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Published online 7 June 2005
Published in J Environ Qual 34:1243-1250 (2005)
DOI: 10.2134/jeq2004.0339
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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TECHNICAL REPORTS

Waste Management

Phosphorus Leaching at Cold Temperatures as Affected by Wastewater Application and Soil Phosphorus Levels

M. Mamoa,*, S. C. Guptab, C. J. Rosenb and U. B. Singhb

a Department of Agronomy and Horticulture, University of Nebraska, Lincoln, NE 68583
b Department of Soil, Water, and Climate, University of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108

* Corresponding author (mmamo3{at}unl.edu)

Received for publication September 2, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Land application of wastewater in the northern-tier United States during winter months has been suggested as a means to reduce cost of building storage lagoons. A study was initiated in 1996 to assess land application of potato-processing wastewater on a 120-ha field at Park Rapids, MN. One objective of this study was to evaluate the effects of soil P levels and temperature on P leaching in soil columns. In this paper, we report the P sorption, desorption, and leaching characteristics of a high-P (>200 mg kg–1) and a low-P (<25 mg kg–1) surface soil from the wastewater irrigation site. The leaching experiment was done with wastewater at 4 ± 2 or 10 ± 2°C. The high-P soil resulted in an equilibrium P concentration of 8.0 mg L–1 compared with 0.14 mg L–1 for the low-P soil. When low-P wastewater was applied to the high-P soil, the soil acted as a P source, and the total phosphorus (TP) concentration in the leachate was 3.5 times higher than the input TP concentration (C0). When high-P wastewater was applied to the high-P soil, the soil acted as a P sink retarding the TP concentration in the leachate by 80%. Phosphorus desorption was higher at 10°C compared with 4°C. The results showed that depending on P levels of the soil and the wastewater, reduction or increase in leachate P will occur below the surface soil. However, further mobility of this P under field conditions will depend on the volume and rate of percolating water as well as the sorption–desorption characteristics of the subsoil.

Abbreviations: BTC, breakthrough curve • C, output or leachate concentration of phosphorus forms and/or bromide • C0, input concentration of phosphorus forms and/or bromide • DPS, degree of phosphorus saturation calculated as Pox/[0.5(Feox + Alox)] • DRP, dissolved reactive phosphorus • DUP, dissolved unreactive phosphorus • EPC, equilibrium phosphorus concentration • FeO-P, phosphorus extracted by iron oxide–impregnated filter paper • MPCA, Minnesota Pollution Control Agency • PEBC, phosphorus equilibrium buffering capacity • Pox, Feox, and Alox, acid ammonium oxalate–extractable phosphorus, iron, and aluminum, respectively • TP, total phosphorus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
PHOSPHORUS TRANSPORT in surface runoff has received much attention because of its importance as a major nonpoint-source pollutant to surface waters such as lakes and rivers. While surface runoff is an important pathway of P losses from agricultural lands, significant P losses can also occur via leaching through sandy soils, preferential flow in fine textured soil, high-organic-matter soils, or soils with a long history of overfertilization and/or use of organic wastes (Sims et al., 1998; Stamm et al., 1998; Sui and Thompson, 2000; Turner and Haygarth, 2000; Toor et al., 2004). In the past, little attention has been given to P losses via leaching because it is understood that oxides of Fe and Al in the clay fraction and free Ca in soils will result in P retention.

The most common agricultural management practice that is conducive to significant downward P movement is the long-term application of organic wastes. Prolonged application of organic waste increases the degree of soil P saturation and thus increases the potential of P leaching (Sharpley and Smith, 1989; Beauchemin et al., 1996; Iyamuremye et al., 1996; Mozaffari and Sims, 1996; Djodjic et al., 1999). While much research has shown significant soil P buildup from manure applications, soil P buildup and subsequent P leaching can also occur from municipal and industrial wastewater applications (Sims et al., 1998, 2000; Toor et al., 2004). The extent of P buildup or P leaching from solid waste or wastewater application on land depends on forms of P applied (organic, inorganic, particulate), rate of P application, soil P sorption characteristics, soil pH, rate of water transport in soil, and plant P uptake (Weaver and Ritchie, 1994; Djodjic et al., 2004). Adriano et al. (1975) reported downward movement of P to a 6.6-m depth in sandy soils after long-term spray application of food processing wastewater. Sommers et al. (1979) observed a significant increase in total P at the 0.30- to 0.60-m depth in a sandy loam soil after 12 yr of irrigation with municipal wastewater. King et al. (1990) observed P movement to about a 0.75-m depth in a loamy sand after 11 yr of swine lagoon effluent application. In more recent research, Johnson et al. (2004) observed an increase in soil test P levels to a depth of 0.30 m in a coarse-textured soil applied with dairy wastewater, while Toor et al. (2004) observed P movement primarily as particulate P through a silt loam soil applied with dairy effluent wastewater.

While the impact of land application of wastewater has been studied in many regions ranging from arid, semiarid, and humid climates, much of the focus has been on wastewater application impacts during the growing season. In general, nutrient leaching losses from wastewater applied during the growing season should be low provided the applications are based on water consumptive use by plants. Very few investigations have characterized P leaching when wastewater is applied during the nongrowing season or in cold temperatures, especially in areas such as the northern-tier states of the United States where soils are seasonally frozen from 0.30 to 0.90 m. This study was part of a comprehensive effort to assess the feasibility and impacts of wastewater application on frozen soils as an alternative to storage lagoons during winter. The objectives of this study were to (i) evaluate the effects of antecedent P on subsequent P sorption and desorption and (ii) determine the effects of temperature and background soil P levels on P leaching from surface soil following application of potato-processing wastewater in soil columns.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil Sampling Site
The sampling site is located on a 120-ha field next to a potato-processing plant at Park Rapids, MN (46°98' N, 95°09' W). The processing plant was built in 1981 and since that time the site has received potato-processing wastewater through a center pivot irrigation system. The site is also near (<1 km) the confluence of two major trout fishing rivers that are directly fed with ground water from the site. In 1996, a temporary permit was granted to allow year-around wastewater application. Restrictions imposed by the Minnesota Pollution Control Agency (MPCA) were that concentrations in the wastewater in the winter period (October–March) could be no higher than 10 mg L–1 NO3–N, 20 mg L–1 total Kjeldahl N, and 6 mg L–1 total phosphorus (TP). In addition to the concentration limitations, the total wastewater application over the entire year could not exceed the agronomically accepted N uptake rates of a given crop. The field was completely bermed to prevent surface runoff moving into nearby ditches and streams. However, during ice and snowmelt period, wastewater ran off and accumulated in depressional areas within the site, resulting in ponds that were as deep as 1.5 m. The cropping systems at the site included corn (Zea mays L.)–rye (Lolium perenne L.) rotation, alfalfa (Medicago sativa L.), and reed canary grass (Phalaris arundinacea L.). On the average, 50% of the N and 26% of the P needs for a specific crop were supplied from wastewater during the winter period.

To meet the MPCA requirements, wastewater was treated using sequential batch reactor techniques where N is denitrified and P is precipitated (Surampalli et al., 1997; Irvine et al., 1987). During the nonwinter period (April–September), concentration limitations were not in effect; thus, a single-stage treated (sequential batch reactor only) wastewater and/or ground water was applied. During the winter periods, however, two-stage treated (sequential batch reactor plus alum) potato-processing wastewater was land-applied.

The soil was a Verndale sandy loam (coarse-loamy, mixed, frigid Udic Argiboroll). The soil profile consists of an Ap horizon from 0 to 0.30 m with 2.5% organic matter, Bt1 and Bt2 horizons from 0.30 to 0.55 m with a distinct and smooth boundary, 2Bw1 and 2Bw2 horizons from 0.55 to 1.0 m with coarse sand and gravel, and a C horizon at >1.0 m with sand and mostly gravel. Samples taken during well drilling showed that the texture of the subsoil to the water table depth was mostly coarse sand and gravel.

Surface soil samples from 0 to 0.15 m were collected from high- and low-P testing areas to evaluate P sorption and P leaching. The high-P testing areas had received wastewater, processing sludge, and horse manure before the monitoring period and had Bray-1 P (i.e., 0.025 M hydrochloric acid and 0.03 M ammonium fluoride–extractable P) concentrations of >200 mg kg–1. The high-P soil represented a large proportion of the 120-ha field (90–95%). The low-P testing areas at the site had not received any wastewater and/or sludge application and had Bray-1 P of <25 mg kg–1. These areas were mainly located in upland positions outside of the pivot irrigation areas. The particle-size distribution of both the high- and low-P soils was 53 g kg–1 clay, 137 g kg–1 silt, and 810 g kg–1 sand. Both soils had a bulk density of 1.70 Mg m–3.

Soil Analyses
Soil samples were air-dried and ground to pass through a 2-mm sieve before soil analyses, sorption, desorption, and leaching experiments. Soil pH was measured at a 1:1 soil to water ratio and organic carbon was obtained by loss-on-ignition (Nelson and Sommers, 1982). Total Kjeldahl N was determined after digestion with sulfuric acid and quantification by conductimetric techniques (Carlson, 1978). Water-soluble P was determined after a 1-h extraction of 1 g of dry equivalent soil with 20 mL of deionized water. Extracts were centrifuged at 5000 x g for 5 min and filtered through a 0.45-µm filter before P analyses. Iron-oxide phosphorus (FeO-P), considered as bioavailable P, was extracted by iron oxide–impregnated filter strips (Chardon, 2000). Acid ammonium oxalate–extractable phosphorus (Pox), iron (Feox), and aluminum (Alox) were obtained by a 2-h extraction of 1 g of dry equivalent soil in 40 mL of 0.2 M acid ammonium oxalate at a pH of 3 (McKeague and Day, 1966). Degree of phosphorus saturation (DPS), which relates P already sorbed by a soil to its P sorption capacity, was calculated as Pox/[0.5(Feox + Alox)] according to van der Zee and van Riemsdijk (1988). Phosphorus in extracts was determined by the ascorbic acid method of Murphy and Riley (1962). Iron and aluminum were measured by atomic absorption spectroscopy.

Phosphorus Sorption
One gram of dry equivalent soil in triplicate was equilibrated with 20 mL of P solution as KH2PO4 for 24 h. The P concentration of the solution was 2.8, 5.3, 11.0, 16.5, and 33.0 mg P L–1 prepared in 0.01 M KCl background solution. Equilibration was achieved by shaking the sample on an end-over-end shaker at 25 ± 2°C. At the end of each equilibration, the supernatant was centrifuged at 5000 x g for 5 min, followed by filtration through a 0.45-µm filter. Phosphorus in the supernatant was measured as described above. Phosphorus adsorbed on the soil was calculated from the difference in P concentration in solution before and after sorption. The sorption data were fit to a sorption isotherm. The equilibrium phosphorus concentration (EPC) for the high- and low-P soils, defined as the solution P concentration where no net P sorption or desorption occurs, was obtained as the x intercept of the linear portion of the sorption isotherm. The phosphorus equilibrium buffering capacity (PEBC) was obtained as the slope of the linear portion of the isotherm.

Phosphorus Desorption
The field site experiences significant ponding in early spring before soil thawing, due to high hydraulic loading from wastewater application. The annual hydraulic load through wastewater application doubled the total amount of precipitation received at the site. Annual precipitation averages 690 mm compared with an average wastewater application of 840 mm. Topographically depressed areas had a high frequency of flooding due to ice and snowmelt runoff from the neighboring areas. Consequently, P desorption at different soil–water contact times and ratios is critical to evaluate potential P loss by leaching.

One gram of dry equivalent soil in triplicate was equilibrated with distilled deionized water for a period of 5, 30, 60, and 180 min. The samples were equilibrated each at liquid to solid ratios of 10 and 100. The sample was mixed on an end-over-end shaker at 25 ± 2°C. The supernatant was separated by centrifugation at 5000 x g for 10 min followed by filtration through a 0.45-µm filter. Phosphorus in the supernatant was determined as described above.

Phosphorus Breakthrough Experiments
Breakthrough curves (BTCs) were run on artificially packed soil columns in a series of experiments that were maintained at 4 ± 2 and 10 ± 2°C. The wastewater used for breakthrough curves had been processed through either a single-stage or two-stage sequential batch reactor. The seasonal average and range of potato-processing wastewater chemical characteristics produced at the wastewater treatment facility are presented in Table 1.


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  		Table 1. Seasonal average and range (in parentheses) of volume-weighted potato-processing wastewater chemistry.

 
Packing of Columns
The soil was packed into 0.40-m-long by 0.16-m-diameter Plexiglass columns with a fine-mesh screen at the bottom. A fine-mesh (approximately 0.1 mm thick) cloth was placed over the screen inside the column, and a 10-mm layer of coarse silica sand was added on top of the cloth. Sand was used to trap the fine soil particles that might move with the leachate during the breakthrough experiment. Soil was then packed in 50-mm depth increments. Packing was done by tapping the soil column on top of the laboratory bench for each increment resulting in a bulk density of 1.4 Mg m–3. After soil packing, about 10 mm of fine sand was uniformly spread at the top of the soil column to prevent soil disturbance during wastewater addition.

Leaching of the Columns
After packing, the soil columns were saturated with distilled water from the bottom up. The columns were then covered with plastic sheets and allowed to stabilize for 2 to 3 d before running the BTCs. A constant 50-mm hydraulic head was maintained at the soil surface at all times during breakthrough experiments using a Marriotte bottle. Breakthrough curves were run at 4 and 10°C with high-P wastewater or deionized water both spiked with Br (70–103 mg L–1). Bromide, a nonreactive tracer, was used to detect the presence of preferential flow within each column. Additional BTCs with low-P wastewater were run at 4°C only. Temperature of the columns and wastewater was maintained by conducting the experiments in cold rooms set at either 4 ± 2 or 10 ± 2°C. Leachate was collected every half pore volume over a period of 14 to 16 h or 2 to 5 pore volume (i.e., 0.33–0.81 m of water or wastewater application). Forms of P measured in leachates according to Beauchemin et al. (1998) are listed in Table 2. Phosphorus was determined by the ascorbic acid method (Murphy and Riley, 1962).


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  		Table 2. Methodological definition of the phosphorus forms measured in soil column leachate following either distilled water or wastewater application.

 
Data Analysis
Sorption, desorption, and BTCs were run on two to three replicates. Regression analysis and standard errors were obtained using SAS Version 8.0 (SAS Institute, 1999). An LSD test at P < 5% was done to evaluate the effect of temperature on leachate P concentrations. Since leachate collection was done at intervals, the method of linear interpolation was used to estimate data points between two consecutive P measurements. For clarity, data points at every 0.10 pore volume are presented in figures.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Bray-1 P, water-soluble P, FeO-P, Pox, Alox, and Feox were higher in the high-P soil compared with that of the low-P soil (Table 3). The increase of Alox is associated with alum (aluminum sulfate) addition to treat wastewater. Although soil Alox and Feox levels were increased, the degree of phosphorus saturation (DPS) was higher in the high-P soil compared with the low-P soil (Table 3). This can be attributed to the 24-fold increase in the Pox or already sorbed P on the high-P soil associated with long-term application of potato-processing wastewater and/or sludge. As an indicator of P sorption capacity, the sum of Alox and Feox for the low-P soil (51 mmol kg–1) is within the range or comparable with that observed by Sims et al. (2002) on 465 soils in Delaware. Organic carbon and total Kjeldahl N were similar in both soils.


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  		Table 3. Chemical and physical characteristics of the 0- to 0.15-m Verndale sandy loam surface soil.

 
Phosphorus Sorption Isotherm
The P sorption of the low-P soil was higher compared with that of the high-P soil (Fig. 1). The low-P soil would be expected to have higher P sorption than the high-P soil since it had not received P from wastewater or sludge and soil minerals and its organic surfaces would have a higher P affinity and faster reaction times (Frossard et al., 1995; Lookman et al., 1995). However, the high-P soil had more desorption than sorption as indicated by the negative values at less than 10 mg L–1 equilibrium P concentration (Fig. 1b). The linear portion of the isotherm, used to determine the EPC, had an R2 of 0.997 for the low-P soil and 0.973 for the high-P soil (Fig. 1). Equilibrium P concentration was 0.14 and 8.0 mg L–1 for the low- and high-P soil, respectively (Table 3). The higher EPC of the high-P soil suggests that the soil can act as a P source when wastewater with low P concentrations (<8.0 mg L–1 P) is applied, such as the winter wastewater, but can act as a sink when wastewater with high P concentrations is applied, such as the growing season wastewater. The lower EPC of the low-P soil indicates that the soil will act primarily as a high affinity sink for wastewater with both high and low P concentrations. Sui and Thompson (2000) had reported similar observations of higher EPC in mollisols applied with biosolids for six years. Sommers et al. (1979) had also observed an increase in EPC from 1.1 to 73 mg L–1 and a decrease in P sorption capacity from 545 to 451 mg P g–1 to a 0.60-m depth 12 yr after municipal wastewater application on a sandy loam soil.



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  		Fig. 1. Linear P sorption isotherm of the (a) low-P soil and (b) high-P soil (N = 3). ***Significant at the 0.001 probability level. Curvilinear lines are fitted Langmuir isotherms through all data points.

 
The high-P soil also has higher DPS compared with that of the low-P soil (12 vs. 1.4%). Increase in DPS is indicative of potential P loss below the surface 0.15 m. Soil with a high DPS has been reported to have a high EPC, and therefore a high potential for P loss by desorption (Zhou and Li, 2001). For example, soils with DPS > 25% have been established to contribute to ground water contamination with P in the Netherlands (Breeuwsma et al., 1995). Nair et al. (2004) suggested that a DPS of 30% was an indicator for potential P loss by leaching from Florida's sandy soils. Although the DPS value was obtained for the surface 0.15 m, the potential risk of P loss below this layer is high, especially considering the average annual hydraulic load through wastewater application of 840 mm. In addition, the PEBC of the high-P soil was lower than that of the low-P soil (Table 3). The high-P soil with low PEBC indicates that the surface soil at this site will maintain a high solution P concentration that could potentially leach below the surface soil (Beauchemin et al., 1996; Sui and Thompson, 2000). The combination of EPC, DPS, and PEBC data clearly indicates that the surface sandy soil has a potential to move P below the surface 0.15 m. Considering the average profile of the soil at this site, which is mostly sandy to gravel, and recognizing the importance of the Bt layer in P sorption, the risk of P loss to the 4- to 6-m-deep ground water is high when combined with a high hydraulic load and the high rate of water movement through the soil profile (Zvomuya et al., 2005). Djodjic et al. (2004) suggested that aside from the soil profile sorption characteristics and the soil P levels, the rate of water movement through the profile was another important variable in assessing risk of P leaching.

Phosphorus Desorption
Phosphorus desorption increased as the ratio of water-to-soil increased, suggesting that higher hydraulic loading especially on the high-P soil could desorb a large amount of P (Fig. 2). With increase in extraction time, the desorption of P from the high-P soil increased slightly but there was no change for the low-P soil. Similar desorption trends have been observed by Maguire et al. (2001) and Sui and Thompson (2000). At the highest soil–water contact time, the amount of P desorbed on the high-P soil was 54 and 88 mg kg–1 for the water to soil ratios of 10 and 100, respectively. The field site experiences high hydraulic load and subsequently high percolation in the spring due to snow and ice melt. Thus, the potential for P desorption increases as the volume and rate of percolating water through the soil increases. In addition, topographically low areas (high-P soils) not only receive high hydraulic load, but the contact time with melt-water is high due to the formation of small ponds, increasing the potential for P desorption.



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  		Fig. 2. Desorption of P from low- and high-P soils at various combinations of equilibration time and liquid to solid ratios (N = 3). Error bars represent standard error of the mean.

 
Breakthrough Experiments
High-Phosphorus Soil with High-Phosphorus Wastewater at 4 and 10°C
Figure 3 shows the BTCs of TP, dissolved reactive phosphorus (DRP), and Br at 4 and 10°C. At about 1.0 pore volume, 50% (i.e, C0/C = 0.5) of the applied Br appeared at the bottom of the soil columns, which is expected for a nonreactive tracer. Appearance of the center of mass (C/C0 = approximately 0.5) of bromide at around 1.0 pore volume is an indicator of the uniform convective transport or lack of a preferential transport. If preferential flow had occurred, Br would have appeared earlier and also at a high concentration or close to the C0 level. Several investigators (Meyer-Windel et al., 1999; Saleem Akhtar et al., 2003) have shown that a peak earlier in the breakthrough curve along with output solute concentration almost equal to the input concentration indicates preferential flow.



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  		Fig. 3. (a) Total phosphorus (TP) and (b) dissolved reactive phosphorus (DRP) leaching from high-P soil following application of single-stage treated potato processing wastewater at 4 ± 2 and 10 ± 2°C (N = 2). C0, input concentration; C, output or leachate concentration.

 
Wastewater used in this experiment had been treated only through the sequential batch reactor and had an initial TP content of 21.2 mg L–1 (DRP = 14.6 mg L–1), which met the MPCA growing season wastewater application guidelines and was within range of seasonal wastewater chemical characteristics presented in Table 1. Phosphorus concentrations in the leachate were generally greater than 2 mg L–1 (i.e, C/C0 > 0.1) for TP and DRP (Fig. 3). Based on the EPC of 8.0 mg L–1, the high-P soil acted as a P sink when single-stage treated wastewater was applied at both temperatures. At 4°C, the high soil P retarded wastewater TP (C0 of TP = 21.2 mg L–1) and DRP (C0 of DRP = 14.6 mg L–1) by up to 80 and 70%, respectively. This retardation was 74% for TP and 66% for DRP at 10°C. This retardation in P suggests that the high-P soil can buffer high-P wastewater and reduce P concentration levels moving below the surface soil layer even at lower temperatures.

At pore volumes of less than 1, P was mostly associated with fine soil sediment initially breaking through the columns. With an increase in pore volume (>1), particulates due to algae from wastewater were qualitatively observed as evidenced by the greenness of the leachate. These particulates possibly contributed much of the TP leached through the column at later stages. Toor et al. (2004) in their study of P leaching under a grassland system irrigated with dairy manure effluent have observed that 69 to 75% of the TP in leachate was mostly unreactive P associated with organic particulates. Although wastewater P concentration was reduced, the P BTCs show that even under conditions of cold temperature, P leaching can occur below the surface soil layer in a high-P, coarse-textured soil. This is especially critical at the site since application of winter wastewater begins in October when the soil is cold but not frozen.

Although the soil acted as a sink when high-P wastewater is applied, its ability to act as a sink was reduced at higher temperatures. For example, there was higher (P < 0.001) TP and DRP in the leachate at 10°C compared with that of 4°C (Fig. 3). The higher leachate P level may be associated with a higher rate of P desorption at 10 than 4°C. Sallade and Sims (1997a) in a laboratory incubation have made similar observations of higher total soluble P at 35 than 7°C. In addition, Sallade and Sims (1997b) have shown higher soluble P concentrations in a drainage ditch in the summer compared with the winter or spring. Based on thermodynamics principles, an increase in temperature should cause not only a faster sorption but also a faster desorption (Aharoni and Ungarish, 1977). Under field conditions, however, desorbed P from the surface soil can be resorbed if water transport rate and water volume are low, giving P sufficient time to react with the subsoil.

High-Phosphorus Soil with Low-Phosphorus Wastewater at 4°C
Wastewater for leaching had been treated both through the sequential batch reactors as well as with alum and met the MPCA guidelines for winter wastewater application with a TP of 1.1 mg L–1 and DRP of 0.52 mg L–1. Breakthrough curves using this wastewater showed TP and DRP leaching higher than the wastewater P (Fig. 4). The ratio of P concentration in the leachate to the P concentration in the wastewater (C/C0) was always greater than 1, indicating that desorption rather than sorption was the dominant process during the leaching event. This result is supported by the EPC of the high-P soil (8 mg L–1), below which desorption dominates sorption. Thus, at this low wastewater P concentration, the high-P soil acted as a source of P. On average, the TP and DRP in leachate were 3.5 and 7.2 times C0, respectively. This suggests that the high-P soil cannot buffer low-P wastewater and has the potential to increase P concentrations moving below the surface soil layer. Further experimentation (data not presented) showed that even when this soil was leached with deionized water spiked with Br, soluble P concentrations in the leachate were 5 to 6 mg L–1 by the time 3.5 to 4 pore volume of water had passed through the soil columns at 4°C, substantiating the notion that nonwastewater irrigation can also enhance the vertical P movement past the surface soil layer.



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  		Fig. 4. Total phosphorus (TP) and dissolved reactive phosphorus (DRP) leaching from high-P soil on application of two-stage treated potato processing wastewater at 4 ± 2°C (N = 3). C0, input concentration; C, output or leachate concentration.

 
Low-Phosphorus Soil with Low-Phosphorus Wastewater at 4°C
The wastewater used for leaching had a total P concentration of 2.5 mg L–1 with 60% of the P present as particulate P, 36% as DRP, and 4% as dissolved unreactive phosphorus (DUP). Figure 5 shows the P leachate concentration from low-P soil columns applied with spiked deionized water (Fig. 5a) and low-P wastewater (Fig. 5b), both at 4°C. In both cases, there was a peak in P concentration at around 0.5 pore volume, due to particulate P associated with some sediment movement. After 1 pore volume, the concentration of all forms of P declined to less than <0.1 mg L–1 in both cases. However, in the soil applied with wastewater, TP concentration in leachate gradually increased at about 4 pore volumes. Most of this increase in leachate P was associated with particulate P as shown by the increase in total particulate phosphorus (TPP). While chlorophyll level in leachate was not quantified, the increase in particulate P at about 4 pore volume and higher appeared to be associated with algae from the wastewater.



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  		Fig. 5. Total phosphorus (TP), dissolved reactive phosphorus (DRP), dissolved unreactive phosphorus (DUP), total dissolved phosphorus (TOTDP), and total particulate phosphorus (TPP) leaching from high-P soil on application of (a) Br-spiked deionized water and (b) two-stage treated potato processing wastewater at 4 ± 2°C (N = 3). Note that P concentration is not presented as C/C0. Bromide breakthrough curves (BTCs) were not run with these columns.

 
Although an increase in leachate TP was observed, it did not exceed wastewater TP concentration. On average, the low-P soil retarded wastewater TP and DRP by 91 and 98%, respectively. This is consistent with the low EPC (0.14 mg L–1) and high PEBC (116 L kg–1) obtained with this low-P soil, suggesting that fast sorption–retention of P is occurring even at this near-freezing temperature.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Results from this study showed that P movement could occur through the highly permeable surface sandy soils at low temperatures (4 and 10°C). Since winter wastewater application is between October and April, the potential of P movement below the soil surface is high especially in the early part of the winter season (October to early December) when soil is cold but not frozen. Sorption isotherms showed that the low-P soil had a lower EPC and higher PEBC, while the high-P soil had a higher EPC and lower PEBC. This suggested that depending on P concentration in the wastewater relative to P concentration in the soil, the soil could serve as a sink or a source of P. Thus, the concentration of P in wastewater on high-P soil will dictate the retardation (sorption > desorption) or increase (desorption > sorption) in leachate P of surface soil. While the P level in surface soil is an important factor in enhancing or reducing P movement from the surface soil, the percolation rate and volume of water moving downward are also critical factors under field conditions in increasing or decreasing P reaction with the subsoil.


   ACKNOWLEDGMENTS
 
The authors wish to thank the Lamb-Weston/RDO potato-processing facility through the Minnesota Pollution Control Agency for funds to support this work.


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


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