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Published in J. Environ. Qual. 33:338-342 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Vadose Zone Processes and Chemical Transport

Bromide and Nitrate Movement through Undisturbed Soil Columns

D. E. Clay*, Z. Zheng, Z. Liu, S. A. Clay and T. P. Trooien

Agricultural and Biosystems Engineering, South Dakota State University, Brookings, SD 57007

* Corresponding author (david_clay{at}sdstate.edu).

Received for publication February 6, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field experiments often assume that Br–, 14NO–3–N, and 15NO–3–N have similar leaching kinetics. This study tested this assumption. Twenty-four undisturbed soil columns (15-cm diameter) were collected from summit–shoulder, backslope, and footslope positions of a no-tillage field with a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation. Each of the landscape positions had a different soil series. After conditioning the columns with 4 L of 0.01 M CaCl2 (2 pore volumes), 15N-labeled Ca(NO3)2 and KBr were applied to the soil surface and leached with 4 L of 0.01 M CaCl2. Leachate was collected, weighed, and analyzed for NO–3–N, NH4+–N, 15N, 14N, and Br–. The total amount of 15NO–3–N and 14NO–3–N collected in 1000, 2000, and 3000 mL of leachate was similar. These data suggest that 15N discrimination during leaching did not occur. Bromide leached faster through the columns than NO–3–N. The more rapid transport of Br– than NO–3–Nwas attributed to lower Br– (0.002 ± 0.036 mg kg–1) than NO–3–N (0.17 ± 0.03 mg kg–1) sorption. Results from this study suggest that (i) if Br– is used to estimate NO–3–N leaching loss, then NO–3–N leaching losses may be overestimated by 25%; (ii) the potential exists for landscape position to influence anion retention and movement in soil; and (iii) 15N discrimination was not detected during the leaching process.

Abbreviations: AEC, anion exchange capacity • CEC, cation exchange capacity • Kd, sorption coefficient


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
A GRICHEMICAL LEACHING RATES are generally related to water flow rate through soil and the strength of sorption to the soil matrix (Toner et al., 1989; Brooks et al., 1998; Ryan et al., 2001). Cation transport through soil is slowed due to sorption to the soil negative charges. However, anions can also be sorbed to soil. Katou et al. (1996) reported that soil containing allophane and other poorly crystallized materials adsorb otherwise inert anions such as Cl– and NO–3–N. Eick et al. (1999) reported that NO–3–N retention in soil was found to depend on the type and quantity of both variable and permanently charged minerals present in the soil, and that acid subsoils high in variable-charge minerals may slow NO–3–N leaching. Anion retention may be completely reversible (Toner et al., 1989) and influenced by texture, with silt loam soils having more anion retention than sandy soils (Vogeler et al., 1997).

Numerous studies have used Br– and 15NO–3–N as a model for estimating 14NO–3–N leaching (Ingram, 1976; Onken et al., 1977; Olson and Cassel, 1999; Ottman et al., 2000). In these studies, Br– is applied to soil and the movement of Br– through soil is monitored. The difference between Br– applied and recovered is estimated to be the amount of NO–3–N subject to leaching (Kessavalou et al., 1996; Schuh et al., 1997; Ressler et al., 1998; Ottman et al., 2000). Advantages of using Br– include the following: (i) it is a conservative tracer that is not subject to microbial transformations and gaseous losses; (ii) it has low concentration in most soils (Bowman, 1984); and (iii) B–r, like NO–3–N, is an anion and, therefore, is repulsed by negatively charged clays. Studies that use Br– or 15NO–3–N as model compounds for 14NO–3–N leaching assume that Br–, 15NO–3–N, and 14NO–3–N have similar leaching kinetics. This assumption has largely been untested. The objective of this study was to test the assumption that Br–, 15NO–3–N, and 14NO–3–N have similar leaching kinetics.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Twenty-four undisturbed soil columns (0.15 m in diameter by 0.3 m long) were collected from the summit–shoulder, backslope, and footslope positions in June 1998 in a eastern South Dakota field with the center coordinate of 44°10' N, 96°37' W. The crop rotation at the site was corn preceded by soybean. No-tillage had been practiced in the field for at least eight years. The soil series in the summit–shoulder area was a Kranzburg silty clay loam (fine-silty, mixed, superactive, frigid Calcic Hapludoll). The soil series at the backslope area was a Waubay silty clay loam (fine-silty, mixed, superactive, frigid Pachic Hapludoll), and the soil series in the footslope was a Cubden silty clay loam (fine-silty, mixed, superactive, frigid Aeric Calciaquoll). The dominate clay mineral at all landscape positions was smectite with small amounts of kaolinite, illite, mica, and quartz.

At each landscape position, soil samples (six subsamples from each landscape position) from the Ap horizon were collected. These samples were analyzed for cation exchange capacity (CEC) using the ammonium acetate method (Soil Survey Staff, 1984); sand, silt, and clay content using the pipette method (Gee and Bauder, 1986); soil organic carbon (National Soil Survey Staff, 1996); pH (0.01 CaCl2); and electrical conductivity (saturated paste). Samples from each landscape position were also analyzed for anion exchange capacity (AEC) following the method of Zelazny et al. (1996). At three sites at each landscape position infiltration was measured using a sprinkler infiltrometer (Ogden et al., 1997). Selected chemical and physical properties of the three landscape positions are shown in Table 1.


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  		Table 1. Physicochemical characteristics of the Ap horizon at the three landscape positions.{dagger}

 
The 24 soil columns were prepared for the experiment by attaching an end cap to each column. To prevent bypass flow between the soil column and PVC tube the interphase between the soil and PVC tube was packed with surface soil collected from the site. The end cap contained a layer of cheesecloth and 0.05 kg acid-washed sand to prevent soil escape. To prevent aggregate destruction, when 0.01 M CaCl2 was applied, the soil was covered with filter paper. Soil was conditioned for the experiment by pre-wetting with 0.5 L purified water, filtered to remove organics and inorganic contaminants, and leached with 4 L 0.01 M CaCl2 (2 pore volumes). Columns were allowed to drain for 24 h. Dilute salt was used (i) to minimize clay dispersion during the experiment; (ii) to remove as much of the background NO–3–N from the columns as possible; (iii) to replace NO–3–N in the soil matrix with Cl–, which reduced the potential for exchange reactions to occur between 15N-enriched NO–3–N applied in the treatment solution and unenriched NO–3–N in the soil matrix; and (iv) because it had a similar ionic strength to that of the resident solution. Soil column conditioning is similar to natural events that occur in the field. For example, fertilizers (KCl), lime (CaCO3), or rainfall enriched with various nutrients may have similar effects on soil properties.

After conditioning, 7.4 mL of a treatment solution was applied in 0.37-mL aliquots with a pipette to 20 points on the soil surface. Treatments were a control (0.01 M CaCl2) (no Br– or NO–3–N added) or treated (0.01 M CaCl2 with 100 kg NO–3–N ha–1 plus KBr). The treatment solution was prepared by dissolving 13.875 g of Ca(NO3)2·4H2O containing 0.6 atom% 15N, 59.9 g of reagent-grade Ca(NO3)2·4H2O, and 7.1 g KBr in 100 mL 0.01 M CaCl2. After applying the treatment, 2 pore volumes of 0.01 M CaCl2 solution (4 L) was applied onto the soil surface. The 0.01 M CaCl2 solution initially was ponded on the soil surface with a maximum head of 22.6 cm. A portion of the solution applied to the columns was saved for NO3––N, 15N, and Br– analysis. In all columns, leachate was collected in about 300-mL increments.

Water samples were weighed and analyzed for NO–3–N and NH+4–N concentration on an Astoria analyzer (Astoria-Pacific, Clackamas, OR). Nitrate and NH+4–N were determined using the Cd reduction and indolphenol blue procedures, respectively (Maynard and Kalra, 1993). Bromide concentrations were measured with a specific ion electrode (9435BN; Thermo Orion, Beverly, MA). The standards (1, 5, 10, 20, 40, 60, and 80 mg L–1) were made in 0.01 M CaCl2. For 15N analysis, a 100-mL aliquot of each sample was concentrated to about 2 mL, which was analyzed for atom% 15N on a Europa Scientific (Crewe, UK) ratio mass spectrometer.

To determine the percentage of 14NO–3–N and 15NO–3–N leached, the amount of 14NO–3–N and 15NO–3–N leached from the control soils was subtracted from the amount of 14NO–3–N and 15NO–3–N leached from treated columns. The ratio between the percentage of NO–3–N and Br– collected in the leachate was determined by dividing the cumulative amount of Br–, 14NO–3–N, and 15NO–3–N collected in the leachate by the amount of Br–, 14NO–3–N and 15NO–3–N applied to the soil, respectively. To evaluate if Br– and 15NO–3–N leached at similar rates the following calculations were made. First, the amount of 15NO–3–N leached from the control soil was subtracted from the 15NO–3–N contained in the leachate in the treated columns. Second, the relative amounts of Br– and 15NO–3–N leached were determined by dividing the cumulative amount of Br– and 15NO–3–N contained in each individual sample by the total amount of leached Br– and 15NO–3–N. Third, the relative leaching rates of Br– and 15NO–3–N were determined by dividing the relative amount of Br– leached by the relative amount of 15NO–3–N leached. A ratio of 1.0 indicated that Br– and 15NO–3–N leached at similar rates. A ratio greater than 1.0 indicated that Br– leached faster than 15NO–3–N. These values were plotted on a curve and a 95% confidence interval for the regression line was determined.

Bromide sorption was determined on 10 g of air-dried, sieved (<2 mm) soil collected from the three landscape positions. The soil was mixed with 20 mL of water containing 0 and 10 mg Br– L–1 of solution. Each treatment was replicated three times and the experiment was repeated twice. The soil solution mixtures were shaken for two hours and centrifuged, and the supernatant was analyzed for Br– using appropriate standards. Bromide was determined colormetrically following the procedure described below. The water samples were buffered to pH 5.6 and mixed with chloramines-T, which oxidized bromide to hypobromous acid. Hyprobromous acid reacted with fluorescein to form tetrabromofluorescein, which was measured with a spectrophotometer at 520 nm. The bromide sorption coefficient (Kd) was calculated using the following equation:

[1]
where Br– sorbed (mg Br L–1) was equal to (Br– applied + Br– soil blank) – (Br– in the solution at equilibrium [mg Br L soil–1]). The Br– soil blank was the concentration in mg Br– L–1 in the 0 mg Br– L–1 treatment.

Nitrate sorption coefficients were determined on soil samples collected from the three landscape positions (Ryan et al., 2001). Ten grams of air-dried sieved (<2 mm) soil from each landscape position was mixed with either 20 mL of water or 20 mL of 1 M KCl spiked with 0 or 20 mg NO3––N L–1, shaken for two hours, and centrifuged. The supernatant was analyzed for NO–3–N using methods described above. Each treatment was replicated four times. The nitrate Kd values were calculated using the equation:

[2]
where NO–3–N sorbed (mg NO–3–N L–1) was equal to the difference between the amount of NO–3–N extracted with 1.0 M KCl and water after shaking for 2 h. The approaches to calculate Br– and NO–3–N Kd values were conceptually identical. For NO–3–N, the total amount of NO–3–N contained in the soil was extracted by 1 M KCl, while for Br– the total amount of Br– contained in the soil was equal to the amount of Br– added plus the Br– contained in the control soil. A t test at the 0.05 level was used to compare Br– and NO–3–N sorption values and analysis of variance (ANOVA) analysis (P = 0.05) was used to determine differences due to landscape position.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Characteristics and Anion Sorption
Soil collected from the summit–shoulder area appeared to have lower CEC, organic matter content, and pH than soil collected from backslope and footslope areas (Table 1). Landscape differences were attributed to weathering and erosion that transports clays, salts, and organic matter from summit–shoulder areas to the foot–toe slope areas. Anion exchange capacities were slightly negative and similar at the three landscape positions. Negative AEC can occur from anion exclusion, previously reported by Sposito (1989).

Bromide sorption coefficients (Kd) were lower in soil collected from the backslope than footslope and summit–shoulder areas (Table 1). The near-zero or slightly negative Br– sorption coefficients were in agreement with the slightly negative AEC. Associated with lower Kd values in the backslope area were moderate pH (6.65) and CEC (32.1 cmolc kg–1). These results are different from conventional theory, which suggests that anion sorption should increase as pH decreases (Sposito, 1989). The negative Kd values in the backslope suggest that anion exclusion may have occurred (Br– soil solution > Br– applied + Br– blank). Negative sorption can be produced by electrostatic repulsion (Sposito, 1989). The factors responsible for landscape-dependent Br– sorption were not determined.

Sorption was less for Br– than NO–3–N. Differences between NO–3–N and Br– sorption may be attributed to differences in the adsorption envelopes and their solubilities with other soil cations (Sposito, 1989). For example, Weast and Astle (1983) reported that at 0°C, 54 g of KBr and 13 g of KNO3 are soluble in 100 mL of water. Herbel and Spalding (1993) had similar results for nitrate and reported that KCl extracts yielded significantly higher NO–3–N concentrations than deionized water. They postulated that NO–3–N not extracted by deionized water was trapped within the microstructure of the swelling clays, and that deionized water rather than KCl extractants may provide a better estimate of ground water loading from nonpoint sources.

Tracer Movement through Soil Columns
In all columns, NO–3–N and Br– were measured in the first samples collected. These results suggest that macropore bypass flow most likely occurred (Beven and Germann, 1982; Jabro et al., 1991; Timlin et al., 1998). The relatively rapid movement of these ions through the columns was attributed to no-tillage that encouraged the development of macropores.

A landscape by tracer interaction was not observed and, therefore, the discussion will focus on the main effects. The points of maximum Br– concentration in the leachate collected from the summit–shoulder, backslope, and footslope were 1500, 1200, and 1870 mL of water, respectively. These results suggest that landscape position or soil type have the potential to influence chemical transport. Similar landscape differences were observed for NO–3–N (data not shown).

The similarity in the cumulative total amounts of 15NO–3–N and 14NO–3–N collected in 1000, 2000, and 3000 mL of leachate indicates that 15N discrimination did not occur during NO–3–N leaching (Table 2). Apparent 15N discrimination can occur if exchange between 15N in the solution and 14N on the exchange sites occurs. Exchange reactions were minimized by conditioning the columns with 0.01 M CaCl2. Discrimination can also result from N addition stimulating mineralization of 14N-labeled organic matter. If either of these processes occurred, then 14NO–3–N would appear to leach faster than 15NO–3–N. These results are different than those reported for other components of the N cycle. For example, 15N discrimination occurs during both denitrification and ammonia volatilization (Delwiche and Steyn, 1970; Mariotti et al., 1981). Isotopic discrimination has been attributed to differences in the molecule size and molecular weight.


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  		Table 2. The influence of the amount of cumulative water collected on the percentage of Br–, 14NO–3–N, and 15NO–3–N leached through the undisturbed soil columns.{dagger}

 
A comparison between the amount of Br– and NO3––N collected in the leachate showed that approximately 41, 36, and 35% more Br– than NO–3–N was collected in 1000, 2000, and 3000 mL of leachate, respectively (Table 2). These results could have resulted from NO–3–N immobilization, denitrification, and/or higher Kd values for nitrate than Br–. Similar leaching patterns have been observed in Hamilton (Reno, NV) Pxp-100 anion exchange and Wescan Anion/R columns (Alltech Associates, Deerfield, IL). To assess if microbial immobilization or denitrification were responsible for the longer retention of nitrate, the ratios were recalculated. In these calculations the numerator was the cumulative amount of Br– and NO–3–N leached, while the denominator was the total amount of Br– and NO–3–N collected in the leachate (Fig. 1) . In the first 1000 mL of leachate collected, the ratio between the relative amounts of Br– to 15NO–3–N collected was greater than 1. Ratios greater than one indicate that Br– leached faster than NO–3–N. Differences between NO–3–N and Br– were not attributed to differences in hydrated diameters, since both have been estimated at 0.3 nm (Kielland, 1937), or denitrification or immobilization, because only leached NO–3–N was included in the calculations. The more rapid movement of Br– than NO–3–N was attributed to Br– having lower sorption than NO–3–N. Jiang et al. (1997) had similar results and reported that Br– and NO–3–N had similar breakthrough characteristics, yet the relative concentration of nitrate was consistently lower than that of Br–.



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  		Fig. 1. The relationship between leachate volume and the relative amount of Br– and 15NO–3–N contained in the leachate. A value greater than 1 indicates that Br– leached faster than NO–3. The different symbols represent different soil columns and the 95% confidence intervals are shown as dashed lines.

 
Smith and Davis (1974) had different results and reported that Br– and NO–3–N had similar leaching characteristics when the potential for immobilization or denitrification to occur was eliminated by autoclaving. Differences between Smith and Davis (1974) and our study may have resulted from several factors. Smith and Davis (1974) conditioned the soil columns by eluting them first with 0.005 M CaSO4, while this study used 0.01 M CaCl2. Differences in the anion used to condition the columns are significant because SO2–4 can be so strongly adsorbed to exchange sites that exchange reactions with NO–3–N or Br– may be minimal (Katou et al., 1996). Differences between anion loss and retention have been reported for other anions. Brooks et al. (1998) reported that even with 475 times more Cl– than Br–, exchange sites on the ferrihydrite [Fe5O7(OH)·4H2O] selectively adsorbed Br–. Rustad et al. (1996) reported that in a field study when HNO3 and H2SO4 were applied to soil, NO–3–N concentration decreased 71 to 93%, while SO2–4 concentration decreased by 50 to 86% after passing through 25 cm of surface soil. Reductions in the SO2–4 concentration were attributed to anion sorption. Changes in the NO–3–N concentration was attributed to NO–3–N immobilization. Differential movement may explain the findings of Kelly and Wilson (2000) and Ottman and Pope (2000). Ottman and Pope (2000) reported that the use of Br– as a model for NO–3–N transport overestimated the depth of 15N movement. Kelly and Wilson (2000) reported that 4 h after flooding approximately 100% of applied Br– and NO–3–N were present in the soil solution. Sixteen hours after flooding only 67% of the applied Br– remained in the soil solution and 84% of the applied 15NO–3–N remained in the soil solution. The lower percentage of Br– remaining in the soil solution may have resulted from either exchange reactions between 14NO–3–N in the soil matrix and 15NO–3–N applied in the treatment solution or differential leaching and sorption as discussed in this paper.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The total amounts of 15NO–3–N and 14NO–3–N collected in 1000, 2000, and 3000 mL of leachate were similar, which suggests that 15N discrimination during leaching did not occur. Bromide leached through the columns faster than NO–3–N. The more rapid transport of Br– than NO–3–N was attributed to lower Br– than NO–3–N sorption. Results from this study suggest that (i) if Br– is used to estimate NO–3–N leaching loss, then estimated leaching may be overestimated by about 25%; (ii) the potential exists for landscape position to influence anion retention and movement in soil; and (iii) 15N discrimination was not detected during the leaching process.


   ACKNOWLEDGMENTS
 
Support provided by the South Dakota Soybean Research and Promotion Council, North Central Soybean and United Soybean Boards, and the USDA Cooperative State Research, Education, and Extension Service (CSREES) National Research Initiative (NRI). South Dakota Experiment Station no. 3348.


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




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