Sulfate-Induced Eutrophication and Phytotoxicity in Freshwater Wetlands LEON P. M. LAMERS,* HILDE B. M. TOMASSEN, AND JAN G. M. ROELOFS Research Group Environm ental Biology, Departm ent of Ecology, University of Nijm egen, Toernooiveld 1, 6525 ED Nijmegen, the Netherlands In recent decades,sulfate concentrations in many European freshwater wetlands have increased by 10-fold or more, due mainly to the use of sulfate-polluted river water to compensate for water shortage in these areas. To test the effect of sulfate enrichment, a mesocosm experiment was set up, using waterlogged soil cores, intact with vegetation, from a mesotrophic fen meadow. During sulfate addition at environmentally relevant levels (0, 2, and 4 mmol L -1 ), phosphate concentration and alkalinity of the pore water rapidly rose due to increased sulfate reduction rates. Free sulfide accumulated to levels toxic to several wetland plant species and biomass regrowth after harvesting was significantly lower on treated soils, especially for Carex species. Eventually, the concentrations of ammonium, phosphate, and potassium increased strongly in the treated soils due to reduced uptake by plants and extra mineralization. Sulfate availability was rate limiting, until the supply of readily decomposable organic matter became limited. It is argued that the significance of the observed changes in free sulfide concentrations and in the rate of nutrient mobilization should be recognized, and that these effects can be as important as direct eutrophi- cation caused by the import of nutrients. The reported changes may severely influence the plant species composition of freshwater wetlands. Introduction Nutrient kinetics in wet soils and sediments rich in organic matter are highly influenced by the rate of microbial mineralization (1). For chemoorganotrophs, the supply of terminalelectron acceptorslike oxygen (duringdesiccation), nitrate, or sulfate (in reduced sediments) is essential, in addition to the availabilityofoxidizable organic compounds derived from readily decomposable organic matter (2). A high mineralization rate directly leads to a higher nutrient availability for plants. Jørgensen (3) showed that sulfate- reducingbacteria playan important role in the mineralization of organic matter in marine sediments. Sulfate reduction, however, also affects nutrient kinetics indirectly. Sulfide, produced bysulfate reduction,interfereswith iron-phosphate binding in soils and sediments due to the formation of iron sulfides. In this way, phosphate is released, both in marine and in freshwater sediments (4-7). The literature shows that the amount of phosphate released depends on the availability of sulfate. In saline systems, this might be the reason why biomass production is generally not limited by phosphate (5). In freshwater systems,sulfate reduction rates are generallylow,because ofthe modest availabilityofsulfate. In many lowland regions of Europe, groundwater and surface water levels have fallen by a few decimeters up to several meters in recent decades. This is due to hydraulic operations for agricultural purposes and increased water extraction for agricultural, industrial, and domestic use. To compensate forthe concomitant shortage ofwater,riverwater is used on a large scale in many freshwater wetlands, particularlyin the regions with large lowland rivers. The use of such water in the restoration of groundwater tables in agricultural areas and nature reserves and for the flooding and waterlogging of desiccating natural wetlands has led to severe changes in soil and surface water quality, including an increase in the abundance of nutrients and macroions becauseofriverpollution (6, 8-11). Initially,the concomitant eutrophication ofmany wetlands was blamed entirely upon the import of nutrients with river water. Recent research, however, suggests that increased import of macroions like sulfate and bicarbonate plays a major role in the observed eutrophication (6, 7, 12, 13). In recent decades, average sulfate concentrations in (incoming) polluted surface- and groundwater offreshwater wetlandshave risen from lessthan 0.1 mmol L -1 to values over 0.5-1.5 mmol L -1 , and even to values over 3 mmol L -1 . This is caused by anthropogenic sulfate input into rivers (including mining activities), in- creased atmospheric sulfur input, and the use of sulfate- containing fertilizers, in addition to the weathering of geological sulfur deposits (6, 14, 15). Furthermore, the desiccation of wetlands strongly promotes the oxidation of reduced sulfur compounds, leading to high sulfate concen- trations in ditches and rivers receiving the drainage water (16, 17). Besidescausingeutrophication,theincreased supply of sulfate may also lead to sulfide toxicity to the roots of aquatic plants (13). Smolders et al. (18) demonstrated that phosphate mobilization and sulfide toxicity in sulfate-rich sediments can be prevented by iron addition, indicating the importance offree iron availability in sediments for binding of both. The aim of the present study was to analyze the effects of increased sulfate pollution on the biogeochemistry of anoxic peaty soils and the consequences of biogeochemical changes for growth and survival of characteristic plant species. Along-term mesocosm experiment wasset up using intact soilcores,includingthe vegetation,from a mesotrophic wetland meadow. We hypothesized that increased sulfate concentrations, at levels similar to those in polluted fresh- water wetlands, would induce the mobilization of nutrients and the accumulation offree sulfide,due to enhanced sulfate reduction rates. Moreover, it was also hypothesized that these changes would influence vegetation growth. Experimental Section Experimental Design and Treatments. The experiment was carried out between November, 1994, and May, 1995. Sods were collected from a mesotrophic wetland meadow in the nature reserve “De Bruuk” near Nijmegen, the Netherlands (51°45′ N, 5°58′ E). The soil at this location was classified as Rhizic Hydromoder. The upper 12 cm, used for the experi- ment, included moderately decomposed peat containing loam and living roots. The grassland is annually mown for hay making in late summer, and the vegetation zone is dominated by Carex nigra. The vegetation can be considered Caricion nigrae (19). In contrast,the adjacent zone isflooded *Corresponding author fax: +31 24 3652134; email: leonl@sci.kun.nl. Environ. Sci. Technol. 1998, 32, 199-205 S0013-936X(97)00362-3 CCC: $15.00  1998 American Chemical Society VOL. 32, NO. 2, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 199 Published on Web 01/15/1998