Published online 1 March 2008
Published in J Environ Qual 37:387-394 (2008)
DOI: 10.2134/jeq2006.0545
© 2008 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Plant and Environment Interactions
Nitrate-Nitrogen Concentrations in the Perched Ground Water under Seepage-Irrigated Potato Cropping Systems
F. Munoz-Arboledaa,
R. Mylavarapub,*,
C. Hutchinsona and
K. Portierc
a Horticultural Sciences Dep., Univ. of Florida, Fifield Hall, PO Box 110690, Gainesville, FL 32611
b Soil and Water Science Dep., McCarty Hall, PO Box 110290, Gainesville, FL 32611
c Statistics & Evaluation Center, American Cancer Society-NHO, 1599 Clifton Road, NE, Atlanta, GA 30329
* Corresponding author (raom{at}ufl.edu).
Received for publication December 18, 2006.
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ABSTRACT
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Excessive nitrogen rates for potato production in northeast Florida have been declared as a potential source of nitrate pollution in the St. Johns River watershed. This 3-yr study examined the effect of N rates (0, 168, and 280 kg ha–1) split between planting and 40 d after planting on the NO3–N concentration in the perched ground water under potato (Solanum tuberosum cv. Atlantic) in rotation with sorghum sudan grass hybrid (Sorghum vulgare x Sorghum vulgare var. sudanese, cv. SX17), cowpea (Vigna unguiculata cv. Iron Clay), and greenbean (Phaseolus vulgare cv. Espada). Soil solution from the root zone and water from the perched ground water under potato were sampled periodically using lysimeters and wells, respectively. Fertilization at planting increased the NO3–N concentration in the perched ground water, but no effect of the legumes in rotation with potatoes on nitrate leaching was detected. Fertilization of green bean increased NO3–N concentration in the perched ground water under potato planted in the following season. The NO3–N concentration in the soil solution within the potato root zone followed a similar pattern to that of the perched ground water but with higher initial values. The NO3–N concentration in the perched ground water was proportional to the rainfall magnitude after potato planting. A significant increase in NO3–N concentration in the perched ground water under cowpea planted in summer after potato was detected for the side-dressing of 168 kg ha–1 N applied to potato 40 d after planting but not at the 56 kg ha–1 N side-dress. Elevation in NO3–N concentration in the perched ground water under sorghum was not significant, supporting its use as an effective N catch crop.
Abbreviations: PSF, potato-sorghum-fallow • PSG, potato-sorghum-greenbean • PCF, potato-cowpea-fallow • PCG, potato-cowpea-greenbean • TCAA, Tri-County Agricultural Area
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INTRODUCTION
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SEEPAGE-IRRIGATED potato is an important spring crop in northeast Florida yielding more than $100 million a year for the past 5 yr (USDA, 2004). Most of the 9000 ha in potato production are located in the Tri-County Agricultural Area (TCAA), a major agricultural area in the St. Johns River Water Management District. Potato production in the TCAA represents 60% of the Florida's annual production (USDA, 2007). Through a voluntary cost-share Best Management Practices program, the St. Johns River Water Management District is trying to control nutrient loading into the St. Johns River from the TCAA (Livingston-Way, 2000) that specifies maximum applied fertilization levels for potatoes.
The potato crop has been targeted for study because of the increased potential for nitrate losses from the root zone due to its high N demand for optimum production (Hill and McCague, 1974; Millburn et al., 1990; Honisch et al., 2002). In Florida, the Best Management Practices nitrogen rate for potatoes has been set at 224 kg N ha–1 (Hochmuth and Hanlon, 2000; Livingston-Way, 2000). However, growers have traditionally applied more N fertilizer than the recommended rate as insurance against weather conditions that promote leaching. Seepage irrigation is a subsurface irrigation system where the perched ground water height is maintained during the potato season by pumping up water from deep wells and perching it on a restricting loamy layer lying approximately 1.5 m below the soil surface. Therefore, continuous pumping of water throughout the season renders the seepage irrigation not only highly inefficient but makes the system highly vulnerable to nitrate leaching from the potato beds.
The sandy nature of Florida soils, excess nitrate in the potato beds, seasons with high rainfall, and seepage irrigation combine to create conditions that can result in N leaching with subsequent eutrophication of water bodies. The maximum contaminant level for nitrate in drinking water is 10 mg L–1 of NO3–N (USEPA, 2003).
The objectives of this study were (i) to quantify nitrate concentrations in the perched ground water during potato season, (ii) to determine the effect of crop rotation with legumes as summer/fall cover and cash crops on NO3–N concentration in the soil solution and perched ground water below the following potato crop as compared with the traditional potato/sorghum/fallow rotation, and (iii) to quantify a possible residual effect of the N applied in winter/spring during potato season on the elevation of NO3–N concentration in the perched ground water under summer cover crops.
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Materials and Methods
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Site Description
The study was conducted during 2001, 2002, and 2003 at the University of Florida/Institute of Food and Agricultural Sciences (UF/IFAS) Plant Science and Education Unit, Hastings Farm in Hastings, FL (29° 42' 60'' N and 81° 30' 34'' W) (Fig. 1
). The soil at the site is classified as sandy, siliceous, hyperthermic Arenic Ochraqualf belonging to the Ellzey series (USDA, 1983). The typical pedon has a fine sand layer from 0 to 1 m and a loamy fine sand restricting layer from 1 to 1.6 m. As a result, the soil profile has very poor drainage even though the saturated hydraulic conductivity in the top 1 m is about 10 cm h–1. The surface soil texture is 940 gm kg–1 sand, 25 gm kg–1 silt, and 35 gm kg–1 clay (Campbell et al., 1978). In its natural state, the perched ground water is within 25 cm of the surface for 1 to 6 mo in most years, and land slopes are less than 2% (USDA, 1983). Seepage irrigation is used to grow crops in this region. The seepage irrigation system uses high row beds to plant potatoes and shallow water furrows to supply irrigation and to remove drainage water. Irrigation and drainage water furrows are arranged every 16 potato rows. Farmers pump up water from deep wells and perch it over the loamy fine sand restricting layer, increasing the natural water table level to continuously supply water to the root zone of the crops by capillary effect. The perched ground water is maintained at an average depth of 60 to 70 cm below the top of the potato row throughout the season. Farmers check soil moisture visually to determine irrigation needs. Seepage irrigation events are graphically reported in Fig. 2
. No information on lateral perched ground water flow is available, but it is expected to be very slow due to the minimal land slope. Water samples were taken from the two central rows of each treatment plot to prevent cross contamination. Perched ground water levels were not measured at the site. Weather data were continuously recorded at the research site through one of the weather stations operated by the Florida Automated Weather Network (http://fawn.ifas.ufl.edu).
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Fig. 2. Crop cycles, sampling schedules, and precipitation pattern over the 3-yr (2001–2003) period of study.
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General Statistical Design and Experimental Layout
The experiment was arranged in a split-plot design with four replicates (blocks), except in the third year when three replicates were used and only three of the five N rates (0, 168, and 280 kg ha–1) were sampled. In the third year, the fourth block was used as a separate experiment to study the effect of subsoiling on root distribution and yield, but those results are not presented in this paper.
The main plot treatment factor was crop rotation, and the subplot treatment factors were five N fertilizer rates applied to potato. Each block consisted of one bed, 16.4 m wide (16 potato rows) by 160-m long. Four main plots inside each bed (block) resulted from dividing lengthwise each bed into two strips of 8.2 m width by 160 m length (eight rows 160 m long) and dividing each one of these strips (8.2 x 160 m) into two parts, resulting in four main plots 8.2 m wide (eight rows) by 80 m long.
A crop rotation plan was randomly assigned for summer and fall to each main plot. Each year in summer, all main plots were planted with sorghum (Sorghum vulgare x Sorghum vulgare, cv. Sudanese, cv. SX17) and cowpea (Vigna unguiculata, cv. Iron Clay), which are referred to collectively as the summer cover crops. In the fall of each year, one of two possible treatments was assigned to a main plot. Main plots were planted with green bean (Phaseolus vulgare, cv. Espada) or left uncultivated and maintained as a weed-free fallow. Before potato (Solanum tuberosum, cv. Atlantic) planting in winter/spring, main plots were subdivided into the five subplots (eight rows x 16 m), and each subplot had its random nitrogen rate applied. Allocation of experimental plots and the assignment of treatments to main plots and subplots were performed once at the beginning of the 3-yr study period. Hence, measurements over time represent repeated measures.
Crop Production Practices
Crop rotations were identified as potato-sorghum-fallow (PSF) (the traditional rotation in the region), potato-sorghum-green bean (PSG), potato-cowpea-fallow (PCF), and potato-cowpea-green bean (PCG). All ground was fumigated with 1,3-dichloropropene (56 L ha–1) for nematode control approximately 4 wk before planting potatoes. Potatoes were planted on 28, 12, and 6 February during the 3 yr, respectively. Seed pieces (
57 g) were planted at an in-row spacing of 20 cm with a 102-cm between-row spacing, resulting in a crop density of 48,216 plants ha–1. Recommended N rate for potatoes in the region is 224 kg ha–1 (Hochmuth and Hanlon, 2000; Livingston-Way, 2000). Ammonium nitrate (NH4NO3), at 112 kg N ha–1, was incorporated in subplots receiving N. Remaining N (NH4NO3), at 0, 0, 56, 112, 168 kg N ha–1, was side-dressed 2 wk after potato plant emergence to complete the final N treatments (0, 112, 168, 224, and 280 kg N ha–1). The no-nitrogen subplot (control) was not fertilized with N. Levels of P and K were reestablished to recommended values in all the plots before potato planting in each production year with application rates of 25 and 139 kg ha–1 of elemental P and K, respectively.
Sorghum/sudan grass and cowpea were seeded in a single row on 102-cm between-row spacing with within-row seed spacing of 2.5 cm, resulting in a density of 392,000 plants ha–1. Cowpea and sorghum were planted 28, 11, and 25 of June in each of the three seasons, respectively. No additional fertilizer was applied to cowpea or sorghum crops. Around mid-August in each year (before the cowpea set seed), sorghum and cowpea were chopped and incorporated into the soil as a green manure.
Bush green bean was planted on 4, 10, and 15 September in each of the three seasons, respectively, in a single drill per row with in-row and between-row spacing of 2.5 and 102 cm, respectively, resulting in a density of 392,000 plants ha–1. Cowpea and green bean were inoculated with specific Rhizobium strains before planting. The green bean crop was fertilized pre-plant and 1 mo after planting with 400 kg ha–1 of 14–1.3–8.3 and 600 kg ha–1 of 14–0–8.3 fertilizer blends of N, P, and K, respectively. Total N, P, and K amounts applied to green bean were 140, 5.2, and 83 kg ha–1, respectively. Therefore, it is important to account for the extra amount of N (140 kg ha–1) applied to green bean in PCG and PSG unlike PCF and PSF rotations and the resultant increased potential for leaching.
Water Sampling
A 10-cm-diameter, top-capped PVC pipe was buried in each potato plot to a depth of 70 cm from the top the row and provided access to sample the perched ground water below each plot. A mechanical post-hole digger driven by a tractor was used to install the PVC wells. Air gaps around the well were packed with soil compacted with a wooden tamping rod to avoid preferential flow pathways around the well. During samplings the well cap was removed, and a perched ground water sample was taken by using a polyethylene vial held with a mechanical grabber. In 2001 and 2002, wells were installed 3 and 8 d before N side-dressing, respectively. In 2003, wells were installed 2 d after side-dressing. In 2001 the perched ground water was sampled weekly, and in 2002 and 2003 the perched ground water was sampled bi-weekly (Fig. 2).
During the 3-yr study, suction lysimeters (Soil Moisture Equipment Corp., Santa Barbara, CA) were buried at the 30-cm depth in one of the two central rows of each subplot to sample the soil solution at the root zone during the potato season. A day before the sampling, a vacuum was created inside the lysimeter by using a pneumatic pump, and the water sample was collected the following day using a silicone hose attached to a syringe. The syringe was rinsed with distilled water between samples. Due to multiple missing samples during the first (2001) and third (2003) years, only data from the second year (2002) were analyzed and are presented in this paper. Suction lysimeters were installed 1 wk before N side-dressing in 2002 (Fig. 2).
Sampling wells were installed in the summer of 2003 to study the residual effect of the nitrogenous fertilization applied to potato in spring on the perched ground water under cowpea- and sorghum-planted areas in subplots where potato had been fertilized with 0, 168, or 280 kg N ha–1. Three bi-weekly water samplings were taken under sorghum and cowpea in summer.
Water samples from wells and lysimeters were collected in 20-mL, high-density polyethylene vials, frozen, and measured for NO3–N concentration at a later date at the UF/IFAS Analytical Research Laboratory. Analysis was conducted on an air-segmented continuous autoanalyzer (Flow Solution IV; OI Analytical, College Station, TX) as described by Mylavarapu and Kennelley (2002).
Statistical Analysis
Regression models were used to describe NO3–N concentration levels in soil solution (lysimeter data) and ground water (well data) as a function of the split-plot N rate applied and days since side-dress. Water samples from each treatment on each sampling date were replicated four and three times for wells and suction lysimeters, respectively. The total number of samples analyzed during the study period was 1440 and 600 for wells and suction lysimeters, respectively. Orthogonal polynomial contrasts allowed partitioning of variability into linear, quadratic, cubic, and quartic components to determine the appropriate polynomial response surface model form. The assumptions of homogeneity of variance and normality of residuals were checked for the final models. Model building, parameter estimation, and assumption checking were performed using SAS (SAS Institute, 2004) and in particular the RSREG procedure. The regression models are presented to facilitate understanding of the response patterns and should not be interpreted as mathematical descriptions of mechanistic or functional processes underlying the NO3–N responses observed.
General linear models were used to examine the effect of residual fertilization on the NO3–N concentration in the perched ground water under sorghum and cowpea planted in summer 2003. Residual fertilization was measured at 40 d after planting or just before the time that side-dress treatments were applied. The analysis model incorporated the split plot nature of the design in which crop type (cowpea or sorghum) was the main plot factor and nitrogen rates applied to potato was the subplot factor. Means were separated using Tukey's honest significant differences multiple comparison method (Ott and Longnecker, 2001), and 95% confidence limits were calculated for each main plot factor and split-plot factor means. Analysis computations were performed with SAS, primarily using the GLM procedure. More details on the design are given in Munoz (2004).
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Results and Discussion
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NO3–N Concentrations in the Perched Ground Water under Potato
The NO3–N concentration patterns over time in each year under the four treatments with N added were similar, so to make a clear graphical representation of observed and predicted values, only the control (0 N) and two of the four N rates applied to potato are presented in this paper (Fig. 3
–6).
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Fig. 3. Predicted and observed NO3–N concentrations in the perched ground water under four crop rotations and three N rates during 2001. PCF, potato-cowpea-fallow; PCG, potato-cowpea-green bean; PSF, potato-sorghum-fallow; PSG, potato-sorghum-green bean. Model significance: *p < 0.05; **p < 0.01; ***p < 0.0001; ns = not significant.
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In the 3-yr period, NO3–N concentration in the perched ground water under the four different crop rotations exhibited a similar pattern within each season, but different trends were observed among seasons. In the first, second, and third years, the maximum predicted values for NO3–N concentration in the perched ground water at the first sampling were 54.5 (±3.3 SE), 5.0 (±5.6 SE), and 54.6 (±2.7 SE) mg L–1 NO3–N, and the averages of its coefficients of determination for the models were 0.77, 0.32, and 0.55, respectively (Fig. 3–5). The relatively low concentration of NO3–N in the perched ground water during the second year produced a tendency for lower values of R2, and therefore a low fit was obtained. This result could be due to many NO3–N concentration values reported as zero when concentration values were below the analytical method detection limit (0.5 mg L–1 of NO3–N). Decaying linear and quadratic patterns in NO3–N concentration starting with higher values before side-dressed application of N were common observations under all rotations and N rates in each one of the 3 yr studied (Fig. 3–5).
The similarity among NO3–N concentration patterns in each year was evidently due to the application of 112 kg ha–1 of N at planting through readily soluble ammonium nitrate to all of the potato plots receiving N treatments. The actual differences among N treatments were set in only at side-dressing when the final supplementary N dose was applied to reach the total N rate as per each of the treatments. Any confounding effect due to the same N rates up to side-dressing stage among treatments may be resolved through a separate study. The fact that N applied at planting is soil incorporated, whereas N at side-dress is surface applied and covered with some soil at hilling to avoid root system damages, should be taken in account. Therefore, increased N rates at side-dressing time could result in increased risk of N leaching or runoff because most of the root system directly involved in nutrient uptake was around the seed piece (Munoz-Arboleda et al., 2006). The difference among years apparently was caused by the amount of precipitation received between potato planting and N side-dressing (Table 1
and Fig. 2). During the first and third years, precipitation received around the planting period was about 50% higher than the historic 25-yr average for the same period in the region (NOAA, 2007), including occurrence of leaching rains, as defined by 75 mm in 3 d or 100 mm in 7 d by Kidder et al. (1992) before the first water sampling (Fig. 2). The high rainfall around planting during the first and the third years probably produced a descendant water flow moving NO3–N deeper into the soil profile. In contrast, precipitation during the second year was 45% below the average. Therefore, the crop was irrigated during the first month after planting, thus generating an ascendant movement of water by capillary action from the perched ground water to the root zone and keeping nitrates at the top layer of the soil profile. As a result, a relatively lower NO3–N concentration was measured in the perched ground water. In the second year, NO3–N concentrations in the perched ground water between planting and side-dressing N fertilizations under all rotations were less than 5 mg L–1 (Fig. 4
) compared with the same period in the first and third years (Fig. 3, 5
), when the highest NO3–N concentrations were close to 50 mg L–1. All treatments were equal because land use and management history up to the beginning of the study were the same. During the third year, control plots had lower NO3–N concentration than fertilized plots, except in the PCG plots, where N fertilization applied to green bean generated residual nitrate (Fig. 5). During the first and third years, the combined effect of high rainfall (Table 1) and the low uptake capacity of the potato root system early after planting resulted in higher leaching of NO3–N to the perched ground water than the second year when rainfall was relatively low.
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Fig. 4. Predicted and observed NO3–N concentrations in the perched ground water under four crop rotations and three N rates during 2002. PCF, potato-cowpea-fallow; PCG, potato-cowpea-green bean; PSF, potato-sorghum-fallow; PSG, potato-sorghum-green bean. Model significance: *p < 0.05; **p < 0.01; ***p < 0.0001; ns = not significant.
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Fig. 5. Predicted and observed NO3–N concentration in the perched ground water under four crop rotations and three N rates during 2003. PCF, potato-cowpea-fallow; PCG, potato-cowpea-green bean; PSF, potato-sorghum-fallow; PSG, potato-sorghum-green bean. Model significance: *p < 0.05; **p < 0.01; ***p < 0.0001; ns = not significant.
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From the concentration patterns described in this study, it is possible to conclude that most of the leached N came from the fertilizer application at planting. Also, it is possible that part of the side-dressed N applied at 40 d after planting could persist in the soil, increasing chances for potential leaching during peak summer rains. A preceding dry potato season may enhance the leaching potential. In fact, during the 3-yr study period, a decreased rainfall was recorded immediately after N side-dressing (Fig. 2). Therefore, depending on the rate of N applied, the side-dressed N could become a secondary source of NO3–N leaching after the potato season, as shown in the following section. This argument is supported by the steady decline in NO3–N concentration in the perched ground water even after the side-dressed fertilizer was applied (Fig. 3–5).
Concentrations of NO3–N in the Soil Solution under Potato
The NO3–N concentration in the soil solution followed a decreasing pattern from 8 d before N side-dressing until 30 to 40 d after N side-dressing when an increase in NO3–N concentration was detected (Fig. 6
). The increase in NO3–N concentration in the soil solution coincides with a period when the seepage irrigation was turned on (Fig. 2), suggesting dissolution of the side-dressed fertilizer up in the potato bed and/or ascendant nitrates as reported in literature (Patel et al., 2001). A concurrent but small increase was noted in the NO3–N concentration in the perched ground water (Fig. 4). These observations provide further support of the hypothesis that N applied at side-dressing could be a potential source for NO3–N leaching because the potato N uptake capacity decreases significantly at the end of the growing season (Voss, 1999) when seepage irrigation is turned off, producing a drop in the perched ground water level. Therefore, a high root zone concentration of NO3–N close to potato harvest leads to increased risk of NO3–N loss by leaching in summer (Fig. 7
).
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Fig. 6. Predicted and observed NO3–N concentrations in the soil solution under four crop rotations and three N rates during 2002. PCF, potato-cowpea-fallow; PCG, potato-cowpea-green bean; PSF, potato-sorghum-fallow; PSG, potato-sorghum-green bean. Model significance: *p < 0.05; **p < 0.01; ***p < 0.0001; ns = not significant.
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Fig. 7. Nitrate-nitrogen concentrations in the perched ground water under sorghum and cowpea at three N rates during 2003. Confidence intervals are at the 0.95 level of significance.
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The concentration of NO3–N in the soil solution under four crop rotations and three N rates during the second year was described by the quadratic regression model with coefficients of determination ranging from 0.47 to 0.89 (Fig. 6). Eight days before N side-dressing, values fluctuating between 156.0 (±8.63 SE) and 63.4 (±5.08 SE) mg L–1 of NO3–N were predicted for fertilized plots planted with PSG and PCG rotations (Fig. 6), and predicted values for the nonfertilized plots planted under the same rotations were 25.0 (±3.4 SE) and 40.4 (11.8 SE) mg L–1 of NO3–N, respectively. In the plots with crop rotations including a fallow-fall period (PSF and PCF), predicted values for fertilized plots ranged from 85.6 (±3.9 SE) to 27.1 (±3.4 SE) mg L–1 of NO3–N. Predicted values for nonfertilized plots with the same rotations were around 17.0 (±3.9 SE) mg L–1 of NO3–N (Fig. 6). Higher NO3–N concentration in plots with green bean in the fall than plots left fallow can be explained by an accumulative effect of residual N from green bean fertilization, residual N from potato side-dressing (Fig. 7), and N from biological fixation and/or mineralization of the soil incorporated green bean residues. A comparison of predicted NO3–N concentrations at first sampling in PSF and PSG under fertilization showed that the concentration in plots with green bean in fall was almost twice as high as the concentration in fallow plots. A similar trend was observed when PCF and PCG were compared.
Concentrations of NO3–N in the Perched Ground Water under Cowpea and Sorghum
In the summer of 2003, the observed NO3–N concentrations in the perched ground water under cowpea increased significantly when the potato N rate was increased from 168 to 280 kg ha–1 in each one of the three samplings. In contrast, the increase in NO3–N concentration in the perched ground water under sorghum was not significantly different among N rates for any of the three samplings (Fig. 7). The NO3–N concentration in the perched ground water under cowpea planted in plots where potato was fertilized with N at 280 kg ha–1 was significantly higher than the NO3–N concentration under plots fertilized with 0 or 168 kg ha–1. There was no difference between plots fertilized at 0 and 168 kg ha–1 of N (Fig. 7); however, sorghum was a better residual NO3–N "catch" crop than cowpea even at the highest N rate applied to potato in spring.
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Conclusions
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Fertilization at planting is the main source of the required N for a good potato production, but it was also shown to be a possible source of nitrate leaching. The high solubility of the N fertilizers typically used for potato production increases the risk of nitrate leaching from the potato beds. Even under low rainfall conditions, application of N at rates greater than recommended can result in nitrate leaching to the perched ground water later in the summer. Side-dressed N fertilization during dry potato seasons could also increase the risk of N leaching after potato harvest by elevating NO3–N concentration in the soil solution at the end of the potato season when potato uptake is declining. Therefore, side-dressed N application could be considered only as an alternative option to restore leached nitrate under high rainfall conditions. The better performance of sorghum compared with cowpea as a nitrogen scavenger suggests the use of sorghum as summer cover crop at least until other cover crops have been evaluated.
This research suggests that the unpredictability of growing season rainfall combined with easily leached NO3–N from soluble fertilizers makes simultaneously optimizing nitrate plant uptake and minimizing ground water pollution risks very difficult. Although much has been done to adjust fertilizer application to maximize yield, this approach does not seem to lead to effective pollution mitigation. Alternatives, such as the use of controlled-release fertilizers and summer cover crops with high scavenging capacity, could reduce the risk of nitrate leaching during and after the potato season by synchronizing the availability of N with plant demand and by recovering any residual N from the potato season, respectively. More needs to be done to manage soil nitrate concentrations immediately after the potato is harvested, and this research suggests that use of sorghum as a cover crop is one viable management option.
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NOTES
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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
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TOP
ABSTRACT
INTRODUCTION
