HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (17) Citing Articles via Google Scholar Google Scholar Articles by Casey, R. E. Articles by Klaine, S. J. Search for Related Content PubMed PubMed Citation Articles by Casey, R. E. Articles by Klaine, S. J. GeoRef GeoRef Citation Agricola Articles by Casey, R. E. Articles by Klaine, S. J. Related Collections Water Quality Wetlands and Aquatic Processes Soil Chemistry

TECHNICAL REPORT
Wetlands and Aquatic Processes

Mechanisms of Nutrient Attenuation in a Subsurface Flow Riparian Wetland

^a Dep. of Chemistry, Environmental Science and Studies Program, Towson Univ., 8000 York Road, Towson, MD 21252
^b Dep. of Environmental Toxicology, Clemson Univ., Clemson, SC 29670

* Corresponding author (racasey{at}towson.edu)

Received for publication January 31, 2000.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Riparian wetlands are transition zones between terrestrial and aquatic environments that have the potential to serve as nutrient filters for surface and ground water due to their topographic location. We investigated a riparian wetland that had been receiving intermittent inputs of NO^-₃ and PO^3-₄ during storm runoff events to determine the mechanisms of nutrient attenuation in the wetland soils. Few studies have shown whether infrequent pulses of NO^-₃ are sufficient to maintain substantial denitrifying communities. Denitrification rates were highest at the upstream side of the wetland where nutrient-rich runoff first enters the wetland (17–58 µg N₂O–N kg soil^-1 h^-1) and decreased further into the wetland. Carbon limitation for denitrification was minor in the wetland soils. Samples not amended with dextrose had 75% of the denitrification rate of samples with excess dextrose C. Phosphate sorption isotherms suggested that the wetland soils had a high capacity for P retention. The calculated soil PO^3-₄ concentration that would yield an equilibrium aqueous PO^3-₄ concentration of 0.05 mg P L^-1 was found to be 100 times greater than the soil PO^3-₄ concentration at the time of sampling. This indicated that the wetland could retain a large additional mass of PO^3-₄ without increasing the dissolved PO^3-₄ concentrations above USEPA recommended levels for lentic waters. These results demonstrated that denitrification can be substantial in systems receiving pulsed NO^-₃ inputs and that sorption could account for extensive PO^3-₄ attenuation observed at this site.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
SEVERAL REPORTS have shown that riparian soils can be effective for the attenuation of nutrients in ground water (Jordan et al., 1993; Lowrance, 1992; Simmons et al., 1992) and surface water (Ambus and Christensen, 1993). Fewer studies have investigated the magnitude and distribution of attenuation mechanisms in riparian systems receiving intermittent pulses of nutrients. Such pulses can occur during storm-generated runoff from areas of intensive fertilizer use.

The turfgrass industry is one potential source of nutrient inputs to surface and ground water. Turfgrass cultivation is increasing in many areas of the USA due to increases in golf course construction, home lawn maintenance, and turf farming. Brown et al. (1982) found that up to 22% of N applied as NH₄NO₃ to golf greens could be lost as NO^-₃ in leachate. In contrast, Linde and Watschke (1997) collected runoff generated from turf plots maintained as golf course fairways and found low levels of NO^-₃ in runoff water ranging from 0 to 1.3 mg NO₃–N L^-1. Phosphate in runoff water ranged from 0.09 to 10.39 mg PO₄ L^-1.

Riparian wetlands are transition zones between terrestrial and aquatic environments that have the potential to serve as nutrient filters for surface and ground water due to their topographic location. One important mechanism for NO^-₃ attenuation in riparian soils is denitrification, in which NO^-₃ serves as an electron receptor during respiration and is ultimately reduced to nitrous oxide (N₂O) or nitrogen gas (N₂). The other mechanism of NO^-₃ removal in wetlands is uptake by microbes and plants and the subsequent conversion of inorganic N to organic biomass.

Among the studies reporting significant levels of NO^-₃ attenuation in wetlands, Simmons et al. (1992) showed that wetland hydric soils were more efficient at NO^-₃ removal from ground water than adjacent soil classes that were dryer and lower in organic matter. In a similar study, Cooper (1990) found that 56 to 100% of the NO^-₃ lost from ground water intercepting a stream occurred in organic riparian soils, despite the fact that these soils comprised only 12% of the stream border. This activity resulted from the location of the wetland soils in hollows that received a disproportionately large percentage of total ground water flow and contained substantial amounts of available C capable of supporting a large denitrifying community.

Several studies have shown a correlation between soil PO^3-₄ sorption and the Fe and Al oxide content of soil. Richardson (1985) showed that PO^3-₄ sorption in six different wetland soils was correlated with the extractable Al content of the soils. Similarly, Sah and Mikkelsen (1989) investigated P sorption in agricultural soils where water levels fluctuated, creating periodic flooding events. They found that equilibrium PO^3-₄ concentrations increased after flooding in soils with high organic matter content. They attributed this to changes in the Fe fractions of the soil. The fact that organic matter was necessary for this process suggested that, under flooded conditions, soils became anaerobic and Fe was reduced to soluble Fe(II) (Sah et al., 1989). After the soil was drained, the dissolved Fe precipitated and formed fresh surfaces for PO^3-₄ sorption.

We have reported research that characterized the composition of turfgrass runoff generated during storm events and subsequent N and P attenuation that occurred when runoff passed through a riparian wetland (Casey and Klaine, 2001). Runoff transported substantial loads of NO^-₃ and PO^3-₄ in all sampled events. Phosphate attenuation in the wetland reached 100% in all sampled events and NO^-₃ attenuation varied between 20 and 100%, depending primarily on the volume and intensity of the runoff event. In this same wetland, we also conducted amendment experiments that simulated runoff events (Casey and Klaine, 2001). We quantified substantial attenuation for both NO^-₃ and PO^3-₄ and determined that attenuation was occurring on the time scale of natural runoff events.

This research was designed to determine whether infrequent pulses of NO^-₃ could sustain substantial denitrification activity in a wetland soil. Denitrification enzyme activity was quantified and its magnitude was compared with literature values reported for wetlands receiving constant sources of nitrate-enriched water. Phosphate sorption parameters were used to help explain the removal of PO^3-₄ observed previously at this site during natural and artificial runoff events. These data also allowed us to determine whether pulsed inputs of nutrients resulted in an unequal distribution of denitrification and PO^3-₄ sorption capacity between the upstream and downstream sides of the wetland.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Site Description
The study site is described in detail in the companion paper (Casey and Klaine, 2001). Briefly, the study site consisted of a natural bottomland wetland that received runoff from portions of a golf course. Runoff generally infiltrated into the sandy A-horizon of an Ailey series soil, which resulted in predominantly subsurface flow through the Johnston (Cumulic Humaquepts) and Dorovan (Typic Haplosaprists) series soils in the remainder of the wetland. Overland flow was limited to high volume or high intensity storm events.

Soil Sampling
Soil samples were obtained from sites adjacent to existing ground water sampler nests. Samples from the sandy shelf area were taken immediately upgradient from Site 1–2. A detailed description of the construction and locations of the samplers is presented in the companion paper (Casey and Klaine, 2001). Briefly, ground water sampling transects were installed perpendicular to the direction of ground water flow with Transect 1 at the upstream side of the wetland next to the golf course and Transect 4 at the downstream side of the wetland. All of the soils from the wetland transects were saturated at the time of sampling due to the perennially high water table in the wetland. At the sandy shelf and Transect 1, samples were obtained using a 2-cm diameter stainless steel soil probe. At Transects 2, 3, and 4 the soils were much less consolidated and required a Macauley (or Russian) peat sampler (Eijkelkamp Agrisearch Equipment, Giesbeek, the Netherlands) to retrieve intact samples from a known depth. This device can be pushed into an unconsolidated soil and then twisted so that a metal flap moves to cover the core. This prevented contamination of the sample from upper soil layers as the core was withdrawn from the borehole. The portion of the core corresponding to 30 to 60 cm below ground surface was composited into an airtight sampling container and immediately placed on ice. Sampling devices were rinsed with distilled water between sites to prevent cross-contamination of soils. Triplicate cores were taken from each site. There were 3 sites per transect and 4 transects for a total of 36 cores taken from the 12 wetland sites plus 3 additional cores were taken from the sandy shelf area. Samples were obtained on 27 Apr. 1999 and 15 June 1999. Denitrification enzyme activity, P speciation, and organic matter content were determined for all of the April samples and for the Transect 1 samples from June. Phosphorus sorption isotherms, inorganic P concentrations, and the C limitation of denitrification were determined for the June samples. Regression analysis was used to quantify relationships between various measured variables. Statistical differences between transects were determined using an analysis of variance ( = 0.05) on the pooled data followed by a Tukey's honestly significant difference comparison of the means ( = 0.05).

Chemical Analysis
Composited cores were homogenized in their storage containers with a metal spatula before subsamples were removed for analysis. Soil moisture was determined gravimetrically after drying for 24 h at 105°C to constant weight. Volatile solids content was determined as the loss of mass after ashing oven-dried soil samples at 550°C until a constant weight was achieved (approximately 2 h). Volatile solids content was used as a surrogate measurement for organic matter content.

Inorganic P was determined by extracting 3 g of field moist soil with 25 mL of 0.5 M sulfuric acid for approximately 16 h on a rotary shaker table. After extraction, the samples were centrifuged at 3000 rpm for 20 min. The supernatant was analyzed for inorganic P by the molybdenum blue ascorbic acid spectrophotometric method (APHA, 1989). Total P was determined using the same method with the exception that the sample was first ashed at 550°C. Ashing was assumed to oxidize all organic P to inorganic P, which could be detected by the ascorbic acid method. Organic P was quantified as the difference between total and inorganic P. Determinations for total and inorganic P were made in duplicate for each homogenized core.

Phosphorus sorption isotherms were generated for selected soils. In a previous experiment where PO^3-₄ was added to the wetland in surface water, we observed that extensive attenuation occurred in the vicinity of Transect 1 (Casey and Klaine, 2001). Little to no PO^3-₄ was measured at Transects 2, 3, or 4. For this reason, sorption isotherms were generated for soil from the sandy shelf area and all sites in Transect 1. Isotherms were generated for only one site each from Transects 2, 3, and 4. Isotherms were prepared using a procedure similar to Richardson (1985). Ten mL of a solution containing 0, 50, 100, 500, or 1000 mg P L^-1 were added to a 2-g soil sample. The P solutions were prepared with KH₂PO₄ and were made up in 0.01 M CaCl₂ to provide equal ionic strength in the samples. Four drops of toluene were added to each sample as an antimicrobial agent. Soils were incubated in the P solutions for 24 h on a rotary shaker table, and then centrifuged. The equilibrium PO^3-₄ concentration was determined using the ascorbic acid method. The amount of PO^3-₄ sorbed to the soil was determined by the mass difference between the equilibrium PO^3-₄ remaining in solution and the initial PO^3-₄ present in the solution. Sorption was calculated based on dry weight equivalent of the soils.

Sorption data were fit to the linear, Freundlich, and Langmuir models. The Freundlich model gave the best fit based on higher R² values. Isotherms were modeled using the linearized from of the Freundlich equation:

where S is the concentration of PO^3-₄ sorbed to the soil sample (mg P kg soil^-1) and C is the equilibrium concentration of PO^3-₄ in solution (mg P L^-1). For the concentration of sorbed PO^3-₄, the amount of PO^3-₄ removed from solution during the incubation (S_i) was determined and added to the initial PO^3-₄ concentration in the soil (S_o) so that S represented total sorbed PO^3-₄ (S_i + S_o); K is the Freundlich sorption coefficient and the constant b is a measure of the nonlinearity of the isotherm.

The equilibrium P sorption concentration (P_E) was calculated as the aqueous concentration at which PO^3-₄ was in equilibrium between the aqueous and sorbed phases and no net sorption or desorption occurred. The P_E was calculated by setting S = S_o and solving for C using the linearized Freundlich equation. Because USEPA (1986) recommends that PO^3-₄ levels remain below 0.05 mg L^-1 for waters entering impoundments, the concentration of sorbed PO^3-₄ that would lead to an equilibrium aqueous PO^3-₄ concentration of 0.05 mg P L^-1 (P_0.05) was also calculated.

Denitrification Assays
Denitrification rates were quantified using an application of the acetylene block method similar to that of Smith and Tiedje (1979). Because soils were saturated and unconsolidated, it was necessary to use soil slurries instead of intact cores. For determination of denitrification rates, 10 g of field moist soil was weighed into a 125-mL Erlenmeyer flask and 9 mL of de-aerated water was added. Headspace was purged with a stream of N₂ gas and the flask was stoppered. A vortex mixer was used to break up any large soil particles and create a uniform slurry. Flasks were placed on a rotary shaker table for a 12-h preincubation to ensure anaerobiosis for the denitrification assay. Nitrate concentrations were below the detection limit (0.1 mg NO₃–N L^-1) in soil samples before the addition of the incubation solution. Thus synthesis and reactivation of denitrifying enzymes were not expected to occur during the preincubation. After preincubation, 10 mL of headspace gas was removed and replaced with 10 mL of acetone-free acetylene. One mL of a concentrated treatment solution was added to each flask. For determination of denitrification enzyme activity, the treatment solution contained 2 g NO₃ L^-1, 4 g dextrose L^-1, and 5 g chloramphenicol L^-1. For determination of C-limited denitrification rates, the treatment solution contained only NO^-₃ and chloramphenicol. The chosen chloramphenicol concentration was near or slightly above its solubility at room temperature, so the solutions were heated to approximately 45°C to dissolve all chloramphenicol.

Gas samples were obtained at a minimum of three time points during the incubation. Due to differences in the magnitude of N₂O production between samples, incubations lasted between 24 and 72 h. Incubations of this length were required to observe N₂O production in samples having low rates given the limit of quantitation for N₂O in our laboratory (1 µL L^-1). Incubations with higher production were thus terminated earlier than incubations with low production. For all samples with measurable N₂O, production was linear for the range of incubation times that was used. Three mL of headspace gas was removed and placed in a 2-mL screw-top vial that had been purged with N₂ gas and then evacuated. Nitrous oxide (N₂O) was quantified using a Hewlett Packard 5890A gas chromatograph equipped with an electron capture detector and a 30-m GS-Q column (J&W Scientific, Folsom, CA). Injections of 0.5 mL were made on-column at 50°C with a column temperature of 40°C and a detector temperature of 300°C. The carrier gas was H₂ at a flow rate of 4.3 mL min^-1. Correction for aqueous N₂O was made using a Bunsen coefficient of 0.67 and the total volume of water in each flask, including soil water added with the samples. Denitrification rates were determined in duplicate for each homogenized core and were based on dry weight equivalent for each soil. Statistical differences based on C additions were determined using paired t-tests ( = 0.05).

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
We observed substantial spatial heterogeneity in several of the physical parameters and mechanisms of nutrient retention within the riparian wetland. Soil moisture and, to a lesser extent, organic matter both increased in a gradient from upstream to downstream in the wetland (Tables 1 and 2). Regression analysis indicated that wetter soils had higher organic matter content (R² = 0.96 between soil moisture and % organic matter). Denitrification enzyme activity (Table 3) was significantly elevated at Transect 1 with respect to other areas of the wetland. Our measurements of denitrification enzyme activities and rates exhibited high variability with coefficients of variation between 22 and 100%. Comparison of denitrification enzyme activities at Transect 1 between sampling dates showed no significant difference in activity, either using pooled data for the entire transect or on a site-by-site basis (P > 0.4). Regression analysis indicated an absence of correlation between denitrification and % solids, % organic matter, or P concentrations in this system.

View larger version (19K): [in this window] [in a new window]   Fig. 1. Denitrification enzyme activities (C-amended) and C-limited denitrification rates at Transect 1 (µg N2O–N kg soil-1 h-1). Error bars represent SD calculated using the means of three cores per site, n = 3; *Significant at the 0.05 probability level.

The Freundlich equation provided good fit for the isotherm data (R² between 0.84 and 0.99). The value of K, which is proportional to the adsorption maximum (Sparks, 1986), was high throughout the wetland with the exception of the sandy shelf (Table 5). Phosphate sorption was dramatically lower in the sandy shelf samples resulting in a much lower value of K. The value of b in the Freundlich isotherms was generally consistent among all of the wetland soils, with the exception of the sandy shelf. In all cases, b was <1.0, suggesting that sorption occurs at a limited number of sites, which can become saturated.

The calculated concentration for aqueous PO^3-₄ to be in equilibrium with P sorbed to soils in the field (P_E) was uniformly low throughout the wetland (Table 5). The calculated concentration was <1 µg L^-1, indicating that conditions in the wetland at the time of sampling strongly favored PO^3-₄ sorption to soils. Thus, the wetland soil should act as a sink for PO^3-₄ concentrations above the P_E. In contrast, the sandy shelf had a P_E over 5 mg P L^-1, a result of high soil PO^3-₄ concentration and low sorption affinity.

The sorbed PO^3-₄ level calculated to occur with an aqueous concentration of 0.05 mg P L^-1 (P_0.05) showed that the wetland has a large capacity for PO^3-₄ retention through sorption. The P_0.05 indicated that the wetland could retain approximately two orders of magnitude more PO^3-₄ through soil sorption than is currently present and still not exceed USEPA recommendations for PO^3-₄ levels in lentic waters (Table 5). Again, the behavior of the sandy shelf soil differed from the wetland soil with a P_0.05 concentration much lower than the PO^3-₄ level in the soil at the time of sampling, indicating that those soils may be acting as a PO^3-₄ source.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
This study was conducted to determine whether denitrification was a viable mechanism for the nutrient attenuation observed in a wetland receiving only intermittent exposures of nitrate. Few studies have shown whether infrequent pulses of NO^-₃ are sufficient to maintain substantial denitrifying communities. In a previous study, we observed considerable NO^-₃ attenuation over a 12-h time span (Casey and Klaine, 2001), which suggested that a microbial process such as denitrification might be important for the observed NO^-₃ removal. The results of the denitrification assay confirmed that intermittent exposures of NO^-₃ in storm runoff were sufficient to support denitrification in the wetland, especially at Transect 1. This transect was closest to the golf course and had the highest exposure to nitrate-enriched runoff water. Transects further down-gradient intercepted lower concentrations of NO^-₃ due to preceding attenuation at upstream transects and dilution with uncontaminated pre-event ground water.

Other reports have noted that organic matter often limits denitrification in many soils. Soils throughout the wetland contained high levels of organic matter that should be able to sustain denitrifying communities. Additionally, dissolved oxygen levels in the wetland ground water were generally below measurable limits in the field (data not shown). The existence of conditions favoring denitrification and the relatively low activity at Transects 2, 3, and 4 indicated that the development of denitrifying communities at those transects was likely limited by NO^-₃ supply. Nitrate limitation may also explain why no significant correlations were observed between denitrification and physical soil parameters at this site, suggesting that the magnitude of denitrification is related to NO^-₃ availability.

Denitrification rates measured at Transect 1 were similar to rates obtained by Groffman and Tiedje (1989) and Groffman et al. (1992) in poorly drained riparian forest soils. However, Cooper (1990) and Ambus and Lowrance (1991) measured denitrification activities up to 2600 and 1317 µg N₂O–N kg soil^-1 h^-1, respectively, in organic riparian soils, which greatly exceed any of the rates measured at this site. The difference between our findings and these may have been due to different NO^-₃ exposure regimes between study sites. Both Cooper (1990) and Ambus and Lowrance (1991) studied areas where NO^-₃ concentrations in the riparian wetland were consistently high from continual ground water NO^-₃ inputs. At this site, NO^-₃ only entered the wetland in surface runoff during storm events and not through continuous ground water inputs. Volume-weighted concentrations ranged from 0.2 to 7.7 mg NO₃–N L^-1 (Casey and Klaine, 2001). The inconsistent pulsed NO^-₃ loading at our study site appears to support less denitrifying activity than riparian areas with constant NO^-₃ inputs.

Ambus and Christensen (1993) determined denitrification rates and potentials in a riparian zone receiving tile drain effluent from agricultural fields. They found that the highest activities occurred when the water level was elevated and runoff from the upland soils took place. Activities were highest near the field border of the plots and decreased toward the stream, similar to our results.

The presence of denitrification as a mechanism of NO^-₃ removal has important consequences for the longevity of NO^-₃ attenuation in the wetland. Denitrification results in conversion of aqueous NO^-₃ to gaseous species that are then removed from both terrestrial and aquatic components of this system. This implies that anthropogenic NO^-₃ inputs from the adjacent golf course are unlikely to lead to saturation of the N removal mechanisms present in this wetland.

The denitrification rates measured in this experiment help explain the NO^-₃ attenuation observed at this site during both natural and artificial runoff events (Casey and Klaine, 2001). The overall mean of denitrification enzyme activities for this wetland was 15.2 µg N₂O–N kg soil^-1 h^-1. Given that the wetland is 200 m by 30 m and estimating several parameters, a range of possible N transformation rates were calculated. The most conservative estimate was obtained using an active depth of 0.3 m (the length of composited core used for the denitrification determinations) and a bulk density of 0.5 g mL^-1 for the organic soils in the wetland. This resulted in an N transformation rate of 13.7 g N h^-1. Assuming the top 1.0 m of the water table as the active depth of denitrification and a bulk density of 1.0 g mL^-1 (because the wetland is a combination of organic and mineral soils), a less conservative rate of 91.2 g N h^-1 was obtained.

For our 30-h artificial runoff experiment, the range of N transformation due to denitrification based on the calculations would be 0.4 to 2.7 kg NO₃–N. This range explains some or all of the observed N attenuation of the 1.1 to 2.2 kg NO₃–N added in these experiments. Similarly, natural storm events were generally sampled for 12 h and had NO₃–N loads ranging from 0.002 to 1.47 kg NO₃–N with observed attenuations near 100%. The expected range of N transformation due to denitrification for an event of this duration is 0.16 to 1.09 kg NO₃–N, which explains much of the observed attenuation.

In previous studies at this site, we observed extensive PO^3-₄ attenuation in the wetland (Casey and Klaine, 2001). In storm events generating runoff from the adjacent golf course, PO₄–P inputs ranged from 1 to 4156 g PO₄–P but PO₄–P was never detected in water leaving the wetland. In addition, when we added high concentrations of PO^3-₄ (>22 mg PO₄–P L^-1) to the wetland in artificial runoff events (total loads of 1.6 and 2.8 kg PO₄–P), attenuation occurred immediately upon PO^3-₄ entering the wetland and was complete by Transect 2. Evidence of this attenuation was present in the distribution of inorganic P in the wetland soils. Phosphate concentrations were highest where turfgrass runoff first entered the wetland and were uniformly low elsewhere in the wetland.

The PO^3-₄ sorption isotherms generated in this study explained the magnitude of attenuation that we observed in both the natural and artificial runoff events. The isotherms demonstrated that these soils have a high capacity for P sorption. The Freundlich constant K is proportional to the adsorption maximum for a material. The values of K that we determined were slightly higher than those reported by Scheinost and Schwertmann (1995) for agricultural soils and those reported by Heikkinen et al. (1995) for peat soils. However, the K values were more than an order of magnitude higher than values reported by Richardson and Vaithiyanathan (1995) for Everglades peat soils.

The values of P_E confirmed that aqueous concentration of PO^3-₄ in the field should have been below our detection limit in the wetland ground water. In previous experiments, we were unable to detect PO^3-₄ in background samples of ground water in this wetland. The P_E values indicated that the sandy shelf would act as a PO^3-₄ source if PO^3-₄ concentrations in runoff dropped <5.4 mg P L^-1. During storm event sampling, we often observed concentrations below that level (Casey and Klaine, 2001).

The P_0.05 parameter represents the soil PO^3-₄ concentration at which aqueous PO^3-₄ reaches 0.05 mg PO₄–P L^-1. If soil PO^3-₄ concentrations exceed the P_0.05, adverse effects may occur when water infiltrating through these soils enters reservoirs and lakes, but below the P_0.05 water quality in most systems should be protected. Calculated PO^3-₄ levels in the wetland were 100 times less than the P_0.05, indicating that these soils can continue to immobilize a substantial amount of PO^3-₄ and protect water quality in downstream systems. It should be noted that quantified extrapolation of the sorption parameters from the isotherms directly to the field would not be appropriate. Under field conditions, transport constraints may prevent PO^3-₄ from reaching all available binding sites. In contrast, the isotherms were generated using well-mixed samples. However, at the time of sampling this golf course had been in operation for 7 yr and runoff from this time period had not caused saturation of the PO^3-₄ binding capacity in the wetland, as demonstrated by the artificial runoff event data (Casey and Klaine, 2001). This suggests that binding will continue to occur for some time into the future.

One of the factors that will influence the capacity of the wetland to act as a P sink is the assimilation of PO^3-₄ into biomass. We observed substantially lower PO^3-₄ concentrations in June samples (Table 5) from Transect 1 and the sandy shelf compared with the April samples (Table 4). The April soil sampling occurred 11 d after a natural runoff event, whereas June samples were obtained 44 d after a natural runoff event. The storm preceding the April sampling transported 54 g PO₄–P into the wetland (Casey and Klaine, 2001). Phosphorous transport was not measured for the storm preceding the June sampling, but no additional P fertilizers had been applied since the April storm. It is possible that the decrease in PO^3-₄ concentrations from the April to June samples occurred because there was increased time for biological assimilation of PO^3-₄ before the June sampling. It is reasonable to suspect that P demands of the vegetation in the wetland would be high during the period of late spring and early summer. Macrophytes and microbes may remove sorbed PO^3-₄ from soil surfaces for use as a nutrient during periods of growth. This would serve to renovate sorption sites in the wetland soils and could decrease the impacts of anthropogenic PO^3-₄ inputs on the wetland soils. However, interactions between biological and soil components of the wetland have yet to be investigated in this system. Further research in this area would facilitate estimates of the lifetime of the system as a P sink.

	ACKNOWLEDGMENTS

 
The authors thank Alper Elci and Augusta Alencar for assistance with the soil sampling. We also thank Neil Westbrook and Rick Smith of Cheraw State Park for their enthusiasm and cooperation with this project. Funding was provided in part by the South Carolina Department of Parks, Recreation and Tourism and the South Carolina Agriculture and Forestry Commission Turfgrass Initiative.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
South Carolina Agric. and Forestry Res. System Clemson Univ. Technical Contribution no. 4560.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

• Ambus, P., and S. Christensen. 1993. Denitrification variability and control in a riparian fen irrigated with agricultural drainage water. Soil. Biol. Biochem. 25:915–923.
• Ambus, P., and R. Lowrance. 1991. Comparison of denitrification in two riparian soils. Soil Sci. Soc. Am. J. 55:994–997.[Abstract/Free Full Text]
• American Public Health Association. 1989. Standard methods for the examination of water and wastewater. 17th ed. APHA, Washington, DC.
• Brown, K.W., J.C. Thomas, and R.L. Duble. 1982. Nitrogen source effect on nitrate and ammonium leaching and runoff losses from greens. Agron. J. 74:947–950.[Abstract/Free Full Text]
• Casey, R.E., and S.J. Klaine. 2001. Nutrient attenuation by a riparian wetland during natural and artificial runoff events. J. Environ. Qual. 30:1720–1731 (this issue).[Abstract/Free Full Text]
• Cooper, A.B. 1990. Nitrate depletion in the riparian zone and stream channel of a small headwater catchment. Hydrobiologia 202:13–26.
• Groffman, P.M., A.J. Gold, and R.C. Simmons. 1992. Nitrate dynamics in riparian forests: Microbial studies. J. Environ. Qual. 21:666–671.[Abstract/Free Full Text]
• Groffman, P.M., and J.M. Tiedje. 1989. Denitrification in north temperate forest soils: Spatial and temporal patterns at the landscape and seasonal scales. Soil Biol. Biochem. 21:613–620.
• Heikkinen, K., R. Ihme, A. Osma, and H. Hartikainen. 1995. Phosphate removal by peat from peat mining drainage water during overland flow wetland treatment. J. Environ. Qual. 24:597–602.[Abstract/Free Full Text]
• Jordan, T.E., D.L. Corell, and D.E. Weller. 1993. Nutrient interception by a riparian forest receiving inputs from adjacent cropland. J. Environ. Qual. 22:467–473.[Abstract/Free Full Text]
• Linde, D.T., and T.L. Watschke. 1997. Nutrients and sediment in runoff from creeping bentgrass and perennial ryegrass turfs. J. Environ. Qual. 26:1248–1254.[Abstract/Free Full Text]
• Lowrance, R. 1992. Groundwater nitrate and denitrification in a coastal plain riparian forest. J. Environ. Qual. 21:401–405.[Abstract/Free Full Text]
• Richardson, C.J. 1985. Mechanisms controlling phosphorus retention capacity in freshwater wetlands. Science (Washington, DC) 228: 1424–1427.[Abstract/Free Full Text]
• Richardson, C.J., and P. Vaithiyanathan. 1995. Phosphorus sorption characteristics of Everglades soils along a eutrophication gradient. Soil Sci. Soc. Am. J. 59:1782–1788.[Abstract/Free Full Text]
• Sah, R.N., and D.S. Mikkelsen. 1989. Phosphorus behavior in flooded-drained soil: I. Effects on phosphorus sorption. Soil Sci. Soc. Am. J. 53:1718–1722.[Abstract/Free Full Text]
• Sah, R.N., D.S. Mikkelsen, and A.A. Hafez. 1989. Phosphorus behavior in flooded-drained soil: II. Iron transformation and phosphorus sorption. Soil Sci. Soc. Am. J. 53:1723–1729.[Abstract/Free Full Text]
• Scheinost, A.C., and U. Schwertmann. 1995. Predicting phosphate adsorption–desorption in a soilscape. Soil Sci. Soc. Am. J. 59:1575–1580.[Abstract/Free Full Text]
• Simmons, R.C., A.J. Gold, and P.M. Groffman. 1992. Nitrate dynamics in riparian forests: Groundwater studies. J. Environ. Qual. 21:659–665.[Abstract/Free Full Text]
• Smith, M.S., and J.M. Tiedje. 1979. Phases of denitrification following oxygen depletion in soil. Soil Biol. Biochem. 11:261–267.
• Sparks, D.J. 1986. Soil physical chemistry. CRC Press, Boca Raton, FL.
• U.S. Environmental Protection Agency. 1986. Quality criteria for water. USEPA Rep. 440/5-86-001. USEPA, Office of Water Regulations and Standards. U.S. Gov. Print. Office, Washington, DC.

This article has been cited by other articles:

				J. H. Davis, S. M. Griffith, W. R. Horwath, J. J. Steiner, and D. D. Myrold Denitrification and Nitrate Consumption in an Herbaceous Riparian Area and Perennial Ryegrass Seed Cropping System Soil Sci. Soc. Am. J., September 1, 2008; 72(5): 1299 - 1310. [Abstract] [Full Text] [PDF]

				A. Bedard-Haughn, K. W. Tate, and C. van Kessel Using Nitrogen-15 to Quantify Vegetative Buffer Effectiveness for Sequestering Nitrogen in Runoff J. Environ. Qual., November 1, 2004; 33(6): 2252 - 2262. [Abstract] [Full Text] [PDF]

				P. M. Groffman and M. K. Crawford Denitrification Potential in Urban Riparian Zones J. Environ. Qual., May 1, 2003; 32(3): 1144 - 1149. [Abstract] [Full Text] [PDF]

				G. Vellidis, R. Lowrance, P. Gay, and R. K. Hubbard Nutrient Transport in a Restored Riparian Wetland J. Environ. Qual., March 1, 2003; 32(2): 711 - 726. [Abstract] [Full Text] [PDF]

				R. E. Casey and S. J. Klaine Nutrient Attenuation by a Riparian Wetland during Natural and Artificial Runoff Events J. Environ. Qual., September 1, 2001; 30(5): 1720 - 1731. [Abstract] [Full Text] [PDF]

This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (17) Citing Articles via Google Scholar Google Scholar Articles by Casey, R. E. Articles by Klaine, S. J. Search for Related Content PubMed PubMed Citation Articles by Casey, R. E. Articles by Klaine, S. J. GeoRef GeoRef Citation Agricola Articles by Casey, R. E. Articles by Klaine, S. J. Related Collections Water Quality Wetlands and Aquatic Processes Soil Chemistry