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Soil Chemistry

Sorption and Desorption of Ammonium from Liquid Swine Waste in Soils

^a Dep. of Natural Resources & Environmental Design, North Carolina A&T State Univ., Greensboro, NC 27411
^b Dep. of Crop and Soil Science, 3111 Miller Plant Sciences Bldg., The Univ. of Georgia, Athens, GA 30602-7272
^c Dep. of Agronomy, Throckmorton Hall, Kansas State Univ., Manhattan, KS 66502-5501

^* Corresponding author (kxia{at}uga.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Understanding the sorption and desorption behavior of NH₄⁺ in soils associated with animal waste is important because of the potential for the formation of NO₃^– and subsequent leaching that affects ground water quality. Batch equilibration experiments were conducted to evaluate the sorption and desorption of NH₄⁺ in two soils exposed to a complex matrix (liquid swine waste) and a simple matrix [aqueous solution of 0.01 M CaCl₂ containing (NH₄)₂SO₄]. Kennebec silt loam (fine-silty, mixed, mesic Cumulic Hapludolls) and Haynie very fine sandy loam (coarse-silty, mixed, calcareous, mesic Mollic Udifluvents) were used. This study revealed that the sorption and desorption behavior of NH₄⁺ in soils exposed to (NH₄)₂SO₄ solutions with a 0.01 M CaCl₂ matrix is significantly different from that in soils exposed to liquid swine waste. Faster sorption rate, lower sorption capacity, and higher desorption capability were observed for NH₄⁺ in soils exposed to the (NH₄)₂SO₄ solution compared with soils exposed to the liquid swine waste. Sequential extraction could not extract nonexchangeable NH₄⁺ in both soils exposed to liquid swine waste, while a significant amount of nonexchangeable NH₄⁺ was extracted from the two soils that were initially exposed to the (NH₄)₂SO₄ solutions. The high dissolved organic C (DOC) content coupled with the high pH in swine waste appears to stimulate the sorption and retard desorption of NH₄⁺ in the two soils. This study revealed that batch equilibrium studies using solutions with simple matrixes may underestimate the sorption or overestimate desorption of NH₄⁺ in soils associated with swine waste.

Abbreviations: CAO, concentrated animal operations • DOC, dissolved organic C • EC, electrical conductivity • SOC, soil organic C • SOM, soil organic matter

	INTRODUCTION

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EARTHEN-LINED WASTE lagoons have been used extensively and have become integral components of many concentrated animal operations (CAO). Kosco and Hall (1999) estimated that there were more than 6600 CAO with more than 1000 animal units in the USA in 1992. Lagoon waste often contains relatively high concentrations of N, P, inorganic cations and anions, organic matter, and other compounds. Ammonium is one of the dominant inorganic chemical constituents in animal waste with concentrations varying from 600 to 2000 mg L^–1 (Ham and DeSutter, 1999a). Ammonium accounts for almost 99% of the soluble N in animal waste. The NH⁺₄ in animal waste can be exported into the soil at the periphery of the compacted lagoon liner at rates between 2000 and 3000 kg ha^–1 yr^–1 (Ham and DeSutter, 1999b). Further, seepage losses of the lagoon contents from the sides and the bottom of the basin as well as the land disposal of lagoon waste can impact soil and ground water quality (Westerman et al., 1995).

Swine waste-derived NH⁺₄ in soil beneath a waste lagoon can either be sorbed on negatively charged exchange sorption sites or trapped in the interlayers of the clay fraction of the soil (Bohn et al., 2001). The release of NH⁺₄ from the sorption sites into the soil solution favors the nitrification process thus increasing NO^–₃ concentration in the soil solution (Kai and Harada, 1969). Miller et al. (1976) detected 530 mg NH⁺₄–N kg^–1 in a sandy loam to silt loam soil at 91 cm below a swine waste lagoon. In the vadose zone, this NH⁺₄ could transform into NO^–₃ under aerobic conditions through nitrification by bacteria such as Nitrosomonas and Nitrobacter. Since NO^–₃ carries a negative charge it is not generally retained in soils with predominantly 2:1 type clay minerals and thus subject to leaching to the ground water.

It has been reported that NO^–₃ formed from NH⁺₄ in animal waste makes a significant contribution to the elevated NO^–₃ levels in ground water in many locations. For instance, Ciravolo et al. (1979) reported that Cl^– and NO^–₃–N concentrations in the ground water around a swine waste lagoon constructed on a sandy subsurface soil in the coastal plain region of Virginia exceeded the USEPA drinking water standard of 10 mg NO^–₃–N L^–1. Karr et al. (2001) found high concentrations of NO^–₃ in ground water from commercial swine farms in the Mid-Atlantic coastal plain. Average annual NO^–₃–N concentrations of 5 to 30 mg L^–1 have been reported in subsurface drainage beneath spray fields in North Carolina (Evans et al., 1984; Sloan et al., 1999). They also reported maximum NO^–₃–N concentrations > 100 mg L^–1 in the same study. Sievers and Fulhage (1992) claimed that 75% of the rural wells tested in Missouri exceeding the USEPA imposed NO^–₃ limit for drinking water were within 150 m of a livestock operation.

Ammonium sorption in soil has been extensively investigated using batch equilibrium techniques with various solutions such as NH₄Cl (Thompson and Blackmer, 1992; Lumbanraja and Evangelou, 1994; Wang and Alva, 2000), NH₄NO₃, (NH₄)₂SO₄, and NH₄H₂PO₄ (Dalal, 1975). However, no study has been found to study the interactions of NH⁺₄ with soils exposed to liquid animal waste, which has a much more complex matrix than the above listed ammonium salt solutions. Liquid animal waste is rich in organic substances and has a relatively higher ionic strength, higher concentrations of major cations and anions, and a higher pH compared with a normal soil solution (Yasuhara et al., 1984; Ham and DeSutter, 1999b). Therefore, one would expect to observe different behaviors in sorption and desorption of NH⁺₄ in soils exposed to liquid swine waste from that exposed ammonium salt solutions. Erroneous conclusions would be likely to occur if one would attempt to predict the environmental behavior of NH⁺₄ from animal waste without adjustment of important characteristics of animal waste.

Soil organic matter (SOM) has long been known to chemically fix NH⁺₄. Burge and Broadbent (1961) reported a positive correlation between the NH⁺₄ fixed and the percentage of C in the soil. Ammonium fixation by SOM has been attributed to phenolic constituents associated with humic and fulvic acids. Under alkaline conditions a wide variety of ketones, aldehydes, reducing sugars, and other carbonyl-containing compounds are known to react with NH⁺₄ (Stevenson, 1994). Yasuhara et al. (1984) identified approximately 50 organic compounds in fresh and decomposed swine manure. Zahn et al. (1997) found more than 90% of the organic compounds in swine manure samples were acetic, propionic, and butanoic acids. The carboxylic groups in those organic acids may deprotonate in the alkaline environment of the swine waste and, therefore, form complexes with cations such as NH⁺₄. Complexation of NH⁺₄ with soil organic compounds and sorption of those organic compounds on soil could occur simultaneously or consecutively. As a result, less free NH⁺₄ ions may be found in the soil solution after equilibrating the soil with liquid animal wastes than with simple ammonium salt solutions.

The primary objective of this study was to understand the sorption and desorption behavior of NH⁺₄ in soils exposed to a liquid swine waste in comparison with a (NH₄)₂SO₄ solution with 0.01 M CaCl₂ as matrix. The effects of DOC and pH on NH⁺₄ sorption and desorption in soils exposed to swine waste will also be studied.

	MATERIALS AND METHODS

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Soil and Swine Waste Samples
Kennebec silt loam and Haynie very fine sandy loam were used in this study. The Kennebec and Haynie soils were collected from the North Agronomy Farm and Ashland Horticulture Farm, respectively, of Kansas State University in Manhattan, KS. The Kennebec soil was sampled from the surface (5–20 cm) of control plots that had received no N fertilizer or manure for 10 yr in a long-term tillage experiment. The Haynie soil was collected from the same depth of non-N fertilized, uncultivated plots. Six soil cores were taken from the plots at random. The soil cores were mixed to obtain a composite sample, air dried, ground, and sieved through a 2-mm sieve to eliminate plant debris and soil clods. Some important characteristics of the two soils are listed in Table 1.

Sorption Study
From this point on, the term "(NH₄)₂SO₄ solution" refers to a solution of 0.01 M CaCl₂ containing (NH₄)₂SO₄ at certain concentration. The (NH₄)₂SO₄ solution and liquid swine waste were used as the sources of NH⁺₄. Both Kennebec and Haynie soils were used. For the sorption kinetic study, the (NH₄)₂SO₄ solution was prepared in 0.01 M CaCl₂ to reach a concentration of 475 mg NH⁺₄–N L^–1, equivalent to the NH⁺₄ concentration of the liquid swine waste. The electrical conductivity (EC) of this solution was approximately 6.3 dS m^–1, similar to the EC for the swine lagoon waste. The 0.01 M CaCl₂ was used to mimic soil solution. Three grams of soil was mixed with 30 mL of (NH₄)₂SO₄ or liquid swine waste solution in a 50-mL polypropylene centrifuge tube, shaken at 180 strokes min^–1 for 6, 12, 24, 36, 48, 60, 72, 84, 96, 108, 120, and 144 h at room temperature, and then centrifuged at 1250 x g for 5 min. The supernatants were filtered through Whatman #42 filter paper and analyzed for NH⁺₄ using a flow injection automated ion analyzer (Lachat QuikChem 8000, Lachat Instrument, Milwaukee, WI). The amount of NH⁺₄ sorbed by the soil was calculated based on the difference between the initial and final NH⁺₄ concentrations in the supernatants. The residual soil samples were stored at 4°C for the later desorption studies.

For the sorption isotherm experiment, the NH⁺₄ in the liquid waste solution was initially removed by adjusting the pH to >10 and then constantly stirring at 80°C to achieve 90% removal, which was later confirmed by measuring the NH⁺₄ concentration in the solutions. The pH of the NH⁺₄–removed liquid waste was then adjusted back to its original level before its NH⁺₄ levels were brought back to concentrations at 75, 165, 240, 510, 1015, and 2085 mg NH⁺₄–N L^–1 by adding appropriate amount of (NH₄)₂SO₄. Solutions of (NH₄)₂SO₄ containing the same series of NH⁺₄ concentrations as that in the above-mentioned liquid waste solutions were prepared in 0.01 M CaCl₂. Three grams of soil was mixed in a 50-mL polypropylene centrifuge tube with 30 mL of (NH₄)₂SO₄ or liquid swine waste solution with different NH⁺₄ concentrations, shaken for 72 h, and then centrifuged using the conditions described for the above kinetic experiments. The supernatants were filtered and analyzed for NH⁺₄. The residual soil samples were stored at 4°C for later desorption studies.

Desorption Study
The residual soil samples from the above-mentioned sorption kinetic experiment and sorption isotherm experiment were mixed with 30 mL of 0.01 M CaCl₂ solution in a 50-mL polypropylene centrifuge tube and then shaken for 2, 4, 8, 16, 24, and 48 h using the same conditions as described for the sorption experiments. Supernatants were obtained by centrifugation, filtered, and then analyzed for NH⁺₄. The residual soil in each centrifuge tube was sequentially extracted five consecutive times by repetitively shaking with 30 mL of 0.01 M CaCl₂ solution for 1 h. The concentrations of NH⁺₄ in the supernatants obtained after each extraction were analyzed. The amount of NH⁺₄ desorbed was calculated based on the concentration of NH⁺₄ in the supernatants after adjusted for the NH⁺₄ in the entrained soil solution from the sorption experiments.

Effect of Dissolved Organic Carbon and pH on NH₄⁺ Sorption and Desorption in the Two Soils
To test the effect of major factors associated with the complex liquid swine waste, sorption and desorption experiments following the above described procedures were conducted using (NH₄)₂SO₄ and liquid swine waste solutions with various adjusted DOC contents and pH levels. Two separate sets of solutions were prepared as described below. For the first set of solutions, the NH⁺₄–depleted liquid swine waste was first diluted with deionized water to achieve DOC concentrations at 246, 493, 739, 986, and 1232 mg C L^–1. The pH and the concentrations of NH⁺₄ and other major cations and anions in each diluted solution were then adjusted back to the same levels as in the original liquid swine waste. For the second set of solutions, the pHs of the (NH₄)₂SO₄ solutions and the liquid swine waste were adjusted to 4.0, 6.0, and 8.0 using HCl or NaOH. Our test showed that the pHs of the soil-solution mixtures were similar to the three target pH levels. It is because of the large solution to soil ratio (10:1) used for the batch studies, resulting insignificant pH buffering effect of the soils.

Statistical Analysis
The experimental data were analyzed using the PROC MIX procedure in Statistical Analysis System (SAS) for Windows Version 6.12 (SAS Inc., Cary, NC). Results were considered significantly different at p < 0.05 unless noted otherwise. Mean separation was performed using least significant difference (LSD) values. The error bars on the graphs represent one standard deviation above and below the mean. Non-overlapping error bars were interpreted as two statistically different treatments.

	RESULTS AND DISCUSSION

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Sorption of NH₄⁺ in Soils Exposed to Two Different Matrixes—Liquid Swine Waste and (NH₄)₂SO₄ Solution
Figure 1 shows the kinetics of NH⁺₄ sorption in soils exposed to 495 mg NH⁺₄–N L^–1 in two matrixes, the liquid swine waste and the (NH₄)₂SO₄ solution. This concentration is below the saturation concentrations for the two soils (Fig. 2) . When exposed to the simple (NH₄)₂SO₄ solution matrix, the sorption of NH⁺₄ in both soils reached equilibrium within 6 h, whereas it took close to 80 h to reach equilibrium when the two soils were exposed to liquid swine waste. Pattern of NH⁺₄ sorption with (NH₄)₂SO₄ solution was similar for the two soils. In contrast, the NH⁺₄ sorption in the two soils exhibited different patterns when exposed to liquid swine waste. In the Kennebec soil NH⁺₄ sorption decreased with time before it reached equilibrium. In the Haynie soil, NH⁺₄ sorption remained stable for the first 60 h and then increased with time before it reached equilibrium at 80 h. The difference of NH⁺₄ sorption in the two soils maybe due to the differences in their soil physicochemical properties other than soil pH (Table 1). Although the initial pH levels of the two soils were different, when the two soils were equilibrated with the swine waste solution, the pH levels of both systems became similar because of the large solution to soil ratio (10:1) used in the study. The measured pH values of the liquid swine waste-soil mixture were 8.0 and 8.2 for Kennebec and Haynie soils, respectively. The pHs of the (NH₄)₂SO₄–soil mixtures were adjusted to the same values as that of the corresponding waste-soil mixtures. It is speculated that initially some NH⁺₄ was sorbed weakly to Kennebec soil from the animal waste solution. After a prolonged period of equilibration time, the weakly sorbed NH⁺₄ was desorbed back to the solution, resulting in a decrease in NH⁺₄ sorption with increased time of equilibration for the Kennebec soil in swine waste solution. Results of desorption study shown in Fig. 3 indicated stronger retention of swine waste-derived NH⁺₄ by the Haynie soil than the Kennebec soil. The stronger retention of swine waste-derived NH⁺₄ by the Haynie soil may have prevented the sorbed NH⁺₄ from being desorbed back to the solution, resulting in different sorption kinetic compared with that for the Kennebec soil. Since sodium azide was added right before the batch incubation experiment, microbial activity is less likely to be significant to affect the NH⁺₄ sorption in the two soils.

View larger version (20K): [in this window] [in a new window]   Fig. 1. Changes of sorption of NH4+ in Kennebec and Haynie soils with time. The two soils were exposed to the liquid swine waste (closed symbols) and the (NH4)2SO4 solutions (open symbols) at a soil/solution ratio of 1:10. The measured pH values of the liquid swine waste–soil mixture were 8.0 and 8.2 for Kennebec and Haynie soils, respectively. The pHs of the (NH4)2SO4–soil mixtures were adjusted to the same values as that of the corresponding waste-soil mixtures. Both solutions initially contained 495 mg NH4+–N L–1.

View larger version (20K): [in this window] [in a new window]   Fig. 2. Langmuir sorption isotherms (solid lines) for NH4+ in the Kennebec and Haynie soils exposed to the liquid swine waste (solid symbols) and the (NH4)2SO4 solutions (open symbols). The dashed lines indicate a different stage of sorption. The initial NH4+ concentrations in both solutions varied from 75 to 2085 mg NH4+–N L–1. Ce and qe are the equilibrium concentrations of NH4+ in the supernatant (mg NH4+–N L–1) and sorbed in the soils (mg NH4+–N kg–1), respectively.

View larger version (20K): [in this window] [in a new window]   Fig. 3. Percentage of sorbed NH4+ desorbed from NH4+–saturated Kennebec and Haynie soils after 2-h extraction with 0.01 M CaCl2 solution. NH4+ was initially sorbed onto the two soils by equilibrating them with the liquid swine waste (black bar) and the (NH4)2SO4 solutions (white bar) both containing 1075 mg NH4+–N L–1 for 72 h. Different letters indicate statistically significant difference (p < 0.05).

When exposed to the (NH₄)₂SO₄ solution, the Kennebec soil sorbed significantly (p < 0.05) greater amounts, almost 60 to 80% more NH⁺₄ than the Haynie soil (Fig. 1). Higher CEC of the Kennebec soil contributed by its high organic matter and clay contents (Table 1) may have caused the increased sorption of NH⁺₄ in the soil. However, this effect may be masked by the complex components in the swine waste solution, resulting in much lesser difference between the NH⁺₄ sorption capacity at equilibrium in both soils when exposed to the swine waste solution.

The two-step sorption behavior for NH⁺₄ in the Kennebec and Haynie soils exposed to both (NH₄)₂SO₄ and swine waste solutions (Fig. 2) suggests that there may be two sets of binding sites in the soils, each with its own binding strength and adsorption maxima. These binding sites have often been classified as exchangeable and nonexchangeable sites (Silva and Bremner, 1966; Mengel and Scherer, 1981; Kudeyarow, 1981; Keeney and Nelson, 1982; Nommik and Vahtras, 1982; Steffens and Sparks, 1999). There exists a dynamic equilibrium between soil clay and exchangeable and nonexchangeable NH⁺₄ (Nommik, 1965; Nommik and Vahtras, 1982). The NH⁺₄ ions adsorbed in clay occupy exchangeable sites first with a fast reaction rate and then transform to nonexchangeable NH⁺₄. Therefore, NH⁺₄ would occupy most of the exchangeable sites before they are fixed on nonexchangeable sites because the slow reaction is the rate-determining process.

The linearized Langmuir model was fitted for the first five data points in Fig. 2 and the resulting parameters (b and K) are presented in Table 3. The last data point was omitted in the above calculation for the Langmuir parameters because it appears to fall in the nonexchangeable region at this NH⁺₄ concentration level. The values for the NH⁺₄ adsorption maxima (b) for the Kennebec and Haynie soils exposed to liquid swine waste were 1000 and 625 mg NH⁺₄–N kg^–1, respectively (Table 3). The corresponding values for the two soils exposed to the (NH₄)₂SO₄ solution were 909 and 217 mg NH⁺₄–N kg^–1. High NH⁺₄ adsorption maxima indicate high NH⁺₄ sorption capacity a soil has. The Kennebec soil showed greater adsorption maxima values as compared with those of the Haynie soil, irrespective of the matrix type (Table 3). Therefore, the Kennebec soil has a higher NH⁺₄ sorption capacity compared with the Haynie soil. Moreover, the Haynie soil showed a greater response to the liquid manure exposure. This is reflected by the greater difference between the adsorption maxima for the soil exposed to the liquid swine waste and that exposed to the (NH₄)₂SO₄ solution. The complex components in the liquid swine waste may have bigger influence on NH⁺₄ sorption in the Haynie soil, which has much lower organic matter and clay content compared with the Kennebec soil.

View this table: [in this window] [in a new window]   Table 3. Constants for the linearized Langmuir isotherms shown in Fig. 2.

Desorption of NH₄⁺ in Soils Exposed to the Two Different Matrixes
Desorption of NH⁺₄ from soils reached equilibrium within 2 h (data not shown). The soils were initially equilibrated with the (NH₄)₂SO₄ and liquid swine waste solutions both containing 1075 mg NH⁺₄–N L^–1. Figure 3 shows that a significantly lower percentage of NH⁺₄ sorbed was desorbed from soils exposed to the liquid swine waste than that exposed to the (NH₄)₂SO₄ solution.

Stronger retention of swine waste-derived NH⁺₄ in soil, compared with the retention of NH⁺₄ from the CaCl₂ solution, may be due to the sorption of DOC from swine waste on soil colloidal surfaces. Ammonium sorption on soil may have occurred with the possible formation of NH⁺₄–DOC chemical complexes (Stevenson, 1994). This supports our observation that more NH⁺₄ can be desorbed from a soil that was exposed to a simple matrix [the (NH₄)₂SO₄ solution] as compared with a complex matrix (the liquid swine waste). Thus, sorption–desorption studies using (NH₄)₂SO₄ solutions may lead to erroneous conclusions about a waste-added soil system, with possible overestimations of NH⁺₄ losses from the exchange complex of the soil.

Stronger retention of swine waste-derived NH⁺₄ by the Haynie soil (Fig. 3) may be largely due to its enhanced response as a result of the exposure to the DOC rich-swine waste. In other words, the Haynie soil may have retained more DOC because it initially contains less SOC (Table 1). This in turn may have lead to the greater NH⁺₄ retention capability. This is also reflected by the significant increase of NH⁺₄ sorption maxima parameter (b) for the Haynie soil (Table 3) when the soil was exposed to the liquid swine waste compared with the (NH₄)₂SO₄ solution.

Results of a sequential extraction procedure performed on the same soil samples used in the above desorption kinetics studies are shown in Fig. 4 . No more NH⁺₄ could be extracted after two consecutive extractions with 0.01 M CaCl₂. Both soils retained significantly (p < 0.05) more NH⁺₄ sorbed from the swine waste than from the (NH₄)₂SO₄ solution. The negative values indicate the release of NH⁺₄ from nonexchangeable pool. When the soils were initially exposed to the liquid swine waste, sequential extraction could not extract NH⁺₄ from the nonexchangeable pool in both soils. Significant amount of NH⁺₄ was extracted out of the nonexchangeable pool in soils that were initially exposed to the (NH₄)₂SO₄ solution, with more from Kennebec soil than Haynie soil.

View larger version (17K): [in this window] [in a new window]   Fig. 4. Ammonium remained sorbed in Kennebec and Haynie soils after sequential extraction with 0.01 M CaCl2 for 1 h. The soils were originally equilibrated with the liquid swine waste (closed symbols) and the (NH4)2SO4 solutions (open symbols) containing 1075 mg NH4+–N L–1 for 72 h.

In both sequential extractions, the Haynie soil eventually had significantly (p < 0.05) more NH⁺₄ retained than the Kennebec soil (Fig. 4). Since this observation was made irrespective of the type of the matrix, this may be due to some soil-related factor such as clay mineralogy. The potential for NH⁺₄ fixation in interlayer positions of mica in the Haynie soil may have played a role. Douglas (1989) and Fanning et al. (1989) pointed out that vermiculite and mica are usually considered to be K⁺ and NH⁺₄ fixing minerals. Although the Kennebec soil contained relatively higher mica content, the interlayer positions of this mica may have already been occupied with NH⁺₄ ions, probably because the Kennebec soil had almost 2.6 times more total N content compared with the Haynie soil (Table 1). However, the exact mechanism of stronger retention NH⁺₄ in the Haynie soil is not clear. Overall, the Haynie soil showed a greater retention capability for NH⁺₄ although the Kennebec soil showed a greater NH⁺₄ sorption capacity (as indicated by the sorption maxima parameters shown in Table 3) compared with the Haynie soil.

Effect of Dissolved Organic Carbon and pH on NH₄⁺ Sorption and Desorption in the Two Soils
The effects of DOC and pH, the major factors associated with the complex liquid swine waste, on NH⁺₄ sorption in the two soils were tested. Ammonium sorption on both soils was positively correlated with the DOC concentration of the solutions to which the soils were exposed (Fig. 5a) . When exposed to the (NH₄)₂SO₄ solutions without DOC, the Kennebec and Haynie soils sorbed about 300 and 185 mg NH⁺₄–N kg^–1 soil, respectively, significantly lower then the amount of NH⁺₄ sorbed when the soils were exposed to the modified swine waste solutions containing DOC. In general, for each 250-mg L^–1 increment in DOC in the solution, there was a 20 to 30% increase in NH⁺₄ sorption. The results of desorption study on the same samples for the sorption study are shown in Fig. 5b. A negative linear correlation exists between the percentage of sorbed NH⁺₄ that can be desorbed and the DOC concentration in the solutions to which the soils were initially exposed during the sorption study. When the soils were exposed to the (NH₄)₂SO₄ solution only, significantly higher percentage of sorbed NH⁺₄ was desorbed compared with that for soils exposed to the swine waste solutions containing DOC (Fig. 5b). When the DOC concentration in the waste solution reached to 1232 mg L^–1, the DOC concentration in the original liquid swine waste (Table 2), the highest amount of NH⁺₄ was sorbed (Fig. 5a) and the least amount was desorbed (Fig. 5b) later on. The DOC appears to stimulate the sorption and retard desorption of NH⁺₄ in the two soils exposed to the swine waste solutions. Presumably, DOC in swine waste may lower the risk of NO^–₃ leaching from soils in swine lagoon environments.

View larger version (30K): [in this window] [in a new window]   Fig. 5. (a) Ammonium sorption and (b) desorption in Kennebec (closed symbol) and Haynie (open symbol) soils exposed to the modified swine waste solutions containing varying dissolved organic C concentrations (circle symbols) and (NH4)2SO4 solutions (square symbols) for 24 h. The initial NH4+ concentration of all the solutions was 250 mg NH4+–N L–1. The pH and the concentrations of all the major cations and anions in the solutions matched that of the liquid swine waste shown in Table 2.

Our results indicate that the high DOC content in the liquid swine waste may be a key factor contributing to the high NH⁺₄ sorption in soils exposed to the waste solutions. The soils used in this study contained montmorillonite as the major 2:1 type clay mineral. The DOC from equilibrating solution may have been sorbed to montmorillonite of the soil clay. Inoue and Wada (1968) showed that the higher charged 2:1 clay minerals, even if present in smaller quantities, may have a substantial contribution toward the DOC adsorption. Further, it may be possible that the sorption of DOC from the swine waste has created more binding sites for NH⁺₄ ions in the two soils. Since the DOC-adjusted equilibrating solutions contained considerable amounts of anions such as Cl^– and SO₄^2– (Table 2), anion exchange may also have played an important role in DOC sorption on soil. Jardine et al. (1989) reported that anion exchange accounted for about 25% of the DOC adsorption in their study.

Although DOC from the liquid swine waste may be a key factor contributing to the high NH⁺₄ sorption in soils exposed to the waste solutions, the degree of the DOC effect on NH⁺₄ sorption in different soils may be different depending on the soil physicochemical properties. For example, the Kennebec soil contains three times as much clay as the Haynie soil, resulting substantially higher DOC adsorption. This in turn may generate higher maximum sorption capacity of animal waste-derived NH⁺₄ by the Kennebec soil compared with the Haynie soil (Table 3, Fig. 2).

Chung and Zasoski (1994) found that the addition of organic matter increased the preference for NH⁺₄ over K⁺ on sorption sites. Ammonium fixation by SOM has long been studied by many researchers. Burge and Broadbent (1961) reported a positive correlation between chemically fixed NH⁺₄ and the percentage of soil C. Ammonium fixation by SOM increases not only with pH but also with an increase in the amounts of NH⁺₄ present in the system (Stevenson, 1994). The pH of equilibrating solutions used in our study was 8.2. A wide variety of ketones, aldehydes, and other carbonyl-containing compounds are known to react with NH⁺₄ under alkaline conditions.

The effect of pH on NH⁺₄ sorption in the two soils exposed to the (NH₄)₂SO₄ and swine waste solutions are illustrated in Fig. 6 . When both soils were exposed to the swine waste solutions, significantly more NH⁺₄ was sorbed at pH = 6 and 8 compared with pH = 4. However, no difference was observed for NH⁺₄ sorption at the two higher pHs. When exposed to the (NH₄)₂SO₄ solutions, no significant differences were observed for the NH⁺₄ sorption in both soils at the three pHs. Chung and Zasoski (1994) reported very small pH effects on the selectivity for the NH⁺₄ ion in soils, although low pH tended to decrease selectivity. The surface charges of the minerals used in our soils are mostly permanent charges, which are not affected by pH. However, pH affects the charges on DOC and, therefore can affect its interactions with NH⁺₄. At higher pH, more acidic functional groups in the DOC fraction of the waste solutions are deprotonated, resulting in more negative charges and more sites that are available for interaction with NH⁺₄ (McBride, 1994). The pH of the swine waste solution used in this experiment is 8.2. The pH of the (NH₄)₂SO₄ solution varies from 5.5 to 6.5 depending on the concentration of NH⁺₄. The results in Fig. 6 suggest that the high DOC content coupled with high pH in the swine waste contribute significantly to the higher sorption of NH⁺₄ in both soils compared with the soils exposed to the (NH₄)₂SO₄ solution.

View larger version (29K): [in this window] [in a new window]   Fig. 6. Ammonium sorption in Kennebec and Haynie soils exposed to the swine waste and the (NH4)2SO4 solutions at pH 4.0, 6.0, and 8.0 for 24 h. The initial NH4+ concentration of the solutions was 250 mg NH4+–N L–1. Different letters within each bar group indicate statistically significant difference (p < 0.05).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

	ACKNOWLEDGMENTS

 
The work was supported by the Kansas Center for Agricultural Resources and the Environment (KCARE) and the Department of Agronomy, Kansas State University. Gratitude is extended to Tom DeSutter for providing assistance on waste and soil sampling. Authors also thank the reviewers.

Received for publication August 10, 2004.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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