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Published online 5 January 2006
Published in J Environ Qual 35:324-333 (2006)
DOI: 10.2134/jeq2005.0092
© 2006 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Waste Management

Nitrogen and Phosphorus Leaching from Growing Season versus Year-Round Application of Wastewater on Seasonally Frozen Lands

Francis Zvomuyaa, Carl J. Rosenb,* and Satish C. Guptab

a Agriculture and Agri-Food Canada, Lethbridge Research Center, P.O. Box 3000, 5403 - 1st Avenue South, Lethbridge, AB T1J 4B1, Canada
b Department of Soil, Water, and Climate, University of Minnesota, 1991 Upper Buford Circle, Room 439, St. Paul, MN 55108-6028

* Corresponding author (crosen{at}umn.edu)

Received for publication March 11, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Land application of wastewater has become an important disposal option for food-processing plants operating year-round. However, there are concerns about nutrient leaching from winter wastewater application on frozen soils. In this study, P and N leaching were compared between nongrowing season application of tertiary-treated wastewater plus growing season application of partially treated wastewater (NGS) vs. growing season application of partially treated wastewater (GS) containing high levels of soil P. As required by the Minnesota Pollution Control Agency (MPCA), the wastewater applied to the NGS fields during October through March was treated such that it contained ≤6 mg L–1 total phosphorus (TP), ≤10 mg L–1 NO3–N, and ≤20 mg L–1 total Kjeldahl nitrogen (TKN). The only regulation for wastewater application during the growing season (April through September) was that cumulatively it did not exceed the agronomic N requirements of the crop in any sprayfield. Application of tertiary-treated wastewater during the nongrowing season plus partially treated wastewater during the growing season did not significantly increase NO3–N leaching compared with growing season application of nonregulated wastewater. However, median TP concentration in leachate was significantly higher from the NGS (3.56 mg L–1) than from the GS sprayfields (0.52 mg L–1) or nonirrigated sites (0.52 mg L–1). Median TP leaching loss was also significantly higher from the NGS sprayfields (57 kg ha–1) than from the GS (7.4 kg ha–1) or control sites (6.9 kg ha–1). This was mainly due to higher hydraulic loading from winter wastewater application and limited or no crop P uptake during winter. Results from this study indicate that winter application of even low P potato-processing wastewater to high P soils can accelerate P leaching. We conclude that the regulation of winter wastewater application on frozen soils should be based on wastewater P concentration and permissible loading. We also recommend that winter irrigation should take soil P saturation into consideration.

Abbreviations: GS, growing season • MPCA, Minnesota Pollution Control Agency • NGS, nongrowing season • TKN, total Kjeldahl nitrogen • TP, total phosphorus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE FOOD-PROCESSING INDUSTRY generates large volumes of wastewater as a result of water requirements to clean food products and processing equipment. In the United States alone, more than 3 x 108 m3 of wastewater are generated annually from fruit- and vegetable-processing (Hang, 2004). Because of increasingly strict wastewater discharge regulations and escalating sewage surcharges by municipal wastewater treatment plants, land disposal has become an attractive wastewater management option. The typical absence of hazardous substances in food-processing wastewater allows its routine application on agricultural land to supply crop nutrients (Nini and Gimenez-Mitsotakis, 1994). Land application, when properly managed, is an efficient way to recycle wastewater nutrients and water within the soil–plant system.

Potato-processing wastewater contains high concentrations of nitrogen (N) and phosphorus (P) (Smith et al., 1975). Therefore, excessive application of this wastewater has the potential to cause NO3 and P contamination of ground water and surface waters. Total N concentrations exceeding 150 mg L–1 (Burgoon et al., 1999) and TP concentrations of up to 72 mg L–1 (Barl and McKenzie, 1995) in potato-processing wastewater have been reported.

Regulatory limits on the rates of wastewater application on cropland are usually based on crop agronomic N requirements. Pretreatment of the wastewater to reduce N concentrations can allow higher rates of wastewater application. However, this practice can increase loading of other nutrients, such as P, to levels exceeding crop requirement or the sorption capacity of soils. The timing and amount of wastewater irrigation are important because the crop's water demand may not necessarily coincide with its nutrient demand (Krauss and Page, 1997). Application of wastewater when the crop N need is low can cause N leaching and ground water contamination.

Nitrogen and P are considered to be the main causes of surface water eutrophication (Danalewich et al., 1998). The potential for N and P leaching to the ground water due to land application of wastewater increases particularly in coarse-textured soils in the northern parts of the United States, where soils are frozen during the winter (hence no plant growth and nutrient uptake). In addition, glacial terrain covering much of the region is characterized by many lakes, wetlands, and streams that receive inflow from ground water (Winter et al., 1998). Wastewater disposal is, therefore, a major challenge to food-processing plants that operate year-round. A typical strategy is to store the wastewater in lagoons during winter (nongrowing season) and then use it to irrigate cropland during the growing season. However, for potato-processing plants that produce large volumes of high nutrient wastewater in the winter, storage costs may be prohibitive.

In Minnesota, temporary permits have been granted by the Minnesota Pollution Control Agency (MPCA) to allow application of treated food-processing wastewater during the winter. The permits specify that winter-applied wastewater should contain ≤6 mg total P L–1, ≤10 mg NO3–N L–1, and ≤20 mg total Kjeldahl nitrogen (TKN) L–1. For growing season wastewater applications, nutrient concentrations of the applied wastewater are not regulated but regulations stipulate that total N loading from all sources shall not exceed the agronomic N requirement of the crop in the sprayfields. Despite the regulations, there are environmental concerns associated with winter wastewater application because of no or limited nutrient uptake by the crop during winter, limited water movement through frozen soils, and a high volume of ice- and snow-melt in early spring (Zvomuya et al., 2005). While many studies have investigated the impacts of wastewater application during the growing season, there has been limited research on the fate of nutrients from wastewater applied on frozen soils.

Laboratory breakthrough tests at low temperature showed that the soil can act as a sink or source of soil solution P depending on its degree of P saturation (Mamo et al., 2005). Similarly, field breakthrough tests on a seasonally frozen outwash soil in central Minnesota showed that P leaches past the 1.5-m depth in high soil test P soils even when land-applied wastewater is low in P concentration (Zvomuya et al., 2005). Although land application of wastewater on frozen soils in Minnesota is managed differently than application during the growing season, there have been no documented studies comparing nutrient leaching under the two management systems.

The overall objective of this study was to compare nutrient losses from winter application of tertiary treated regulated wastewater plus growing season application of partially treated nonregulated wastewater vs. the growing season application of partially treated nonregulated wastewater. The hypothesis tested in this study was that the extent of nutrient leaching from these two application scenarios was similar.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description
The study was conducted during 2001–2003 in Park Rapids, MN (46°98' N, 95°09' W). The soil at the site was a Verndale sandy loam (coarse-loamy over sandy, mixed, frigid Udic Argiboroll) with an average bulk density of 1.63 Mg m–3. Wastewater from the potato-processing plant was applied on 560 ha of land using a center pivot irrigation system. The sprayfields received unknown amounts of potato sludge and wastewater between 1981 and 1995. In 1996, the MPCA issued a permit allowing winter irrigation of potato-processing wastewater on 120 of the 560 ha. The 120 ha permitted for winter irrigation also received wastewater applications during the growing season, but are designated as NGS sprayfields for discussion purposes in this paper. The NGS sprayfields were bermed to ensure wastewater did not run off from the fields. All five fields at the NGS site (120 ha) and two fields (107 ha) at the GS site were included in the study (Fig. 1 ). The GS sprayfields were seeded with alfalfa (Medicago sativa L.) in April 2001 and the NGS sprayfields with reed canarygrass (Phalaris arundinacea L.) in May 2001. In May 2003, the GS and NGS sites were interseeded with alfalfa and reed canarygrass, respectively, using a no-till drill. Reed canarygrass was harvested two times and alfalfa three times during the growing season. A different crop was used for each system because alfalfa was not able to survive the ice buildup during winter wastewater application. Because of its ability to take up large amounts of nutrients, alfalfa is a preferred crop to grow for wastewater application under conventional systems.



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  		Fig. 1. Sketch map showing the location of the growing season (GS) and nongrowing season (NGS) sprayfields, lysimeters, and monitoring wells up-gradient and down-gradient of the sprayfields.

 
A nonirrigated (control) site just outside the NGS sprayfields was included for comparison. Unlike the GS and NGS sprayfields, the control site was not seeded. Vegetation at the control site included a mixture of timothy (Phleum pratense L.), smooth bromegrass (Bromus inermis Leyss.), and alfalfa originally planted to the site before 1995, plus other species, including perennial ryegrass (Lolium perenne L.) and quackgrass [Elytrigia repens (L.) Desv. ex Nevski].

Lysimeter Construction and Installation
The experiment was set up as a side by side comparison with drainage lysimeters randomly installed in each field to monitor nutrient leaching past the 1.5-m root zone. The drainage lysimeters were installed in the NGS sprayfields (26 lysimeters) in 1996 and in the GS sprayfields (8 lysimeters) in 2000. Nine lysimeters installed in 1996 at a nonirrigated area just outside the NGS sprayfields were used as controls. Lysimeter installation sites are shown in Fig. 1. Details of lysimeter construction and installation are given in Zvomuya et al. (2005). Briefly, each lysimeter consisted of a minimally disturbed subsoil core (28.9 cm in diameter x 120 cm long) in a polyvinyl chloride (PVC) tube (Fig. 2 ) that was taken from areas bordering the sprayfields. Before taking the subsoil core, surface soil (30 cm) was removed with a backhoe. Collection of soil cores was facilitated by a cutting steel ring attached to the bottom of the PVC tube, which was then pushed downward with a backhoe. The 23-L leachate-collection vessel consisted of a PVC tube glued to one PVC cap at the bottom and another PVC cap at the top. An additional smaller PVC cap with an open end in the upright position was bolted on the top of the larger upper PVC cap. The upper PVC caps had a drainage hole to allow dripping of percolate solution from the soil core into the collection vessel. A second hole on the side of the upper PVC cap was connected to a 2.5-m-long flexible PVC electrical conduit, which served as an access tube for pumping the leachate out of the collection vessel. Unbraided fiberglass wick and landscaping mesh fabric glued inside the smaller upper PVC cap facilitated water collection into the vessel. The minimally disturbed subsoil core was attached to the small PVC cap in the upright position. Lysimeter installation involved removing the topsoil (30-cm depth), digging a 60-cm-diameter, 2-m-deep hole with a power auger, lowering the assembled lysimeter into the hole, backfilling the hole with subsoil around the lysimeter assembly, sealing the lysimeter edges with bentonite, and then backfilling the space above the lysimeter with topsoil from the site.



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  		Fig. 2. Schematic diagram of the drainage lysimeter used to monitor nutrient leaching.

 
Although lysimeters at the NGS sprayfields were installed 4 yr earlier than at the GS sprayfields, we assumed the differences in timing of lysimeter installation between NGS and GS sites had minimal impact on subsequent nutrient leaching. The main reasons that support this assumption are:

All lysimeters were seeded with the same crop, at the same time, and using the same land preparation methods as the rest of the sprayfields in which they were located. The lysimeters were also harvested at the same time as the sprayfields.

Ground Water Monitoring Wells
Nutrient concentrations were monitored in monitoring wells installed at the GS and NGS sites (Fig. 1) in 1995. The eight monitoring wells used in the study were screened across the water table. Two monitoring wells were located up-gradient and two wells down-gradient of each site along the path of ground water flow. Water table elevation for the up-gradient and down-gradient wells averaged 435 and 433 m at the GS site and 429 and 425 m, respectively, at the NGS site. Corresponding depths to the water table for the up-gradient and down-gradient wells were 8.6 and 10.4 m at the GS site and 8.8 and 11.8 m at the NGS site.

Wastewater Application
Phosphorus and N concentrations in wastewater land-applied during the winter or nongrowing season months (October through March) were regulated by the MPCA. The MPCA specifications were that winter-applied wastewater should contain no more than 6 mg total P L–1, ≤10 mg NO3–N L–1, and ≤20 mg total Kjeldahl nitrogen (TKN) L–1. These criteria were met by using a tertiary treatment system, which included precipitating P with alum and denitrifying N in a sequencing batch reactor during the wastewater treatment process. During the growing season (April through September), nutrient concentrations were not regulated, but total N loads from the wastewater and other sources were not permitted to exceed the agronomic N requirements of the irrigated crop. For the alfalfa crop at the GS site and the reed canarygrass at the NGS site during the study, the annual N application limit was 336 kg ha–1 (Rehm et al., 1995). The wastewater was, therefore, lightly treated during the GS to reduce N concentrations and thus maximize wastewater application. The GS sprayfields also received sludge applications from the wastewater treatment plant during the growing season.

Soil Sampling and Analysis
For soil sampling purposes, each pivot was subdivided into two sampling units, giving a total of four sampling units at the GS site and 10 sampling units at the NGS site. Soil samples were taken from the 0- to 30-cm depth in May each year from 20 different locations within each sampling unit and mixed into one composite sample. The soils were tested for pH (1:1 soil to water suspension), inorganic bicarbonate-extractable (Olsen) P (Olsen et al., 1954), and organic matter (Nelson and Sommers, 1996). Since the main focus of the study was to compare leaching losses between GS versus NGS sprayfields, soils from the nonirrigated sites (control) were only analyzed for pH, organic carbon, Bray-1 P (Sims, 2000), and water-soluble P (Self-Davis et al., 2000) before the start of the present study. Sampling locations at the control sites are indicated in Fig. 1.

Sampling and Analysis of Leachate and Well Water
Lysimeters were checked weekly for the presence of leachate. If the leachate was present in a given lysimeter then the leachate volume was measured and a 500-mL sample was taken for nutrient analysis. When conditions were drier or soils were frozen, some of the lysimeters did not yield any leachate. Leachate samples were stored frozen until laboratory analysis.

Each sample was analyzed for total P, NO3–N, NH4–N, and TKN. Ammonium and NO3–N were measured by the diffusion-conductivity method based on the gaseous diffusion of ammonia (NH3) across a gas permeable membrane in the presence of excess base (KOH) and subsequent conductivity detection (Carlson et al., 1990). Total P in the water samples was determined by the ascorbic acid method (Murphy and Riley, 1962) following digestion with perchloric acid on a block digester. Total Kjeldahl N was determined by conductimetric techniques (Carlson, 1978) after digestion with sulfuric acid.

Samples were collected monthly from the ground water monitoring wells and independently analyzed for TP, NO3–N, NH4–N, TKN, and pH by the Lamb-Weston/RDO Wastewater Treatment Plant laboratory (Park Rapids, MN) and the Minnesota Valley Testing Laboratories (Ulm, MN), and also by our laboratory. Well data reported in this manuscript are the combined data from all three analytical laboratories. Interlaboratory crosscheck using split sample analysis indicated no statistical difference between results from different laboratories.

Data Analysis
Data from the control, GS, and NGS sites were compared using the Statistical Analysis Systems software (SAS Institute, 1999). Summary statistics were obtained and tabulated using PROC TABULATE. Since the Kolmogorov–Smirnov test from the PROC UNIVARIATE procedure of SAS indicated that most of the analyte data did not conform to a normal or lognormal distribution, we used the Kruskal–Wallis nonparametric method to compare medians for the three sites (control, GS, NGS) using the PROC NPAR1WAY procedure. The Satterthwaite test, which does not require the equal variance assumption, was used to compare median concentrations in the compliance monitoring wells up-gradient and down-gradient of the sprayfields using the PROC TTEST procedure (SAS Institute, 1999). Mean separation of ranked data was done using the Bonferroni test. Statistical significance was tested at P ≤ 0.05.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Weather
Weather data were collected at a station within approximately 3 km of the sprayfields. Growing season and nongrowing season precipitation values during the study were mostly below the long-term (1971–2000) means for the site, with the exception of the 2002 growing season, which had above normal rainfall and snowfall, and the 2003 growing season, which had above normal snowfall (Table 1). The 2002 growing season was the wettest of the three, receiving a total of 604 mm compared with 386 mm in 2001 and 404 mm in 2003. Seasonal temperatures were within 1.5°C of the long-term means for the site.


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  		Table 1. Precipitation and temperature data collected during the study, and long-term (1971–2000) averages for the site.

 
Soil Properties
Soil pH in the GS sprayfields was slightly acidic (6.6) and 1.1 pH units lower than that in the NGS sprayfields (Table 2). The higher pH in the NGS sprayfields may be related to larger volumes of higher pH wastewater applied on these sprayfields (Table 3) or inherent differences in the soils at the two sites, which were 4.5 km apart. Acidification from nitrification of NH4–N mineralized from sludge and crop residue may also have contributed to lower soil pH in the GS sprayfields. Bicarbonate-extractable P (Olsen P) was higher in the GS than the NGS sprayfields in May 2001, whereas it tended to be higher in the NGS in 2002 and 2003. Mean Olsen P concentrations over the three years, however, were similar for the GS (mean 99 mg kg–1) and NGS (mean 100 mg kg–1) sites. For agronomic purposes, the Olsen P concentrations were above the optimum range recommended for the alfalfa and reed canarygrass grown at the GS and NGS sites, respectively (Rehm et al., 1995). Across all sprayfields, clay, silt, and sand fractions in the 0- to 60-cm depth averaged 9, 17, and 74%, respectively. Organic matter contents were also similar for the two sites.


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  		Table 2. Selected chemical properties measured in composite soil samples taken early spring each year from the 0- to 30-cm depth at the growing season (GS) and nongrowing season (NGS) sites.

 

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  		Table 3. Potato-processing wastewater and sludge loading rates and mean (n = 24–60) analyte concentrations and loadings at the growing season (GS) and nongrowing season (NGS) sites.

 
Soil chemical properties measured at nonirrigated control sites before the start of the present study (Mamo et al., 2005) were pH, 6.0; organic carbon (Nelson and Sommers, 1996), 15 g kg–1; Bray-1 P (Sims, 2000), 19 mg kg–1; and water-soluble P (Self-Davis et al., 2000), 1.9 mg kg–1. Year-to-year changes in soil properties were not expected because there were no further inputs other than natural precipitation to this area.

Wastewater and Sludge Properties
Wastewater and sludge pH values throughout the study were slightly alkaline (Table 3). The pH was slightly lower for the sludge (mean 7.2) than the wastewater (mean 7.4). Total P concentrations (individual observations) ranged from 2 to 44 mg L–1 in the wastewater and from 32 to 332 mg L–1 in the sludge. Wastewater was higher in NO3–N and lower in TKN and NH4–N concentrations than sludge. Nitrate N concentration accounted for 27 to 58% of total N concentration in the wastewater and averaged 3% in the sludge. Mean NH4–N and organic N concentrations ranged from 17 to 46% and 17 to 37% of total wastewater N concentration, respectively. Corresponding values for sludge were 20 to 29% NH4–N and 67 to 79% organic N.

It is noteworthy that mean wastewater NO3–N and TKN concentrations during the growing season were below the MPCA limits for nongrowing season irrigation. This was because the growing season wastewater was lightly treated for N to allow for higher hydraulic loading of wastewater without exceeding the agronomic N rate set by the MPCA. Since the MPCA did not set a similar agronomic limit for P addition, wastewater was not treated for P during the growing season and thus there was no corresponding reduction in wastewater TP concentration.

Wastewater, Sludge, and Nutrient Loading Rates
Total wastewater applications in the GS and NGS sprayfields during the 33-mo study were 441 x 106 L (412 mm) and 3071 x 106 L (2559 mm), respectively (Table 3). The GS sprayfields also received 341 x 106 L (319 mm) of sludge during the same period. Total P, NO3–N, and total N (i.e., TKN + NO3–N) loading rates in the GS sprayfields were 669, 79, and 599 kg ha–1, respectively, during the 33 mo. The corresponding loading rates in the NGS sprayfields were 257, 235, and 658 kg ha–1, respectively.

Leachate Total Phosphorus Concentration and Loss
The Kruskal–Wallis test indicated that the overall differences in leachate water quality among the control, GS, and NGS sites were highly significant (P < 0.001). Median total P concentration in the leachates from the NGS sprayfields (3.56 mg L–1) (Table 4) across all sampling dates was nearly sevenfold that from the GS sprayfields (0.52 mg L–1) or the control sites (0.52 mg L–1).


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  		Table 4. Median analyte concentrations measured in lysimeter leachates collected at a l.5-m depth during 2001 through 2003.

 
Throughout the sampling period, total P concentrations in the leachate were significantly higher (P < 0.001) in the NGS sprayfields than the control sites and GS sprayfields (Fig. 3 ). A similar pattern was observed for total P loss (Table 5). A huge leaching event occurred during the 2002 growing season, resulting in the loss of 8.6 kg TP ha–1 between 13 August and 23 September (Fig. 3). This was related to the high rainfall received in June through August that year (293 mm). The 2002 growing season was the wettest of the three years, receiving a total of 604 mm compared with 386 mm in 2001 and 404 mm in 2003. Median total P loss, summed over the entire 33-mo sampling period, was significantly higher from the NGS sprayfields (57 kg ha–1) than from the GS (7.4 kg ha–1) or control sites (6.9 kg ha–1).



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  		Fig. 3. Temporal variation in (a) median concentrations and (b) leaching losses of total phosphorus (TP) at control sites and at sprayfields receiving wastewater application during the growing season and nongrowing season. The nongrowing season sprayfields also received partially treated wastewater during the growing season.

 

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  		Table 5. Median analyte losses in lysimeter leachates collected at a l.5-m depth during 2001 through 2003.

 
These results indicate that wastewater management at the NGS site has accelerated P leaching past the root zone compared with low P soils receiving no wastewater (control) or the conventional system where wastewater is applied on cropland during the growing season only. There is also evidence from compliance monitoring well data that current wastewater management during winter months may be contributing to elevated ground water P levels at the NGS site (Fig. 4 ). Median TP concentration in the samples from the wells at the NGS site (Fig. 4) was significantly higher (P = 0.037) in down-gradient wells (median 0.06 mg L–1) than in up-gradient wells (0.03 mg L–1). In contrast, there was no significant difference (P = 0.95) in ground water total P concentration between up-gradient (median 0.02 mg L–1) and down-gradient wells (median 0.01 mg L–1) at the GS site (Fig. 4).



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  		Fig. 4. Temporal variation in total P concentration in ground water compliance monitoring wells up-gradient and down-gradient of (a) growing season (GS) sprayfields and (b) nongrowing season (NGS) sprayfields.

 
Fractionation into inorganic, particulate, and organic P was not done in this study. However, a related study (Zvomuya et al., 2005) at the NGS site indicated that leachate P collected from the 1.5-m depth was predominantly soluble reactive P (80.1%), with particulate P accounting for 15.4% and dissolved organic P for 4.75% of TP concentration. Corresponding fractions in the wastewater were 65.3% soluble reactive P, 31.4% particulate P, and 3.29% dissolved organic P.

Although the two systems were under different crops (i.e., alfalfa at the GS site and reed canarygrass at the NGS site), it is well documented that annual N and P uptake are similar for the two crops, although P uptake tends to be slightly higher for reed canarygrass (USDA Natural Resources Conservation Service, 1996; USEPA, 1981). However, winter stress at the NGS site significantly decreased reed canarygrass stand, yield, and nutrient uptake (C.C. Sheaffer, personal communication, 1999). Annual dry matter yields during the study averaged 4.2 Mg ha–1 for alfalfa at the GS site and 2.5 Mg ha–1 for reed canarygrass at the NGS site (data not presented). Based on dry matter P concentration of 4.1 g kg–1 for alfalfa and 4.4 g kg–1 for reed canarygrass (C.C. Sheaffer, personal communication, 1999), P uptake was 17.2 kg ha–1 yr–1 at the GS site and 11.0 kg ha–1 yr–1 at the NGS site. These translate to 51.6 kg ha–1 P at the GS site and 33 kg P ha–1 at the NGS site over the 33-mo study period. The lower P uptake at the NGS site suggests that, for a given P application rate, more P was susceptible to leaching loss at the NGS than at the GS site.

Over the duration of the study, total P loadings were 669 kg ha–1 at the GS site and 257 kg ha–1 at the NGS site (Table 3). While all P loadings on the NGS sprayfields were added with the wastewater, 85% of the P added to the GS sprayfields was in the sludge. Much of the P in winter wastewater was tied up by the alum used in the wastewater treatment process. The P loadings at the two sites, in conjunction with the P uptake and P loss data presented above, indicate that the 0- to 1.5-m soil layer served as a sink for 610 kg P ha–1 at the GS site and 167 kg ha–1 at the NGS site during the 33-mo study. Batch equilibration sorption tests on soils (0- to 1.5-m depth) from the NGS sprayfields indicated a mean Langmuir sorption maximum (b) of 184 mg P kg–1, which translates to a P storage capacity of 4500 kg ha–1, based on a bulk density of 1630 kg m–3 for the 0- to 1.5-m depth (Zvomuya et al., 2005). This appears to suggest that, based on current loading rates and assuming constant crop P uptake and P loss, wastewater could be applied in the NGS sprayfields for up to 80 yr without exceeding the maximum P storage capacity of the soil. The degree of P saturation estimated for the NGS soil using oxalate-extractable Al, Fe, and P suggested that 12% of the sorption sites in the 0- to 0.3-m soil layer were saturated (Zvomuya et al., 2005). Significant P leaching has been reported to occur above a threshold degree of P saturation of 25% (Breeuwsma et al., 1995; Sinaj et al., 2002). However, our results indicate that there is a high risk of P leaching at the NGS site even below such a threshold.

The higher P leaching at the NGS site was due primarily to higher hydraulic loading relative to winter crop water requirements and lower plant P uptake. These P losses have important implications on the long-term application of winter wastewater on these seasonally frozen glacial outwash soils where a large part of the precipitation comes as snow. Hydrographs from ground water monitoring well data at the NGS site indicate that a significant portion of ground water recharge at the site occurs from snowmelt in the spring (Rosen et al., unpublished data, 2004). This corroborates earlier findings by Sharratt (2001), who reported that, on average, recharge in that region is highest during ice- and snow-melt in spring. It is during snow-melt and after heavy rainfall that the risk of polluting ground water is greatest. This is because the impeded water infiltration into frozen soils often results in substantial amounts of water and associated contaminants being transported into topographic depressions, thus leading to focused recharge (more water per unit area) in the depressions. Since P solubility increases under reducing environments (Sah and Mikkelsen, 1986), prolonged saturation of the soil in depressions during the winter and early spring possibly further decreases P sorption, thereby increasing P leaching. Higher P concentrations were observed in leachate from lysimeters located in depressions in the NGS sprayfields (Rosen et al., unpublished data, 2004). In upland areas, P leaching in the nongrowing season is also likely accelerated by wetter soil conditions and the lack of growing vegetation during spring thaw.

Current recommendations for wastewater application on the GS and NGS sprayfields during the growing season are based on agronomic N requirements and crop water use. To maximize wastewater disposal, wastewater applied during the growing season is often partially treated to reduce N concentrations. Since wastewater P concentration is not regulated during the growing season, more P is added to the soils when large quantities of such low-N wastewater are applied compared to untreated wastewater, which would be added in much lower volumes (Table 3). The assumption is made that much of the P applied will be taken up by the crop. However, our results show that the annual average P recovered in the crop (11 kg P ha–1 yr–1) during the 3-yr study was only 13 and 7.7% of total wastewater P applied at the NGS (86 kg ha–1 yr–1) and the GS (223 kg ha–1 yr–1) sites, respectively. Thus, there is a high potential of further P accumulation and loss if high P wastewater is applied to sprayfields having elevated soil P levels.

Agronomic N-based wastewater loading rates at the NGS site during the growing season added 93 kg total P ha–1 annually to soils that were already high in P. Thus, the risk of P leaching continues to increase unless measures to limit P leaching, such as P-based wastewater application rates during the growing season or application of P-binding materials such as alum or ferric chloride (Zvomuya et al., 2006), are implemented. Soil P saturation should also be considered in developing wastewater winter application regulations. Frequent reestablishment of reed canarygrass to maintain an adequate perennial stand will also help to increase nutrient uptake rates. However, slow establishment of reed canarygrass under harsh winter conditions may make this goal difficult to achieve (C.C. Sheaffer, personal communication, 1999).

Nitrate and Total Kjeldahl Nitrogen
During the 33-mo monitoring period, NO3–N concentrations were lowest in the control lysimeters. For a greater part of 2001, leachate NO3–N concentrations were highest in the NGS sprayfields. However, in 2002 and 2003, leachate NO3–N concentrations were highest in the GS sprayfields (Fig. 5 ). Nitrogen mineralization from the sludge applied during the growing season may have contributed to the high NO3–N concentrations in the leachate from the GS sprayfields. Median NO3–N concentrations over the 33-mo monitoring period differed significantly (P < 0.05) among the sites. Median concentrations were 2.69, 20.2, and 8.68 mg L–1, for the control, GS, and NGS sites, respectively (Table 4). The corresponding median NO3–N losses were 14, 299, and 174 kg ha–1, respectively (Table 5 and Fig. 5). Nitrate N losses from the GS and NGS sprayfields were not significantly different (P > 0.05) but were significantly higher than those from the control. Heavy rainfall episodes on 23 June (117 mm), 8 Aug. (46 mm), and 28 Aug. 2002 (34 mm) contributed significantly to leaching loss of 120 kg ha–1 NO3–N from the GS sprayfields between 3 May and 25 July (Fig. 5). This huge loss also likely reflects the high N mineralization from potato-processing sludge application and crop residues in the GS sprayfields.



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  		Fig. 5. Temporal variation in (a) median concentrations and (b) leaching losses of NO3–N at control sites and at sprayfield sites receiving wastewater application during the growing season and nongrowing season. The nongrowing season sprayfields also received partially treated wastewater during the growing season.

 
Monthly data from monitoring wells indicated no significant difference (P = 0.96) in NO3–N concentration between wells up-gradient (median 17.5 mg L–1) and those down-gradient (20.1 mg L–1) at the GS sprayfields (Fig. 6 ). At the NGS site, however, NO3–N concentrations were significantly higher (P = 0.001) in up-gradient (median 13.5 mg–1) than in down-gradient wells (10.7 mg L–1) (Fig. 6). This could be due partly to dilution of background ground water NO3–N concentration by NGS wastewater having lower NO3–N concentrations (Table 3).



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  		Fig. 6. Temporal variation in NO3–N concentration in ground water compliance monitoring wells up-gradient and down-gradient of (a) growing season (GS) sprayfields and (b) nongrowing season (NGS) sprayfields.

 
Total Kjeldahl N concentrations and losses were similar for the GS (medians 2.7 mg L–1 and 40.0 kg ha–1, respectively) and NGS (2.6 mg L–1, 53.2 kg ha–1) sites but significantly lower for the control (1.76 mg L–1, 18.9 kg ha–1) (Tables 3 and 4). Temporal trends indicate that both TKN concentrations and loads (Fig. 7 ) tended to be higher for the NGS site during most of 2001; however, site differences diminished during the remainder of the study. Ammonium N concentrations under all three systems were, on average, <1 mg L–1 and constituted a very small percentage of total inorganic N species in the leachates (Table 4). Organic N forms, estimated as the difference between TKN and NH4–N, constituted 39, 11, and 23% of leachate N losses at the control, GS, and NGS sites, respectively.



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  		Fig. 7. Temporal variation in median concentrations (a) and leaching losses (b) of total Kjeldahl nitrogen (TKN) at control sites and at sprayfield sites receiving wastewater application during the growing season and nongrowing season. The nongrowing season sprayfields also received partially treated wastewater during the growing season.

 
These results indicate that wastewater management at the NGS site did not significantly increase NO3–N leaching compared to conventional, growing season application of nonregulated wastewater. Although NO3–N concentrations in the leachate from the GS sprayfields were significantly higher, this did not result in higher NO3–N losses compared to the NGS site mainly because of higher hydraulic loading and thus greater drainage at the NGS site.

Sheaffer (personal communication, 1999) reported the dry matter N concentration of reed canarygrass and alfalfa at 15.5 and 29.8 g kg–1, respectively. Based on mean dry matter yields of 2.5 Mg ha–1 yr–1 for reed canarygrass at the NGS site and 4.2 Mg ha–1 yr–1 for alfalfa at the GS site, this corresponds to N uptake of 38.8 kg ha–1 yr–1 for reed canarygrass and 125 kg ha–1 yr–1 for alfalfa. With wastewater total N (TKN+NO3–N) loading rates of 200 (NGS site) and 219 kg ha–1 yr–1 (GS site) (Table 3), the plant N recoveries were thus 19 and 57% of N applied at the NGS and GS sites, respectively. The lower plant N recovery at the NGS site was due to the poor survival and growth of reed canarygrass during the winter.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Results from this study indicate accelerated P leaching from high P soils irrigated with treated potato-processing wastewater during the cold winter months and partially treated wastewater during the growing season compared with soils receiving only partially treated wastewater during the growing season. Both TP concentration and TP loss were at least seven times higher in the NGS sprayfields than in the GS sprayfields. Nitrate and TKN losses, however, were similar for the two wastewater management sites. Current MPCA regulations require that total P concentration should not exceed 6 mg L–1 in wastewater applied during the winter months, October through March. However, the regulations do not take into consideration the soil P saturation or sorption capacity of the irrigated sprayfields. During the growing season, the NGS sprayfields at the site receive wastewater applications based on agronomic N needs and not on wastewater P loading. Although this practice did not increase NO3 leaching at the NGS site compared with the GS site, there is concern that nonregulated wastewater P loading during the growing season at the NGS site will continue to increase soil P levels and further saturate the soil's P sorption capacity. Reduced yield and P uptake by reed canarygrass due to winter stress at the NGS site along with excessive hydraulic loading on high P soils are likely to exacerbate the problem. We recommend that winter wastewater application on frozen soils should be based on P loading as well as soil P saturation. In addition, the recommendations should reflect realistic crop yield goals and P uptake potential, taking into consideration the negative effects of winter stress.


   ACKNOWLEDGMENTS
 
Partial funding for this research was provided in part by Lamb-Weston/RDO Frozen through the Minnesota Pollution Control Agency. We thank Paul Conklin, Matt McNearney, Monica Carrasco, and Andrea McElhone for technical and laboratory assistance.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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