USDA Conservation Practices Increase Carbon Storage and Water Quality Improvement Functions: An Example from Ohio

We compared potential denitrification and phosphorus (P) sorption in restored depressional wetlands, restored riparian buffers, and natural riparian buffers of central Ohio to determine to what extent systems restored under the U.S. Department of Agriculture's Wetland Reserve Program (WRP) and Conservation Reserve Program (CRP) provide water quality improvement benefits, and to determine which practice is more effective at nutrient retention. We also measured soil nutrient pools (organic C, N, and P) to evaluate the potential for long‐term C sequestration and nutrient accumulation. Depressional wetland soils sorbed twice as much P as riparian soils, but had significantly lower denitrification rates. Phosphorus sorption and denitrification were similar between the restored and natural riparian buffers, although all Natural Resources Conservation Service (NRCS) practices had higher denitrification than agricultural soils. Pools of organic C (2570–3320 g/m²), total N (216–243 g/m²), and total P (60–71 g/m²) were comparable among all three NRCS practices but were greater than nearby agricultural fields and less than natural wetlands in the region. Overall, restored wetlands and restored and natural riparian buffers provide ecosystem services to the landscape that were lost during the conversion to agriculture, but the delivery of services differs among conservation practices, with greater N removal by riparian buffers and greater P removal by wetlands, attributed to differences in landscape position and mineral soil composition. At the landscape, and even global level, wetland and riparian restoration in agricultural landscapes will reintroduce multiple ecosystem services (e.g. C sequestration, water quality improvement, and others) and should be considered in management plans .     

Predictive models for phosphorus retention in wetlands

The potential of wetlands to efficiently remove (i.e., act as a nutrient sink) or to transform nutrients like phosphorus under high nutrient loading has resulted in their consideration as a cost-effective means of treating wastewater on the landscape. Few predictive models exist which can accurately assess P retention capacity. An analysis of the north American data base (NADB) allowed us to develop a mass loading model that can be used to predict P storage and effluent concentrations from wetlands. Phosphorus storage in wetlands is proportional to P loadings but the output total phosphorus (TP) concentrations increase exponentially after a P loading threshold is reached. The threshold P assimilative capacity based on the NADB and a test site in the Everglades is approximately 1 g m^–2 yr^–1. We hypothesize that once loadings exceed 1 g m^–2 yr^–1 and short-term mechanisms are saturated, that the mechanisms controlling the uptake and storage of P in wetlands are exceeded and effluent concentrations of TP rise exponentially. We propose a One Gram Rule for freshwater wetlands and contend that this loading is near the assimilative capacity of wetlands. Our analysis further suggests that P loadings must be reduced to 1 g m^–2 yr^–1 or lower within the wetland if maintaining long-term low P output concentrations from the wetlands is the central goal. A carbon based phosphorus retention model developed for peatlands and tested in the Everglades of Florida provided further evidence of the proposed One Gram Rule for wetlands. This model is based on data from the Everglades areas impacted by agricultural runoff during the past 30 years. Preliminary estimates indicate that these wetlands store P primarily as humic organic-P, insoluble P, and Ca bound P at 0.44 g m^–2 yr^–1 on average. Areas loaded with 4.0 g m^–2 yr^–1 (at water concentrations>150 g·L^–1 TP) stored 0.8 to 0.6 g m^–2 yr^–1 P, areas loaded with 3.3 g m^–2 yr^–1 P retained 0.6 to 0.4 g m^–2 yr^–1 P, and areas receiving 0.6 g m^–2 yr^–1 P retained 0.3 to 0.2 g m^–2 yr^–1. The TP water concentrations in the wetland did not drop below 50 g·L^–1 until loadings were below 1 g m² yr^–1 P.     

Hydrological and nutrient budgets of freshwater and estuarine wetlands of Taylor Slough in Southern Everglades, Florida (U.S.A.)

Hydrological restoration of the Southern Everglades will result in increased freshwater flow to the freshwater and estuarine wetlands bordering Florida Bay. We evaluated the contribution of surface freshwater runoff versus atmospheric deposition and ground water on the water and nutrient budgets of these wetlands. These estimates were used to assess the importance of hydrologic inputs and losses relative to sediment burial, denitrification, and nitrogen fixation. We calculated seasonal inputs and outputs of water, total phosphorus (TP) and total nitrogen (TN) from surface water, precipitation, and evapotranspiration in the Taylor Slough/C-111 basin wetlands for 1.5 years. Atmospheric deposition was the dominant source of water and TP for these oligotrophic, phosphorus-limited wetlands. Surface water was the major TN source of during the wet season, but on an annual basis was equal to the atmospheric TN deposition. We calculated a net annual import of 31.4 mg m^–2 yr^–1 P and 694 mg m^–2 yr^–1N into the wetland from hydrologic sources. Hydrologic import of P was within range of estimates of sediment P burial (33–70 mg m^–2 yr^–1 P), while sediment burial of N (1890–4027 mg m^–2 yr^–1 N) greatly exceeded estimated hydrologic N import. High nitrogen fixation rates or an underestimation of groundwater N flux may explain the discrepancy between estimates of hydrologic N import and sediment N burial rates.     

Impact of drying and re-wetting on N,P and K dynamics in a wetland soil

As increased nutrient availability due to drainage is considered a major cause of eutrophication in wetlands rewetting of drained wetlands is recommended as a restoration measure. The effect of soil drying and rewetting on the contribution of various nutrient release or transformation processes to changed nutrient availability for plants is however weakly understood. We measured effects of soil drying and re-wetting on N mineralization, and denitrification, as well as on release of dissolved organic nitrogen (DON), phosphorus, and potassium in incubated soil cores from a wet meadow in southern Sweden. Additionally, the impact of re-wetting with sulphate-enriched water was studied. Soil drying stimulated N mineralization (3 times higher) and reduced denitrification (5 times lower) compared to continuously wet soil. In the wet cores, denitrification increased to 20 mg N m^–2 d^–1, which was much higher than denitrification measured in the field. In the field, increased inorganic-N availability for plants due to drainage seemed primarily to be caused by increased N mineralization, and less by decreased denitrification. Soil drying also stimulated the release of DON and K, but P release was not affected. Re-wetting of dried soil cores strongly stimulated denitrification (up to 160 mg N m^–2 d^–1), but N mineralization was not significantly decreased, neither were DON or K release. In contrast, the extractable P pool increased upon soil wetting. Re-wetting with sulphate-enriched water had no effect on any of the nutrient release or transformation rates. We conclude that caution is required in re-wetting of drained wetlands, because it may unintendently cause internal eutrophication through an increased P availability for plants.     

Co-development of wetland soils and benthic invertebrate communities following salt marsh creation

The development of wetland soil characteristics andbenthic invertebrate communities were evaluated increated Spartina alterniflorasalt marshes inNorth Carolina ranging in age from 1 to 25 years-old.A combination of measurements from different-agecreated marshes as well as periodic measurements overtime on two marshes were used to (1) document rates ofwetland pedogenesis, especially soil organic matter,and, (2) explore relationships between soil andbenthic invertebrate community development. Soilmacro-organic matter (MOM, the living and dead rootand rhizome mat), organic C and N increased and bulkdensity decreased during the 25 years following marshestablishment. The most dramatic changes in bulkdensity, MOM, C and N occurred within the upper 10 cmof the soil with lesser changes below this depth.Created marshes were sinks for organic C (90–140g·m^-2·yr^-1) and N (7–11g·m^-2·yr^-1) but not for P (0–1g·m^-2·yr^-1). The density of benthicinvertebrates (>250 m) and subsurface-depositfeeding oligochaetes also increased over time oncreated salt marshes. Invertebrate and oligochaetedensity were strongly related to MOM content(r²= 0.83–0.87) and soil organic C(r²= 0.52–0.82) and N (r²= 0.62–0.84). Thesefindings suggest that, in created salt marshes,development of the benthic invertebrate community istied to marsh soil formation, especially accumulationof organic matter as MOM and soil. Field studies thatmanipulate the quantity and quality of soil organicmatter are needed to elucidate the relationshipbetween salt marsh pedogenesis and benthicinvertebrate community development.     

The biogeochemistry of nitrogen in freshwater wetlands

The biogeochemistry of N in freshwater wetlands is complicated by vegetation characteristics that range from annual herbs to perennial woodlands; by hydrologic characteristics that range from closed, precipitation-driven to tidal, riverine wetlands; and by the diversity of the nitrogen cycle itself. It is clear that sediments are the single largest pool of nitrogen in wetland ecosystems (100's to 1000's g N m^-2) followed in rough order-of-magnitude decreases by plants and available inorganic nitrogen. Precipitation inputs (< 1–2 g N m^-2 yr^-1) are well known but other atmospheric inputs, e.g. dry deposition, are essentially unknown and could be as large or larger than wet deposition. Nitrogen fixation (acetylene reduction) is an important supplementary input in some wetlands (< < 1–3 g N m^-2 yr^-1) but is probably limited by the excess of fixed nitrogen usually present in wetland sediments.Plant uptake normally ranges from a few g N m^-2 yr^-1 to 35 g N m^-2 yr^-1 with extreme values of up to 100g N m^-2 yr^-1 Results of translocation experiments done to date may be misleading and may call for a reassessment of the magnitude of both plant uptake and leaching rates. Interactions between plant litter and decomposer microorganisms tend, over the short-term, to conserve nitrogen within the system in immobile forms. Later, decomposers release this nitrogen in forms and at rates that plants can efficiently reassimilate.The NO₃ formed by nitrification (< 0.1 to 10 g N m^-2 yr^-1 has several fates which may tend to either conserve nitrogen (uptake and dissimilatory reduction to ammonium) or lead to its loss (denitrification). Both nitrification and denitrification operate at rates far below their potential and under proper conditions (e.g. draining or fluctuating water levels) may accelerate. However, virtually all estimates of denitrification rates in freshwater wetlands are based on measurements of potential denitrification, not actual denitrification and, as a consequence, the importance of denitrification in these ecosystems may have been greatly over estimated.In general, larger amounts of nitrogen cycle within freshwater wetlands than flow in or out. Except for closed, ombrotrophic systems this might seem an unusual characteristic for ecosystems that are dominated by the flux of water, however, two factors limit the opportunity for N loss. At any given time the fraction of nitrogen in wetlands that could be lost by hydrologic export is probably a small fraction of the potentially mineralizable nitrogen and is certainly a negligible fraction of the total nitrogen in the system. Second, in some cases freshwater wetlands may be hydrologically isolated so that the bulk of upland water flow may pass under (in the case of floating mats) or by (in the case of riparian systems) the biotically active components of the wetland. This may explain the rather limited range of N loading rates real wetlands can accept in comparison to, for example, percolation columns or engineered marshes.     

A catchment-oriented and cost-effective policy for water protection

Governmental programmes and international agreements to counteract eutrophication have largely not attained agreed objectives (e.g. reduction by half of the anthropogenic nitrogen load on Swedish coastal waters). Important components of such programmes are improved removal of nitrogen in municipal treatment plants and changed agricultural practices. In addition, increased N-removal during runoff, i.e. restoration of ponds and wetlands, is an important strategy. One explanation of the fact that the objectives have yet not been achieved might be that the most effective step to counteract diffuse pollution has not been fully implemented. It is therefore important to stress the potential of effective measures and find ways to fully implement them at the watershed level. It is important to avoid excessive applications of fertilizers because this leads to an exponential increase in leaching. Field experiments indicate that the use of winter crops or an undersown catch crop outside the main cropping season has reduced nitrate losses by up to 75% in single years, and by nearly 50% over successive years. In southern Sweden, the area of wetlands has been reduced considerably (more than 90%) by melioration activities. In a recent project, budget studies with restored ponds verified the importance of ponds and wetlands in nitrogen retention. Per unit area, increased nitrogen loading implied increased nitrogen retention, but often a decrease in the percent retained. Ponds with depths of 0.4–2.0 m and hydrological loads of 0.14–5.2 m³ m⁻² day⁻¹ were created. One hundred and fifty to seven thousand kg N ha⁻¹ year⁻¹ was removed in ponds loaded by streams dominated by agricultural run off. A pond receiving pre-treated municipal wastewater removed 8000 kg N ha⁻¹ year⁻¹. The upper limit for N-removal is set by the hydrological conditions. Sedimentation of organic material must be favoured in order to obtain adequate conditions for denitrification. To achieve the governmental objective in nitrogen load reduction changed cultivation practices within the agricultural sector must be combined with restoration of ponds/wetlands.     

Treatment of an artificial sulphide containing wastewater in subsurface horizontal flow laboratory-scale constructed wetlands

In general, treatment wetlands seem to be a potential method of tackling the sulphide problem of post-treatment of anaerobic digester effluents.Because of insufficient practical experience and lack of knowledge of sulphide removal, sulphur transformation was investigated, particularly in horizontal subsurface flow constructed wetlands (depth of 35 cm) under laboratory-scale conditions with artificial wastewater.The plants affected a clear stimulation of the sulphide and ammonia removal rates. Sulphide concentration in the range of 1.5–2.0 mg l⁻¹ was tolerated by the plants and completely removed in the planted model wetlands; sulphide concentration of >2.0 mg l⁻¹ caused instabilities in sulphide and nitrogen removal. Area-specific sulphide removal rates of up to 94 mg sulphide m⁻² d⁻¹ were achieved in the planted beds at hydraulic retention times of 2.5 d. Sulphate affected the sulphide removal. While in the unplanted control bed an almost stable removal in the range of 150–300 mg N m⁻² d⁻¹ was observed variations of hydraulic retention time, sulphide and sulphate concentrations influenced the ammonia removal rate within the planted beds in a broader range (600–1400 mg N m⁻² d⁻¹).These results showed that nitrification, sulphide oxidation, denitrification and sulphate reduction can occur simultaneously in the rhizosphere of treatment wetlands caused by dynamic redox gradients (aerobic–anaerobic) conditions.     

A first generation ecosystem model of the Des Plaines River experimental wetlands

A first generation wetland ecosystem model is developed from first-year field data for four constructed freshwater marshes at the Des Plaines River Wetland Demonstration Project in northeastern Illiois, USA. The model, which includes hydrology, sediment and phophorus algorithms common for all four wetlands, was used for the integration of data collected as part of the many different aspects of the demonstration project and for prediction of wetland function under varying hydrologic and chemical loadings. Stepwise calibration was completed on the model for the hydrology, sediment, productivity, and phosphorus submodels. Separate sedimentation coefficients were necessary for each wetland land and each season. Preliminary simulations investigate the role of changing flow conditions in the wetlands on phosphorus retention. As simulated inflow increased from 34 to 136 cm/wk, phosphorus retention increased from 1.5 to 3.3 g P m⁻² yr⁻¹ while retention efficiency decreased from 67 to 37%.     

Evaluation of relative importance of different microbial reactions on organic matter removal in horizontal subsurface-flow constructed wetlands using a 2D simulation model

In this study, we used a two-dimensional (2D) mechanistic mathematical model in order to evaluate the relative contribution of different microbial reactions to organic matter removal (in terms of COD) in horizontal subsurface-flow constructed wetlands that treated urban wastewater. We also used the model to analyse the effect of increasing or decreasing the organic loading rate (changing the hydraulic loading rate (HLR) at a constant influent organic matter concentration, or changing the organic matter concentration at a constant HLR) on both the removal efficiency and the relative importance of the microbial reactions. The model is based on the code RetrasoCodeBright, which we modified to include the main microbial processes related to organic matter and nitrogen transformations in the wetlands: hydrolysis, aerobic respiration, nitrification, denitrification, sulphate reduction and methanogenesis. The model was calibrated and validated with data from two wetlands (each with a surface area of 55 m²) located in a pilot plant near Barcelona (Spain). According to the simulations, anaerobic processes (methanogenesis and sulphate reduction) are more widespread in the wetlands and contribute to a higher COD removal rate (60–70%) than anoxic (denitrification) and aerobic reactions do. These model results are confirmed by experimental observations. In all the cases tested, the reaction that most contributed to COD removal was methanogenesis (33–52%). According to our simulations, decreasing the HLR (for example, from 40 to 25 mm/d) while maintaining a constant COD influent concentration has a clear positive impact on COD removal efficiency (which increases from 65% to 89%). Changing influent COD concentration (for example, from 290 to 190 mg/L) while maintaining a constant HLR has a smaller impact, causing efficiency to increase from 79% to 84%. Changes in influent COD concentration (at a constant HLR) affect the relative contribution of the microbial reactions to organic matter removal. However, this trend is not seen when the HLR changes and the COD influent concentration remains constant.     

Stable soil nitrogen accumulation and flexible organic matter stoichiometry during primary floodplain succession

Large increases in nitrogen (N) inputs to terrestrial ecosystems typically have small effects on immediate N outputs because most N is sequestered in soil organic matter. We hypothesized that soil organic N storage and the asynchrony between N inputs and outputs result from rapid accumulation of N in stable soil organic pools. We used a successional sequence on floodplains of the Tanana River near Fairbanks, Alaska to assess rates of stable N accumulation in soils ranging from 1 to 500+ years old. One-year laboratory incubations with repeated leaching separated total soil N into labile (defined as inorganic N leached) and stable (defined as total minus labile N) pools. Stable N pools increased faster (2 g N m^–2 yr^–1) than labile N (0.4 g N m^–2 yr^–1) pools during the first 50 years of primary succession; labile N then plateaued while stable and total N continued to increase. Soil C pools showed similar trends, and stable N was correlated with stable C (r² = 0.95). From 84 to 95 % of soil N was stable during our incubations. Over successional time, the labile N pool declined as a proportion of total N, but remained large on an aerial basis (up to 38 g N m^–2). The stoichiometry of stable soil N changed over successional time; C:N ratios increased from 10 to 22 over 275 years (r² = 0.69). A laboratory ¹⁵N addition experiment showed that soils had the capacity to retain much more N than accumulated naturally during succession. Our results suggest that most soil N is retained in a stable organic pool that can accumulate rapidly but is not readily accessible to microbial mineralization. Because stable soil organic matter and total ecosystem organic matter have flexible stoichiometry, net ecosystem production may be a poor predictor of N retention on annual time scales.     

Benithic-pelagic coupling of nutrient metabolism along an estuarine eutrophication gradient

The shallow, brackish (11–18% salinity) Roskilde Fjord represents a eutrophication gradient with annual averages of chlorophyll, ranging from 3 to 25 mg chl a m^–3. Nutrient loadings in 1985 were 11.3–62.4 g N m^–2 yr^–1 and 0.4–7.3 g P m^–2 yr^–1. A simple one-layer advection-diffusion model was used to calculate mass balances for 7 boxes in the fjord. Net loss rates varied from –32.2 to 17.9 g P m^–2 yr^–1 and from –3.3 to 66.8 g N m^–2, corresponding to 74% of the external P-loading and 88% of the external N-loading to the entire estuary.Gross sedimentation rates measured by sediment traps were between 7 and 52 g p m^–2 yr^–1 and 50 and 426 g N M^–2 yr^–1, respectively. Exchangeable sediment phosphorus varied in annual average between 2.0 and 4.8 g P m^–2 and exchangeable sediment nitrogen varied from 1.9 to 33.1 g N m–1. Amplitudes in the exchangeable pools followed sedimentation peaks with delays corresponding to settling rates of 0.3 m d^–1. Short term nutrient exchange experiments performed in the laboratory with simultaneous measurements of sediment oxygen uptake showed a release pattern following the oxygen uptake, the changes in the exchangeable pools and the sedimentation peaks.The close benthic-pelagic coupling also exists for the denitrification with maxima during spring of 5 to 20 mmol N m^–2 d–1. Denitrification during the nitrogen-limited summer period suggests dependence on nitrification. Comparisons with denitrification from other shallow estuaries indicate a maximum for denitrification in estuaries of about 250 µmol N m^–2 h^–2 achieved at loading rates of about 25–125 g N m^–2 yr^–1.     

Retention of nutrients in river basins

In Denmark, as in many other European countries, the diffuse losses of nitrogen (N) and phosphorus (P) from the rural landscape are the major causes of surface water eutrophication and groundwater pollution. The export of total N and total P from the Gjern river basin amounted to 18.2 kg ha^–1 and 0.63 kg P ha^–1 during June 1994 to May 1995. Diffuse losses of N and P from agricultural areas were the main nutrient source in the river basin contributing 76% and 51%, respectively, of the total export.Investigations of nutrient cycling in the Gjern river basin have revealed the importance of permanent nutrient sinks (denitrification and overbank sedimentation) and temporary nutrient storage in watercourses. Temporary retention of N and P in the watercourses thus amounted to 7.2–16.1 g N m^–2 yr^–1 and 3.7–8.3 g P m^–2 yr–1 during low-flow periods. Deposition of P on temporarily flooded riparian areas amounted from 0.16 to 6.50 g P m^–2 during single irrigation and overbank flood events, whereas denitrification of nitrate amounted on average to 7.96 kg N yr^–1 per running metre watercourse in a minerotrophic fen and 1.53 kg N yr^–1 per linear metre watercourse in a wet meadow. On average, annual retention of N and P in 18 Danish shallow lakes amounted to 32.5 g N m^–2 yr^–1 and 0.30 g P m^–2 yr^–1, respectively, during the period 1989–1995.The results indicate that permanent nutrient sinks and temporary nutrient storage in river systems represent an important component of river basin nutrient budgets. Model estimates of the natural retention potential of the Gjern river basin revealed an increase from 38.8 to 81.4 tonnes yr^–1 and that P-retention increased from –0.80 to 0.90 tonnes yr^–1 following restoration of the water courses, riparian areas and a shallow lake. Catchment management measures such as nature restoration at the river basin scale can thus help to combat diffuse nutrient pollution.     

Influence of bed media characteristics on ammonia and nitrate removal in shallow horizontal subsurface flow constructed wetlands

Two bed media were tested (gravel and Filtralite) in shallow horizontal subsurface flow (HSSF) constructed wetlands in order to evaluate the removal of ammonia and nitrate for different types of wastewater (acetate-based and domestic wastewater) and different COD/N ratios. The use of Filtralite allowed both higher mass removal rates (1.1 g NH₄–N m⁻² d⁻¹ and 3 g NO₃–N m⁻² d⁻¹) and removal efficiencies (>62% for ammonia, 90–100% for nitrate), in less than 2 weeks, when compared to the ones observed with gravel. The COD/N ratio seems to have no significant influence on nitrate removal and the removal of both ammonia and nitrate seems to have involved not only the conventional pathways of nitrification–denitrification. The nitrogen loading rate of both ammonia (0.8–2.4 g NH₄–N m⁻² d⁻¹) and nitrate (0.6–3.2 g NO₃–N m⁻² d⁻¹) seem to have influenced the respective removal rates.     

Influence of hydrological connectivity of riverine wetlands on nitrogen removal via denitrification

Wetland ecosystems in agricultural areas often become progressively more isolated from main water bodies. Stagnation favors the accumulation of organic matter as the supply of electron acceptors with water renewal is limited. In this context it is expected that nitrogen recycling prevails over nitrogen dissipation. To test this hypothesis, denitrification rates, fluxes of dissolved oxygen (SOD), inorganic carbon (DIC) and nitrogen and sediment features were measured in winter and summer 2007 on 22 shallow riverine wetlands in the Po River Plain (Northern Italy). Fluxes were determined from incubations of intact cores by measurement of concentration changes or isotope pairing in the case of denitrification. Sampled sites were eutrophic to hypertrophic; 10 were connected and 12 were isolated from the adjacent rivers, resulting in large differences in nitrate concentrations in the water column (from <5 to 1,133 μM). Benthic metabolism and denitrification rates were investigated by two overarching factors: season and hydrological connectivity. SOD and DIC fluxes resulted in respiratory quotients greater than one at most sampling sites. Sediment respiration was coupled to both ammonium efflux, which increased from winter to summer, and nitrate consumption, with higher rates in river-connected wetlands. Denitrification rates measured in river-connected wetlands (35–1,888 μmol N m^?2 h^?1) were up to two orders of magnitude higher than rates measured in isolated wetlands (2–231 μmol N m^?2 h^?1), suggesting a strong regulation of the process by nitrate availability. These rates were also significantly higher in summer (9–1,888 μmol N m^?2 h^?1) than in winter (2–365 μmol N m^?2 h^?1). Denitrification supported by water column nitrate (D_W) accounted for 60–100% of total denitrification (Dtot); denitrification coupled to nitrification (D_N) was probably controlled by limited oxygen availability within sediments. Denitrification efficiency, calculated as the ratio between N removal via denitrification and N regeneration, and the relative role of denitrification for organic matter oxidation, were high in connected wetlands but not in isolated sites. This study confirms the importance of restoring hydraulic connectivity of riverine wetlands for the maintenance of important biogeochemical functions such as nitrogen removal via denitrification.     

Constructed wetlands for the treatment of landfill leachates: The Slovenian experience

Constructed wetlands are widely recognized as an economical, efficient and environmentally acceptable means to treat many different types of wastewater. Six systems have been constructed in Slovenia for the treatment of landfill leachates. This paper describes the early stages of two treatment systems operating from 1990 to 1993 that were used to treat leachates from municipal landfills. System S1 consisted of a sedimentation lagoon and a 600 m² reed bed. System S2 was designed with a sedimentation lagoon and two reed beds (total=450 m²). The subsurface flow in both was horizontal and fluctuated widely in S2, but was a constant 0.2 1 s^–1 in S1. Peat, soil, sand and gravel were in system S2, while gravel with a hydraulic conductivity of 5×10^–4 m s^–1, was in S1.The unanticipated fluctuations of hydraulic and organic loadings influenced their performance. The efficiency in reduction of organic matter, N, P, metals and fecal coliforms varied through the year. In 3 years of operation, the average removal efficiencies for COD, BOD₅ and TSS were 38%, 61%, and 81%, respectively, in S1, and 53%, 45%, and 47%, respectively, in 1.5 years of operation for S2. The reduction of ammonium did not reach the effluent standard of 10 mg l^–1 for either S1 or S2. As, Zn, Pb, and Cu accumulated in roots, and Cu, Fe, As, Ni and Pb accumulated in rhizomes after one year of operation in S2.     

Phosphorus sorption by sediments in a subtropical constructed wetland receiving stormwater runoff

This study investigated the potential of using a mixture of fishpond bund material, completely decomposed granite and river sand as substrate in a constructed wetland for phosphorus removal. Core samples were collected from the newly constructed Hong Kong Wetland Park (HKWP) receiving influent stormwater from a nearby new town, and batch incubation experiments were conducted to determine the P sorption characteristics of sediments. The HKWP sediments adsorbed the majority of available P in the initial 20 min of incubation, with a first-order rate constant of 1.01–2.11 h^?1. Sediments in the reedbeds and freshwater marshes possessed a great capacity for P adsorption with the high Langmuir sorption maxima (478–858 mg kg^?1) and Freundlich adsorption constants (417–672 L kg^?1) obtained, attributable to the high amorphous iron and aluminium concentrations compared to other constructed wetlands. Moreover, sediment equilibrium P concentrations were generally low (4.6–23.6 μg L^?1), facilitating a net P adsorption by sediments under moderate P loadings. Yet, the amount of P adsorbed by the HKWP sediments was limited by the low ambient porewater P concentrations and there was even a risk of P desorption when sediments in the freshwater marshes were resuspended into the water column. While substrates in the HKWP demonstrated a great potential for P adsorption, consideration should also be given to P loadings in influent water to fully utilize the P sorption capacity of sediments and enhance the P removal efficiency of constructed wetlands.     

Seasonal denitrification in flooded and exposed sediments from the Amazon floodplain at Lago Camaleão

Denitrification processes were measured by the acetylene-blockage technique under changing flood conditions along the aquatic/terrestrial transition zone on the Amazon floodplain at Lago Camaleão, near Manaus, Brazil. In flooded sediments, denitrification was recorded after the amendment with NO ₃ ^– (100 mol liter^–1) throughout the whole study period from August 1992 to February 1993. It ranged from 192.3 to 640.7 mol N m^–2 h^–1 in the 0- to 5-cm sediment layer. Without substrate amendment, denitrification was detected only during low water in November and December 1992, when it occurred at a rate of up to 12.2 mol N m^–2 h^–1 Higher rates of denitrification at an average rate of 73.3 mol N m^–2 h^–1 were measured in sediments from the shallow lake basin that were exposed to air at low water. N₂O evolution was never detected in flooded sediments, but in exposed sediments, it was detected at an average rate of 28.3 mol N m^–2 h^–1 during the low-water period. The results indicate that under natural conditions there is denitrification and hence a loss in nitrogen from the Amazon floodplain to the atmosphere. Rates of denitrification in flooded sediments were one to two orders of magnitude smaller than in temperate regions. However, the nitrogen removal of exposed sediments exceeded that of undisturbed wetland soils of temperate regions, indicating a considerable impact of the flood pulse on the gaseous turnover of nitrogen in the Amazon floodplain.     

Denitrification in the periphyton associated with plant shoots and in the sediment of a wetland system supplied with sewage treatment plant effluent

Seasonal variation in denitrification and major factors controlling this process were determined in sediment, microbial communities attached to plant shoots (periphyton) and in the water of a Phragmites and an Elodea-dominated stand of a constructed wetland system between May 1997 and February 1998. The wetland was supplied with effluent from a sewage treatment plant. The denitrification rate in periphyton on plants shoots (expressed per shoot area) was always considerably higher than in the sediment and varied with the chlorophyll-a content of the periphyton in the course of the year. The algae in the periphyton provided attachment surfaces and probably also organic compounds to the denitrifying bacteria. Decreases in periphyton biomass and denitrification rate in the Phragmites and Elodea-dominated stands during the growing season were associated with enhanced shading by Phragmites shoots or a floating layer of macro-algae and Lemna spp., respectively. Light availability and the denitrification rate of periphyton increased again after the Phragmites shoots were cut in October. Nitrate appeared to limit the denitrification rate in the sediment. Periphyton denitrification rates were mostly lower on Elodea shoots than on Phragmites shoots, in spite of the higher living algal biomass on Elodea shoots. This difference was associated with lower nitrate concentrations in the Elodea-dominated stand. In the two stands, the daily denitrification rates in periphyton on shoots of Phragmites australis (44.4–121 mg N m^–2 stand area d^–1) and Elodea nuttallii (14.8–33.1 mg N m^–2 d^–1) were clearly more important than rates in the sediment (0.5–25.5 mg N m^–2 d^–1) or the water (0.4–3.9 mg N m^–2 d^–1). The presence of few bacteria attachment sites or low organic carbon availability possibly resulted in low denitrification rates in the water. Denitrification appeared to be a major process in nitrate removal from the through-flowing water in this wetland system.     

Whole-system estimation of denitrification in a plains river: a comparison of two methods

Whole-system denitrification in the South Platte River was measured over a 13-month period using an open-channel N₂ method and mass-balance measurements. Concentrations of dissolved N₂ were measured with high precision by membrane-inlet mass spectrometry and estimates of denitrification were based on the mass flux of N₂, after correction for reaeration and groundwater flux. Open-channel estimates of denitrification ranged from 0 to 3.08 g N m^–2 d^–1 and the mean annual rate was 1.62 g N m^–2 d^–1, which corresponds to removal of approximately 34% of the nitrate transported by the river over a distance of 18.5 km. Over the same period of time, estimates of denitrification based on mass-balance measurements ranged from 0.29 to 5.25 g N m^–2 d^–1 and the mean annual rate was 2.11 g N m^–2 d^–1. The two methods revealed similar seasonal patterns of denitrification the highest rates were measured from late April to August and the lowest rates were in winter. Both methods provide whole-system estimates of denitrification in running waters; where reaeration rate coefficients are low and flux of groundwater is well quantified, the open-channel method has fewer sources of uncertainty and is easier to implement.