-ring analysis of spruce (Picea abies (L.) Karst.) and pine (Pinus sylvestris L.) has been based on comparisons between regions differently stressed by acid precipitation and also between sites presumed to differ in sensitivity to acidification. No effect that can be related to acid precipitation has yet been detected on diameter growth.     

Leaching of nitrogen from small forest catchments having different deposition and different stores of nitrogen

This investigation describes storage and leaching dynamics of nitrogen in three small forest catchments of the Swedish Integrated Monitoring Programme (IM). The sites, situated in North and South Sweden, have similar types of boreal forests and acid podsols although some properties of the catchments are very dissimilar. Total nitrogen deposition ranges from 0.2 to 2 g m^?2 yr^?1. Reported measurements include soil stores, tree biomass, soil water and stream water chemistry. The investigated catchments only leach small amounts of nitrogen. NH₄ ⁺ and NO₃ ^? concentrations are commonly low with sudden concentration peaks. Signs of developing nitrogen saturation are clearly visible at one of the sites. These signs are a low C/N ratio, nearly 20, in the upper part of the mineral soil and an unseasonal behaviour of NH₄ ⁺ and NO₃ ^? peaks in the soil solution. Properties of the soil organic matter, which contains huge stores of nitrogen, and the timing of hydrologic events, are crucial for the future leaching losses.     

The effects of drying and re-wetting and increased temperature on sulphate release from upland and wetland material

In central Ontario, elevated SO₄ concentrations and export have been measured in both upland and wetland-draining catchments following summer droughts, although the source of excess SO₄ is unclear. The objective of this study was to determine the effects of drying and re-wetting and temperature, respectively, on the release of SO₄ from the primary S pools in wetlands (Sphagnum and peat) and uplands (forest floor and mineral soil), using material collected from the PC1 catchment in Haliburton County, and from catchment S50 in the Turkey Lakes Watershed. Peat exhibited the most marked response to drying of the four materials considered, and within 24 h of re-wetting dried peat from both catchments released 3-4 times more SO₄ (50-67 mg kg⁻¹ S-SO₄) than continuously moist peat (16 mg kg⁻¹ S-SO₄), although temperature had only a marginal effect on SO₄ concentrations. There was no immediate response of Sphagnum to either drying or temperature, although S-SO₄ concentrations in Sphagnum tended to increase over the 30-day (d) incubation. There was a small but immediate increase in S-SO₄ concentrations in forest floor material (LFH) from both catchments within the first 24 h of incubation, which was greatest in treatments that were dried and/or incubated at a higher temperature. In contrast, neither temperature nor drying appeared to affect SO₄ release from mineral soil collected from either site. Results of laboratory incubations suggest that increases in SO₄ concentration that have been reported in wetland-draining streams immediately following summer dry periods may be quantitatively explained by drying and re-wetting of peat rather than increased mineralization in Sphagnum. Similarly, the higher SO₄ concentrations that have been measured in upland streams following summer droughts may in part be due to enhanced SO₄ release from the forest floor following drying and re-wetting. In contrast, while the mineral soil constitutes a large pool of total S, it does not appear to be responsive to changes in moisture or temperature in the short-term (<30 d) and therefore likely does not contribute to reported climate-related temporal variations in stream SO₄.