EFFECTS OF ACID PRECIPITATION ON NUTRIENT CYCLING PROCESSES AND WATER QUALITY IN FOREST ECOSYSTEMS OF SOUTHERN APPALACHIAN AND PIEDMONT WATERSHEDS Task Group Project Code: E1-6.3 and F10-20 Wayne T. Swank, J. Bruce Wallace,2 John W. Fitzgerald,2 H. Larry Ragsdale,^ Bruce Haines,2 Coweeta Hydrologic Laboratory^ Southeastern Forest Experiment Station, Forest Service, USDA; ^University of Georgia, 3Emory University ABSTRACT The Coweeta Hydrologic Laboratory is a Forest Service site of long-term interdisciplinary research that currently encompasses a wide spectrum of integrated forest ecosystem studies involving Forest Service staff, university, and other Agency senior scientists working under formal cooperative agreements. The National Acid Precipitation Assessment Program is one important component of the scientific effort. This research is focused on detailed studies of nutrient cycling on selected undisturbed and managed watersheds and the research approach involves a continuum of theory, experimentation, and application. Long-term measurements of precipitation and stream chemistry of forest ecosystems are combined with process-level studies to elucidate cause and effect relationships. The current effort seeks to define trends in acidity and consequent changes in terrestrial and aquatic ecosystem structure and function. Over 10 years of continuous precipitation chemistry records indicate no significant trend toward increasing acidity. Stream records of equal length also show no consistent trend of changing acidity but there does appear to be small increases in sulfate concentrations. Hydrologic budgets of sulfate indicate large annual net accumulations of sulfur in undisturbed hardwood forests. Sulfate adsorption by Fe and Al oxides in soil is an important process and more recent findings indicate that microbial metabolism of sulfate into organic forms of sulfur is also a major process of sulfur immobilization that regulates sulfate and cation leaching. The influence of soil ph and other factors on processes of sulfate incorporation and organic sulfur mobilization are being studied. Other studies are testing hypotheses on the amount and composition of organic acids in hydrologic compartments, the relative importance of sulfur emission from hardwood species, aluminum biogeochemistry, stream community structure and secondary production, mechanisms of deposition exchange in forest canopies, and short-term changes in stream chemistry during storm events. Much of this research is being concentrated on a high elevation watershed which is expected to be more sensitive to acid deposition.
EFFECTS OF ACID PRECIPITATION ON NUTRIENT CYCLING PROCESSES AND ATER QUALITY IN FOREST ECOSYSTEMS OF SOUTHERN APPALACHIAN AND PIEDMONT WATERSHEDS Task Group Project Code: E1-6.3 and F10-20 Wayne T. Swank,'' J. Bruce Wallace,^ John W. Fitzgerald, H. Larry Ragsdale,3 Bruce Haines,^ 'Coweeta Hydrologic Laboratory, Southeastern Forest Experiment Station, Forest Service, USDA; ^University of Georgia, 3Emory University IMPORTANCE The Coweeta Hydrologic Laboratory is a 2250 ha Forest Service site of integrated, interdisciplinary, long-term research on hydrologic and nutrient cycling processes in managed and undisturbed (control) watersheds. Research on the effects of man's use of the forest on the quality and quantity of streamflow in reference to baseline characteristics spans a period of 50 years. Detailed nutrient cycling research was initiated 16 years ago. Acid deposition is one component of a larger research effort to quantify nutrient budgets and cycling of nutrients through selected watersheds and to determine changes in cycles produced by natural disturbances, management, or changes in atmospheric input. The integration of acid deposition research with other major studies and topical areas of investigation in progress at Coweeta are illustrated in Figure 1 along with primary sources of support. This report includes only Terrestrial and Aquatic Effects projects wholly or partly supported by NAPAP.through Forest Service funding. A. Terrestrial HYPOTHESES (1) Bulk precipitation ph shows no consistent seasonal trends or annual trends in decreasing or increasing acidity or other chemical constituents. (2) Microbial transformations of inorganic sulfate in forest floor and soil layers are important processes in regulating the sulfur cycle and SO^, mobility of forest ecosystems but are unaffected by changes in soil ph. (3) Leaching of water-soluble organic acids is insignificant in forest ecosystems at Coweeta and does not contribute to stream water acidity. (4-) Emissions of reduced sulfur gases are significant for tree species in the Southern Appalachians but rates are independent of light intensity, soil SO^. concentration, and S02 fumigation.
Effects of Acid Precipitation on Nutrient Cycling Processes and Water Quality in Forest Ecosystems Project 1651 (Forest Service) Evaluation of ILWAS Model at Coweeta (EPRI) Long-Term Ecological Research Program (NSF) (FS) Comparative Evaluation of Acid Precipitation on Forest Soils (EPA) A'cid Rain Interactions with Forest Canopies (FS) Effects of Atmospheric Deposition on Canopy and Soil Processes - Forest Ecosystems (EPRI) Sulfur Transformations in Forest Soils (NSF) (FS) Comparative Aluminum Biogeochemistry Analysis (EPRI)(EPA) Characterization of Organic Acids (FS) _L Aluminum Distribution in Hardwood Forest Ecosystems (FS) Potential Sulfur Emission From Forest Trees (FS) Population Biology and Acid tolerance of Crayfish (FS) Invertebrate Population Structure - Monitoring of Acid Precipitation (FS) Figure 1. Topical areas of acid precipitation research in progress at Coweeta Hydrologic Laboratory and primary sources of support.
(5) Aluminum mobilization and leaching is an unimportant process in response to acidic deposition in a high elevation watershed. (6) Rates and amounts of chemical leaching from hardwood leaves are regulated by leaf surface H +, and dry deposition of and NO X. B. Aquatic (1) Invertebrate community structure, secondary production, and detritus processing of a high elevation stream can be used to document long-term effects of acid rain on stream ecosystems. (2) Concentrations of solutes in stream water and the adjacent saturated soil zone do not change appreciably during a storm event. (3) Changes in population structure of crayfish species common to Coweeta streams provides a sensitive indicator of the impacts of decreasing ph levels in surface waters. Stream solutes show no trends in relation to atmospheric deposition. A. Terrestrial TECHNICAL APPROACH A network of eight precipitation stations located within the Coweeta basin has been maintained for the past 11 years. Samples are collected weekly for volume and chemical analysis of the major inorganic solutes. These same parameters are measured on weekly grab samples from gaged streams draining six disturbed and seven undisturbed watersheds ranging from 9 to 760 ha in area. Data are currently undergoing rigorous statistical analyses to determine trends in concentrations and fluxes. Other studies are examining changes in water chemistry during storm events by measuring solute concentrations in precipitation, throughfall, soil water, seep water, and runoff. Investigations of microbial incorporation of into organic matter utilize 35so^ in laboratory incubations of forest floor and soil samples collected from a permanent plot design on a control watershed. A variety of biochemical tests, extractions, and analytical methods are used to determine S compounds. Similar methods are applied to research on mineralization of organic forms of S. In situ experiments have been conducted to verify findings of laboratory incubations. Physical and chemical factors which regulate incorporation and mineralization processes are also examined. A portable gas exchange system is used to collect sulfur emission samples from hardwood leaves; these are subsequently analyzed with a variety of laboratory separations and analyses. Field measurements are supplemented with greenhouse
Swa,nk et al. experiments to examine.the influence of varying levels of light intensity, soil SO^. concentrations, and S02 fumigation on rates of sulfur emissions. An Al budget for a high elevation forest is being estimated by sampling ridge, mid-slope, and stream sites for aluminum fractions in precipitation, throughfall, soil, and stream water in addition to total Al in vegetation components. Another study is oriented toward leaf precipitation and stem precipitation interactions in a mixed hardwood forest to determine relationships between leaf surfaces, precipitation ph, dry deposition, rainfall duration and intensity, leaf age and species specificity in the resulting throughfall and stemflow. Major solutes are being examined for storm events and sampling methods include wet-dryfall samplers for precipitation and throughfall, deposition plates for dryfall and impaction, and laboratory investigations involving leaf washing methods. B. Aquatic Stream samples are collected weekly from six treated and seven undistrubed gaged watersheds and analyzed for major solutes. Stream and precipitation chemistry are combined with water volumes to construct nutrient budgets. Rigorous statistical analyses are currently being applied to both concentration and budget data to assess long-term trends. In other research in a high elevation stream, monthly densities and standing stock estimates for all invertebrate taxa are sampled in four bottom substrate types, Macroinvertebrates are classified by functional group in terms of number and biomass and secondary production estimates are derived for each microhabitat. Estimates of leaf litter breakdown rates for four species are derived from open mesh bags; all sampling is keyed to permanently established sites. In crayfish studies, random samples are taken in a variety of habitats in conjunction with population sampling. Trapping and quadrat sampling provides quantitative data for abundance estimates. A variety of physical measurements are used to characterize specimens and the relative abundance of juveniles and sex ratios are statistically analyzed for comparisons between sites. Research emphasis is placed on a high elevation stream which also is the site of invertebrate studies. Acid tolerance of crayfish is determined in laboratory experiments at ph levels of /4-0, 5.0 and 6.0. In other research, organic acidic functional groups are being characterized in precipitation, throughfall, soil water, and stream samples using a variety of extraction and analytical methods. RESULTS A. Terrestrial Studies on foliar sulfur emissions, ecosystem Al budgets, canopy leaching processes, and organic acid characterization were recently initiated and results are not available at this time. Analysis of H + concentration of bulk precipitation indicates no significant
trend in annual weighted concentrations over an 1 1 year period. Acidity of bulk precipitation differed significantly between years with 1977 being the year of lowest acidity (ph 4. 70) and 1978 the year of highest acidity (ph 4-30). Both years reflected belowaverage annual precipitation values. Both episodic events and chronic trends of long-grange transport and deposition originating from man's activities and other sources can be expected in seemingly isolated locations. For example, orthophosphate inputs in bulk precipitation at Coweeta are typically low and relatively uniform throughout the year (Figure 2) However, during the second week of April 1977, a dust storm originating in Texas visibly blanketed much of the Southern Appalachian region. During the next three weeks, with only 13 cm of rainfall, PO^ input was 30 percent of the amount normally received for the entire year; dryfall accounted for 90 percent of the input. Concentrations of N03, NH^_ and SO^. were also substantially higher. The atmosphere is a significant source of plant nutrients that partially replenishes losses due to timber harvesting (Swank, 1984-). Nitrogen in bulk precipitation is equivalent to at least 70 percent of the N incorporated annually into aboveground woody tissues of some temperate hardwood forests. Atmospheric sources of Ca and K supply between 20 and 4.0 percent of the nutrients sequestered in woody increments. m 120 100 80 60 40 20 0 O 150 125 O Figure 2. J J 1976 DATE J J 1977 S O N O Concentration and input of orthophosphate in bulk precipitation at Coweeta Hydrologic Laboratory, North Carolina (Swank, 1984),
Acid precipitation represents a major source of 30^2- and at Coweeta this anion comprises 68 percent of precipitation anions in bulk precipitation. The average annual hydrologic budget for hardwood covered watersheds at Coweeta during the past 10 years shows apparent accumulations of 30^2- ranging from 5-9 to 8.9 kg ha~' year"' of S (Table 1). Part of this accumulation is due to SO^- adsorption to Fe and Al oxides in the soil profile (Johnson et al. 1980). Our recent research indicates that microbial metabolism of 80^2- to organic S forms in forest floor and soil is also a major process of potential S immobilization (Fitzgerald et al. 1983; Swank et al. 1984). The incorporation of -^SQ. supplied to A-] soil samples from a hardwood forest in incubation studies, increased with increases in exogenous SO/2- concentrations and temperature (Figure 3} Incorporation of S into non-salt extractable fractions involves the formation of covalent sulfur linkages which is microbially mediated. Convincing evidence for microbial incorporation of sulfate into organic matter was obtained from samples treated prior to incubation with compounds which stimulate or inhibit microbial metabolism (Strickland and Fitzgerald 1984) Incorporation activity was observed in 01 and 02 layers of the forest floor and throughout the soil profile (Swank et al. 1984). Based on sampling of all horizons throughout the year, an annual incorporation rate of 30 kg ha~' was estimated (Table 2). However, other research at Coweeta has shown that sulfur incorporated into organic matter can be mobilized; i.e., it undergoes a solubilization process of depolymerization and/or desulfation (Strickland et al. 1984)- Mobilization experiments showed substantial release of incorporated sulfur (Table 3) and the process appears to be mediated by preformed enzymes. These initial results indicate that incorporation exceeds mobilization; thus, there is a net accumulation of organic sulfur in the soil which reduces the supply and mobility of sulfate. Table 1. Mean annual (1973-1982) sulfate-sulfur budgets for four mixed hardwood watersheds at Coweeta Hydrologic Laboratory. Watershed number Area (ha) Input Sulfate-Sulfur (kg ha~ year ) Output Net difference 2 12.1 9-5 1.3 + 8.2 18 12.5 10.5 1.6 + 8.9 27 38.8 13.2 7.0 + 6.2 36 48.6 11.7 5.8 + 5.9
20 20 C 1.2 0-8 0-4 n moui ' 5 SO^ Supplied Figure 3- Rates of sulfate incorporation into organic matter in A- soil samples as a function of temperature and exogenous sulfate concentrations. Table 2. Estimates of annual rates of sulfate incorporation into organic sulfur forms by microbial populations in forest floor and soil horizons of a forest ecosystem at Coweeta Hydrologic Laboratory.^ Location 02 A/B Incorporation rate (kg 5 ha" 1 year -- 0.2 0.2 10.7 6.1 11.1 Total 29. After Swank et al. 1984
o have identified biological transformations of 30^ and organic S as major processes in the sulfur cycle of some forests which potentially reduce sulfate mobility and cation leaching. How important are these processes in other ecosystems and what factors regulate the balance between incorporation and mobilization? What is the relative influence of 30^2- adsorption versus biological transformations in soil 30^2- accumulation and dynamics? The amount, composition, and interaction of dry deposition constituents in forest canopies is a critical information gap in acid precipitation research. The buffering effects of the canopy are well known but mechanisms of ion exchange and sources of these ions must be examined to evaluate physiological and nutrient cycling impacts. Equally important is the documentation of the population structure and dynamics of primary fauna in streams of baseline ecosystems which are responsive to changes in stream acidity. CONCLUSIONS 1. Eleven years of weekly H" 1 " measurements in bulk precipitation and stream water of undisturbed watersheds in the Coweeta basin show no apparent trend toward increasing or decreasing acidity. 2. Hydrologic budgets of 30^2- inputs and outputs indicate apparent accumulations of 30^2- ranging from 5-9 to 8.9 kg ha-1 year""' of 3. Adsorption of SO^- to Fe and Al oxides in the soil profile account 'for part of the accumulation. Microbial metabolism of 30^2- to organic 3 forms in forest floor and soil is also a major process of 30^2- immobilization which reduces the supply and mobility of the sulfate anion and, hence, reduces cation leaching. 3. Sulfate in stream water of undisturbed watersheds has shown recent trends of increasing concentrations but annual weighted concentrations are weakly correlated with annual bulk precipitation concentrations. Conclusive reasons for increased stream 30/2- concentrations are unknown at this time. REFERENCES Fitzgerald, J. W., Ash, J. T., Strickland, T. C., Swank,. T. 1983- Formation of organic sulfur in forest soils: a biologically mediated process. Canadian Journal of Forest Research 13: 1077-1082. Johnson, D.., Hornbeck, J.., Kelly, J. M., Swank,. T., Todd, D. E., Jr. 1980. Regional patterns of soil sulfate accumulation: relevance to ecosystem sulfur budgets, pp. 507-520. In: Shriner, D. S., Richmond, C. R., Lindberg, 3. E., eds. Atmospheric sulfur deposition: environmental impact and health effects. Ann Arbor Science Publishers, Inc., Ann Arbor, MI.
Strickland, T. C., Fitzgerald, J.. Formation and mineralization of organic sulfur in forest soils. In: Role of sulfur in ecosystem processes, Proceedings ESA Symposium, Biogeochemistry 1 (in press) Strickland, T. C., Fitzgerald, J.., Swank,. T. 1984.. Mobilization of recently formed forest soil organic sulfur. Canadian Journal of Forest Research H: 63-67, Swank,. T. 1984- Atmospheric contributions to forest nutrient cycling. ater Resources Bulletin 20: 313-321. Swank,. T., Fitzgerald, J.., Ash, J. T. 1984.. Microbial transformation of sulfate in forest soils. Science 223: 182-184.. DELIVERABLES Swank,. T. 1984-. Atmospheric contributions to forest nutrient cycling. ater Resources Bulletin 20: 313-321. Swank,. T., Fitzgerald, J.., Ash, J. T. 1984.. Microbial transformations of sulfate in forest soils. Science 223: 182-184.. Fitzgerald, J.., Ash, J. T., Strickland, T. C., Swank,. T. 1983. Formation of organic sulfur in forest soils: a biologically mediated process. Canadian Journal of Forest Research 13: 1077-1082. a. Publications completed fiscal year 1984- Strickland, T. C., Fitzgerald, J.., Swank,. T. 1984.. Mobilization of recently formed forest soil organic sulfur. Canadian Journal of Forest Research 14-: 63-67. Swank,. T., Swank,. T. S. 1984-. Dynamics of water chemistry in hardwood and pine ecosystems, pp. 335-34-6. In: Burt, T. P., Walling, D. E., eds. Catchment experiments in fluvial geomorphology: Proceedings of International Geographical Union Commission on Field Experiments in Geomorphology. Geo Books, Norwich, England. b. Continuation of long-term precipitation and stream chemistry data sets. c. Publications planned fiscal year 1985 Swank,. T., aide, J. B. Characterization of baseline precipitation stream chemistry and watershed nutrient budgets. Book chapter in Coweeta Symposium. Haines, B. L., Swank,. T. Acid precipitation studies at Coweeta. Book chapter in Coweeta Symposium.
d. Completion of baseline studies on crayfish and stream invertebrates; preliminary data on organic acid characterization, atmospheric deposition interactions with the forest canopy, sulfur emission and Al biogeochemistry projects. RELATIONSHIP TO OTHER ORK This work is part of the Forest Service effort at the Coweeta Hydrologic Laboratory and is partially supported by the Southeastern Forest Experiment Station and partially supported by other sources as indicated in Figure 1. As part of the Long-Term Ecological Research (LTER) program at Coweeta, a major segment of studies related to acid deposition is supported by the National Science Foundation CNSF). A number of shorter term projects, which contribute directly to acid deposition interpretations, are also funded by NSF. Other interagency cooperative projects are partially funded by DOE, EPA, and Electric Power Research Institute. ACTIVITIES See Deliverables Section; also the following presentations: 1. Swank et al. 198/4. Transformations of sulfur in forest floor and soil of a forest ecosystem. IHP- orkshop on Hydrological and Hydrochemical Mechanisms and Model Approaches to the Acidification of Ecological Systems, September 15-16, 1984, Uppsala, Sweden. 2. Swank,. T., Fitzgerald, J. W. 1983. Microbial transformations of sulfate in forest soils: acid rain implications. Acid Rain and Forest Resources Conference, Quebec City, Canada. 3. Swank,. T. S., Swank,. T., Blood, E. J984. Sulfate dynamics in a loblolly pine stand on the South Carolina Coast, ESA Annual Meeting, August 5-9, 1984, Fort Collins, CO. 4. Ragsdale, H. L., Berish, C.., Swank, W. T. 1984. Chemical element comparisons between control watersheds using LTER reference plots at the Coweeta Hydrologic Laboratory. ESA Annual Meeting, August 5-9, 1984, Fort Collins, CO. SUPPORT Forest Service - Terrestrial, $110K; Aquatic, $55K National Science Foundation- $100,000,1983-84; EPA - $15,000, 1983-84
PERSONNEL W. T. Swank, Principal Ecologist; L.. Swift, Jr., Forest Meteorologist: J. B. aide, Research Ecologist; and T. J. Harshbarger, Research Biologist, USDA, Forest Service, Southeastern Forest Experiment Station, Coweeta Hydrologic Laboratory, Otto, North Carolina. D. A. Crossley, Jr. Professor; J. B. Wallace, Professor; J.. Fitzgerald, Professor; B. Haines, Assistant Professor, University of Georgia. H. L. Ragsdale, Professor, and C.. Berish, Research Associate, Emory University. M. Perdue, Professor, Georgia Institute of Technology. Forest Service staff: 70,4-524-2128 TELEPHONE CONTACTS