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a Biological and Agricultural Engineering Dep., Univ. of Georgia, Tifton, GA 31793-0748
b Southeast Watershed Research Laboratory, USDA-ARS, Tifton, GA 31793
* Corresponding author (yiorgos{at}tifton.uga.edu)
Received for publication April 29, 2002.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONS FOR RIPARIAN BUFFER...REFERENCES |
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Abbreviations: DRP, dissolved reactive phosphorus • LIFE, low-impact flow event • TKN, total Kjeldahl nitrogen
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONS FOR RIPARIAN BUFFER...REFERENCES |
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Surface runoff and subsurface flow from farm animal waste land application sites can provide significant loadings of nutrients to receiving waters unless appropriate management techniques are employed. Grass buffer zones or vegetative filter strips (VFS) have been investigated as a means of reducing nutrient loadings in streams by minimizing pollutant concentration in manure-polluted runoff (Doyle et al., 1977; Dickey and Vanderholm, 1981; Overcash et al., 1981). The buffer zones reduce runoff by increasing infiltration, which may increase the nutrient loading of ground water. Recent studies have focused on the use of grass and forest buffers as treatment areas for liquid manure (Hubbard et al., 1998b). Dillaha et al. (1989) and Magette et al. (1989) found that VFS with buffer strip to waste area length ratios of 0.25 and 0.5 were effective in removing sediment from runoff, but did not reliably reduce nutrient losses from agricultural areas. A buffer strip to waste area length ratio of 1.0 is usually required to reduce nutrient loading, thus making the cost of the buffer zone a major factor in the cost–benefit analysis of land application waste treatment (Bingham et al., 1980).
Riparian ecosystems can also be used to control nonpoint pollution. Lowrance et al. (1984)(1985) and Peterjohn and Correll (1984) demonstrated that riparian forest ecosystems of coastal plain agricultural watersheds are excellent nutrient sinks that buffer the nutrient discharge from surrounding agroecosystems. Lowrance et al. (1984), Peterjohn and Correll (1984), and Lowrance (1992) showed that nutrient uptake and removal by soil and vegetation in the riparian forest ecosystem prevented agricultural upland outputs from reaching stream channels. They concluded that the riparian ecosystem can serve as both a short- and long-term nutrient filter and sink if aboveground vegetative biomass is periodically harvested to ensure a net uptake of nutrients. Forested riparian wetlands have also been shown to function as nutrient sinks and filters for land-treated waste application of municipal sewage (Turner et al., 1976; Sloey et al., 1978). Scientific studies of restoration efforts are relatively recent and few in number and causal relationships between restoration techniques and long-term responses of particular ecosystem process are poorly understood (Kusler and Kentula, 1990).
Information on storage and removal of nutrients migrating through the riparian zone is still lacking for riparian areas during the first years following restoration. Although mature riparian forests have been shown to be excellent nutrient sinks and buffers, little research has been conducted on the effectiveness of newly established riparian buffers. Licht and Schnoor (1990) used densely planted poplar (Populus spp.) trees to provide a riparian buffer strip for conventional row crop agricultural land in Iowa. They found that in some instances, the reestablished riparian forest decreased nitrate concentrations in the shallow soil profile from 25 to 5 mg NO3–N kg-1 dry soil during the first year of reestablishment. Clausen et al. (2000) studied nutrient transport and developed N budgets for a restored fescue (Festuca spp.) buffer in Connecticut. They found that loads and concentrations of nitrate, total Kjeldahl nitrogen (TKN), and total P were reduced in runoff and that the concentration of nitrate in ground water was reduced by 35% compared with the control, which was an unrestored riparian corn (Zea mays L.) field. Oelbermann and Gordon (2000) found that a 12-year-old riparian forest buffer restored with northern hardwood species in Ontario provided large quantities of N in litterfall to the adjacent stream. They hypothesized that the restored forest buffer converted inorganic N inputs to organic N outputs to the stream, an observation made for mature buffers by Lowrance et al. (1983).
Although a number of studies of riparian forest buffer restoration were begun in the 1990s (Schultz et al., 1995; Schuetz et al., 1994; Vellidis et al., 1994), and some components of these studies have been reported (Tufekcioglu et al., 2001, Lee et al., 1999., Lowrance et al., 1995), there are still very few estimates of the ability of restored riparian forest buffers to remove nutrients from agricultural runoff and shallow ground water. Plot-scale studies of mature forest buffers have provided guidance on the use of buffers for water quality renovation, but studies are needed that examine the effects of riparian buffer restoration on stream nutrient levels (Dosskey, 2000) and compare these stream nutrient levels to suggested water quality standards.
Because much of the emphasis in buffer zone studies has been on N, there are still substantial unanswered questions about the effects of forest buffers and wetlands on P transport. The function of riparian buffers in control of P is particularly important in manure application areas that might already be overloaded with P and because the near-stream area is known to be the most critical for P discharge (Heathwaite et al., 2000; Smil, 2000; Sharpley et al., 1999; Hubbard et al., 1998a).
The purpose of this study was to determine the effects of a restored forested riparian wetland buffer system on surface and subsurface transport of nitrogen and phosphorus entering the buffer from adjacent upland agricultural production sites, including a liquid manure application area and a pasture. The site was also used for studies of denitrification removal of nitrate (Lowrance et al., 1995) and herbicide transport through the riparian buffer (Vellidis et al., 2002).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONS FOR RIPARIAN BUFFER...REFERENCES |
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On the upland southwest of the Dairy Wetland (Fig. 2) , a 5.6-ha center pivot irrigation system applies liquid manure derived from flush cleaning of a 120-dairy cow facility at a nitrogen application rate of 600 kg N ha-1 yr-1 (Vellidis et al., 1996). Manure application began in July 1991. Liquid manure is applied year-round on a biweekly schedule and results in an additional 760 mm yr-1 of water to the field. The cropping system under the pivot is overseeding of ‘Abruzzi’ rye (Secale cereale L.) into ‘Tifton 44’ bermudagrass [Cynodon dactylon (L.) Pers.] sod in the fall, followed by minimum tillage planting of silage corn into the bermudagrass and rye stubble in the spring, followed by summer crops of hay or silage from the residual bermudagrass. During the summer, the hay is harvested on a monthly schedule. The year-round sod cover minimizes erosion and reduces runoff by promoting infiltration.
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Dairy Wetland Hydrology
The Dairy Wetland, approximately 1 ha in size, is located on an intermittent first-order stream. The upland areas surrounding the Dairy Wetland typically have a plinthic layer at a depth of 1 to 1.5 m. As in the wetland, the plinthite acts as an aquitard and, during periods of high rainfall, is responsible for the formation of transient perched water tables. From May to December, surface runoff generally occurs during intense rainfall events. During the winter and early spring months, when the soil profile is often saturated, ground water seeps along the southern perimeter of the wetland. The seepage, in combination with frequent runoff events, results in stream flow through the wetland during the winter and early spring. The boundaries of the restored area (Fig. 2) are similar to the boundaries of the area delineated by the USDA Natural Resources Conservation Service as a wetland based on hydrology, hydric soils, and wetland vegetation. This fits the general pattern of the Coastal Plain, where all or parts of riparian forests are frequently delineated as wetlands.
Dairy Wetland Restoration
The mature riparian forest of the Dairy Wetland was logged in 1985 and replaced with a wet pasture. Ditches were dug after logging to facilitate drainage into a farm pond constructed downslope. Two smaller ditches in the headland met to form a larger ditch that carried intermittent streamflow. Over the next five years, the ditches filled in with eroded sediment.
Restoration began in February 1991, when a three-zone riparian buffer system, as prescribed by USDA Forest Service specifications (Welsch, 1991), was established by planting hardwoods in Zone 1 and slash pine in Zone 2 (Fig. 3) (Vellidis et al., 1993). Zone 1 is a 10-m-wide band of trees with mixed hardwoods in rows including swamp blackgum (Nyssa sylvatica Marshall), green ash (Fraxinus pennsylvanica Marshall), and yellow poplar (Liriodendron tulipifera L.). Zone 2 is a 20-m-wide band of slash pine (Pinus elliottii Engelm.) in rows. Rows were about 2 m apart. No trees were planted under the overhang of the pivot. This area is considered to be a modified Zone 2. Vegetation in the modified Zone 2 was all volunteer and consisted of a mixture of silverling (Baccharis halimifolia L.), black willow (Salix nigra Marshall), grasses (especially crowngrass [Paspalum spp.]), and other herbaceous species. On the west, south, and east sides of the Dairy Wetland, Zone 3 is an 8-m-wide strip of ‘Tifton 44’ bermudagrass. The entire three zone buffer averages 38 m in width.
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Monitoring Well Network
The original nutrient monitoring well network consisted of 63 PVC wells, 50 mm in diameter, and screened with a 0.25-mm well screen from 0.1 to 0.8 m below the soil surface. Twenty-three wells in six transects were installed on the west slope, 18 wells in three transects were installed on the south slope, and 22 wells in six transects were installed on the east slope of the Dairy Wetland. Within a transect, wells were installed 5 m apart near the upland–wetland interface and 10 m apart within the wetland. These wells were installed using the methods described by Vellidis et al. (1994). Henceforth they are referred to as "original shallow wells" (0.1–0.8 m).
Beginning in January 1993 a network of 72 monitoring wells appropriate for pesticide transport studies was established on the east slope of the Dairy Wetland, replacing the 22 original east slope nutrient wells. The network consisted of 36 shallow and 36 deep wells. Shallow wells were screened from 0.1 to 0.6 m below the soil surface while deep wells were screened from 0.6 to 2.0 m below the soil surface. Henceforth these wells are referred to as "new shallow wells" (0.1–0.6 m) and "deep wells" (0.6–2.0 m). Wells were assembled from threaded PVC pipe and screened with a 0.25-mm well screen. The wells were installed using the methods described by Lowrance et al. (1997). Six well transects were installed with each transect containing 12 wells—a shallow and deep well pair at each of six positions within the transect. The transect positions, in meters from the upslope edge of the Zone 3 grass buffer were: 0, 8 (the upslope edge of the Zone 2 managed forest buffer [pines]), 13, 18, 28 (upslope edge of the Zone 1 forest buffer [hardwoods]), and 38 m (near the stream channel) (Fig. 4) .
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Surface Runoff Collector Network
Surface flow through the Dairy Wetland was monitored with a large H-flume at the wetland outlet and two types of runoff collectors. Paired gutter and flume collectors installed on the west and south slopes of the site were later supplemented with low-impact flow event runoff samplers installed on the east slope of the Dairy Wetland. The runoff collectors were designed to provide specific information on nutrient uptake and removal by wetland soil and vegetation types as nutrient fronts from the upland migrated through the wetland. The outlet flume provided data on the overall effectiveness of the Dairy Wetland in attenuating nonpoint source pollution in intermittent streamflow.
Gutter and Flume Runoff Collectors.
In 1992, a total of four gutters were installed in pairs on the west and south slopes, with each pair consisting of an upslope and a downslope gutter. The gutters on the west slope sampled water entering and leaving the forested Zone 2—a distance of approximately 25 m. The gutters on the south slope sampled water entering and leaving the modified Zone 2—a distance of approximately 20 m. Because the upslope gutter of this pair was located within the seepage area at the southern perimeter of the Dairy Wetland, some of the overland flow it intercepted originated as seepage. Runoff was collected in a 3.6-m-long galvanized sheet metal gutter, passed through a 200-mm modified Tucson flume, and redistributed through a 3.6-m-long slotted gutter.
A 450-mm H-flume installed at the wetland outlet was used to measure surface water quality and quantity discharged into the farm pond. Tapered earthen berms approximately 10 m long were constructed on either side of the H-flume to route surface flow (Fig. 2).
The five flumes were equipped with 5-FW1 strip chart stage recorders (Belfort Instrument, Baltimore, MD) and 96-h charts. Discharge was determined by manually digitizing runoff event hydrographs and integrating beneath the resulting curves to obtain total flow volume per event. Composite water samples of runoff events were collected from the four modified Tucson flumes and the H-flume with battery-powered peristaltic pumps using the methods described by Vellidis et al. (1994). During runoff events, the pumps were switched on by electro-optic liquid level switches installed in the flume stilling wells. Multiple events in a day were not collected separately. The 13-L composite sample jars were returned to the laboratory and a well-mixed 500-mL subsample was poured from the jar into a 500-mL glass bottle for storage at 4°C until analyzed. If less than 500 mL was collected, the entire sample was stored.
Low-Impact Flow Event Runoff Samplers.
During 1993, as part of the herbicide study, three transects of four 0.3-m-wide low-impact flow event (LIFE) samplers (Sheridan et al., 1996) were installed to sample surface runoff on the east slope of the Dairy Wetland (Vellidis et al., 2002). In each transect, the collectors were installed at the upslope edge of Zones 1, 2, and 3 and at the midpoint of Zone 2 (Fig. 5) . Two transects had instruments that retained 10% of the collected sample. The third transect had instruments that retained 1% of the collected sample. This design ensures that a measurable volume can be collected over a wide range of runoff events. The 10% collection is made by splitting the flow into 10 pathways at the back of the collector and collecting flow from one pathway. The 1% sample is collected by connecting two 10% samples in series. The sample receptacle is large enough to contain runoff from approximately a 10-year return interval event in the 1% samplers (Vellidis et al., 2002). The samplers were positioned so as not to interfere with surface runoff collection at the next zonal interface. The three samplers at the upslope end of Zone 3 are designated Position 1; the upslope end of Zone 2 is Position 2. The middle of Zone 2 is Position 3 and the upslope end of Zone 1 is Position 4 (Fig. 5). Having two types of samplers (10 and 1%) allowed both large and small runoff events to be sampled and runoff volumes obtained. Samples from all collectors that had volumes greater than 100 mL were used for each surface runoff event.
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All ground water and surface runoff samples were analyzed for NO3–N, NH4–N, dissolved reactive phosphorus (DRP), and Cl- with USEPA-approved colorimetric techniques (Clesceri et al., 1998). The TKN was quantified with digestion and titration techniques adapted from USEPA-approved methods (Clesceri et al., 1998). Total P was quantified with digestion and USEPA-approved colorimetric techniques (Clesceri et al., 1998). Total N was the sum of nitrate plus TKN.
Data Analysis
For both ground water and surface runoff, data were summarized seasonally with winter consisting of data from January, February, and March; spring consisting of April, May, and June; summer consisting of July, August, and September; and autumn consisting of October, November, and December.
Shallow Ground Water
For each monitoring well, mean seasonal concentration data were developed from the entire period of record (1992–1999). These seasonal means for each of the 114 wells were used to develop nutrient surface concentration maps and loading rates.
Surface concentration maps were developed with the kriging geostatistical gridding method. Kriging is a flexible gridding method that incorporates anisotropy and underlying trends in an efficient and natural manner. Kriging attempts to express trends suggested in the data, so that, for example, high points might be connected along a ridge rather than isolated by bull's eye–type contours (Deutsch and Journel, 1992; Cressie, 1991; Journel, 1989; Isaaks and Srivastava, 1989; Journel and Huijbregts, 1978).
To quantify nitrogen attenuation in shallow ground water within the Dairy Wetland, seasonal NO3–N loads were developed for wells along the perimeter of the wetland and for downslope wells near the stream. The loads were calculated by applying average seasonal concentrations for the wells to Darcian flows calculated through the saturated thickness of the soil profile within the effective sampling depth of the wells (Bouwer, 1978) and applied to the contributing area assigned to each well (Fig. 6) . Bail tests (Freeze and Cherry, 1979) of each well conducted during autumn 1998 and winter and spring 1999, and the Hvorslev (1951) method were used to determine saturated hydraulic conductivity of the soil around each well. Hydraulic gradient was calculated using water table elevations of the perimeter wells and the downslope wells near the stream. Loads were calculated only for the autumn, winter, and spring seasons as bail tests could not be performed during the summer. Inputs to the Dairy Wetland were calculated based on the upper tiers of wells along the perimeter of the riparian zone. Ground water outputs from the wetland were calculated from the near-stream tiers of wells.
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Nutrient loads entering the Dairy Wetland in surface runoff were estimated by calculating a load per unit length of interface (g-1) for the west, south, and east slopes and applying that load to the total length of the perimeter. The length of interface was 87, 52, and 160 m for the west, south, and east slopes, respectively (Fig. 6).
Nutrient loads leaving the Dairy Wetland in surface flow were calculated by applying measured concentrations to flows measured at the wetland outlet H-flume. Outputs were calculated from both storm events and baseflow with most baseflow occurring during the winter and early spring. For all runoff collectors, individual runoff event loads were calculated by multiplying concentration by volume of each measured runoff event. Annual loads were calculated by summing individual event loads during each year. Mean annual loads were determined by averaging available annual loads.
Water and Nutrient Mass Balances
Water and nutrient mass balances were calculated from entering surface runoff, entering subsurface flow, precipitation, exiting streamflow, exiting subsurface flow, and evapotranspiration. The length of record was different for the gutter and flume samplers and the LIFE samplers. Therefore, all mass balance components were available only for the 1993 to 1994 time period when the LIFE collectors were in place. Although the time periods for different components of the mass balance differ, we used mean annual loads for inputs and outputs to construct an average mass balance for the Dairy Wetland. Mean annual subsurface flow and surface runoff inputs and outputs were calculated as described above. Data from two National Atmospheric Deposition Program (NADP) network collectors (GA50 and GA99) were used to estimate precipitation amounts, nitrogen deposition, and chloride deposition for the site for 1992–1999 (http://nadp.sws.uiuc.edu/nadpdata/; verified 15 Oct. 2002). The NADP collectors are located about 3 km from the Dairy Wetland. Data collected on Little River watershed in 1979–1981 were used to estimate the total P input to the dairy wetland in precipitation (Lowrance et al., 1985). Evapotranspiration losses were estimated from earlier studies of water balance in riparian buffers on the Little River watershed (Lowrance et al., 1983). Balances were calculated by subtracting water volume or nutrient mass measured leaving the Dairy Wetland from water volume or nutrient mass measured entering the wetland. Percent retention and removal was defined as the difference divided by water volume or nutrient mass measured entering the wetland. After estimates were made for all the mass balance components, the chloride (Cl) inputs and outputs were balanced by increasing Cl output. This adjustment was then applied to both water and nutrients to obtain a conservative estimate of nutrient retention and removal.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONS FOR RIPARIAN BUFFER...REFERENCES |
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Surface Runoff
Mean nutrient concentrations in surface runoff as measured by the trough (west and south slopes) and dustpan (east slope) collectors and the H-flume at the Dairy Wetland outlet are presented in Table 2. Significance level based on the Kruskal–Wallis test is presented for each grouping of surface runoff collectors. Differences with a probability level greater than 0.01 are counted as no significant difference for this test.
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The LIFE samplers had less reduction of N concentrations from the field edge to the downslope position than in the runoff troughs. Nitrate and TKN were not significantly different but ammonium was significantly lower in the downslope position. Both DRP and total P were lower at Position 4 (downslope) than Position 1 (upslope) for the LIFE samplers.
This study provides the first example for a restored Coastal Plain buffer, where it was possible to compare the surface runoff concentrations from the field edge to outputs in streamflow. Most results were as expected with edge of field concentrations significantly higher than the streamflow outputs. The exception was ammonium, for which streamflow output concentrations (1.20 mg NH4–N L-1) were greater than the surface runoff input concentrations (0.96 mg NH4–N L-1).
Water and Nutrient Mass Balances
An annual water balance and nutrient mass balance of the Dairy Wetland is presented in Table 3. Water and nutrient loadings to the west and south edges of the Dairy Wetland can be attributed to the liquid manure land application site, while loadings to the east edge can be attributed to the pasture. With the exception of NO3–N, surface runoff dominates nutrient mass entering the Dairy Wetland. Entering water volumes are dominated by runoff and precipitation. The highest influx of water in surface runoff was along the south edge of the wetland, with approximately 109 m3 yr-1. This was much higher than the comparable values of 22 and 41 m3 yr-1 along the west and east edges, respectively. The difference can be attributed to increased seepage and runoff resulting from the land application of liquid dairy manure onto the upland crop production area. Incoming surface runoff nitrate, DRP, and total P loads, which were at least three to five times higher along the south edge than the west and east edges, were also increased due to the large increase in water input.
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Water left the Dairy Wetland primarily as surface flow through the H-flume or evapotranspiration. Ground water accounted for only 5% of water leaving (Table 3). Retention and removal rates ranged from a high of 83% for nitrate to a low of 24% for chloride. If not all outputs from the system were accounted for at the H-flume, there would be an apparent retention of water and chloride in the system. Not accounting for all outputs would lead to overestimates of nutrient retention and removal. If all inputs and outputs of chloride are estimated correctly, the mass of chloride entering the system should equal the mass of chloride leaving the system. Yet, our mass balance showed 75.3 kg of Cl retained by the system. Since our water volume balance also showed approximately the same retention rate, we attributed the difference to measurement errors at the H-flume. These errors probably occurred because of missed flows when the H-flume was submerged during a few large storm events in the summers of 1994 and 1996. To provide a more conservative estimate of the nutrient retention and removal in the Dairy Wetland we balanced the Cl budget and applied this correction to water and nutrient export through the H-flume. This correction increased the mass of water, Cl, and nutrients leaving the system by 31.3%, the amount needed to balance the Cl budget. This increase exactly balanced the chloride budget and balanced the water budget within <1% (Table 3). These conservative estimates of mass retention and removal for nitrate, ammonium, total N, DRP, and total P provide estimates of the minimum percent nutrient retention and removal by the Dairy Wetland (bottom three rows of Table 3).
Final retention and removal rates for nitrogen species ranged from a high of approximately 78% for nitrate to a low of 52% for ammonium. Final retention rates for both DRP and total P were 66%. Although measurement of all processes of N and P retention and removal was beyond the scope of this study, denitrification losses of N were measured from the site during the first two years of the Dairy Wetland restoration (April 1991–May 1993) (Lowrance et al., 1995). The average annual denitrification rate based on 2480 intact cores taken in the top 24 cm of soil was 68 kg N ha-1 yr-1. The denitrification estimate was about 71% of the unbalanced N retention and removal and 83% of the balanced N retention and removal. The remainder of the N retention and removal and most of the P retention would be accounted for by vegetation uptake and soil storage of N and P. Although neither mechanisms of P retention nor vegetation uptake of N and P were the subject of this study, other studies provide some information on the possible mechanisms of nutrient retention. Studies of other riparian forests near the Dairy Wetland showed that N and P storage in woody vegetation would account for 63% of the N retention and 28% of the P retention found in the Dairy Wetland (Lowrance et al., 1984). Future studies of the Dairy Wetland will involve harvesting of whole trees to determine nutrient accumulation in woody biomass. Nitrate and DRP retention rates (80 and 74%, respectively) were reported for a riparian wetland receiving storm runoff from a golf course in the Sandhills region of South Carolina (Casey and Klaine, 2001). These rates are similar to the Dairy Wetland rates. In a companion study of the South Carolina riparian wetland, Casey et al. (2001) reported that denitrification was important for N removal and that soil sorption of P could account for the extensive phosphate retention found.
Mean annual concentrations of total N and total P leaving the site determined by dividing the balanced mean annual load leaving by the balanced mean annual volume leaving were 1.98 and 0.24 mg L-1, respectively. These can be compared with similar calculations for the large watershed (1593 ha) that contains the Dairy Wetland (Lowrance et al., 1985). Flow-weighted concentrations for this watershed were 1.01 and 0.27 mg L-1, respectively, for total N and total P (Lowrance et al., 1985). Although for different time periods, this comparison indicates that the watershed of the Dairy Wetland that includes the liquid manure application area, the adjacent fertilized pasture, and the Dairy Wetland itself has N loads and flow-weighted concentrations that are about twice that of the larger watershed and that the P loads and flow-weighted concentrations are about the same as the larger watershed.
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IMPLICATIONS FOR RIPARIAN BUFFER RESTORATION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONS FOR RIPARIAN BUFFER...REFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONIMPLICATIONS FOR RIPARIAN BUFFER...REFERENCES |
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