AQUATIC EFFECTS OF ACIDIC DEPOSITION - CHAPTER 8 pot

175 8 Experimental Manipulation Studies The scientific and political value of experimental field studies of acidifica- tion processes have been well recognized for some time (Wright, 1991). Other sources of quantitative information, including survey results, moni- toring, laboratory studies, and modeling are insufficient, on their own, as a foundation for understanding and predicting acidification and recovery responses. The results of surveys of water quality in areas impacted by acidic deposition, as well as areas not impacted by acidic deposition (c.f., Sullivan, 1990) have been used for two decades as evidence of acidification effects. Interpretation of such data is always compromised, however, by dif- ferences between the impacted and unimpacted areas that are independent of acidic deposition. Such differences may include aspects of soils, geology, climate, land use, and hydrology that in some cases can overwhelm the effects of S or N deposition. Acidification of aquatic and terrestrial ecosystems operates on time scales of many years to many decades. There are few time series of monitoring data available with long enough period of record to confirm the validity of our understanding of key acidification processes. Furthermore, interpretation of time series data is often uncertain because a variety of mechanisms can pro- vide plausible explanations of observed responses. Concurrent changes in climate, land use, disturbance, or other factors confound the interpretation of monitoring results. There has been a large increase during the past decade in the amount of experimental research being conducted on the environmental effects of atmo- spheric deposition, especially of N. This research has been initiated mostly in Europe; little comparable work has been conducted in the U.S. The experi- mental approach has shifted heavily into the area of whole-ecosystem exper- imental manipulations that have been and are being conducted across gradients of atmospheric deposition and other environmental factors throughout northern Europe (Sullivan, 1993). Individual investigators have, in many cases, been working at a variety of sites, thus enhancing the compa- rability of the resulting databases. Manipulations have focused primarily on coniferous forest ecosystems, and have involved 1416/frame/C08 Page 175 Wednesday, February 9, 2000 2:18 PM © 2000 by CRC Press LLC 176 Aquatic Effects of Acidic Deposition 1. Increasing deposition of S and/or N. 2. Excluding the previously existing deposition via construction of roofs over entire forested plots or mini-catchments. 3. Manipulating climatic factors, especially water availability. This research has been highly interdisciplinary, and experiments were designed to continue for relatively long periods of time (i.e., 5 to 10 years). Manipulation studies conducted thus far have clearly demonstrated that a long-term commitment is an essential component of whole-ecosystem research. With the notable exception of the watershed manipulation projects at Bear Brook and Coweeta and several smaller scale projects elsewhere, research of this type and scope has been generally lacking in the U.S. The whole-ecosystem manipulation experiments in Europe have been aug- mented by a number of detailed, process-level studies at the various manip- ulation sites. Key aspects included stable isotope ( 15 N) tracer studies to quantify the partitioning of N into various ecosystem pools (i.e., soil, litter, trees, ground vegetation) and to measure changes in the quantities of stored N in these pools. Other studies focused on quantifying the rates of important ecosystem processes, including the N conversion processes of denitrification and mineralization. Results of both the broad-scale and detailed studies have been used to build, test, and validate mathematical models that simulate N processing, nutrient cycling, and water regulation in coniferous forest ecosystems under varying depositional and climatic regimes. Ultimately, these models will be used to predict N saturation, estimate the critical loads of N for European for- ests, and to specify emission controls needed to protect European forests from the detrimental effects of excess N deposition. Such large-scale, controlled whole-ecosystem experiments have become an increasingly important tool in environmental research regarding the effects of atmospheric pollutants. It is now realized that all parts of the ecosystem are involved in the response to an environmental perturbation such as atmo- spheric N or S input. Key processes must be evaluated in the broader context of whole-ecosystem structure and function. It is not possible to understand environmental impacts on the basis of isolated process studies alone. A holis- tic approach is required. In addition, whole-ecosystem experimental manip- ulations are needed across gradients of atmospheric deposition, climate, and other important factors. It has also become increasingly evident in recent years that it is not pos- sible to separate research on ecosystem effects attributable to acidic deposi- tion from the effects of other ecosystem stressors. Climatic fluctuations, especially precipitation input and its effects on water availability, act syner- gistically with a variety of indirect effects of acidic deposition. The obvious linkages between short-term climatic fluctuation and anthropogenic inputs of N and S were incorporated into the experimental approach followed by the European EXMAN program (Beier and Rasmussen, 1993). Both drought 1416/frame/C08 Page 176 Wednesday, February 9, 2000 2:18 PM © 2000 by CRC Press LLC Experimental Manipulation Studies 177 and also N and S inputs were evaluated alone and in combination under a variety of conditions. The linkage with climatic change was taken further still in the European CLIMEX project that entailed simultaneous whole-eco- system manipulation of temperature, atmospheric CO 2 , and acidic deposi- tion (Jenkins et al., 1992). Thus, not only short-term climatic fluctuations, but also long-term climatic trends (hypothesized global climate change) have been under investigation as they relate to ecosystem responses to acidic deposition. Some of the most important contributions to the state of scientific under- standing of the aquatic effects of acidic deposition during the past decade have been made in the area of N effects. Much of this research has been con- ducted in Europe and has involved experimental manipulation of atmo- spheric inputs to small catchments or forested plots. Although it is well beyond the scope of this review to attempt to cover all of the important research findings of European studies in recent years, it is helpful to sum- marize some of the key elements of the experimental ecosystem manipula- tion research. The European scientists have concluded that it is important to study N questions as large multidisciplinary, multi-investigator research teams. This is because of 1. The complexities of the N cycle. 2. The multitude of scientific disciplines involved in its study. 3. The emerging importance of very expensive, large-scale, whole- system manipulations as a tool for studying N effects. A high degree of international and inter-institutional cooperation has devel- oped during the last decade within Europe. This spirit of cooperation has been evident in several recent international umbrella projects on N effects, especially NITREX and EXMAN (Wright and van Breemen, 1995; Rasmus- sen, 1990; Tietema and Beier, 1995). The NITRogen Saturation EXperiments (NITREX) project was a large, international, interdisciplinary research program that focused on the impacts of NO 3 - and NH 4 + on forest ecosystems (Wright and Van Breemen, 1995). NITREX included 11 separate large-scale N addition or removal experiments at 9 sites that span the European gradient in N deposition, from less than 5 kg N/ha per year in western Norway to greater than 50 kg N/ha per year in The Netherlands (Figure 8.1). In general, the same team of investigators and the same techniques were used across sites. At each site, precipitation, throughfall, soil, soil solution, and runoff (catch- ments only) were monitored, before and after initiation of the experimental manipulations. Nutrient status and nutrient cycling were studied by period- ically examining litterfall, needle composition, soil organic matter composi- tion and mass, and fine root biomass. Nitrogen-15 tracer studies were conducted at several sites to follow the fate of added N through the forest 1416/frame/C08 Page 177 Wednesday, February 9, 2000 2:18 PM © 2000 by CRC Press LLC 178 Aquatic Effects of Acidic Deposition ecosystems. The overall objective of the program was to obtain direct exper- imental data at the ecosystem level of N-saturation from atmospheric depo- sition, and subsequent ecosystem recovery. Questions regarding the threshold for N saturation, critical loads, and reversibility were addressed by means of a matrix of experimental manipulations that included increasing N inputs to some forest ecosystems and excluding ambient N (and S) inputs to other forest ecosystems. The NITREX sites clearly separate into those that receive less than 150 meq/m 2 per year of N (approximately 30 kg N/ha per year) input and do not FIGURE 8.1 Location of NITREX research sites as described by Emmett et al. (1998). 1416/frame/C08 Page 178 Wednesday, February 9, 2000 2:18 PM © 2000 by CRC Press LLC Experimental Manipulation Studies 179 have NO 3 - leaching (Sogndal, Aber, Gårdsjön, Klosterhede) and those that receive greater than 200 meq/m 2 per year (approximately 40 kg N/ha per year) and leach significant amounts of NO 3 - (Speuld, Solling, Ysselsteyn; Dise and Wright, 1995). Other aspects of ecosystem acidification also illustrate the gradient in the NITREX sites. For example, leaching of Ca 2+ and Al n + are low at Sogndal and Aber, intermediate at Klosterhede and Gårdsjön, and high at Solling, Speuld, and Ysselsteyn. A potential complication in interpreting these data, however, is that S deposition in Europe follows approximately the same gradient as N deposition. It is unclear as to what extent the N data may be confounded by the effects of S. The EXperimental MANipulations of Forest Ecosystems in Europe (EXMAN) project conducted experimental manipulations of five forest sites in Denmark, Germany, and The Netherlands, with an unmanipulated control site in Ireland. Major objectives of the program were quantification of ele- ment biogeochemical cycling, biomass turnover, and the effects of atmo- spheric deposition on forest ecosystems. Comparable manipulations have been conducted within similar forest types across a range of atmospheric inputs. Experimental approaches and methods were generally standardized. Treatments included simulated summer drought, irrigation, optimal nutri- tion and water, fertilization, liming, and exclusion of ambient atmospheric deposition via roof emplacement (Rasmussen, 1990). 8.1 Whole-System Nitrogen and/or Sulfur Enrichment Experimental Manipulations There are a number of research sites where ambient N, and in some cases also S, deposition has been augmented. Typically, the experimental approach involved acid application during rainfall events by means of sprinkler sys- tems, using chemically altered water from a nearby lake or spring as a carrier for the acid or acid precursor addition. In some cases, ammonium sulfate was applied periodically by helicopter to the experimental watershed. Several of these studies are highlighted below. 8.1.1 Gårdsjön, Sweden At the Gårdsjön experimental manipulation catchment included within NITREX (Catchment G2), about 35 kg N/ha per year was added to the ambient deposition (12 kg N/ha per year) as NH 4 NO 3 . The sum of the experimentally added N plus the ambient deposition in this 0.52-ha catch- ment was in the range of deposition received by damaged forest ecosystems in central Europe, but much higher than the deposition levels in sensitive areas of North America. Data have been collected since 1988 and the treat- 1416/frame/C08 Page 179 Wednesday, February 9, 2000 2:18 PM © 2000 by CRC Press LLC 180 Aquatic Effects of Acidic Deposition ment began in April 1991. Data were routinely collected of meteorology, deposition, throughfall, soil solution, hydrology (tensiometers), runoff, soil chemistry, root vitality, rhizosphere soil chemistry, micorrhizal fungi, litter, vegetation, foliage chemistry, and rates of mineralization, nitrification, and denitrification. Fish toxicity studies were conducted with runoff from the manipulated catchment. The forest is a mixture of Norway spruce ( Picea abies ) and some Scots pine ( Pinus sylvestris ) less than 100 years old. Soils are mostly acidic silty and sandy loams to an average depth of 38 cm. Moldan et al. (1995) presented results from input-output measurements at the Gårdsjön manipulation and reference sites for the first 2 years of treatment. During year 1, slightly elevated levels of NO 3 - in discharge were found during the first 2 weeks of treatment in April 1991 and again during late fall and win- ter. Loss of NO 3 - continued during the second year of treatment, including increased losses during the growing season. However, the watershed retention of deposited N during year 2 was still quite high (98.9 ± 0.1%). In the untreated reference catchment, N retention was about 99.9% of the total inorganic N inputs (Moldan et al., 1995). The monthly mean volume-weighted concentra- tions of NO 3 - in runoff increased from near zero during the 2-year pre-treat- ment period to values typically in the range of 5 to 17 µ eq/L during year 2. Moldan et al. (1995) also conducted intensive sampling for a 2-week period during which three experimental NH 4 NO 3 additions occurred. They found that NO 3 - concentrations in runoff consistently exhibited a sharp increase to concentrations in the range of 15 to 35 µ eq/L immediately following N addi- tion, followed by a recession to below the NO 3 - detection limit (0.4 µ eq/L) within 48 h. Thus, the N retention capacity was only exceeded for short periods of time associated with the experimental treatments, and even then only by a relatively small amount. This happened despite the very high N inputs. After 2 years, NO 3 - appeared in soil solution at shallow depth, and after 4 years at all soil depths (Moldan and Wright, 1998a). Nevertheless, the annual loss of inor- ganic N was only about 5% of the incoming N. The cumulative effect of the N addition was apparent when NO 3 - concentration was plotted by Moldan and Wright (1998a) as a function of stream discharge during the autumn periods of 1994, 1995, and 1997. Nitrate concentrations reached higher values at a given discharge as the experimental acidification proceeded. Discharge rate was the most important factor influencing NO 3 - leaching loss. Peak NO 3 - concentra- tions in discharge (approximately 20 to 100 µ eq/L) corresponded temporally with either times of experimental NH 4 NO 3 addition or high discharge (Moldan and Wright, 1998a). During the first 3 years of experimental N addition, NO 3 - concentrations in discharge were only high during winter. During the fourth and fifth years, however, elevated concentrations also were observed during summer months. The inorganic N lost in discharge, as a percentage of input, was 0.6%, 1.1, 5.0, 5.7, and 4.5%, respectively, during the 5 years of treatment (Moldan and Wright, 1998b). The somewhat reduced N loss during year 5 was attrib- uted to drought and consequent low runoff during that year. 1416/frame/C08 Page 180 Wednesday, February 9, 2000 2:18 PM © 2000 by CRC Press LLC Experimental Manipulation Studies 181 Total N deposition at the experimental Gårdsjön site is about 3 to 5 times higher than ambient N deposition in the high-deposition areas of the north- eastern U.S. It may be, however, that additional time, rather than higher N dose, may be required before NO 3 - leaching shows a more dramatic response at this site. One of the first biological changes attributed to the experimental treatment was a change in the ectomycorrhizal fungus flora. Brandrud (1995) reported that the micorrhizal fruit body production was reduced after 1 1/2 years of treatment, especially for the dominant genera Cortinarious and Russula . A decrease in the amount of fine roots, especially in the upper soil horizons, was also observed for the Vaccinium -dominated portion of the study area (Clemensson-Lindell and Persson, 1995). 8.1.2 Sogndal, Norway The Sogndal site in western Norway was part of the Reversing Acidification in Norway (RAIN) project (Wright et al., 1993). One of the small catchments (SOG4) received a 1 : 1 mixture of sulfuric and nitric acid (50 meq/m 2 per year each) additions since 1984. The region receives only 4 kg S and 2.5 kg N/ha per year of ambient atmospheric deposition. Located at 900 m elevation in western Norway, the Sogndal site has gneissic bedrock, thin and patchy soils averaging about 30 cm depth, and alpine vegetation. Shrub vegetation is dominated by birch ( Betula verrucosa , B. nana ), juniper ( Juniperus communis ), and willow ( Salix hastata ), with a ground cover of Calluna vulgaris , Empetrum nigrum , several species of Vaccin- ium , grasses, mosses, and lichens. Addition of H 2 SO 4 and HNO 3 at SOG4 has caused large changes in runoff chemistry. During the first 5 years of treatment, NO 3 - concentrations in runoff at SOG4 were elevated above concentrations at the control sites (SOG1 and SOG3) only immediately after acid applications. Since 1989, however, the NO 3 - concentration in runoff has been chronically high. Alkalinity and pH decreased in parallel fashion at the H 2 SO 4 treated catchment (SOG2) and the H 2 SO 4 + HNO 3 treated catchment (SOG4; Wright et al., 1994). The RAIN project ended in 1991, and the site was at that point included within NITREX. Sogndal represents the catchment receiving lowest deposi- tion within the NITREX framework, and is also the only nonforested (alpine) catchment in the project. The experiment at Sogndal represents the only long- term study of chronic N addition to an alpine site. Results of 9 years of N deposition at a level of 9 kg N/ha per year were summarized by Wright and Tietema (1995). As was found by Moldan et al. (1995) at Gårdsjön, the general pattern of NO 3 - concentration in runoff was one of sharp peaks during and immediately after each acid addition, fol- lowed by a rapid decline to concentrations near zero. It was only during the last few years of treatment that the decline in NO 3 - concentration in runoff following experimental N additions proceeded more slowly, and runoff 1416/frame/C08 Page 181 Wednesday, February 9, 2000 2:18 PM © 2000 by CRC Press LLC 182 Aquatic Effects of Acidic Deposition between additions also contained elevated concentrations of NO 3 - . Ecosys- tem N saturation was not achieved after 9 years of N loading at a rate of 9 kg N/ha per year. More data of this type are needed to improve our under- standing of the susceptibility of alpine soils and vegetation communities to N saturation. What is particularly noteworthy about the Sogndal study is the fact that the total N deposition (7 kg/ha per year experimental plus 2 kg N/ha per year ambient) is in the range of deposition found in many parts of the U.S. The results of the Sogndal research are, therefore, perhaps more relevant to the situation in the U.S. than are the results at many of the other NITREX sites. The input–output budget for N at the treatment catchment (SOG4), sum- marized over the 9-year period indicated that 88% of the total N input of 72 kg N/ha was retained in the catchment. The percent N retention at the untreated reference catchment SOG1 was identical (88%), although the total N input was much lower, only about 20 kg N/ha. Wright and Tietema (1995) concluded that there was little evidence that the 9 years of N deposition at a level of about 9 kg N/ha per year had induced N-saturation. Most of the NO 3 - leaching occurred during the early phases of snowmelt and immediately during or following experimental N addition. They attributed the increased leaching loss to insufficient time or capacity to immobilize the NO 3 - flux during times of high flow and high input concen- trations and emphasized that the total N deposition at Sogndal was near the 10 kg N/ha per year apparent threshold for N saturation proposed by Gren- nfelt and Hultberg (1986) and Dise and Wright (1995). 8.1.3 Lake Skjervatjern, Norway The Humic Lake Acidification Experiment (HUMEX) was initiated by the Norwegian Institute for Water Research (NIVA) in 1987. The principal goals of HUMEX were to evaluate the role of humic substances in the acidification of surface waters and the effects of S and N deposition on the properties of humic substances in watershed soils and surface waters (Gjessing, 1992). HUMEX is an investigation of the interaction between acid deposition and natural organic acids by means of acid addition to the entire catchment of a pristine humic lake in western Norway. Skjervatjern is a small (2.4 ha), pris- tine, naturally acidic, humic lake located near Førde, western Norway. The lake has pH 4.6 with average concentrations of TOC of about 9 mg C/L, non- marine base cations of about 30 µ eq/L, and Al i of less than 50 µ g/L. Lake Skjervatjern is located in an area of western Norway that receives low levels of anthropogenically derived atmospheric deposition of S and N. The 6.5 ha catchment is underlain by granitic bedrock, covered by histosols in the lower portions and podsols developed on thin glacial till in the upland areas. Annual precipitation at the site is about 2 m. The lake was divided in 1988 by a plastic curtain that effectively sepa- rated the lake and its drainage basin into two systems, a manipulated side 1416/frame/C08 Page 182 Wednesday, February 9, 2000 2:18 PM © 2000 by CRC Press LLC Experimental Manipulation Studies 183 (Basin A) and control side (Basin B). A 105 m plastic curtain was installed from the middle of the natural outlet to the opposite shore. The bottom edge of the curtain was pressed into the soft upper lake sediments by sand bags that minimized water movement between the two lake halves. Water qual- ity was monitored for two years prior to treatment and three years during which N and S were applied to one-half of the lake and its respective drain- age basin. Artificial acidification of the treatment side of the catchment (Basin A) was initiated 2 years after installation of the dividing curtain. A sprinkling system consisting of 50 sprinklers (15 m sprinkling radius) was mounted at the top of the taller trees throughout the treatment basin. A combination of H 2 SO 4 and NH 4 NO 3 was applied at pH 3.0 to 3.2 weekly, in a volume equivalent to approximately 10% of ambient precipitation, using water pumped from nearby Lake Åsvatn. Annual target loadings for SO 4 2- and total N were 63 to 66 and 17 to 32 kg/ha, respectively. Water chemistries in the treatment and reference sides of the lake were monitored weekly, for 2 years prior to initia- tion of the artificial acid additions and for 5 years during which N and S were applied to one-half of the lake and its respective drainage basin (Gjessing, 1994; Lydersen et al., 1996). The physical division of Lake Skjervatjern into two basins had some effects on the water chemistry of the lake, likely due to small differences in the terrestrial catchments that drain into the two lake halves. Lake water in the treatment side had equivalent or lower concentrations of all ions and lower electrical conductivity than did lake water in the reference side. The most pronounced differences prior to chemical manipulation were for Na + (-4 µ eq/L), Cl - (-2 µ eq/L), SO 4 2- (-1 µ eq/L), and K + (-1 µ eq/L). Lake water pH was slightly higher on the experimental side (approximately 0.03 pH units) and TOC was 0.67 mg C/L lower (Gjessing, 1992, 1994). During the first 2 years of treatment, 8.5 g m -2 of H 2 SO 4 and 6.7 g m -2 of NH 4 NO 3 were applied to the catchment and lake surface of the experimental side (A) (Gjessing, 1992). About 4% of the total chemicals were sprayed directly on the lake surface, and the balance would have received some con- tact with the terrestrial catchment prior to entering the lake. The majority of the increased lake water SO 4 2- concentration (greater than 80%) was attribut- able to SO 4 2- that had made some contact with the catchment. Nevertheless, a considerable amount of the added SO 4 2- was apparently retained in the ter- restrial system. The amount of SO 4 2- applied to the experimental catchment during the first 2 years of treatment should have caused an increase in lake-water SO 4 2- con- centration of about 44 µ eq/L above the premanipulation concentrations, assuming steady-state conditions and average annual runoff of about 1950 mm. The observed increase in lake-water SO 4 2- concentration was 15 µ eq/L (Gjessing, 1992), suggesting that about two-thirds of the S added during the first 2 years were retained in the watershed. Over the 5-year treatment period reported by Lydersen et al. (1996), the mean SO 4 2- concentration in the treat- ment catchment increased by 16 µ eq/L. 1416/frame/C08 Page 183 Wednesday, February 9, 2000 2:18 PM © 2000 by CRC Press LLC 184 Aquatic Effects of Acidic Deposition The increase in lake-water NO 3 - concentration was fairly small at Skjer- vatjern, only about 3 µ eq/L. A substantial portion of that increase can be attributed to NO 3 - added directly to the lake surface, even without assuming that some of the NH 4 + applied to the lake surface would have been converted to NO 3 - in the lake water. Total N also increased in the treatment side of the lake, however, and by an amount considerably greater than the total N applied to the lake surface (Gjessing, 1994). This suggested that at least some of the N applied to the terrestrial portion of the catchment also reached the lake. Nevertheless, about 90% of the added N was retained in the terrestrial system, lake sediments, and/or biota, and did not contribute to increased concentrations of NO 3 - and NH 4 + in lake water. Lydersen et al. (1996) used randomized intervention analysis (RIA) to test for differences between runoff chemistry from the two basins before and after the artificial acidification treatment. Significantly higher concentrations were found of SO 4 2- , NO 3 - , Ca 2+ , Mg 2+ , H + , NH 4 + , and Al i in Basin A after treatment compared with the control basin. The average ANC increased in the control basin during the course of the study, and this was attributed by Lydersen et al. (1996) to the long-lasting effect of Na + leakage after storms having high inputs of sea salts. During a hurricane in January 1993, the concentration of Cl - in rainfall exceeded 400 µ eq/L at the nearby weather station. During that event, the lowest runoff pH (4.25) and ANC (-62 µ eq/L) values were recorded in the control basin. ANC remained unchanged in Basin A. Acidifi- cation of Basin A was observed as a gradual change in the difference in ANC between the two basins. Highest concentrations of SO 4 2- were observed during summer, likely related to low flow conditions and consequent reoxidation of S stored in wet- land soils. One of the most dramatic results of the acidification experiment was the observed decrease in the anion deficit (an estimate of the organic acid anion concentration) in Basin A. The difference in average anion deficit between the 2-year pre-acidification period and the 5-year post-acidification period in Basin A was nearly as large as the corresponding change in base cat- ion concentrations (Lydersen et al., 1996). Thus, organic acid anions became more protonated in the treatment basin compared with the control basin as a consequence of the experimental treatment. 8.1.4 Aber, Wales Most forests in the U.K. are thought to immobilize a high proportion of incoming atmospheric N. One known exception is the Beddgelert forest in North Wales, where N outputs are higher than inputs in bulk precipitation. The Aber forest study component of NITREX was designed to examine how a forest in this region would process additional loadings of N. The experi- mental site is located at an elevation of 300 m, 10 km from the North Wales coast. Originally moorland, the site was planted in Sitka spruce ( Picea sitch- ensis ) in 1960. Ambient deposition includes about 17 kg N/ha per year. 1416/frame/C08 Page 184 Wednesday, February 9, 2000 2:18 PM © 2000 by CRC Press LLC [...]... in part, the low rates of nitrification observed (Gundersen and Rasmussen, 1990, 1995) © 2000 by CRC Press LLC 1416/frame/C 08 Page 186 Wednesday, February 9, 2000 2: 18 PM 186 8. 1.6 Aquatic Effects of Acidic Deposition Bear Brook, ME The Bear Brook watershed covers the top 210 m of the southeast slope of Lead Mountain (475 m) in eastern Maine Ambient loading of inorganic N in deposition is about 9 kg/ha... February 9, 2000 2: 18 PM 188 Aquatic Effects of Acidic Deposition prevent throughfall and stemflow from passing through the holes Runoff from the roof is collected, chemically altered, and reapplied beneath via sprinklers The technique allows simulation of drought and decreased atmospheric deposition to entire terrestrial systems 8. 2.1 Gårdsjön, Sweden Whole-catchment exclusion of incoming ambient atmospheric... real-time watering in 1992 © 2000 by CRC Press LLC 1416/frame/C 08 Page 190 Wednesday, February 9, 2000 2: 18 PM 190 Aquatic Effects of Acidic Deposition The reduced input of N and S to the “clean” roof plot at Ysselsteyn resulted in reduced NH4+, NO 3-, and SO4 2- concentrations in soil solution Between 1990 and 1992, the NO 3- concentration was reduced 45% and NH4+ concentration 80 % compared with the roof... above-ground processes were the slowest components to react to the treatment in a temporal cascade of soil solution leading to fine roots leading to above-ground stand © 2000 by CRC Press LLC 1416/frame/C 08 Page 192 Wednesday, February 9, 2000 2: 18 PM 192 8. 2.5 Aquatic Effects of Acidic Deposition Risdalsheia, Norway The Risdalsheia site in southernmost Norway receives a high loading of acidic deposition. .. ground vegetation; mineralization of soil organic matter; soil fauna; and biologically mediated © 2000 by CRC Press LLC 1416/frame/C 08 Page 194 Wednesday, February 9, 2000 2: 18 PM 194 Aquatic Effects of Acidic Deposition processes, and the quality and quantity of runoff water Process-oriented models will be developed to link aquatic and terrestrial processes Several of the roof experiments in Europe have... played a major role in the acid–base chemistry of runoff at the site and in moderating pH change following reduction in acid deposition This site is particularly important because of its long period of record (greater than 10 years) and large change in the concentrations of SO4 2- and NO 3- in both the deposition and drainage water Major results of the acidic deposition exclusion experiment at Risdalsheia... manipulation of West Bear Brook has included a 2 1/2-year calibration period (1 987 –1 989 ), 9 years of chemical addition of (NH4)2SO4, and will soon be followed by a recovery period Chemical additions of 180 0 eq of SO4 2- and NH4+ per hectare per year effectively increased total atmospheric loading about 200% for S and 300% for N (Norton et al., 1999) Prior to the manipulation, stream-water chemistry of both... activities and atmospheric deposition are unknown It is not possible to separate research on ecosystem effects attributable to acidic deposition from the effects of other ecosystem stressors Climatic fluctuations, especially precipitation input and its effect on water availability, act synergistically with a variety of indirect effects of acidic deposition The obvious linkages between short-term climatic fluctuation... detail by Wright and co-workers (e.g., Wright, 1 989 ; Wright et al., 1 986 , 1 988 b, 1990) Average stream-water SO4 2- concentrations were reduced from 92 µeq/L in the 1 985 water year to 28 µeq/L in 1992 This reduction in SO4 2- was compensated primarily by decreased base cation concentrations, from 136 to 104 µeq/L (F = 0.5) In addition, the average H+ concentration decreased from 87 to 61 µeq/L and the... µeq/L Roof manipulation studies, such as those described previously, have proven valuable for investigating the environmental effects of reduced deposition of S and N and for testing of mathematical models that predict the effects of abatement strategies However, a variety of unintended changes have also been caused by the roof construction and experimental design in some cases (Beier et al., 19 98) These . 2: 18 PM © 2000 by CRC Press LLC 176 Aquatic Effects of Acidic Deposition 1. Increasing deposition of S and/or N. 2. Excluding the previously existing deposition via construction of roofs. state of scientific under- standing of the aquatic effects of acidic deposition during the past decade have been made in the area of N effects. Much of this research has been con- ducted in Europe. rates of nitrification observed (Gundersen and Rasmussen, 1990, 1995). 1416/frame/C 08 Page 185 Wednesday, February 9, 2000 2: 18 PM © 2000 by CRC Press LLC 186 Aquatic Effects of Acidic Deposition 8. 1.6